Burls, Natalie J., C Reason, P Penven, and S G H Philander, November 2011: Similarities between the tropical Atlantic seasonal cycle and ENSO: An energetics perspective. Journal of Geophysical Research: Oceans, 116, C11010, DOI:10.1029/2011JC007164. Abstract
The tropical Pacific and Atlantic Oceans have similar mean states - easterly winds and a zonally sloping thermocline which shoals in the east - but strikingly different sea surface temperature variability. Seasonal and interannual variations have comparable amplitudes, and correspond to different modes of oscillation in the Pacific. In the Atlantic, the seasonal cycle is dominant and has properties of both those modes. The Bjerknes feedback between the wind and SST, and its associated delayed, negative feedback constitute a free (interannual) mode of the Pacific, but in the Atlantic influence the seasonal cycle. This difference between the two oceans is attributable to the smaller dimensions of the Atlantic. An energetic analysis shows a circular relationship between available potential energy and the work done by the wind from April until September in the Atlantic. This energetics perspective suggests that a seasonally excited thermocline mode of coupled variability plays an active role in the seasonal cycle during these months, after which point seasonal forcing regains control.
Philander, S G., 2009: Where are you from? Why are you here? An African perspective on global warming. Annual Review of Earth and Planetary Sciences, 37, 1-18. Abstract
Global warming, although usually associated with imminent environmental disasters, also presents splendid opportunities for research and education and for collaboration between the rich and the poor that will benefit both groups. Unfortunately, efforts to take advantage of these opportunities are handicapped by the misperception that scientific disputes concerning imminent global climate changes have been settled—that the science “is over”—in addition to a failure to appreciate that some people face problems more urgent than adaptation to and mitigation of the climate changes scientists predict. Changes over the past few decades in the way we conduct our affairs in the Earth sciences, specifically the atmospheric and oceanic sciences, contribute to these misunderstandings. This is a subjective, personal account of those changes.
Barreiro, M, Alexey Fedorov, Ronald C Pacanowski, and S G H Philander, 2008: Abrupt climate changes: How freshening of the northern Atlantic affects the thermohaline & wind-driven oceanic circulations. Annual Review of Earth and Planetary Sciences, 36, 33-58. Abstract
Leading hypotheses for abrupt climate changes are focused on the ocean response to a freshening of surface waters in the north Atlantic. The degree to which such a freshening affects the deep, slow thermohaline, rather than the shallow, swift, wind-driven circulations of the ocean, and hence the degree to which that freshening affects climate in high rather than low latitudes, differ from model to model, depending on factors such as the treatment of diffusive processes in the oceans. Many comprehensive climate models are biased and confine the influence mainly to the thermohaline circulation and northern climates. Simulations of paleoclimates can provide valuable tests for the models, but only some of those climates provide sufficiently stringent tests to determine which models are realistic.
A decrease in cloud cover over higher latitudes—a decrease in the extratropical albedo—especially over the Southern Ocean, can result in an extratropical and tropical warming with the intensity of the equatorial cold tongues in the Pacific and Atlantic basins decreasing. These results, obtained by means of a coupled ocean–atmosphere model of intermediate complexity that allow the prescription of atmospheric cloud cover, are relevant to future global warming, and also to conditions during the Pliocene some 3 million years ago. The mechanisms responsible for the response of the tropics to changes in the extra-tropics include atmospheric and oceanic connections. This tropical adjustment can be interpreted from the constraint of a balanced heat budget for the ocean: A change in the albedo of the Southern Hemisphere causes the ocean to lose less heat there, so that it has to gain less heat in the tropics. As a consequence the cold tongues are reduced, particularly in the eastern Pacific where a decrease in the zonal tilt of the equatorial thermocline significantly weakens the east-west sea surface temperature gradient. The total adjustment time scale of the equatorial Pacific to the extratropical perturbation is of the order of interdecadal to centennial time scales, and thus represents a new mechanism of rapid climate change.
Fedorov, Alexey, M Barreiro, G Boccaletti, Ronald C Pacanowski, and S G H Philander, April 2007: The freshening of surface waters in high latitudes: Effects on the thermohaline and wind-driven circulations. Journal of Physical Oceanography, 37(4), DOI:10.1175/JPO3033.1. Abstract
The impacts of a freshening of surface waters in high latitudes on the deep, slow, thermohaline circulation have received enormous attention, especially the possibility of a shutdown in the meridional overturning that involves sinking of surface waters in the northern Atlantic Ocean. A recent study by Federov,et al. has drawn attention to the effects of a freshening on the other main component of the oceanic circulation—the swift, shallow, wind-driven circulation that varies on decadal time scales and is closely associated with the ventilated thermocline. That circulation, too, involves meridional overturning, but its variations and critical transitions affect mainly the Tropics. A surface freshening in mid- to high latitudes can deepen the equatorial thermocline to such a degree that temperatures along the equator become as warm in the eastern part of the basin as they are in the west, the tropical zonal sea surface temperature gradient virtually disappears, and permanently warm conditions prevail in the Tropics. In a model that has both the wind driven and thermohaline components of the circulation, which factors determine the relative effects of a freshening on the two components and its impact on climate? Studies with an idealized ocean general circulation model find that vertical diffusivity is one of the critical parameters that affect the relative strength of the two circulation components and hence their response to a freshening. The spatial structure of the freshening and imposed meridional temperature gradients are other important factors.
During the early and mid-Pliocene, the period from 5 to 3 million years ago, approximately, the Earth is believed to have been significantly warmer than it is today, but the reasons for the higher temperatures are unclear. This paper explores the impact of recent findings that suggest that, at that time, cold surface waters were absent from the tropical and subtropical oceanic upwelling zones. El Niño was in effect a perennial rather than intermittent phenomenon, and sea surface temperatures in low latitudes were essentially independent of longitude. When these conditions are specified as the lower boundary condition for an atmospheric GCM, we find that the trade winds along the equator, and hence the Walker Circulation, collapse. The low-level stratus clouds in low latitudes diminish greatly, thus reducing the albedo of the Earth. The atmospheric concentration of water vapor increases, and enhanced latent heat release due to stronger evaporation warms up the tropical atmosphere, particularly between 40°S and 20°N. Moreover, teleconnection patterns from the Pacific induce a warming over North America that is enhanced by surface albedo feedback, a process that may have helped to maintain this region ice-free before 3 Ma. The results presented here indicate that the suggested absence of cold surface waters from the tropical and subtropical oceanic upwelling zones could have contributed significantly to the Pliocene warmth
During the early Pliocene, 5 to 3 million years ago, globally averaged temperatures were substantially higher than they are today, even though the external factors that determine climate were essentially the same. In the tropics, El Niño was continual (or "permanent") rather than intermittent. The appearance of northern continental glaciers, and of cold surface waters in oceanic upwelling zones in low latitudes (both coastal and equatorial), signaled the termination of those warm climate conditions and the end of permanent El Niño. This led to the amplification of obliquity (but not precession) cycles in equatorial sea surface temperatures and in global ice volume, with the former leading the latter by several thousand years. A possible explanation is that the gradual shoaling of the oceanic thermocline reached a threshold around 3 million years ago, when the winds started bringing cold waters to the surface in low latitudes. This introduced feedbacks involving ocean-atmosphere interactions that, along with ice-albedo feedbacks, amplified obliquity cycles. A future melting of glaciers, changes in the hydrological cycle, and a deepening of the thermocline could restore the warm conditions of the early Pliocene.
Philander, S G., 2006: Sextant to Satellite: The education of a land-based oceanographer In Physical Oceanography: Developments Since 1950, Jochum, M., and R. Murtugudde, eds., New York, Springer, 153-163.
Can atmospheric forcing of the ocean in high latitudes induce decadal variability in low latitudes? Most theoretical studies that have considered this question assign a critical role to adiabatic, advective, subsurface oceanic links between the tropics and extra-tropics. Observational evidence of such links is proving elusive. This study posits that given the constraint of a balanced heat budget for the ocean in a state of equilibrium, atmospheric forcing over a broad spectrum of frequencies in high latitudes can force decadal variability in low latitudes without any explicit evidence of oceanic links. The oceanic response to an abrupt change in diabatic forcing, a sudden increase in heat loss in high latitudes say, is characterized by two time-scales. The one, tw, is relatively short and is associated with planetary and coastal waves that propagate from the disturbed region to the equator (and then back to higher latitudes.) The other, td, is on the order of a few years and depends on diabatic processes responsible for increasing the oceanic heat gain in low latitudes. Through these processes the system is driven towards a new balanced heat budget in which the heat gain, mainly in the equatorial upwelling zones, equals the heat loss in high latitudes. When the forcing, rather than abrupt, is sinusoidal with period P, then the amplitude of the response depends on the ratio P/td. The response is modest when that ratio is small because the period P is too short for the ocean to adjust. As P gets larger compared to td, the amplitude increases, but explicit evidence of the waves that connect high and low latitudes is very hard to detect. The ocean acts as a low pass filter to the forcing with characteristic timescale td.
The salient feature of the oceanic thermal structure is a remarkably shallow thermocline, especially in the Tropics and subtropics. What factors determine its depth? Theories for the deep thermohaline circulation provide an answer that depends on oceanic diffusivity, but they deny the surface winds an explicit role. Theories for the shallow ventilated thermocline take into account the influence of the wind explicitly, but only if the thermal structure in the absence of any winds, the thermal structure along the eastern boundary, is given. To complete and marry the existing theories for the oceanic thermal structure, this paper invokes the constraint of a balanced heat budget for the ocean. The oceanic heat gain occurs primarily in the upwelling zones of the Tropics and subtropics and depends strongly on oceanic conditions, specifically the depth of the thermocline. The heat gain is large when the thermocline is shallow but is small when the thermocline is deep. The constraint of a balanced heat budget therefore implies that an increase in heat loss in high latitudes can result in a shoaling of the tropical thermocline; a decrease in heat loss can cause a deepening of the thermocline. Calculations with an idealized general circulation model of the ocean confirm these inferences. Arguments based on a balanced heat budget yield an expression for the depth of the thermocline in terms of parameters such as the imposed surface winds, the surface temperature gradient, and the oceanic diffusivity. These arguments in effect bridge the theories for the ventilated thermocline and the thermohaline circulation so that previous scaling arguments are recovered as special cases of a general result.
Studies of the effect of a freshening of the surface waters in high latitudes on the oceanic circulation have thus far focused almost entirely on the deep thermohaline circulation and its poleward heat transport. Here it is demonstrated, by means of an idealized general circulation model, that a similar freshening can also affect the shallow, wind-driven circulation of the ventilated thermocline and its heat transport from regions of gain (mainly in the upwelling zones of low latitudes) to regions of loss in higher latitudes. A freshening that decreases the surface density gradient between low and high latitudes reduces this poleward heat transport, thus forcing the ocean to gain less heat in order to maintain a balanced heat budget. The result is a deepening of the equatorial thermocline. (The deeper the thermocline in equatorial upwelling zones is, the less heat the ocean gains.) For a sufficiently strong freshwater forcing, the poleward heat transport all but vanishes, and permanently warm conditions prevail in the Tropics. The approach to warm oceanic conditions is shown to introduce a bifurcation mechanism for the north–south asymmetry of the thermal and salinity structure of the upper ocean.
Philander, S G., 2004: Our Affair with El Niño: How We Transformed an Enchanting Peruvian Current into a Global Climate Hazzard, Princeton, NJ: Princeton University Press, 296 pp.
Nobody anticipated that El Niño would be weak and prolonged in 1992, but brief and intense in 1997/98. Why are various El Niño episodes so different, and so difficult to predict? The answer involves the important role played by random atmospheric disturbances (such as westerly wind bursts) in sustaining the weakly damped Southern Oscillation, whose complementary warm and cold phases are, respectively, El Niño and La Niña. As in the case of a damped pendulum sustained by modest blows at random times, so the predictability of El Niño is limited, not by the amplification of errors in initial conditions as in the case of weather, but mainly by atmospheric disturbances interacting with the Southern Oscillation. Given the statistics of the wind fluctuations, the probability distribution function of future sea surface temperature fluctuations in the eastern equatorial Pacific can be determined by means of an ensemble of calculations with a coupled ocean–atmosphere model. Each member of the ensemble starts from the same initial conditions and has, superimposed, a different realization of the noise. Such a prediction, made at the end of 1996, would have assigned a higher likelihood to a moderate event than to the extremely strong event that actually occurred in 1997. (The rapid succession of several westerly wind bursts in early 1997 was a relatively rare phenomenon.) In late 2001, conditions were similar to those in 1996, which suggested a relatively high probability of El Niño appearing in 2002. Whether the event will be weak or intense depends on the random disturbances that materialize during the year.
Philander, S G., 2003: El Niño: A predictable climate fluctuation In Global Climate, Rodó, X., and F. A. Comín, eds., Berlin, Springer-Verlag, 34-40.
Philander, S G., 2003: Of dipoles and spherical cows. Bulletin of the American Meteorological Society, 84(10), 1424. PDF
Philander, S G., 2003: Why global warming is a controversial issue In Global Climate, Rodó, X., and F. A. Comín, eds., Berlin, Springer-Verlag, 25-33.
Philander, S G., and Alexey Fedorov, 2003: Is El Niño sporadic or cyclic?Annual Review of Earth and Planetary Sciences, 31, 579-594. Abstract
Is El Niño one phase of a continual, self-sustaining natural mode of the coupled ocean-atmosphere that has La Niña as the complementary phase? Or is El Niño a temporary departure from "normal" conditions "triggered" by a random disturbance such as a burst of westerly winds? A growing body of evidence—stability analyses, studies of the energetics, simulations that reproduce the statistics of sea surface temperature variations in the eastern equatorial Pacific—indicates that reality corresponds to a compromise between these two possibilities: The observed Southern Oscillation between El Niño and La Niña corresponds to a weakly damped mode that is sustained by random disturbances. This means that the predictability of El Niño is limited by the continual presence of "noise" so that forecasts should be probabilistic. The Southern Oscillation is also subject to decadal modulations. How it will be influenced by global warming is a matter of considerable uncertainty.
Philander, S G., and Alexey Fedorov, 2003: Role of tropics in changing the response to Milankovich forcing some three million years ago. Paleoceanography, 18(2), 1045, DOI:10.1029/2002PA000837. Abstract
Throughout the Cenozoic the Earth experienced global cooling that led to the appearance of continental glaciers in high northern latitudes around 3 Ma ago. At approximately the same time, cold surface waters first appeared in regions that today have intense oceanic upwelling: the eastern equatorial Pacific and the coastal zones of southwestern Africa and California. There was furthermore a significant change in the Earth's response to Milankovich forcing: obliquity signals became large, but those associated with precession and eccentricity remained the same. The latter change in the Earth's response can be explained by hypothesizing that the global cooling during the Cenozoic affected the thermal structure of the ocean; it caused a gradual shoaling of the thermocline. Around 3 Ma the thermocline was sufficiently shallow for the winds to bring cold water from below the thermocline to the surface in certain upwelling regions. This brought into play feedbacks involving ocean-atmosphere interactions of the type associated with El Niño and also mechanisms by which high-latitude surface conditions can influence the depth of the tropical thermocline. Those feedbacks and mechanisms can account for the amplification of the Earth's response to periodic variations in obliquity (at a period of 41K) without altering the response to Milankovich forcing at periods of 100,000 and 23,000 years. This hypothesis is testable. If correct, then in the tropics and subtropics the response to obliquity variations is in phase with, and corresponds to, El Niño conditions when tilt is large and La Niña conditions when tilt is small.
Fedorov, Alexey, and S G H Philander, 2001: A stability analysis of tropical ocean-atmosphere interactions: Bridging measurements and theory for El Niño. Journal of Climate, 14(14), 3086-3101. Abstract PDF
Interactions between the tropical oceans and atmosphere permit a spectrum of natural modes of oscillation whose properties—period, intensity, spatial structure, and direction of propagation—depend on the background climatic state (i.e., the mean state). This mean state can be described by parameters that include the following: the time-averaged intensity J of the Pacific trade winds, the mean depth (H) of the thermocline, and the temperature difference across the thermocline ()I). A stability analysis by means of a simple coupled ocean-atmosphere model indicates two distinct families of unstable modes. One has long periods of several years, involves sea surface temperature variations determined by vertical movements of the thermocline that are part of the adjustment of the ocean basin to the fluctuating winds, requires a relatively deep thermocline, and corresponds to the delayed oscillator. The other family requires a shallow thermocline, has short periods of a year or two, has sea surface temperature variations determined by advection and by entrainment across the thermocline, and is associated with westward phase propagation. For the modes to be unstable, both families require that the background zonal wind exceed a certain intensity. An increase in )I , and in H beyond a certain value, are stabilizing. For intermediate values of H, between large values that favor the one mode and small values that favor the other, the modes are of a hybrid type with some properties of each family. The observed Southern Oscillation has been of this type for the past few decades, but some paleorecords suggest that, in the distant past, the oscillation was strictly of the delayed oscillator type and had a very long period on the order of a decade.
Liu, Zhengyu, and S G H Philander, 2001: Tropical-extratropical oceanic exchange pathways In Ocean Circulation and Climate: Observing and Modelling the Global Ocean, San Diego, CA, Academic Press, 247-257.
Philander, S G., 2001: Why global warming is controversial. Science, 294(5549), 2105-2106. PDF
Philander, S G., 2001: Atlantic Ocean Equatorial Currents In Encyclopedia of Ocean Sciences, 188-191. PDF
Philander, S G., 2001: El Nino Southern Oscillation (ENSO) Models In Encyclopedia of Ocean Sciences, 827-832. PDF
Recent advances in observational and theoretical studies of El Niño have shed light on controversies concerning the possible effect of global warming on this phenomenon over the past few decades and in the future. El Niño is now understood to be one phase of a natural mode of oscillation—La Niña is the complementary phase—that results from unstable interactions between the tropical Pacific Ocean and the atmosphere. Random disturbances maintain this neutrally stable mode, whose properties depend on the background (time-averaged) climate state. Apparent changes in the properties of El Niño could reflect the importance of random disturbances, but they could also be a consequence of decadal variations of the background state. The possibility that global warming is affecting those variations cannot be excluded.
Goddard, L M., and S G H Philander, 2000: The energetics of El Niño and La Niña. Journal of Climate, 13(9), 1496-1516. Abstract PDF
Data from a realistic model of the ocean, forced with observed atmospheric conditions for the period 1953-92, are analyzed to determine the energetics of interannual variability in the tropical Pacific. The work done by the winds on the ocean, rather than generating kinetic energy, does work against pressure gradients and generates buoyancy power, which in turn is responsible for the rate of change of available potential energy (APE). This means interannual fluctuations in work done by the wind have a phase that leads variations in APE. Variations in the sea surface temperature (SST) of the eastern equatorial Pacific and in APE are highly correlated and in phase so that changes in the work done by the wind are precursors of El Niño. The wind does positive work on the ocean during the half cycle that starts with the peak of El Niño and continues into La Niña; it does negative work during the remaining half cycle.
The results corroborate the delayed oscillator mechanism that qualitatively describes the deterministic behavior of ENSO. In that paradigm, a thermocline perturbation appearing in the western equatorial Pacific affects the transition from one phase of ENSO to the next when the perturbation arrives in the eastern equatorial Pacific where it influences SST. The analysis of energetics indicates that the transition starts earlier, during La Niña, when the perturbation is still in the far western equatorial Pacific. Although the perturbation at that stage affects the thermal structure mainly in the thermocline, at depth, the associated currents are manifest at the surface and immediately affect work done by the wind. For the simulation presented here, the change in energy resulting from adjustment processes far outweighs that due to stochastic processes, such as intraseasonal wind bursts, at least during periods of successive El Niño and La Niña events.
Bush, A B., and S G H Philander, 1999: The climate of the last glacial maximum: results from a coupled atmosphere-ocean general circulation model. Journal of Geophysical Research, 104(D20), 24,509-24,525. Abstract PDF
Results from a coupled atmosphere-ocean general circulation model simulation of the Last Glacial Maximum reveal annual mean continental cooling between 4º and 7°C over tropical landmasses, up to 26° of cooling over the Laurentide ice sheet, and a global mean temperature depression of 4.3°C. The simulation incorporates glacial ice sheets, glacial land surface, reduced sea level, 21 ka orbital parameters, and decreased atmospheric CO2. Glacial winds, in addition to exhibiting anticyclonic circulations over the ice sheets themselves, show a strong cyclonic circulation over the northwest Atlantic basin, enhanced easterly flow over the tropical Pacific, and enhanced westerly flow over the Indian Ocean. Changes in equatorial winds are congruous with a westward shift in tropical convection, which leaves the western Pacific much drier than today but the Indonesian archipelago much wetter. Global mean specific humidity in the glacial climate is 10% less than today. Stronger Pacific easterlies increase the tilt of the tropical thermocline, increase the speed of the Equatorial Undercurrent, and increase the westward extent of the cold tongue, thereby depressing glacial sea surface temperatures in the western tropical Pacific by ~5°-6°C.
Gu, D, and S G H Philander, 1999: A theory for interdecadal climate fluctuations In Beyond El Niño : Decadal and Interdecadal Climate Variability, Berlin, Germany, Springer-Verlag, 301-308.
Masina, S, and S G H Philander, 1999: An analysis of tropical instability waves in a numerical model of the Pacific Ocean 1. Spatial variability of the waves. Journal of Geophysical Research, 104(C12), 29,613-29,635. Abstract PDF
Unstable oscillations confined within the mixed layer close to the equator are generated in wind-forced experiments performed in a multilevel general circulation model configured for the tropical Pacific Ocean. The experiments indicate that the waves develop preferentially in the eastern Pacific along the northern temperature front. However, there is clear evidence of a second unstable region along the southern temperature front in the central Pacific. In both regions, the instabilities propogate westward, but in the central Pacific their phase speed is considerably smaller. The differences between the wave characteristics in the eastern and central Pacific are closely correlated to the differences in the time mean conditions of the flow. The eastern instabilities have a structure with two peaks in amplitude: one located on the equator and the other a few degrees north of it. Their dispersion characteristics show many similarities to those of tropical instability waves (TIWs) observed in the Pacific Ocean, while the instabilities which grow in the central Pacific do not have any known observed correspondents. We explore the spatial variability of the simulated waves through a wavelet analysis, which provides detailed results on how the period and wavelength of the instabilities change as a function of longitude, latitude, and depth. The wavelet analysis reveals that in the eastern Pacific and close to the surface the TIWs have a phase speed of -48 cm/s, while in the central Pacific they have a phase speed of -11 cm/s. In particular, the change in the phase speed is due to a change in the dominant period of the TIWs: The period of the central Pacific instabilities is considerably longer than the period of the instabilities present in the eastern Pacific.
Masina, S, S G H Philander, and A B Bush, 1999: An analysis of tropical instability waves in a numerical model of the Pacific Ocean 2. Generation and energetics of the waves. Journal of Geophysical Research, 104(C12), 29,637-29,661. Abstract PDF
The instability processes which generate unstable waves with characteristics similar to observed tropical instability waves in the Pacific Ocean are examined through a local energy analysis based on deviations from the time mean flow. Numerical experiments indicate that the waves develop preferentially in the eastern Pacific along the northern temperature front and have a westward phase speed and a structure with two peaks in amplitude: one located on the equator and the other a few degrees north of it. The energy analysis shows that the "two-peak" structure of the eastern waves is explained by two different instability processes which occur at different latitudes. In the time mean sense the region north of the equator is baroclinically unstable, while barotropic instability prevails at the equator. The life cycle of the waves is revealed by the time evolution of the energetics. Baroclinic instability is the dominant triggering mechanism which induces growth of the waves along the northern temperature front. The eddy pressure fluxes radiate energy south of the equator where the meridional shear between the Equatorial Undercurrent and the South Equatorial Current becomes barotropically unstable. From the numerical simulations, there is evidence of a second unstable region in the central Pacific south of the equator where the instabilities have a lower phase speed. The energy analysis also shows that these waves grow from both barotropic and baroclinic conversions.
El Niño is so versatile and ubiquitous -- he causes torrential rains in Peru and Ecuador, droughts and fires in Indonesia, and abnormal weather globally -- that the term is now part of everyone's vocabulary; it designates a mischievous gremlin. Hence, if the stock market in New York is erratic, or the traffic jams in London are exceptionally bad, it must be El Niño. This is consistent with our practice of using meteorological phenomena as metaphors in our daily speech: the president is under a cloud, the examination was a breeze. We know exactly what these statements mean because we have a life-long familiarity with clouds and breezes.
Philander, S G., 1999: A review of tropical ocean-atmosphere interactions. Tellus, 51A-B(1), 71-90. Abstract PDF
Enormous strides have been made towards the goal of operational predictions of seasonal and interannual climate fluctuations, especially as regards the phenomenon El Niño. To initialize models, measurements are available from an impressive array of instruments that monitor the tropical Pacific continually; coupled general circulation models of the ocean and atmosphere are already capable of reproducing many aspects of the earth's climate, its seasonal cycle, and the Southern Oscillation. These achievements crown the studies, over the past few decades, that describe, explain and simulate the atmospheric response to sea-surface temperature variations, the oceanic response to different types of wind fluctuations, and the broad spectrum of coupled ocean-atmosphere modes that results from interations between the two media. Those modes, which are involved not only in the Southern Oscillation but also in the seasonal cycle and the climatology, differ primarily as regards the main mechanisms that determine sea-surface temperature variations in the central and eastern tropical Pacific: advection by surface currents, and vertical movements of the thermocline induced by either local winds or, in the case of the delayed oscillator mode, by non-local winds. The observed Southern Oscillation appears to be a hybrid mode that changes from one episode to the next so that El Niño can evolve in a variety of ways -- advection and nonlocally generated thermocline displacements are important to different degrees on different occasions. The extent to which random disturbances, such as westerly wind bursts over the western equatorial Pacific, influence El Niño depends on whether the southern oscillation is self-sustaining or damped. Attention is now turning to the factors that determine this aspect of the Southern Oscillation, its decadal modulation whih causes it to be more energetic in some decades than others. Those factors include interactions between the tropics and extratropics that affect the mean depth of the thermocline, and the intensity of the climatological trade winds.
Bush, A B., and S G H Philander, 1998: The role of ocean-atmosphere interactions in tropical cooling during the Last Glacial Maximum. Science, 279(5355), 1341-1344. Abstract PDF
A simulation with a coupled atmosphere-ocean general circulation model configured for the Last Glacial Maximum delivered a tropical climate that is much cooler than that produced by atmosphere-only models. The main reason is a decrease in tropical sea surface temperatures, up to 6°C in the western tropical Pacific, which occurs because of two processes. The trade winds induce equatorial upwelling and zonal advection of cold water that further intensify the trade winds, and an exchange of water occurs between the tropical and extratropical Pacific in which the poleward surface flow is balanced by equatorward flow of cold water in the thermocline. Simulated tropical temperature depressions are of the same magnitude as those that have been proposed from recent proxy data.
Delecluse, P, M K Davey, Y Kitamura, S G H Philander, M J Suarez, and L Bengtsson, 1998: Coupled general circulation modeling of the tropical Pacific. Journal of Geophysical Research, 103(C7), 14,357-14,373. Abstract PDF
During the Tropical Ocean-Global Atmosphere (TOGA) program substantial progress was made in the development of coupled general circulation models with regard to representation of the tropical mean state and climate variability. This paper provides a review of the main developments, focusing on the tropical Pacific region. Early coupled general circulation models were relatively crude; with coarse resolution and limited physical parameterizations and poor surface fluxes, the model drift from the observed mean state was often substantial, and their use for investigating climate variability such as El Niño-Southern Oscillation (ENSO) was limited. Improvements in resolution and parameterizations led to rapid progress. Through the TOGA program it has been possible to assess coupled model mean states and variability against high-quality observations, particularly for the tropical Pacific. Both components of the coupled system (ocean and atmosphere) have benefited from an improved understanding of the physics. Coupled experiments have revealed deficiencies in each model component that were concealed in separate forced runs. Several general systematic errors have yet to be eliminated, especially in the east Pacific. The variety of behavior obtained with coupled models provides evidence that more than one mechanism is active in the generation and evolution of ENSO events. There are also indications that interannual variability is linked to the mean structure of the equatorial Pacific.
Philander, S G., 1998: Is the Temperature Rising: The Uncertain Science of Global Warming, Princeton, NJ: Princeton University Press, 240 pp.
Philander, S G., 1998: Learning from El Niño. Weather, 53(9), 270-274.
Gu, D, and S G H Philander, 1997: Interdecadal climate fluctuations that depend on exchanges between the tropics and extratropics. Science, 275(5301), 805-807. Abstract PDF
The unexpected and prolonged persistence of warm conditions over the tropical Pacific during the early 1990s can be attributed to an interdecadal climate fluctuation that involves changes in the properties of the equatorial thermocline arising as a result of an influx of water with anomalous temperatures from higher latitudes. The influx affects equatorial sea-surface temperatures and hence the tropical and extratropical winds that in turn affect the influx. A simple model demonstrates that these processes can give rise to continual interdecadal oscillations.
Gu, D, S G H Philander, and Michael J McPhaden, 1997: The seasonal cycle and its modulation in the Eastern Tropical Pacific Ocean. Journal of Physical Oceanography, 27(10), 2209-2218. Abstract PDF
Data for the period from 1985 to 1993 from TAO moorings along 110°W (5° S-5°N) and 140°W (2°S - 9°N) describe the vertical, meridional, and temporal structure of the seasonal cycle of several variables. The results have a number of interesting features. The amplitude of the seasonal cycle is relatively constant in the surface layers but varies considerably at the depth of the equatorial thermocline where it was small before 1989, large thereafter. Also, vertical seasonal movements of the thermocline have little effect on sea surface temperatures. These seasonal variations are consistent with a westward propagating coupled ocean-atmosphere mode in the surface layers. Conversely, the low-frequency modulation of the seasonal cycle in the thermocline is associated with changes in the seasonal cycle of the zonal wind in the central and western tropical Pacific and might be attributable to equatorial Kelvin waves forced resonantly by the surface winds.
Li, Tim, and S G H Philander, 1997: On the seasonal cycle of the equatorial Atlantic Ocean. Journal of Climate, 10(4), 813-817. Abstract PDF
Although the seasonal cycle of the equatorial Atlantic and Pacific Oceans have many similarities, for example, an annual signal is dominant at the equator even though the sun "crosses" the equator twice a year, different processes determine the seasonal cycles of the two oceans and in the Atlantic different processes are important in the east and west. In the Gulf of Guinea in the eastern equatorial Atlantic, the seasonal cycle of surface winds is primarily in response to seasonal variations in land temperatures so that annual changes in sea surface temperatures are, to a first approximation, the passive response of the ocean to the winds. The seasonal cycle of the western equatorial Atlantic has similarities with that of the equatorial Pacific--both are strongly influenced by ocean-atmosphere interactions in which the surface winds and sea surface temperature patterns depend on each other--but only in the western equatorial Atlantic are the seasonal variations in sea surface temperature influenced by vertical excursions of the thermocline. These results are obtained by means of a general circulation model of the atmosphere and a relatively simple coupled ocean-atmosphere model.
Philander, S G., 1997: Review of "Currents of Change: El Niño's Impact on Climate and Society". Nature, 385(6611), 194. PDF
Li, Tim, and S G H Philander, 1996: On the annual cycle of the eastern equatorial Pacific. Journal of Climate, 9(12), 2986-2998. Abstract PDF
Although the sun "crosses" the equator twice a year, the eastern equatorial Pacific has a pronounced annual cycle, in sea surface temperature and in both components of the surface winds for example. (This is in contrast to the Indian Ocean and western Pacific where a semiannual oscillation of the zonal wind is the dominant signal on the equator.) Calculations with a relatively simple coupled ocean-atmosphere model indicate that the principal reason for this phenomenon is the marked asymmetry, relative to the equator, of the time-averaged climatic conditions in the eastern tropical Pacific. The important asymmetries are in surface winds, oceanic currents, and sea surface temperature: The time-averaged winds and currents have northward components at the equator and the warmest waters are north of the equator. Because of those asymmetries, seasonally varying solar radiation that is strictly antisymmetric relative to the equator can force a response that has a symmetric component. The amplitude of the resultant annual cycle at the equator depends on interactions between the ocean and atmosphere, and on positive feedbacks that involve low-level stratus clouds that form over cold surface waters.
Philander, S G., 1996: El Niño, La Niña, and the Southern Oscillation In Encyclopedia of Climate and Weather, Vol. 1, New York, Oxford University Press, 273-277.
Although the distribution of sunshine is symmetrical about the equator, the earth's climate is not. Climatic asymmetries are prominent in the eastern tropical Pacific and Atlantic Oceans where the regions of maximum sea surface temperature, convective cloud cover, and rainfall are north of the equator. This is the result of two sets of factors: interactions between the ocean and atmosphere that are capable of converting symmetry into asymmetry, and the geometries of the continents that determine in which longitudes the interactions are effective and in which hemisphere the warmest waters and the intertropical convergence zone are located. The ocean-atmosphere interactions are most effective where the thermocline is shallow because the winds can readily affect sea surface temperatures in such regions. The thermocline happens to shoal in the eastern equatorial Pacific and Atlantic, but not in the eastern Indian Ocean, because easterly trade winds prevail over the tropical Atlantic and Pacific whereas monsoons, with a far larger meridional component, are dominant over the Indian Ocean. That is how the global distribution of the continents, by determining the large-scale wind patterns, causes climatic asymmetries to be prominent in some bands of longitude but not others. The explanation for asymmetries that favor the Northern rather than Southern Hemisphere with the warmest waters and the ITCZ involves the details of the local coastal geometries: the bulge of western Africa to the north of the Gulf of Guinea and the slope of the western coast of the Americas relative to meridians. Low-level stratus clouds over cold waters are crucial to the maintenance of the asymmetries.
Gu, D, and S G H Philander, 1995: Secular changes of annual and interannual variability in the tropics during the past century. Journal of Climate, 8(4), 864-876. Abstract PDF
Wavelet transforms, which can unfold signals in both time and frequency domains, are used to analyze the Comprehensive Ocean and Atmospheric Data Sets for the period 1870-1988. The focus is on secular changes in the interannual variability and the annual cycle of selected equatorial regions. The amplitude of El Niño/Southern Oscillation (ENSO) is found to be large from 1885-1915, to be small during the period 1915-1950, and to increase rapidly after about 1960. Surprisingly, the decadal variations in the amplitude of ENSO are not matched by similar decadal variations in the amplitude of the annual cycle. Wavelet transforms, which can unfold signals in both time and frequency domains, are used to analyze the Comprehensive Ocean and Atmospheric Data Sets for the period 1870-1988. The focus is on secular changes in the interannual variability and the annual cycle of selected equatorial regions. The amplitude of El Niño/Southern Oscillation (ENSO) is found to be large from 1885-1915, to be small during the period 1915-1950, and to increase rapidly after about 1960. Surprisingly, the decadal variations in the amplitude of ENSO are not matched by similar decadal variations in the amplitude of the annual cycle. On short timescales of 2-5 years, ENSO stronly influences the annual cycle in certain parts of the central and eastern tropical Pacific where the thermocline is shallow. The annual cycle is weak in warm El Niño years and is strong in cold La Niña years. This result suggests that the amplitude of the seasonal cycle is affected by interannual variations in the depth of the thermocline and in the intensity of the trade winds.
Liu, Zhengyu, and S G H Philander, 1995: How different wind stress patterns affect the tropical-subtropical circulations of the upper ocean. Journal of Physical Oceanography, 25(4), 449-462. Abstract PDF
An oceanic GCM is used to investigate the response of the tropical and subtropical thermocline circulation and structure to different wind stress patterns. Although the subtropical winds do not affect the transport or the speed of the Equatorial Undercurrent significantly, they do change the equatorial temperature field in the lower part of the equatorial thermocline significantly. A weaker subtropical wind curl causes a cooling of the subsurface equatorial region and, hence, an intensification of the equatorial thermocline. A weakening of the subtropical wind curl by a factor of 2 cools the equatorial lower thermocline water by 2°C.
Mechoso, C R., and S G H Philander, et al., 1995: The seasonal cycle over the tropical pacific in coupled ocean-atmosphere general circulation models. Monthly Weather Review, 123(9), 2825-2838. Abstract PDF
The seasonal cycle over the tropical Pacific simulated by 11 coupled ocean-atmosphere general circulation models (GCMs) is examined. Each model consists of a high-resolution ocean GCM of either the tropical Pacific or near-global oceans coupled to a moderate- or high-resolution atmospheric GCM, without the use of flux correction. The seasonal behavior of sea surface temperature (SST) and eastern Pacific rainfall is presented for each model. The results show that current state-of-the-art coupled GCMs share important successes and troublesome systematic errors. All 11 models are able to simulate the mean zonal gradient in SST at the equator over the central Pacific. The simulated equatorial cold tongue generally tends to be too strong, too narrow, and extend too far west. SSTs are generally too warm in a broad region west of Peru and in a band near 10°S. This is accompanied in some models by a double intertropical convergence zone (ITCZ) straddling the equator over the eastern Pacific, and in others by an ITCZ that migrates across the equator with the seasons; neither behavior is realistic. There is considerable spread in the simulated seasonal cycles of equatorial SST in the eastern Pacific. Some simulations do capture the annual harmonic quite realistically, although the seasonal cold tongue tends to appear prematurely. Others overestimate the amplitude of the semiannual harmonic. Nonetheless, the results constitute a marked improvement over the simulations of only a few years ago when serious climate drift was still widespread and simulated zonal gradients of SST along the equator were often very weak.
Philander, S G., 1995: Commentary on the paper of Cane et al In Natural Climate Variability on Decade-to-Century Time Scales, Washington, DC, National Academy Press, 456.
Philander, S G., 1995: Comments on "Global Climate Change and Tropical Cyclones" by J. Lighthill, et al. Bulletin of the American Meteorological Society, 76(3), 380.
Chang, Ping, and S G H Philander, 1994: A coupled ocean-atmosphere instability of relevance to the seasonal cycle. Journal of the Atmospheric Sciences, 51(24), 3627-3648. Abstract PDF
Recent observational studies have suggested that interactions between the atmosphere and the ocean play an important role in the pronounced annual cycle of the eastern equatorial Pacific and Atlantic Oceans. The key to this atmosphere-ocean interaction is a positive feedback between the surface winds and the local SST gradients in the cold tongue/ITCZ complex regions, which leads to an instability in the coupled system. By means of linear instability analyses and numerical model experiments, such an instability mechanism is explored in a simple coupled ocean-atmosphere system. The instability analysis yields a family of antisymmetric and symmetric unstable SST modes. The antisymmetric mode has the most rapid growth rate. The most unstable anti-symmetric mode occurs at zero wavenumber and has zero frequency. The symmetric SST mode, although its growth rate is smaller, has a structure at annual period that appears to resemble the observed westward propagating feature in the annual cycle of near-equatorial zonal wind and SST. Unlike the ENSO type of coupled unstable modes, the modes of relevance to the seasonal cycle do not involve changes in the thermocline depth. The growth rates of these modes are linearly proportional to the mean vertical temperature gradient and inversely proportional to the depth of mean thermocline in the ocean. Because of the shallow thermocline and strong subsurface thermal gradients in the eastern Pacific and Atlantic Oceans, these coupled unstable modes strongly influence the seasonal cycles of those regions. On the basis of theoretical analyses and the observational evidence, it is suggested that the antisymmetric SST mode many be instrumental in rapidly reestablishing the cold tongues in the eastern Pacific and Atlantic Oceans during the Northern Hemisphere summer, whereas the symmetric SST mode contributes to the westward propagating feature in the annual cycle of near-equatorial zonal winds and SST.
Koberle, C, and S G H Philander, 1994: On the processes that control seasonal variations of sea surface temperatures in the tropical Pacific Ocean. Tellus A, 46A, 481-496. Abstract PDF
In the tropical Pacific Ocean, the cold phase of both interannual and seasonal sea surface temperature variations is characterized by cold waters off the coast of South America and a pronounced equatorial tongue of cold surface waters in the eastern Pacific. The warm phase, in both cases, is marked by the weakening or complete absence of these features. Despite these striking similarities, very different physical processes are dominant on seasonal and interannual time scales. Interannually, a horizontal redistribution of warm surface waters, the dynamical response of the ocean to changes in the winds, is of primary importance. What matters most seasonally are two local processes: seasonal upwelling associated with a divergence of surface currents, and the seasonal modulation of mixing processes, by heat fluxes, that control to what extent upwelling induced by the mean winds influences sea surface temperatures. These results shed light on the different requirements that coupled ocean-atmosphere models should meet if they are to reproduce both seasonal and interannual variability. The results also make a case for measurements, along a meridian in the eastern tropical Pacific, that focus on the relations between sea surface temperature changes, heat flux variations and mixing processes.
Liu, Zhengyu, S G H Philander, and Ronald C Pacanowski, 1994: A GCM study of tropical-subtropical upper-ocean water exchange. Journal of Physical Oceanography, 24(12), 2606-2623. Abstract PDF
Experiments with an oceanic general circulation model indicate that the tropical and subtropical oceanic circulations are linked in three ways. Far from coasts in the oceanic interior, equatorial surface waters flow poleward to the southern part of the subtropical gyre, and then are subducted and returned in the thermocline to the upper part of the core of the Equatorial Undercurrent. Experiments with an oceanic general circulation model indicate that the tropical and subtropical oceanic circulations are linked in three ways. Far from coasts in the oceanic interior, equatorial surface waters flow poleward to the southern part of the subtropical gyre, and then are subducted and returned in the thermocline to the upper part of the core of the Equatorial Undercurrent. There is, in addition, a surface western boundary current that carries waters from the equatorial region to the northern part of the subtropical gyre. After subduction, that water reaches the equator by means of a subsurface western boundary current and provides a substantial part (2/3 approximately) of the initial transport of the Equatorial Undercurrent. The eastward flow in the Equatorial Undercurrent is part of an intense equatorial cell in which water rises to the surface at the equator, drifts westward and poleward, then sinks near 3° latitude to flow equatorward where it rejoins the undercurrent.
Matano, R P., and S G H Philander, 1994: On the decay of the meanders of eastward currents. Journal of Physical Oceanography, 24(2), 298-304. Abstract PDF
The separation of western boundary currents from the coast and their eastward extensions into the open ocean are characterized by the presence of quasi-stationary, large-scale meanders. These meanders result when the western boundary current overshoots the latitude of zero wind-stress curl. The wavelength and the scale of decay of those meanders have previously been estimated by considering the superposition of a westward Rossby wave and an eastward mean flow. While observations indicate that the wavelength of the meanders is in good agreement with theory, the decay scale is much shorter than that indicated by scaling arguments. The purpose of this article is to show that the rapid decay of the meanders of eastward currents can be related to the effect of the meridional shear of the current.
Xie, Shang-Ping, and S G H Philander, 1994: A coupled ocean-atmosphere model of relevance to the ITCZ in the eastern Pacific. Tellus A, 46A(4), 340-350. Abstract PDF
The intertropical convergence zone (ITCZ) stays in the northern hemisphere over the Atlantic and eastern Pacific, even though the annual mean poaition of the sun is on the equator. To study some processes that contribute to this asymmetry about the equator, we use a two-dimensional model which neglects zonal variations and consists of an ocean model with a mixed layer coupled to a simple atmospheric model. In this coupled model, the atmosphere not only transports momentum into the ocean, but also directly affects sea surface temperature by means of wind stirring and surface latent heat flux. Under equatorially symmetric conditions, the model has, in addition to an equatorially symmetric solution, two asymmetric solutions with a single ITCZ that forms in only one hemisphere. Strong equatorial upwelling is essential for the asymmetry. Local oceanic turbulent processes involving vertical mixing and surface latent heat flux, which are dependent on wind speed, also contribute to the asymmetry.
Chao, Y, and S G H Philander, 1993: On the structure of the southern oscillation. Journal of Climate, 6(3), 450-469. Abstract PDF
A realistic oceanic general circulation model is forced with winds observed over the tropical Pacific between 1967 and 1979. The structure of the simulated Southern Oscillation is strikingly different in the western and eastern sides of the basin, because the principal interannual zonal-wind fluctuations are confined to the west and are in the form of an equatorial jet. This causes thermocline displacements to have maxima off the equator in the west (where the curl of the wind is large) but on the equator in the east. Zonal phase and propagation, both on and off the equator, is at different speeds in the west and the east. The phase pattern is complex, and there is, on interannual time scale, no explicit evidence of individual equatorial waves. These results lead to a modification of the "delayed oscillator" mechanism originally proposed by Schopf and Suarez to explain a continual Southern Oscillation. The results also permit an evaluation of the various coupled ocean-atmosphere models that simulate the Southern Oscillation and indicate which measurements are necessary to determine which models are most relevant to reality.
Matano, R P., and S G H Philander, 1993: Heat and mass balances of the South Atlantic Ocean calculated from a numerical model. Journal of Geophysical Research, 98(C1), 977-984. Abstract
The general circulation model of Bryan (1969), modified by the introduction of open boundary conditions at the Drake Passage and between Africa and Antarctica, has been used to study the mass and heat budgets of the South Atlantic Ocean. The model was initialized with the climatological annual mean values of temperature and salinity of Levitus (1982) and forced at its surface with the climatological wind stress data of Hellerman and Rosentstein (1983). After 3 years of integration the model reached a quasi-stationary state. A heat balance shows that the model transports 0.19 PW of heat toward the north across 30°S. While a large part of this heat is supplied by the atmosphere and involves the conversion of intermediate waters into surface waters, a comparison with climatological data of atmospheric heat fluxes suggests that an extra source of heat is necessary to maintain the northward heat flux.
Philander, S G., 1993: Ocean-atmosphere interactions in the tropics In Modelling Oceanic Climate Interactions, NATO ASI Series I, Vol. 11, Heidelberg, Germany, Springer-Verlag, 35-65.
A 140-year simulation of the ocean-atmosphere climate system has been performed by the GFDL Climate Dynamics Project using a low-resolution coupled general circulation model (GCM). The model was subjected to annually averaged insolation throughout the integration. This coupled system exhibits well-defined fluctuations in the tropical Pacific, with a preferred time scale of 3-4 years. The characteristics of these recurrent anomalies were examined by applying an extended empirical orthogonal function (EEOF) analysis to selected model variables. These results indicate that the simulated oscillations are accompanied by coherent changes in the atmospheric and oceanic circulation.
The spatial patterns associated with the leading EEOF mode indicate that SST anomalies make their first appearance off the Peru-Ecuador coast and then migrate steadily westward, with an average transit time of 12-15 months. The arrival and eventual decay of SST fluctuations in the western Pacific is typically followed by the initiation of anomalies of the opposite polarity along the American coasts. The space-time evolution of various meteorological and oceanographic signals exhibits well-defined phase relationships with the SST perturbations. Some aspects of the model behavior during these warm and cold episodes are reminiscent of observed phenomena associated with the El Niño-Southern Oscillation (ENSO).
Analysis of the climatological heat budget for the top ocean layer indicates a near balance between horizontal and vertical temperature advection by the time-mean flow, vertical diffusion, and heat input from the overlying atmosphere. Contributions of transient effects to this balance are negligible. The principal mechanisms associated with the simulated ENSO-like cycles were then studied by examining the local heat budget for the SST perturbations. It is shown that the relative importance of various linear advective processes in the heat budget exhibits a notable dependence on geographical location and on the specific phase of the ENSO-like cycle.
Neelin, J D., Ngar-Cheung Lau, and S G H Philander, et al., 1992: Tropical air-sea interaction in general circulation models. Climate Dynamics, 7, 73-104. Abstract
An intercomparison is undertaken of the tropical behavior of 17 coupled ocean-atmosphere models in which at least one component may be termed a general circulation model (GCM). The aim is to provide a taxonomy - a description and rough classification - of behavior across the ensemble of models, focusing on interannual variability. The temporal behavior of the sea surface temperature (SST) field along the equator is presented for each model, SST being chosen as the primary variable for intercomparison due to its crucial role in mediating the coupling and because it is a sensitive indicator of climate drift. A wide variety of possible types of behavior are noted among the models. Models with substantial interannual tropical variability may be roughly classified into cases with propagating SST anomalies and cases in which the SST anomalies develop in place. A number of the models also exhibit significant drift with respect to SST climatology. However, there is not a clear relationship between climate drift and the presence or absence of interannual oscillations. In several cases, the mode of climate drift within the tropical Pacific appears to involve coupled feedback mechanisms similar to those responsible for El Niño variability. Implications for coupled-model development and for climate prediction on seasonal to interannual time scales are discussed. Overall, the results indicate considerable sensitivity of the tropical coupled ocean-atmosphere system and suggest that the simulation of the warm-pool/cold-tongue configuration in the equatorial Pacific represents a challenging test for climate model parameterizations.
A global atmospheric general circulation model (GCM) coupled to an oceanic GCM that is dynamically active only in the tropical Pacific simulates variability over a broad spectrum of frequencies even though the forcing, the annual mean incoming solar radiation, is steady. Of special interest is the simulation of a realistically irregular Southern Oscillation between warm El Niño and cold La Niña states. Its time scale is on the order of 5 years. The spatial structure is strikingly different in the eastern and western halves of the ocean basin. Sea surface temperature changes have their largest amplitude in the central and eastern tropical Pacific, but the low-frequency zonal wind fluctuations are displaced westward and are large over the western half of the basin. These zonal wind anomalies are essentially confined to the band of latitudes 10°N to 10°S so that they form a jet and have considerable latitudinal shear. During El Niño the associated curl contributes to a pair of pronounced minima in thermocline depth, symmetrically about the equator in the west, near 8°N and 8°S. In the east, where the low-frequency wind forcing is at a minimum, the deepening of the thermocline in response to the winds in the west have a very different shape-an approximate Gaussian shape centered on the equator.
The low-frequency sea surface temperature and zonal wind anomalies wax and wane practically in place and in phase without significant zonal phase propagation. Thermocline depth variations have phase propagation; it is eastward at a speed near 15 cm s-1 along the equator in the western half of the basin and is westward off the equator. This phase propagation, a property of the oceanic response to the quasi-periodic winds that force currents and excite a host of waves with periods near 5 years, indicates that the ocean is not in equilibrium with the forcing. In other words, the ocean-atmosphere interactions that cause El Niño to develop at a certain time are countered and, in due course, reversed by the delayed response of the ocean to earlier winds. This "delayed oscillator" mechanism that sustains interannual oscillations in the model differs in its details from that previously discussed by Schopf and Suarez and others. The latter investigators invoke an explicit role for Kelvin and Rossby waves. These waves cannot be identified in the low-frequency fluctuations of this model, but they are energetic at relatively short periods and are of vital importance to a quasi-resonant oceanic mode with a period near 7 months that is excited in the model. The similarities and differences between the results of this simulation and those with other models, especially the one described in a companion paper, are discussed.
Chao, Y, and S G H Philander, 1991: On the structure of the southern oscillation In Eighth Conference on Atmospheric and Oceanic Waves and Stability, Boston, MA, American Geophysical Union, J5-J8.
Philander, S G., and Y Chao, 1991: On the contrast between the seasonal cycles of the equatorial Atlantic and Pacific Oceans. Journal of Physical Oceanography, 21(9), 1399-1406. Abstract PDF
Although the winds on the equator at 28°W in the Atlantic and 140°W in the Pacific have similar seasonal variations, the current fluctuations have pronounced differences. In the Pacific, the maximum speed of the Equatorial Undercurrent, attained in the northern spring, can exceed 140 cm s-1, while the minimum speed, in the autumn, is less than 80 cm s-1. In the Atlantic the maximum speed of 80 cm s-1 hardly varies seasonally, although it tends to be largest in the autumn. Analyses of results from a realistic simulation of the equatorial currents indicate that the larger zonal extent of the Pacific, and the seasonal variations of the winds over the western Pacific, which can be out of phase with those in the east, are the principal reasons for the differences between the Atlantic and Pacific.
Ravelo, A C., R G Fairbanks, and S G H Philander, 1990: Reconstructing tropical Atlantic hydrography using planktontic foraminifera and an ocean model. Paleoceanography, 5(3), 409-431. Abstract PDF
In the tropical Atlantic, planktonic foraminifera species are vertically distributed with highest abundances occurring in the photic zone (approximately 0-100 m). The tropical Atlantic thermocline dips from east to west and varies seasonally due to changes in the southeast and northeast trade winds. In the east, the thermocline is in the photic zone, and in the west, the well-mixed surface layer extends below the photic zone most of the year. As expected from species vertical distributions in plankton tows, the species assemblages on the seafloor are correlated to the hydrographic conditions of the overlying surface ocean layer. A new technique to reconstruct past tropical Atlantic (20°N to 20°S) photic zone hydrography and surface wind field uses faunal assemblage data from deep-sea cores. Planktonic foraminifera abundances in core tops correlate with observations of modern photic zone hydrography defined here as seasonal temperature variation and mixed layer depth. The hydrography is mathematically described using empirical orthogonal function (EOF) analysis of annual temperature range as a function of depth. Factor analysis of 29 species of planktonic foraminifera from 118 core tops produces three factors. The factors correlate to mixed layer depth and the two EOF modes. The ocean model of the Atlantic ocean produces similar map patterns of the EOF modes. Therefore the model can be used to simulate hydrographic changes to compare with faunal predicted past hydrographic changes. Since the ocean model is wind driven, this approach provides a way of evaluating the validity of estimates of past wind stress changes and the contribution of these changes to the faunal changes in the past. A double wind stress run indicates that the central and eastern equatorial and southeast regions of the study area are most sensitive to wind stress increases. Factor analysis of the foraminifera abundances from the last glacial maximum (LGM) shows that species associations change downcore and demonstrates how the methods developed in this study can be applied. Comparison of the double wind stress experiment and the LGM faunal changes indicates some areas of significant agreement suggesting that faunal changes may reflect thermocline structure response to the LGM wind field. Discrepancies may reflect the fact that uniform changes in the north and south trade wind strengths did not occur at the LGM.
Chang, Ping, and S G H Philander, 1989: Rossby wave packets in baroclinic mean currents. Deep-Sea Research, Part I, 36(1), 17-37. Abstract
A WKB description of the propagation of Rossby wave packets in a shallow water model of the tropical oceans indicates that the presence of the baroclinic mean currents can modify the characteristics of wave propagation significantly. For currents with weak latitudinal shear the effect of the current itself is less important than the effect of the associated variations in the depth of the thermocline, except near critical layers where waves are absorbed. For example, a westward current, and the associated shoaling of the thermocline towards the equator, can cause the speed of the long Rossby waves to decrease with decreasing latitude. (The speed increases towards the equator in the absence fo mean currents.) Westward currents inhibit meridional propagation, but eastward currents enhance it. The amplification and decay of a wave packet as it propagates through a mean current are described in terms of the conservation of wave action. Implications of these results for the propagation of Rossby wave in the real ocean are discussed.
Halpern, D, R A Knox, D S Luther, and S G H Philander, 1989: Estimates of equatorial upwelling between 140 degrees and 110 degrees W during 1984. Journal of Geophysical Research, 94(C6), 8018-8020. Abstract PDF
The equation of continuity is used to estimate profiles of vertical velocity between 25 and 120 m from moored current measurements in arrays nested within the triangle with vertices located at 1 degree 30' S, 140 degrees W and along the equator at 140 degrees and 110 degrees W during December 1983 through March 1984 and May-September 1984. All directions of the 4- to 5-month mean values were upward. The ensemble averaged mean vertical velocity was 2.2 x 10-5 ms-1. Upwelling speeds decreased eastward.
Katz, E J., S G H Philander, and P L Richardson, 1989: SEQUAL/FOCAL - A study of the equatorial Atlantic Ocean. EOS, 70(2), 18-19, 23.
Two different coupled ocean-atmosphere models simulate irregular interannual fluctuations that in many respects resemble El Niño Southern Oscillation phenomena. For example, the spatial structure of various fields at the peaks of the warm El Niño and cold La Niña phases of the oscillation are realistic. This success indicates that the models capture certain aspects of the interactions between the ocean and atmosphere that cause the Southern Oscillation. The principal difference between the models, namely the prominence of oceanic Kelvin waves in one but not the other, causes the two models to differ significantly in the way El Niño episodes evolve, and in the mechanisms that cause a turnabout from El Niño to La Niña and vice versa. It is possible that the different processes that determine the properties of the simulated oscillations all play a role in reality, at different times and in different regions. Each of the models captures some aspects of what is possible. However, reality is far more complex than any model developed thus far and additional processes not yet included are also likely to have a significant influence on the observed Southern Oscillation.
Philander, S G., and William J Hurlin, 1988: The heat budget of the tropical Pacific Ocean in a simulation of the 1982-83 El Niño. Journal of Physical Oceanography, 18(6), 926-931. Abstract PDF
The heat budget of a model that realistically simulates the 1982-83 El Niño indicates that the enormous changes in the winds during that event failed to disrupt the usual seasonal variations in meridional heat transport. Cross-equatorial transport towards the winter hemisphere continued as in a regular seasonal cycle. The key factor was the continued seasonal migrations of the ITCZ during El Niño. In early 1983 the ITCZ strayed farther south than usual and remained near the equator longer than usual thus causing an increase in the northward heat transport. This, together with an increase in the evaporative heat loss because of higher sea surface temperatures, resulted in a large loss of heat from the band of latitudes approximately 12°N - 12°S during El Niño.
Philander, S G., and Ngar-Cheung Lau, 1988: Predictability of El Niño In Physically-based Modelling and Simulation of Climate and Climatic Change, Part II, Dordrecht, The Netherlands, Kluwer Academic Publishers, 967-982.
A general circulation model of the tropical Pacific Ocean, which realistically simulates El Niño of 1982-83, has been used to determine how different initial conditions affect the model. Given arbitrary initial conditions (not in equilibrium with the wind) the model takes almost a year to return to a state in which the currents and density gradients are in equilibrium with the winds. Errors in the absolute value of the temperature persist far longer, however, indicating that accurate density data are essential initial conditions. If the correct density field is specified initially, but no information is provided about the currents, then the model recovers the currents within an inertial period, except for the eastern equatorial region. That region is affected by equatorial Kelvin waves which are excited because the model is initially in an unbalanced state. The currents associated with these waves are relatively modest and do not affect the density field significantly. Because of the large zonal scale of the thermal field in the tropical Pacific, three or four high resolution meridional density sections appear adequate for the initialization of the model. This result, however, takes into account neither the energetic waves, with a scale of 1000 km, that are associated with instabilities of the equatorial currents nor other high frequency fluctuations in the ocean.
Philander, S G., William J Hurlin, and A D Siegel, 1987: Simulation of the seasonal cycle of the tropical Pacific Ocean. Journal of Physical Oceanography, 17(11), 1986-2002. Abstract PDF
In a general circulation model of the tropical Pacific Ocean forced with climatological seasonally varying winds, equatorial upwelling and downwelling in adjacent latitudes play central roles in closing the oceanic circulation. The transport of the eastward North Equatorial Countercurrent decreases in a downstream direction because fluid is lost to downwelling into the thermocline where there is equatorward motion. Although this fluid converges onto the Equatorial Undercurrent, the latter's transport decreases because of equatorial upwelling. The upwelling, on the other hand, enhances the transport of the westward South Equatorial Current. Seasonally, the Countercurrent and South Equatorial Current are intense during the Northern Hemisphere summer and fall, at which time the thermocline has pronounced trough near 3°N and a ridge near 10°N, and are weak in the spring when latitudinal thermal gradients are small and when the southeast trades are relatively weak. These variations are out of phase with those of the Equatorial Undercurrent, which is most intense in the spring.
The seasonal changes are associated with considerable variations in the meridional heat transport, especially across 9°N. The heat transport is always towards the winter hemisphere. During the northern winter, Ekman drift in the central Pacific affects the northward transport of warm surface waters. During the northern summer, when the ITCZ is near 9°N and the winds there are weak, the Ekman drift across 9°N is small. The relatively steady southward flow of warm surface waters across 9°N in the far western Pacific now contributes significantly to the southward heat transport. Seasonally there is both this meridional and a zonal redistribution of warm surface waters in the upper tropical Pacific Ocean. The zonal redistribution, from west to east, contributes to high sea surface temperatures in the east in April when the Equatorial Undercurrent surges eastward and attains its highest speed and transport during the period of weak southwest tradewinds. Increased heat flux across the ocean surface at this time also contributes to the warming of the upper equatorial ocean. Seasonal wind variations west of the dateline have little effect on the eastern tropical Pacific in the model.
Nonlinearities have a large effect on the circulation of the tropical Atlantic Ocean within a few hundred kilometers of the equator, both in the surface layers and at depth. Qualitative features of a nonlinear model that are absent from a linear model include energetic unstable waves in the western equatorial Atlantic and a westward surface jet that penetrates to considerable depths between the equator and 3°N. The largest quantitative difference between the nonlinear and linear models is the intensity of the westward surface flow at the equator. In a linear model it can be twice as fast as in a nonlinear model. Motion below the equatorial themocline, though sufficiently slow to be linear, is related to the surface forcing in a nonlinear manner because it is forced to a large extent by vertical movements of the thermocline. (Linear models assume that disturbances reach the deep ocean by propagating through a fixed thermocline.) In addition to the equatorial zone, nonlinearities affect the coastal zone of Africa south of the equator, where alongshore currents and zonal pressure gradients in linear and nonlinear models are different.
Richardson, P L., and S G H Philander, 1987: The seasonal variations of surface currents in the tropical Atlantic Ocean: A comparison of ship drift data with results from a general circulation model. Journal of Geophysical Research, 92(C1), 715-724. Abstract PDF
Historical ship drifts from the tropical Atlantic Ocean are compared with surface currents from a general circulation model forced with monthly mean climatological winds. The model accurately reproduces the spatial structure of the currents and its time dependence, which varies considerably from the eastern side of the basin, where a semiannual harmonic is prominent, to the western side, where an annual harmonic is dominant. However, the amplitude of the simulated surface currents in the western side of the basin is too large. Mixing processes in the model appear to be too weak, especially when the winds are weak in regions where the thermocline is deep. High-frequency fluctuations of the winds need to be taken into account, and parameterization of the mixing needs to be improved, especially when the Richardson number is small.
Philander, S G., 1986: Predictability of El Niño. Nature, 321(6073), 810-811.
Philander, S G., 1986: Unusual conditions in the tropical Atlantic Ocean in 1984. Nature, 322(6076), 236-238. Abstract PDF
During the first half of 1984, oceanic and atmospheric conditions in the tropical Atlantic were, in many respects, similar to conditions in the Pacific Ocean during El Niño: the upper ocean was unusually warm in the eastern part of the basin, rainfall was heavy over the normally arid regions to the south of the Equator, and coastal upwelling was inhibited in regions (southwestern Africa) where this is a seasonal phenomenon. The change in sea-surface temperatures, which resulted when unusual eastward currents to the south of the Equator transported warm surface waters towards Africa, contributed to the change in the sea-surface temperature.
In a general circulation model of the tropical Atlantic Ocean, the northwestward flowing Brazilian Coastal Current is fed by the westward South Equatorial Countercurrent and in turn loses water to the eastward Equatorial Undercurrent and the eastward North Equatorial Countercurrent. The transport of the countercurrent decreases in a downstream direction primarily because of downwelling and then equatorward flow, in the thermocline, into the undercurrent. Some of the countercurrent water penetrates into the Gulf of Guinea, where it flows into the southern hemisphere. The transport of the Equatorial Undercurrent decreases because upwelling, which is most intense in the western side of the basin, transfers fluid into the surface layers to sustain divergent Ekman drift which is swept westward by the South Equatorial Undercurrent. The model has northward heat transport across all latitudes in the tropics. Seasonal variations in the transport are modest to the south of 5 degrees S and to the north of 15 degrees N. Across 8 degrees N, however, the transport varies from 1.5 x 1015 W in January and February to -0.1 x 1015 W in August. This result implies that the zonal bands 5 degrees S to 8 degrees N and 8 degrees N to 15 degrees N act as capacitors that are out of phase. In July, August, and September the heat gained from the southern hemisphere is stored in the 5 degrees S to 8 degrees N band where the thermocline deepens. During this period (when the Brazilian Coastal Current turns offshore near 5 degrees N) the thermocline between 8 degrees N to 15 degrees N rises as heat is lost across 15 degrees N. When the Brazilian Coastal Current flows continuously along the coast into the Gulf of Mexico, from December into May, it transports heat from the band 5 degrees S to 8 degrees N to replenish the heat stored between 8 degrees N and 15 degrees N and to sustain the heat flux across 15 degrees N.
In general circulation models of the seasonal cycle, westward propagating waves, with an approximate wavelength of 1000 km and period of 3 to 4 weeks, in the western equatorial Atlantic and eastern equatorial Pacific derive their energy from the kinetic and potential energy of the mean flow. There is intense downwelling the cold crests of the wave and upwelling in the warm troughs. The local meridional heat flux associated with the waves is of the order of 100 W m-2, but their contribution to the net heat transport across the equator is small. The waves are highly nonstationary in time and inhomogenous in space.
Philander, S G., and Ronald C Pacanowski, 1986: A model of the seasonal cycle in the tropical Atlantic Ocean. Journal of Geophysical Research, 91(C12), 14,192-14,206. Abstract
In the western tropical Atlantic, seasonal variations in the surface winds and in the ocean are dominated by an annual harmonic. A simulation with a general circulation model indicated that the response in the western side of the basin is an equilibrium one practically in phase with the local winds. It includes the following: large vertical excursions of the thermocline that have a 180 degree change in phase across 8 degrees N approximately; a change in the direction of the North Brazilian Coastal Current, which flows continuously along the coast between December and May but which veers offshore near 5 degrees N to feed the North Equatorial Countercurrent during the other months; and a seasonal reversal of the countercurrent. To the east of 30 degrees W, seasonal changes in the model have a prominent semiannual harmonic in phase with the local winds but only partially attributable to forcing at that frequency. The transients excited by the abrupt intensification of the southeast trade winds in May happen to have a phase essentially the same as that of the semiannual forcing. These transients decay by the end of the calendar year, so that the seasonal cycle that starts with the intensification of the winds in May can be treated as an initial value problem as far as the upper ocean, above the thermocline, is concerned. The winds along the equator determine the response of the surface equatorial layer in the Gulf of Guinea but play a minor role in the seasonal upwelling along the coast near 5 degrees N. That upwelling is strongly influenced by changes in both components of the wind, and in the curl of the wind, over the Gulf of Guinea.
Garzoli, S L., and S G H Philander, 1985: Validation of an equatorial Atlantic simulation model using inverted echo sounder data. Journal of Geophysical Research, 90(C5), 9199-9201. Abstract
Changes in the vertically averaged temperature of water column, observed over a 12-month period at eight locations in the tropical Atlantic with inverted echo sounders, are described in terms of empirical orthogonal functions. These modes are compared with those from a similar analysis for data from a general circulation model of the tropical Atlantic. The method proves to be useful for the validation of models.
Philander, S G., 1985: El Niño and La Niña. Journal of the Atmospheric Sciences, 42(23), 2652-2662. Abstract PDF
El Niño and La Niña are the two complementary phases of the Southern Oscillation. During El Niño, the area of high sea surface temperatures increases, while the atmospheric convection zones of the tropical Pacific expand and merge so that there is a tendency toward spatially homogeneous conditions. La Niña is associated with low sea surface temperatures near the equator, with atmospheric convergence zones that are isolated from each other, and with spatial scales smaller than those of El Niño. It is proposed that both phases of the Southern Oscillation can be attributed to unstable interactions between the tropical ocean and atmosphere. During El Niño, the increased release of latent heat to the atmosphere drives the instability. During La Niña, when the heating of the atmosphere decreases, the compression of the convection into smaller and smaller areas permits an instability that intensifies the trade winds and the oceanic currents. The unstable air-sea interactions are modulated by the seasonal movements of the atmospheric convergence zones, and this determines some of the characteristics of the perturbations that can be amplified. The zonal integral of winds along the equator, rather than winds over a relatively small part of the Pacific such as the region west of the date line, is identified as a useful indicator of subsequent developments in the Pacific.
Philander, S G., 1985: Tropical oceanography. Advances in Geophysics, 28A, 461-477.
Philander, S G., and E Rasmusson, 1985: The Southern Oscillation and El Niño. Advances in Geophysics, 28A, 197-215.
Philander, S G., and A D Siegel, 1985: Simulation of El Niño of 1982-1983 In Coupled Ocean-Atmosphere Models, Amsterdam; The Netherlands, Elsevier Science Publishers, 517-541. Abstract
A general circulation model of the ocean simulates El Niño of 1982-1983 with reasonable success and provides the following results. The massive eastward transfer of warm surface waters from the western to the eastern Pacific was accomplished by unusual eastward surface currents which, by November 1982, extended from 9 degrees S to 9 degrees N across 120 degrees W. Further east, the persistence of the southeast trades over the eastern tropical Pacific inhibited eastward surface flow at, and to the south of the equator at that time, but the eastward flow between 3 degrees and 8 degrees N penetrated right to the coast of Central America. The relaxation of the trades and the changes in the curl of the wind stress, that caused the redistribution of heat in the upper ocean, occurred so gradually between June and November 1982 that the response of the ocean was approximately an equilibrium one. The zonal pressure gradient along the equator and the intensity of the Equatorial Undercurrent, for example, decreased gradually as the trade winds weakened. In December 1982, the anomalous eastward winds west of the dateline suddenly changed to northerly winds. The westward pressure force which the eastward winds had established was left unbalanced. This excited an eastward travelling equatorial Kelvin wave which elevated the thermocline and accelerated the equatorial currents westward. The wave front dispersed downwards as it propagated eastward and there is no evidence of its reflection at the South American coast affecting the surface layers of the ocean. Eastward winds in the eastern equatorial Pacific Ocean in March and April interrupted the recovery from El Niño and generated an intense local eastward surface jet. The reappearance of the tradewinds in May 1983 signaled the end of El Niño and the gradual return to normal conditions.
Philander, S G., et al., 1985: Long waves in the equatorial Pacific Ocean. EOS, 66(14), 154-156.
Yamagata, T, and S G H Philander, 1985: The role of damped equatorial waves in the oceanic response to winds. Journal of the Oceanographical Society of Japan, 41(5), 345-357.
Carton, J A., and S G H Philander, 1984: Coastal upwelling viewed as a stochastic phenomenon. Journal of Physical Oceanography, 14(9), 1499-1509. Abstract PDF
Four years of winds from the northeastern Pacific are used to drive a reduced-gravity ocean model which includes a high-resolution eastern coastal zone that spans 17 degrees longitude and 30 degrees latitude. Spectra of the alongshore velocity and interface height, measured in the coastal zone, are red to 100-day periods. At periods less than 50 days, 1) the circulation is strongly trapped within a radius of deformation of the coast and 2) the alongshore current is well correlated with the alongshore wind stress. At periods longer than 50 days, wind-stress curl becomes important. The alongshore pressure gradient becomes well correlated with the alongshore wind stress. Much of the ocean variability is at periods longer than 10 days. At periods longer than 100 days the alongshore currents begin to weaken and disperse away from the eastern boundary in a series of jets alternating northward and southward.
Simulation of the seasonal cycle in the tropical Atlantic Ocean with a multi- level primitive equation numerical model yields remarkably realistic results including the separation of the Brazilian Current from the coast and the reversal of the Countercurrent.
Philander, S G., and E Rasmusson, 1984: On the evolution of El Niño. Tropical Ocean-Atmosphere Newsletter, 24, 16.
During El Niño Southern Oscillation events modest anomalies amplify spatially and temporally until the entire tropical Pacific Ocean and the global atmospheric circulation are affected. Unstable interactions between the ocean and atmosphere could cause this amplification when the release of latent heat by the ocean affects the atmosphere in such a manner that the altered surface winds induce the further release of latent heat. Coupled shallow water models are used to simulate this instability which is modulated by the seasonal movements of the atmospheric convergence zones.
Delecluse, P, and S G H Philander, 1983: Variability of coastal zones in low latitudes (with application to the Somali Current, the Gulf of Guinea and the El Niño Current) In Hydrodynamics of the Equatorial Ocean, Amsterdam, The Netherlands, Elsevier Science Publishers, 219-235. Abstract
Southerly winds in low latitudes drive very intense currents along the western coast and much slower currents along the eastern coast. Just under the surface layer, the currents are opposite to the wind in the eastern part (they flow upwind, following the pressure gradient force); they are in the same direction as the surface flow along the western coast. These different properties are due to the Ekman drift in the surface layer, which is maximal within 3 degrees off the equator and drives strong zonal currents that give outer conditions to coastal currents. Nonlinearities intensify the western currents but have small effects on the eastern side. When the wind relaxes, southward current appears along the coast. The relevance of these results for the Somali Current, the Gulf of Guinea and the eastern Pacific currents is examined.
Philander, S G., 1983: Anomalous El Niño of 1982-83. Nature, 305(5929), 16.
At intervals that vary from 2 to 10 years sea-surface temperatures and rainfall are unusually high and the tradewinds are unusually weak over the tropical Pacific Ocean. These Southern Oscillation El Niño events which devastate the ecology of the coastal zones of Ecuador and Peru, which affect the global atmospheric circulation and which can contribute to severe winters over northern America, often develop in a remarkably predictable manner. But the event which began in 1982 has not followed this pattern.
Philander, S G., 1983: Little and large Kelvin waves. Tropical Ocean-Atmosphere Newsletter, 18, 11-13.
Philander, S G., and P Delecluse, 1983: Coastal currents in low latitudes (with application to the Somali and El Niño currents). Deep-Sea Research, Part I, 30(8A), 887-902. Abstract
Near the equator, the directly wind-driven coastal jet along the western boundary of an ocean basin differs from its counterpart along the eastern boundary in several respects. (1) The western boundary current is more intense by an order of magnitude, because it flows in the direction of the pressure force associated with density gradients whereas the eastern boundary current is opposed by this pressure force. (2) In the west the flow at the depth of the thermocline is still in the direction of the wind, but in the east the pressure force drives an undercurrent that is practically as strong as the surface current. (3) Variability on time scales from a week to months is much higher in the east than the west. (4) On time scales longer than a month long Rossby waves disperse the eastern boundary current westward, but the western current remains a coastal jet. (5) A relaxation of the wind causes a prompt reversal in the direction of the flow in the east but in the western region, where the current flows 'downhill', there is only a gradual deceleration. The relevance of these results to the Somali Current in the western Indian Ocean and El Niño Current in the eastern equatorial Pacific Ocean is discussed.
Gonella, J, M Fieux, and S G H Philander, 1982: Evidence for equatorial Rossby waves in the Indian Ocean. Tropical Ocean-Atmosphere Newsletter, 10, 4-5.
Philander, S G., and J-H Yoon, 1982: Eastern boundary currents and coastal upwelling. Journal of Physical Oceanography, 12(8), 862-879. Abstract PDF
The adjustment of the eastern coastal zone of an inviscid ocean with vertical walls to a change in wind conditions occurs in two stages. After the propagation of a Kelvin wave across the forced region in a time Tk which is of the order of a day or two, the coastal upwelling zone is temporarily in equilibrium with the wind. Further adjustment occurs after a time TR, which is of the order of a few months, when westward Rossby dispersion of the coastal jet becomes important. These time scales define three frequency ranges that characterize the response to fluctuating winds with period P.1) At high frequencies short Kelvin waves can destroy coherence between the forcing and response, alongshore coherence of oceanic variables is small, and the spectrum of the response is red even if that of the forcing is white. The offshore scale of the response is the radius of deformation. Poleward phase propagation at Kelvin wave speed c in unforced regions and at speed 2c in the forced region is prominent in this frequency range and at all lower frequencies. 2) At intermediate frequencies long Kelvin waves from the boundary of the forced region establish an equilibrium response so that the ocean and atmosphere are practically in phase, but Kelvin waves excited by remote winds could destroy this coherence. Alongshore correlations are high and the spectrum of the response is much less red than at higher frequencies. The offshore scale exceeds the radius of deformation and increases with decreasing frequency. 3) At low frequencies the offshore scale is the distance Rossby waves travel in time P. A complex system of northward and southward currents appears near the eastern boundary of the basin. It is proposed that the California Current system is generated in this manner.
Yoon, J-H, and S G H Philander, 1982: The generation of coastal undercurrents. Journal of the Oceanographical Society of Japan, 38(4), 215-224. Abstract
Equilibrium conditions in an f-plane ocean evolve as follows after the sudden onset of winds parallel to a coast. At first the flow is two-dimensional-- spatial variations are confined to a plane perpendicular to the coast--and the salient features in the forcing region are acceleration of a coastal jet in the surface layers in the wind direction, and offshore Ekman drift that causes coastal upwelling. Kelvin waves excited at the edge of the forced region establish equilibrium conditions by creating an alongshore pressure gradient that balances the wind so that the acceleration stops. The vertical structure corresponding to each vertical mode differs from that of the wind- driven coastal jet so that the arrival of the barotropic Kelvin wave starts to accelerate a coastal undercurrent in a direction opposite to that of the wind. Subsequent baroclinic Kelvin waves modify the vertical structure of the coastal current so that the undercurrent in the subsurface layer is accelerated. In an inviscid model there is a singularity in the surface layers at the coast because the Kelvin modes with small offshore and vertical scales travel slowly and take a very long time to make their contribution to the establishment of equilibrium conditions. A modest amount of friction eliminates this problem. Nonlinearities are important in the heat equation and affect sea surface temperatures significantly but their effect on the momentum balance is secondary.
Measurements indicate that mixing processes are intense in the surface layers of the ocean but weak below the thermocline, except for the region below the core of the Equatorial Undercurrent where vertical temperature gradients are small and the shear is large. Parameterization of these mixing processes by means of coefficients of eddy mixing that are Richardson-number dependent, leads to realistic simulations of the response of the equatorial oceans to different windstress patterns. In the case of eastward winds results agree well with measurements in the Indian Ocean. In the case of westward winds it is of paramount importance that the nonzero heat flux into the ocean be taken into account. This heat flux stabilizes the upper layers and reduces the intensity of the mixing, especially in the east. With an appropriate surface boundary condition, the results are relatively insensitive to values assigned to constants in the parameterization formula.
Philander, S G., 1981: The response of equatorial oceans to a relaxation of the trade winds. Journal of Physical Oceanography, 11(2), 176-189. Abstract PDF
The trade winds over the central Pacific are observed to weaken several months after the appearance of anomalously warm surface waters in the eastern equatorial Pacific Ocean. The following results obtained with a numerical model indicate how this relaxation of the winds affect the later stages of El Niño. A weakening of the westward trade winds causes a zonal redistribution of heat in the equatorial oceans and a warming of the eastern part of the basin. The warming depends on the zonal extent of the region over which the winds relax, and on the length of time T for which the winds relax. As T increases the warming in the east increases until it asymtotes to a maximum value when T exceeds the adjustment time of the basin (which is ~400 days in the case of the Pacific Ocean). Maximum heating is associated with a permanent weakening of the winds, unless the winds reverse direction and become eastward. Even weak eastward winds for a short period can cause disproportionately large temperature increases (because of nonlinear mechanisms).
In the region where the winds relax, the heating is due to convergence of surface waters on the equator, and advection by accelerating eastward surface currents. As the time scale T increases, the acceleration becomes less pronounced. East of the region where the winds relax, Kelvin waves suppress the thermocline but leave the sea surface temperature unchanged in linear models. In nonlinear models advection by eastward currents in the wake of Kelvin waves can cause a warming, even at the surface. For winds with a realistic spatial and temporal structure the identification of these waves is difficult.
Philander, S G., and Ronald C Pacanowski, 1981: The oceanic response to cross-equatorial winds (with application to coastal upwelling in low latitudes). Tellus, 33, 201-210. Abstract
Sea surface temperature variations observed in the eastern equatorial Atlantic and Pacific Oceans on seasonal, and possibly interannual (El Niño) time scales, may to a large extent be due to the variability of the local meridional winds. In a numerical model of the ocean, southerly winds cause low sea surface temperatures in the southeastern part of the basin because the coastal upwelling zone is extended far westward by (1) advection and (2) Rossby wave propagation which is important on time scales greater than a month. North of the equator sea surface temperatures are high. The thermocline has a trough near 3 degrees N where there is an intense eastward jet. A relaxation of the southerly wind causes a warming in the southeastern part of the basin primarily because of a zonal redistribution of heat by the South Equatorial Current and Countercurrent.
Oscillating wind with a period P induce variability with the following characteristics in the upper few hundred meters of the equatorial zone (5 degrees N to 5 degrees S) of the ocean. (1) P < 10 days: these winds fluctuate too rapidly to generate strong currents and excite primarily waves. (2) 10 days < P < 50 days: At these periods the winds generate intense equatorial jets in the upper 50 m, but at greater depths the variability has a small amplitude. Nonlinear eastward jets are more intense, are narrower, and are deeper than the corresponding westward jets so that winds with a zero mean value give rise to a mean eastward surface current. If the wind is always westward, then its fluctuating component intensifies the eastward equatorial undercurrent maintained by the mean winds. The surface flow is eastward and convergent when winds that are always westward go through a weak phase. (3) 50 days < P < 150 days: An eastward presure force exists sufficiently long to generate an intense eastward equatorial undercurrent. Variability has a large amplitude in the surface layers and in the thermocline. Eastward phase propagation associated with Kelvin waves is prominent in the upper ocean because the nonlinear currents impede the Rossby waves. (4) P > 150 days: The amplitude of variability is almost independent of frequency. An equilibrium response which is in phase with the forcing and which corresponds to a succession of steady states is approached asymptotically. These time scales are for a basin 5000 km wide. If the width of the basin exceeds 5000 km, then the 150 daytime scale increases. In the deep ocean below the thermocline, motion corresponds to propagating waves generated by the divergence of the nonlinear currents in the upper ocean.
Philander, S G., and Ronald C Pacanowski, 1981: Variability of SST in eastern equatorial oceans. Tropical Ocean-Atmosphere Newsletter, 6, 1, 7.
Philander, S G., 1980: The Equatorial Undercurrent revisited. Annual Review of Earth and Planetary Sciences, 8, 191-204.
Philander, S G., and W Düing, 1980: The oceanic circulation of the tropical Atlantic, and its variability, as observed during GATE In Equatorial and A-Scale Oceanography, GATE-2, New York, NY, Pergamon Press, Inc., 1-27. Abstract PDF
The principal components of the surface current system in the tropical Atlantic are the westward South Equatorial Current south of 3 degrees N, the eastward Equatorial Countercurrent between 3 degrees N and the westward North Equatorial Current north of 10 degrees N. The subsurface, eastward Equatorial Undercurrent is confined to about 1-1/2 degrees latitude and is centered on the equator. These mean currents are subject to fluctuations over a spectrum of frequencies. Very high-frequency, turbulent fluctuations are of major importance in the mixed layers at the ocean surface and below the core of the Undercurrent. One-dimensional models cannot simulate these mixed layers because they are strongly influenced by the divergence of large-scale currents. Fluctuations with periods less than the inertial period correspond to inertia-gravity waves but their spectral properties, near 8 degrees N, are unusual in two respects:
(i) at periods between 1/2 hr. and 10 minutes there are very energetic oscillations associated with a therocline-trapped internal mode;
(ii) amplitudes of inertial waves below the thermocline are correlated with the intensity of surface winds. Near 5 degrees N the inertial peak in the spectrum disappears and equatorward of 5 degrees latitude equatorially trapped waves dominate the spectrum at periods longer than 10 days. There are hints that wave-like fluctuations do not have a universal spectrum in low latitudes. Particularly energetic equatorial oscillations observed during GATE include the following: 3- to 5-day equatorially trapped inertia-gravity waves which were forced by atmospheric disturbances with the same period; 16-day meanders of the Equatorial Undercurrent which may be related to atospheric fluctuations with the same period; 30-day, 1000 km waves which appear to be due to instabilities of the surface currents. Superimposed on these oscillations is a trend that is part of the seasonal cycle: for example, the zonal pressure gradient in the equatorial plane increased throughout GATE, practically in phase with the intensification of the tradewinds. This information sheds light on the seasonal upwelling in the Gulf of Guinea which is not correlated with changes in the local winds.
In response to the sudden onset of zonal winds the surface layers of the ocean accelerate in the direction of the wind. Motion is most intense near the equator where a jet forms within a week. The next stage in the evolution of equilibrium conditions is associated with wave fronts, excited initially at the coasts, that propagate across the ocean basin and establish zonal density gradients. Wave modes trapped in and above the strong shallow tropical thermocline because of internal reflection there are responsible for the adjustment of the upper ocean in low latitudes. These thermocline-trapped modes extend over a depth greater than that of the wind-driven surface currents and hence give rise to an undercurrent in the thermocline. This undercurrent is zonal and particularly intense near the equator, where it appears in the wake of an eastward traveling Kelvin or westward traveling Rossby wave after about 1 month. In the case of eastward winds, nonlinearities intensify the eastward equatorial surface jet and weaken the westward undercurrent. In the case of westward winds a different nonlinear mechanism intensidies the eastward Equatorial Undercurrent and weakens the westward surface flow. In a 5000-km winde basin, equilibrium equatorial currents are established about 150 days after the onset of the winds. The response time of the ocean below a depth of a few hundred meters is much longer. Winds with no spatial and a simple temporal structure generate currents with a complex vertical structure in the deep ocean. Closed current systems are possible in a confined forced region of an unbounded ocean; meridional coasts are not essential for their maintenance. The intensity of equatorial current is sensitive to dissipation parameters.
This paper is an analytical and numerical study of the response of the ocean to the fluctuating component of the wind stress as computed from twice-daily weather maps for the period 1973 to 1976. The results are described in terms of (time) mean and rms fields, frequency spectra and horizontal cross spectra, and local cross spectra between oceanic and atmospheric variables.
A forcing function with scales strictly larger than A forcing function with scales strictly larger than O(100 km) induces oceanic motion that is depth independent at periods between the inertial period and O(100 km) induces oceanic motion that is depth independent at periods between the inertial period and ~300 days. The dynamics is essentially linear so that rectified currents are small, the associated rectified transport amounts to at most 1-2 Sv in the western boundary layer. Root-mean-square currents are typically a few centimeters per second and are most intense in the western part of the basin, and near major topographic features. Fluctuations in the transport of the western boundary layer can be as large as 20-30 Sv. Three distinct frequency bands characterize the wind-induced barotropic fluctuations: 1) At periods between the inertial period and about one week the energy density increases steeply with decreasing frequency. Current spectra have a slope between -2 and -4. These forced waves can show an (imperfect) coherence between wind stress and the corresponding current components, and between atmospheric pressure and subsurface pressure. But spatial inhomogeneities in the wind field or bottom topography can destroy this coherence. 2) At periods between a week and a month planetary (or topographic) Rossby waves are dominant so that westward phase propagation is prominent. 3) At longer periods westward phase propagation is less evident and there is a time-dependent Sverdrup balance between meridional (cross-isobath) currents and wind stress curl. The spectra at these long periods are frequency independent (white) and the zonal (along-isobath) velocity component is more energetic than the meridional (cross-isobath) component.
Despite the high degree of idealization in the models, local coherence between oceanic and atmospheric variables is virtually nonexistent (except possibly at periods between 1 and 10 days) because of the wavelike structure of the oceanic response, the broadband stochastic character of the atmospheric variability, and inhomogeneities in the wind field and bottom topography.
It is proposed that fluctuations observed at site D north of the Gulf Stream are primarily atmospherically forced. At the MODE central mooring, however, there must be an additional energy source.
Philander, S G., 1979: Equatorial waves in the presence of the equatorial undercurrent. Journal of Physical Oceanography, 9(3), 254-262. Abstract PDF
Because of the narrow region over which it has high speeds, the Equatorial Undercurrent has little effect on long waves with large phase speeds, such as long Kelvin and equatorially trapped inertia-gravity modes with equivalent depths greater than approximately 30 cm. The Rossby branch of the Rossby-gravity family, and the gravest Rossby modes, have phase velocities comparable to the maximum speed of the Undercurrent and are significantly modified by this current. Meanders of the Undercurrent that are due to superimposed neutral (non-amplifying) waves must have westward phase propagation; standing or eastward traveling meanders are posible only if the Equatorial Undercurrent is unstable.
Philander, S G., 1979: Nonlinear coastal and equatorial jets. Journal of Physical Oceanography, 9(4), 739-747. Abstract PDF
Nonlinearities weaken westward equatorial jets and cause them to be shallower and broader than their linear counterparts. Nonlinear eastward equatorial jets, on the other hand, are more intense, deeper and narrower than linear jets. Since nonlinear effects are important on time scales longer than about one week, winds that fluctuate on such time scales introduce hysteresis effects and can generate flow with a complicated vertical structure in the surface layers of the equatorial oceans. Coastal jets differ from equatorial jets in that they are only weakly influenced by nonlinearities; this result could change if alongshore pressure forces are taken into account.
Upwelling along the northern coast of the Gulf of Guinea occurs only between June and October even though the local winds are favorable for upwelling throughout the year and have no seasonal variability. Away from the coast, near the equator for example, the winds do vary seasonally and cause large scale oceanographic conditions in the Gulf of Guinea to change seasonally. One of these changes is an intensification of the eastward Guinea current north of the equator, during the summer. Because the current is in geostrophic balance it is associated with a thermocline that shoals in a northward direction, particularly so during the summer. Hence conditions along the northern coast are more favorable for upwelling during the summer than during the rest of the year. The cross-equatorial winds, that have a strong seasonal variation at the equator but not at the coast, are shown to contribute to the intensification of the Guinea current during the summer.
Philander, S G., 1979: Variability of the tropical oceans. Dynamics of Atmospheres and Oceans, 3, 191-208.
Philander, S G., 1978: Forced oceanic waves. Reviews of Geophysics & Space Physics, 16(1), 15-46. Abstract
This paper concerns the linear response of the ocean to forcing at a specified frequency and wave number in the absence of mean currents. It discusses the details of the forcing function, the general properties of the equations of motion, and possible simplifications of these equations. Two representations for the oceanic response to forcing are described in detail. One solution is in terms of the normal modes of the ocean. The vertical structure of these modes corresponds to that of the barotropic and baroclinic modes; their latitudinal structure corresponds to that of inertia- gravity and Rossby waves. These waves are eigenfunctions of Laplace's tidal equations (LTE) with the frequency as eigenvalue. The description in terms of vertically standing modes is particularly useful if the forcing is nonlocal, because only these modes can propagate into undisturbed regions. The principal result is that it is extremely difficult for baroclinic (but not barotropic) disturbances to propagate horizontally away from a forced region. Instabilities of the Gulf Stream excite disturbances that are confined to the immediate neighborhood of the current; disturbances due to instabilities of equatorial currents do not propagate far latitudinally. A second representation of the oceanic response to forcing is in terms of vertically propagating, or vertically trapped, latitudinal modes. These modes are eigenfunctions of LTE with the equivalent depth h (not the frequency) as eigenvalue. Both positive and negative eigenvalues h are necessary for completeness. The modes with h > 0 consist of an infinite set of intertia- gravity waves and a finite set of Rossby waves which either propagate vertically or form vertically standing modes. The latitudinally gravest modes are equatorially trapped and have been observed in the Atlantic and Pacific oceans. The modes with h < 0 are necessary to describe the oceanic response to nonresonant forcing. In the vertical this response attenuates with increasing distance from the forcing region. Because of the shallowness of the ocean the large eastward traveling atmospheric cyclones in mid-latitudes and high latitudes force a response down to the ocean floor. Interaction with the bottom topography will result in smaller scale disturbances and will affect the frequency spectrum of the response when bottom-trapped waves are excited.
Philander, S G., 1978: Instabilities of zonal equatorial currents, 2. Journal of Geophysical Research, 83(C7), 3679-3682. Abstract PDF
A stability analysis of a realistic meridional profile of the surface currents in the equatorial Atlantic and Pacific oceans reveals that the most unstable waves are westward propagating and have a period of about 1 month, a wavelength of approximately 1100 km, and an e folding time near 2 weeks. This is proposed as an explanation for waves with these scales that have recently been observed in the Atlantic and Pacific oceans.
Moore, D W., and S G H Philander, 1977: Modeling of the tropical oceanic circulation In Marine Modeling, The Sea, Vol. 6, Chichester, UK, John Wiley & Sons, 319-361.
Philander, S G., 1977: The effects of coastal geometry on equatorial waves (forced waves in the Gulf of Guinea. Journal of Marine Research, 35(3), 509-523. Abstract PDF
The response of a stratified, semi-infinite equatorial ocean, bounded by a zonal coast close to the equator, to forcing at a given frequency and zonal wavenumber, is considered. If the coast is distant from the equator, the vertically propagating waves that are excited could include an infinite set of inertia-gravity waves, a finite set of Rossby waves, a Rossby-gravity wave and a coastally or equatorially trapped Kelvin wave. If the coast is close to the equator all these waves are modified except the equatorially trapped Kelvin wave. Most severely affected by the coast are the coastally trapped Kelvin wave and the equatorially trapped mixed Rossby-gravity wave. On a dispersion diagram the lines corresponding to the latter two waves are deformed so as to give rise to a Kelvin-gravity and a Rossby-Kelvin mode, each with a point at which the zonal component of the group velocity vanishes. The modification of these waves is most severe in the neighborhood of this point, particularly for small vertical wavenumbers.. Examples of waves that have been, or are likely to be observed in the Gulf of Guinea (where there is a nearly zonal coast to the equator) are discussed.
Philander, S G., 1976: Instabilities of zonal equatorial currents. Journal of Geophysical Research, 81(21), 3725-3735. Abstract
A two-layer model is used to study the effect of divergence on the inertial (barotropic) stability of zonal jets on a rotating sphere. Both the beta effect and divergence stabilize eastward jets, but both these effects can destabilize westward jets. The following are the principal are the principal results concerning the stability of zonal currents in the tropical oceans. Instabilities of the Equatorial Undercurrent are unlikely in the Indian and Atlantic oceans but may occur in the central Pacific in March and April. (The undercurrent has been found to be at its most intense in this location at this time.) The unstable waves, which will cause the undercurrent to meander about the equator, have a wavelength of about 900 km, a period of approximately 40 days, and an e folding time of more than 2 weeks. Such an instability could explain measurements in the Pacific by Taft, et al. (1974). Meanders of the undercurrent may also be caused by instabilities of the surface currents: the westward South Equatorial Current and the adjacent eastward North Equatorial Countercurrent. The westward propagating amplifying waves have a zonal wavelength in excess of 2000 km and a period and e folding time of 2-3 weeks. These results are in reasonable agreement with measurements in the Atlantic. Currents with a given vertical shear and stability that are baroclinically unstable in mid-latitudes are shown to be stabilized as they are moved equatorward. The only zonal equatorial current that could be baroclinically unstable is the westward North Equatorial Current between 10 degrees N and 20 degrees N.
Philander, S G., 1976: A note on the stability of the tropical easterlies. Journal of the Meteorological Society of Japan, 54(5), 328-330.
In the temporary absence of surface winds, density gradients in the ocean can give rise to an eastward equatorial current which has a width, depth and maximum speed comparable to that of the equatorial undercurrent. This is offered as an explanation for the deeper of the two eastward currents (the one in the thermocline) which HISARD, MERLE and VOITURIEZ (1970) observed at the equator in the western Pacific. It also explains the eastward surface current observed at the equator when the trade winds are weak or absent.
One striking feature of the meridional circulation is the occurrence of downwelling. It is suggested that this accounts for the downward spreading of the equatorial thermocline and for the deep penetration of water of high oxygen and low phosphate concentration at the equator.
Philander, S G., 1973: Equatorial undercurrent: Measurements and theories. Reviews of Geophysics & Space Physics, 11(3), 513-570. Abstract
An eastward jet in the equatorial thermocline and, below it, a weaker westward current extending to a depth of about 1200 meters have been observed at practically all longitudes. In the western Pacific the flow in the deep (150 meter) mixed layer above the thermocline is westward at and near the surface and eastward at greater depths when the winds are westward. These shallow currents appear to reverse direction when the monsoons do. At longitudes where a deep mixed layer is absent, the Atlantic and eastern part of the Pacific, the undercurrent in the thermocline is symmetric about the equator, has its downstream flow in geostrophic balance, and is marked by deep penetration of warm water of high oxygen concentration when the winds are light. When the southeast trades gain in strength, the core of the undercurrent moves upwind, its zonal flow becomes ageostrophic, the westward surface flow becomes stronger, the subsurface eastward flow becomes weaker, the ridging of isotherms at the equator becomes more pronounced, and the troughing becomes less pronounced. The evidence in favor of these variations occurring systematically is very tenuous. For a constant-density model to be relevant to the motion observed in the mixed layer of the western Pacific, it must be nonlinear and the vertical diffusion of momentum must be important at all depths. The results of such models are in reasonable agreement with observations if the winds are westward. To explain the eastward flow in the thermocline below the mixed layer, a stratified model is necessary. According to one such model, density gradients, in the absence of local winds, give rise to an eastward surface current in the equatorial thermocline and below it to a westward current. The meridional circulation is marked by equatorial downwelling. The modification of these 'thermally' driven currents by local winds is consistent with the earlier description of the flow at longitudes where a mixed surface layer is absent.
