Large-scale Ocean Dynamics
Changes in the large-scale ocean circulation, such as the Atlantic Meridional Overturning Circulation (AMOC), have a profound impact on global and regional climate systems, including Sahel and Indian summer monsoon rainfall, Atlantic hurricane activities, and Arctic climate. The large-scale ocean dynamics have played a key role in modern and paleo climate change and low frequency variability in coupled ocean-atmosphere systems. Understanding the dynamics of large-scale ocean circulation is crucial for prediction of low frequency climate variability and future climate change, especially abrupt climate change. Large-scale ocean dynamics also have important influence on the uptake of heat and carbon dioxide by the ocean, and global biogeochemical cycling.
One focus of GFDL’s research is the dynamics of large-scale ocean circulation, such as AMOC, and its climate implications. Examples of GFDL research in this area are described below.
Modeling Biases in AMOC:
Recent research at GFDL mapped the AMOC’s upwelling pathways as the it moves water northward through the Atlantic and found that much of the northward flow is brought up to the surface in the narrow upwelling zones off the coast of Africa (Toggweiler et al., manuscript in preparation). This helps explain why much of the northward flow is at the surface as the AMOC exits from the tropics off Florida. A parallel simulation of the surface 14C in GFDL’s coupled model CM2Mc (Galbraith et al., 2011) found that very little of the northward flow in the model is brought up to the surface in the tropics, as observed. This is not surprising as these upwelling zones are not resolved very well in current climate models. The missing upwelling has important implications for the northward heat transport in climate models.
GFDL scientists have examined the sensitivity of the AMOC and water masses to biases in the convergence of moisture into the basin using two different general circulation models and suggested that shortcomings in the models? ability to reproduce realistic bulk water mass properties are due to an overestimation of the inter-basin moisture export from the tropical Atlantic. Freshwater forcing in the coupled models due to combined precipitation, evaporation and land runoff was compared to oceanic freshwater export estimated from historical transect data. The implied Atlantic freshwater imbalance in the models, based on their heat and salt biases, were found to be in rough agreement with those estimates.
Upwelling at Eastern Boundaries:
A GFDL ocean model was used to examine the impact of winds along the Pacific South American coast (Barry Klinger, Paul Schopf, and Harrison, manuscript in preparation). Ekman theory would suggest that the resultant coastal up-welling is balanced by down-welling at the same latitude. They demonstrated that the coastal winds drive a cross-equatorial overturning circulation with buoyancy driven down-welling in the North-East Equatorial Pacific and sub-surface Tsuchiya jets supplying the up-welling to the coast. This research helps us to better understand and model the climate and variability in this region, which is a focal point for the El-Nino Southern Oscillation.
Interaction of AMOC and Carbon cycle:
In evaluating the influence of changing ocean currents on climate change, GFDL scientists compared an earth system model’s response to increased CO2 with and without an ocean circulation response. Inhibiting the ocean circulation response has a much larger influence on the heat storage pattern than on the carbon storage pattern. The heat storage pattern without circulation changes resembles carbon storage (either with or without circulation changes) more than it resembles the heat storage when currents are allowed to respond.
GFDL scientists have also investigated the role of AMOC on glacial terminations. Near the end of the last ice age, a resurgent precessional cycle produced inputs of meltwater to the North Atlantic that lasted for thousands of years. The meltwater inputs suppressed the AMOC, flattened the temperature contrast between the hemispheres, and produced a redistribution of heat from north to south that warmed Antarctica and the Southern Ocean. The same factors caused the level of CO2 in the atmosphere to rise along with the temperatures in Antarctica.
Changes in anthropogenic radiative forcing cannot account for the observed anti-correlation between detrended multidecadal surface and subsurface temperature variations in the tropical North Atlantic, while changes in the strength of AMOC can induce such anti-correlated variations. Such anticorrelated change is a distinctive signature of AMOC variations.
Altimeter data is highly correlated with instrumental subsurface ocean temperature data in the North Atlantic, and both show opposite signs between the subpolar gyre and the Gulf Stream path. Such a dipole pattern is a distinctive fingerprint of AMOC variability. Contrary to previous interpretations, the recent slowdown of the subpolar gyre is a part of a multidecadal variation and suggests a strengthening of the AMOC. The fingerprint could be used to the reconstruct past AMOC multidecadal variations and monitor future AMOC variations.
Meridional coherence/propagation of AMOC:
The Nordic Sea overflow plays an important role in the large-scale North Atlantic Ocean circulation, and it is crucial for climate models to have a correct representation of the Nordic Sea overflow. This can be achieved by using a high-resolution eddy-permitting global coupled ocean-atmosphere model. AMOC changes induced by changes in the Nordic Sea overflow propagate on the slow tracer advection time scale. A stronger and deeper-penetrating Nordic Sea overflow leads to stronger and deeper AMOC, contracted subpolar gyre (SPG), westward shift of the North Atlantic Current (NAC) and southward shift of the Gulf Stream, warmer SST east of Newfoundland and colder SST south of the Grand Banks. This underscores the importance of realistic overflow simulations in climate models. GFDL scientists have developed an innovative approach to represent topography in coarse resolution ocean models to improve overflow simulations.
Interior pathways of North Atlantic Deep Water from Flemish Cap to Cape Hatteras have been directly observed, and AMOC variations estimated in density space have been shown to propagate with the advection speed in this region, resulting in a much longer lead time (several years) between subpolar and subtropical AMOC variations — providing a more useful predictability. This suggests that AMOC variations have significant meridional coherence in density space, and monitoring AMOC variations in density space at higher latitudes might reveal a stronger signal with a several-year lead time.
Mechanism of Deep Convection:
GFDL researchers have tested the effect of various climatic forcings on the strength and structure of the overturning circulation using an idealized geometry ocean general circulation model coupled to an Earth system model, and showed that without winds or a high vertical diffusivity, the ocean does not support deep convection. Once deep convection and overturning set in, the distribution of convection centers is determined by the relative strength of the thermal and haline buoyancy forcing.
A dynamically cold ocean is globally less ventilated than a dynamically warm ocean. With dynamic cooling, convection decreases markedly in regions that have strong haloclines, while increases in the North Atlantic.
AMOC and Gulf Stream Separation:
The path of the Gulf Stream and the formation of the northern recirculation gyre (NRG) are sensitive to both the magnitude of lateral viscosity and the strength of the deep western boundary current (DWBC), the deep branch of the AMOC and showed that bottom vortex stretching induced by a downslope DWBC leads to the formation of a cyclonic NRG and keeps the path of Gulf Stream separated from the North American coast.
- Zhang, Rong, April 2015: Mechanisms for low-frequency variability of summer Arctic sea ice extent. Proceedings of the National Academy of Sciences, 112(15), DOI:10.1073/pnas.1422296112.
- Winton, Michael, Whit G Anderson, Thomas L Delworth, Stephen M Griffies, William J Hurlin, and Anthony Rosati, December 2014: Has Coarse Ocean Resolution Biased Simulations of Transient Climate Sensitivity? Geophysical Research Letters, 41(23), DOI:10.1002/2014GL061523.
- Wang, He, Sonya Legg, and Robert Hallberg, February 2015: Representations of the Nordic Seas overflows and their large scale climate impact in coupled models. Ocean Modelling, 86, DOI:10.1016/j.ocemod.2014.12.005.
- Danabasoglu, G, Stephen M Griffies, and Bonita L Samuels, et al., January 2014: North Atlantic simulations in Coordinated Ocean-ice Reference Experiments phase II (CORE-II). Part I: Mean states. Ocean Modelling, 73, DOI:10.1016/j.ocemod.2013.10.005.
- Harrison, Matthew J., Alistair Adcroft, and Robert W Hallberg, 2014: Atlantic watermass and circulation response to persistent freshwater forcing in two coupled general circulation models. Climate Dynamics, 42(1-2), DOI:10.1007/s00382-013-1798-5.
- Zhang, Rong, Thomas L Delworth, R Sutton, D Hodson, Keith W Dixon, Isaac M Held, Y Kushnir, D Marshall, Yi Ming, Rym Msadek, J Robson, Anthony Rosati, Mingfang Ting, and Gabriel A Vecchi, April 2013: Have Aerosols Caused the Observed Atlantic Multidecadal Variability? Journal of the Atmospheric Sciences, 70(4), DOI:10.1175/JAS-D-12-0331.1.
- Adcroft, Alistair, 2013: Representation of topography by porous barriers and objective interpolation of topographic data. Ocean Modelling, 67, DOI:10.1016/j.ocemod.2013.03.002.
- Winton, Michael, Stephen M Griffies, Bonita L Samuels, Jorge L Sarmiento, and Thomas L Frolicher, 2013: Connecting Changing Ocean Circulation with Changing Climate. Journal of Climate, 26(7), DOI:10.1175/JCLI-D-12-00296.1.
- Zhang, Rong, Thomas L Delworth, Anthony Rosati, Whit G Anderson, Keith W Dixon, Hyun-Chul Lee, and Fanrong Zeng, December 2011: Sensitivity of the North Atlantic Ocean circulation to an abrupt change in the Nordic Sea overflow in a high resolution global coupled climate model. Journal of Geophysical Research, 116, C12024, DOI:10.1029/2011JC007240.
- Toggweiler, J R., and D W Lea, 2010: Temperature differences between the hemispheres and ice age climate variability. Paleoceanography, 25, PA2212, DOI:10.1029/2009PA001758.
- Zhang, Rong, 2010: Latitudinal dependence of Atlantic Meridional Overturning Circulation (AMOC) variations. Geophysical Research Letters, 37, L16703, DOI:10.1029/2010GL044474.
- De Boer, A M., J Robert Toggweiler, and D M Sigman, 2008: Atlantic dominance of the meridional overturning circulation. Journal of Physical Oceanography, 38(2), DOI:10.1175/2007JPO3731.1.
- Zhang, Rong, 2008: Coherent surface-subsurface fingerprint of the Atlantic meridional overturning circulation. Geophysical Research Letters, 35, L20705, DOI:10.1029/2008GL035463.
- De Boer, A M., D M Sigman, J Robert Toggweiler, and J L Russell, 2007: Effect of global ocean temperature change on deep ocean ventilation. Paleoceanography, 22, PA2210, DOI:10.1029/2005PA001242.
- Zhang, Rong, 2007: Anticorrelated multidecadal variations between surface and subsurface tropical North Atlantic. Geophysical Research Letters, 34, L12713, DOI:10.1029/2007GL030225.
- Zhang, Rong, and Geoffrey K Vallis, 2007: The role of bottom vortex stretching on the path of the North Atlantic Western Boundary Current and on the Northern Recirculation Gyre. Journal of Physical Oceanography, 37(8), DOI:10.1175/JPO3102.1.