Posted on April 24th, 2014 in Isaac Held's Blog
Observed (HADCRUT4) surface temperature trends from 1980-2010, compared to the estimate of the forced response over this time frame obtained from the multi-model mean of the CMIP5 models. From Knutson et al 2013.
In the late 80’s, Mark Cane, Steve Zebiak and colleagues wrote a series of papers – Zebiak and Cane 1987 is one of the first – about a simple oscillatory atmosphere-ocean model of the tropical Pacific, with the goal of capturing the essence of ENSO evolution and providing dynamical predictions of ENSO. In 1996 Clement et al subjected this model’s surface temperatures to a forced warming tendency and showed that it then evolves towards a state that favors La Nina, and a cold Eastern Pacific, over the warm El Nino state.
It is easy enough to understand why the Cane-Zebiak model tilts towards la Nina as it warms. On the ocean side, this is a model of the waters above the thermocline in the equatorial Pacific. Crucially, the temperature of the water upwelling into this layer from below is fixed as a boundary condition. Most of the upwelling occurs in the eastern Pacific. When the waters of the surface layer are warmed, the upwelling of water from deeper layers, assumed to be unaffected by the warming, retards the warming in the East but not the West, increasing the east-west temperature gradient across the Pacific. One can then envision the basic mechanism underlying ENSO kicking in to enhance the temperature gradient. Known as the Bjerknes feedback, a stronger east-west temperature gradient generates a precipitation distribution (more rain in the west, less in the east) that enhances the strength of the trade winds along the equator, pushing surface waters westward and enhancing the upwelling of cold waters in the east. The manner in which different negative feedbacks then develop due to slower transfers of heat between equatorial and off-equatorial waters is a main focus of ENSO theory, and these complicate matters, but presumably you can still think of la Nina conditions as being favored by upwelling waters that have not yet experienced warming.
The path taken by this water that upwells in the eastern Pacific is intricate. The major pathway is part of the shallow wind-driven overturning circulation. Subduction and last contact with the surface is primarily in the subtropics, mostly in the eastern half of the basin from where water masses can more easily drift westward and equatorward, typically reaching the western boundary first, where they proceed equatorward below the surface, eventually feeding the equatorial undercurrent which rises as it moves back eastward, mixing with surface waters in the east. An early paper describing the theory and modeling of this circulation is McCreary and Lu, 1994. See also the schematic in Fig. 3 of England et al, 2014. I was skeptical of the Clement et al result when it came out because of the extreme assumption that the upwelling waters are assumed not to have warmed at all. The time-scale of this shallow circulation is at most a decade or two, so one would have to visualize this modest delay being large enough to drive the system preferentially towards la Nina.
Waters subducted further polewards than the subtropics can also move equatorward and get caught up in the equatorial undercurrent and coastal (Peruvian) upwelling. Radiocarbon in tropical corals – Toggweiler et al 1991 – suggests that these denser source waters come from as far away as the Southern ocean north of the circumpolar current. This would lengthen the time lag, and maybe make it more plausible that the subsurface plumbing that emerges in our ocean models might be deficient.
Models don’t typically generate a la Nina like forced response, as seen in the figure at the top. The discrepancy does not just affect the usual hiatus period, the past 15 or so years, but as shown in the figure it affects trends over the full satellite era (causing the discrepancy between models’ and satellite (MSU) estimates of tropical tropospheric warming trends among other things). One possibility of course is that internal variability is the cause of this discrepancy between the observed and the forced component of the model trends. But the question here is whether the models could be missing a la Nina-like tendency in their forced responses.
In Held et al 2010, we tried to separate the response in our CM2.1 model, (in an ensemble of 20th century +A1B scenario simulations with stabilization of forcing agents after 2100) into fast and slow components with different spatial structures. We did this by returning all anthropogenic forcing agents to their pre-industrial values instantaneously at three times (2100, 2200, 2300). In response there is a fast cooling with e-folding time of just a few years, followed by a much slower “recalcitrant” cooling back to the pre-industrial climate. The slow part is computed by looking at what’s left 20 years following the instantaneous return to pre-industrial forcing, long enough for the fast part to have decayed away. The slow component can be thought of as the effect of the warming of the sub-surface waters on surface temperatures. The upshot is that the temperature response at any time is decomposed into two components, . The patterns and are normalized to integrate to unity over the sphere, so that the global mean temperature is . Post #8 discusses the magnitudes of these two components. Their spatial patterns and are shown below . (We didn’t try to estimate the slow part at 2100 because its amplitude is too small to get a good handle on its spatial structure — we were only using a single realization.)
The fast part resembles la Nina, with larger warming in the western than in the eastern tropical Pacific. The slow part provides a complimentary El-Nino like pattern, more or less as one would expect from the dynamic retardation argument of Clement et al (I am avoiding the word “thermostat” because this mechanism is not maintaining a particular temperature.) You also get the sense of the different tropical responses imprinting themselves on the North Pacific as expected from the known responses to ENSO. I am not sure why this distinction between the equatorial Pacific structure of the fast and slow responses shows up clearly here and not so clearly within the 20th century part of these simulations, which should be dominated by the fast response. (The oversimplification of there being only two effective time scales is probably to blame — ie, some of the equatorial response in the slow component may not be as slow as the global mean recalcitrant component discussed in post #8.) I am pretty confused about the whole range of issues related to forced responses and free multi-decadal variability in the tropical Pacific. But maybe there is something to the simple idea that when warming starts kicking in rapidly enough, the eastern equatorial Pacific holds it back temporarily.
[The views expressed on this blog are in no sense official positions of the Geophysical Fluid Dynamics Laboratory, the National Oceanic and Atmospheric Administration, or the Department of Commerce.]