Volume 16, Number 16: 17 April 2013
In the introduction to their study of the origins of tropical-wide sea surface temperature (SST) biases in CMIP multi-model ensembles, Li and Xie (2012) write that "state-of-the-art coupled ocean-atmosphere general circulation models (CGCMs) suffer from large errors in simulating tropical climate, limiting their utility in climate prediction and projection." And describing the size of the several errors, they say they "are comparable or larger in magnitude than observed interannual variability and projected change in the 21st century."
Continuing, the two researchers state that what they call "well-known errors" include: (1) "too weak a zonal SST gradient along the equatorial Atlantic," citing Davey et al. (2002) and Richter and Xie (2008), (2) "an equatorial cold tongue that penetrates too far westward in the Pacific," citing Mechoso et al. (1995) and de Szoeke and Xie (2008), and (3) "too warm SSTs over the tropical Southeast Pacific and Atlantic, and a spurious double intertropical convergence zone," citing Lin (2007). What is even more disturbing, they indicate that "these errors have persisted in several generations of models for more than a decade [italics added]."
Closer inspection of zonal SST profiles along the equator is additionally said by Li and Xie to reveal "basin-wide offsets, most obvious in the Indian (Saji et al., 2006) but visible in the Pacific (de Szoeke and Xie, 2008), and Atlantic (Richter and Xie, 2008) Oceans." And they add that "it is unclear whether such basin-wide offset errors are limited to individual tropical basins or shared among all the basins."
At this point, Li and Xie go on to "examine the Climate of the Twentieth Century (20C3M) simulations (also termed as 'historical' runs) from 22 CMIP3 and 21 CMIP5 CGCMs for a 30-year period of 1970-99, together with their available Atmospheric Model Intercomparison Project (AMIP) runs that are forced with the observed SST." And in doing so, they find two types of tropical-wide offset biases. One of them "reflects the tropical mean SST differences from observations and among models, with a broad meridional structure and of the same sign across all basins of up to 2°C in magnitude," while the other "is linked to inter-model variability in the cold tongue temperatures in the equatorial Pacific and Atlantic."
In further describing their findings, the two researchers write that "the first-type offset error is ascribed to atmospheric model representation of cloud cover, with cloudy models biasing low in tropical-wide SST," while "the second type originates from the diversity among models in representing the thermocline depth, with deep thermocline models biasing warm in the equatorial cold tongues."
In light of the magnitude and significance of these two major errors, we can only hope that it will not take the world's climate modelers more than another decade to remedy them.
Sherwood, Keith and Craig Idso
References
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de Szoeke, S.P. and Xie, S.P. 2008. The tropical eastern Pacific seasonal cycle: Assessment of errors and mechanisms in IPCC AR4 coupled ocean-atmosphere general circulation models. Journal of Climate 21: 2573-2590.
Li, G. and Xie, S.-P. 2012. Origins of tropical-wide SST biases in CMIP multi-model ensembles. Geophysical Research Letters 39: 10.1029/2012GL053777.
Lin, J.L. 2007. The double-ITCZ problem in IPCC AR4 coupled GCMs: Ocean-atmosphere feedback analysis. Journal of Climate 20: 4497-4525.
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Richter, I. and Xie, S.-P. 2008. On the origin of equatorial Atlantic biases in coupled general circulation models. Climate Dynamics 31: 587-595.
Saji, N.H., Xie, S.-P. and Yamagata, T. 2006. Tropical Indian Ocean variability in the IPCC twentieth century climate simulations. Journal of Climate 19: 4397-4417.