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Simulating the Equatorial Pacific Cold Tongue

Reference
Zheng, Y., Lin, J.-L. and Shinoda, T. 2012. The equatorial Pacific cold tongue simulated by IPCC AR4 coupled GCMs: Upper ocean heat budget and feedback analysis. Journal of Geophysical Research 117: 10.1029/2011JC007746.
Zheng et al. (2012) introduce their study by writing that "the equatorial Pacific is observed to have a minimum sea surface temperature (SST) that extends from the west coasts of the Americas into the central Pacific," which "extension of cool water is commonly referred to as the cold tongue (Wyrtki, 1981)." And they say that "it is generally argued that the Pacific cold tongue is maintained by horizontal advection of cold water from the east and by upwelling of cold water from the subsurface." The three researchers then proceed to examine "the contribution of ocean dynamics to sea surface temperature biases in the eastern Pacific cold tongue region in fifteen coupled general circulation models (CGCMs) participating in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4)," analyzing "twenty years (1980-1999) of the twentieth-century climate simulations from each model."

In describing what the three researchers learned, they are quoted directly below; but italics have been added to the many words they use that are indicative of various failures of the models to do specific things correctly.

"Errors in both net surface heat flux and total upper ocean heat advection significantly contribute to the excessive cold tongue in the equatorial Pacific." "The stronger heat advection in the models is caused by overly strong horizontal heat advection associated with too strong zonal currents, and overly strong vertical heat advection due to excessive upwelling and the vertical gradient of temperature." "The Bjerknes feedback in the coupled models is shown to be weaker than in observations, which may be related to the insufficient response of surface zonal winds to SST in the models and an erroneous subsurface temperature structure," such that "the cold tongue becomes colder than the cold tongue in the observations."

In final parting words the authors say that "more work is needed on the role of the ocean model and ocean-atmosphere feedback in the growth of the double-ITCZ pattern." And, it might be added, this is but one of many aspects of the challenge of attempting to correctly chart the development of global climate change over the coming decades and centuries.

Additional Reference
Wyrtki, K. 1981. An estimate of equatorial upwelling in the Pacific. Journal of Physical Oceanography 11: 1205-1214.

Archived 9 October 2012