The tropical eastern Pacific seasonal cycle: Assessment of errors and mechanisms in IPCC AR4 coupled general circulation models

Warmer SST and more rain in the northern hemisphere are observed year-round in the tropical eastern Pacific. Southerly wind blows across the equator toward this atmospheric heating. Southerlies are minimal during boreal spring, when two precipitation maxima straddle the equator. The climate of the eastern tropical Pacific Ocean and atmosphere is analyzed in 15 coupled ocean-atmosphere models from 8 nations. Coupled models variously reproduce the observed meridional march with the seasons of sea surface temperature (SST) and intertropical convergence zone (ITCZ) precipitation. Most models have too much rain in the southern hemisphere so that rain is nearly symmetric about the equator in the annual mean.

In 8 out of 15 models the ITCZ alternates symmetrically between the hemispheres with the seasons. This seasonally alternating ITCZ error generates two wind speed maxima per year, one northerly and one southerly, resulting in a spurious equatorial cooling of SST in March, a semiannual cycle SST, and a cool bias of the equatorial ocean. March cold tongue temperature among the models is correlated to the scalar speed of cross-equatorial wind at -0.6. Models that simulate too weak a meridional asymmetry have weaker than observed meridional wind. The weak meridional wind explains a warm SST error within 1000 km of the South American coast, with a correlation of -0.66 among models.

Northeasterly wind jets blow over the Central American isthmus in winter and cool the SST in the eastern Pacific warm pool. In some models the strength of these winds contributes to the early demise of the northern ITCZ relative to observations. The February-April northerly wind bias on the equator is correlated to the antecedent December-February Central American Pacific wind speed at -0.87, suggesting an atmospheric connection between the Central American northeasterlies and the southward shift of the ITCZ.

The various representations of stratus clouds among models clearly affect the underlying SST, but their effect on the meridional atmospheric circulation is difficult to discern from the multi-model ensemble. This study identifies several features to be improved in atmospheric and coupled GCMs, including the northeasterly cross-Central American wind in winter and the meridional wind on the equator. It suggests that both the double ITCZ error and equatorial SST errors in the eastern Pacific could be alleviated by reducing errors in the climatology of meridional wind.