The tropical eastern Pacific seasonal cycle: An assessment of coupled ocean-atmosphere general circulation models
Simon de Szoeke, ESRL/PSD
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 and seasonal march of sea surface temperature (SST) and intertropical convergence zone (ITCZ) precipitation. The warmest SST and the preponderance of precipitation are observed in the northern hemisphere year-round. A brief season during boreal spring has two precipitation maxima astride the equator. The double ITCZ bias identified in coupled models more than 10 years ago persists in 3 of the models examined, while in 8 models the ITCZ alternates symmetrically between the hemispheres with the seasons. Only 3 models maintain the strongest precipitation in the northern hemisphere year-round as observed. Simple metrics are introduced to diagnose each models fidelity. The spread of the ensemble of models elucidates the coupled physics the various model solutions.
Wind on the equator follows the atmospheric heating, with a southerly maximum in September when the northern tropics are warmest, and with a minimum in March when the ITCZ straddles the equator. The seasonally alternating ITCZ bias generates two wind speed maxima per year, one northerly and one southerly, resulting in a cool bias of the equatorial ocean by upwelling, mixing, and evaporation. The equatorial cold tongue temperature among the models is correlated to the equatorial scalar wind speed at 0.6.
Within 1000 km of the South American coast the zonal wind is small and meridional wind affects the ocean. SST in the Nino 1+2 region (80-90W,0-10S) is correlated to meridional wind at 0.6 among models. Cool eastern equatorial SST can propagate westward by atmospheric Rossby waves and the coupled Bjerknes feedback. Biases in the meridional wind can explain the bias of a westward-displaced cold tongue.
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