HR: 09:30h
AN: A31D-06    [Abstracts]
TI: The Tropical Eastern Pacific Seasonal Cycle: Assessment of Errors and Mechanisms in IPCC AR4 Coupled Ocean-Atmosphere General Circulation Models
AU: * de Szoeke, S P
EM: simon.deszoeke@noaa.gov
AF: National Oceanic & Atmospheric Administration, Earth System Research Laboratory, Physical Sciences Division 3, 325 Broadway , Boulder, CO 80305, United States
AU: Xie, S
EM: xie@hawaii.edu
AF: University of Hawaii, International Pacific Research Center, 2525 Correa Road, Honolulu, HI 96822, United States
AU: Fairall, C
EM: chris.fairall@noaa.gov
AF: National Oceanic & Atmospheric Administration, Earth System Research Laboratory, Physical Sciences Division 3, 325 Broadway , Boulder, CO 80305, United States
AB: 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. Year-round, warmer SST and more precipitation are observed in the northern hemisphere. Briefly during boreal spring two precipitation maxima straddle the equator. This "double ITCZ" error 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 stronger precipitation in the northern hemisphere year- round. Simple metrics are introduced to diagnose model fidelity. Multi-model ensemble spread elucidates the coupled physics of various model solutions. Year-round southerly wind on the equator follows the atmospheric heating, maximum in September when the northern tropics are warmer and minimum in March when the ITCZ straddles the equator. The seasonally alternating ITCZ error generates two wind speed maxima per year, one northerly and one southerly, resulting in a cool bias of the equatorial ocean. The equatorial cold tongue temperature among the models is correlated to the equatorial scalar wind speed at -0.6. Errors in the meridional wind within 1000 km of the South American coast explain warm SST errors. SST on the equator (80-90° W, 2° S-2° N) is correlated to meridional wind speed at -0.66 among models. Northeasterly wind jets blowing over Central American isthmus in winter cool the SST in the eastern Pacific warm pool, contributing 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 there is an atmospheric connection linking the Central American northwesterlies and the southward shift of convergence. 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. This study 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.
DE: 3310 Clouds and cloud feedbacks
DE: 3337 Global climate models (1626, 4928)
DE: 3339 Ocean/atmosphere interactions (0312, 4504)
DE: 3360 Remote sensing
DE: 4227 Diurnal, seasonal, and annual cycles (0438)
SC: Atmospheric Sciences [A]
MN: 2007 Joint Assembly