A TROPICAL INFLUENCE ON GLOBAL CLIMATE

A potential influence of tropical sea surface temperature on the globa l climate response to a doubling of the CO2 concentration is tested us ing an atmospheric general circulation model coupled to a slab mixed l ayer ocean. The warming is significantly reduced when sea surface temp eratures in the eastern equatorial Pacific cold tongue region between latitudes 2.25 degrees N and 2.25 degrees S are held at the control si mulation values. Warming of the global mean temperature outside of the cold tongue region is reduced from 2.4 degrees C in the unconstrained case to 1.9 degrees C when the sea surface temperature constraint is applied. The decrease in the warming results from a positive net heat flux into the ocean cold tongue region and implicit heat storage in th e subsurface ocean, induced by horizontal atmospheric heat fluxes. The reduced surface temperature warming outside of the cold tongue region is due to reduction in the downward longwave radiative flux at the su rface, caused in turn by reduced atmospheric temperature and moisture. The global mean surface temperature responds to the heat storage in t he ocean as if the global mean radiative forcing due to the doubled CO 2 (approximately 4 W m(-2)) was reduced by the value of che global mea n heat flux into the ocean. This mechanism also provides a possible ex planation for the observed high correlation on interannual timescales between the global mean tropospheric temperature and sea surface tempe rature in the eastern tropical Pacific. The results emphasize the impo rtance of correctly modeling the dynamical processes in the ocean and atmosphere that help determine the sea surface temperature in the equa torial eastern Pacific, in addition to the thermodynamical processes, in projecting global warming.