A Tropical Influence on Global Climate

A potential influence of tropical sea surface temperature on the global climate response to a doubling of the CO2 concentration is tested using an atmospheric general circulation model coupled to a slab mixed layer ocean. The warming is significantly reduced when sea surface temperatures in the eastern equatorial Pacific cold tongue region between latitudes 2.258N and 2.258S are held at the control simulation values. Warming of the global mean temperature outside of the cold tongue region is reduced from 2.48C in the unconstrained case to 1.98C 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 the 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 surface, caused in turn by reduced atmospheric temperature and moisture. The global mean surface temperature responds to the heat storage in the ocean as if the global mean radiative forcing due to the doubled CO2 (approximately 4 W m22) was reduced by the value of the global mean heat flux into the ocean. This mechanism also provides a possible explanation for the observed high correlation on interannual timescales between the global mean tropospheric temperature and sea surface temperature in the eastern tropical Pacific. The results emphasize the importance of correctly modeling the dynamical processes in the ocean and atmosphere that help determine the sea surface temperature in the equatorial eastern Pacific, in addition to the thermodynamical processes, in projecting global warming.