The aqua-planet experiment (APE): Response to changed meridional SST profile

Abstract Th is paper explores the sensitivity of Atmospheric General Circulation Model (AGCM) sim ulations to changesin the meridional distri bution of sea surface tem perattu-e (SST). The sim ulatio ns are for an aqua-planet, a watercovered Eaith with no land, orography or sea-ice and with specified zonally symmetric SST. Sim ulations from 14AGCMs developed for Nu merical Wea ther Prediction and climate applications are compai·ed. Four experimentsare performed to study the sensitivity to the meridional SST profile . These profiles range from one in which theSST gradient continues to the equator to one which is flat approaching the equator, a ll with the sa me maximu mSST at the equator.The zonal me.ai1 circu lation of all models shows strong sensitivity to latitudinal distribution of SST. The Hadleycirculation weakens and shifts polewai·d as the SST profile fla ttens in the tropics. One q uestion of interest is theformation of a double versus a single ITCZ. There is a large varia tion between models of the strength of the ITCZand where in the SST experiment sequence they transition from a single to double ITCZ. The SST profiles a redefi ned such tha t as the equatorial SST grad ient flattens, the ma ximum gradient increases and moves poleward.Th is leads to a wea kening of the m id-latitude jet accompa nied by a poleward sh ift of the jet core. Also consid-ered ai·e tropical wave activity a nd tropical precipitation frequency distributions. The details of each va ry greatlybetwe.en models , both with a given SST and in the response to the change in SST.One additional experiment is included to exam ine the sensitiv ity to an off-equatorial SST maximum . The u p-wa rd branch of the Had ley circu lation follows the SST maximum off the equator. The models tha t form a singleprecipita tion max imum when the ma ximum SST is on the equator shift the precipitation ma ximu m off equator andkeep it centered over the SST maxim um. Those that form a double w ith mini mum on the equatorial maximu mSST shift the double structu re off the equator, keeping the m inim um over the maximum SST. In both situationsonly modest cha nges appear in the shi fted profile of zonal average precipitation. When the upwai·d branch of the Hadley circulation moves into the hemisphe.re with SST maxim um, the zona l average zona l, meridiona l andvertical winds all indicate that the Hadley cell in the other hemisphere dominates.Keywords comparison of atmospheric general circulation models (GCMs); sea sw·face temperature profile; Hadleycirculation; intertropical convergence zone (ITCZ); precipitation

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