The variety of forced atmospheric structure in response to tropical SST anomaly in the aqua-planet experiments

In this paper, we examine the steady state responses of models participating in the Aqua-Planet Experiment Project (APE) to the zonal asymmetry of equatorial sea surface temperature (SST) anomalies (SSTAs). Experiments were performed using three different SSTA distributions, i.e., two localized SSTAs with a common shape but different intensities, and an SSTA that varied with zonal wavenumber one. The structure of the responses obtained differs significantly among the models; however, some common features are also present. The principal features of the responses to the localized SSTAs are a positive precipitation anomaly over the warm SSTA, a widespread negative precipitation anomaly along the intertropical convergence zone, a pair of Rossby wavetrains along the equatorward flanks of mid-latitude westerly jets originating from a pair of upper tropospheric anticyclones that develop to the east of the warm SSTAs, and zonally wavelike precipitation and geopotential anomalies along the baroclinic zones. The structure of the tropical responses differs considerably from the Matsuno-Gill pattern, and the magnitude of the responses is almost proportional to the intensity of the localized SSTA in each of the models. The responses to the zonal wavenumber one SSTA are dominated by zonal wavenumber one structures. Around the longitudes of the warm (cold) SSTA, tropical precipitation increases (decreases). At longitudes east of the positive precipitation anomaly, the region of nearly zero absolute vorticity near the equator in the upper troposphere expands polewards, and the midlatitude westerly jets become narrower and stronger. To the west of the positive precipitation anomaly, the upper tropospheric region of nearly zero absolute vorticity shrinks, and the mid-latitude jets become weaker but broader, so that the regions of westerly winds extends to the equator, which results in the development of a zonal mean westerly wind anomaly around the equator. The longitudinal shift of the upper tropospheric westerly zonal wind anomaly relative to the precipitation anomaly is in marked contrast to that associated with the Walker circulation and the convection center around the Maritime Continent.