Varying stratospheric responses to tropical Atlantic SST forcing from early to late winter

Abstract

Using multiple reanalysis datasets and model simulations, we begin in this study by isolating the tropical Atlantic Ocean (TAO) sea surface temperature (SST) signals that are independent from ENSO, and then investigate their influences on the northern winter stratosphere. It is revealed that TAO SST forcing does indeed have significant effects on the northern winter stratosphere, but these effects vary from early to late winter in a way that explains the overall insignificant effect when the seasonal average is considered. The stratospheric polar vortex is anomalously weaker/warmer in November–December, stronger/colder in January–March, and weaker/warmer again in April–May during warm TAO years. The varying impacts of the TAO forcing on the extratropical stratosphere are related to a three-stage response of the extratropical troposphere to the TAO forcing during cold season. The tropospheric circulation exhibits a negative North Atlantic Oscillation–like response during early winter, an eastward propagating Rossby wave pattern in mid-to-late winter, and a meridional dipole over North America in spring. Associated with this is varying planetary wave activity in the stratosphere, manifested as an increase in early winter, a decrease in mid-to-late winter, and an increase again in spring. The varying modulation of stratospheric circulation by TAO forcing is consistently confirmed in three reanalysis datasets, and model simulations (fully coupled model and its component AGCM). The exception to the robustness of this verification is that the circumpolar wind response in the fully coupled model is relatively weaker, and that in its component AGCM appears a month later than observed.