Sensitivities of Stratospheric Transport to Tropical Sea Surface Temperature (SST) Warming in an Idealized Aqua-planet Model

Tropical sea surface temperatures (SSTs) can impact stratospheric transport at different time scales. In this study, the sensitivities of the stratospheric transport to different patterns of tropical SST warming are investigated in an idealized aqua-planet model. The stratospheric transport is quantified with three metrics: the residual mean meridional circulation, the equivalent length of isentropic mixing and the stratospheric Age of Air (AOA). Among the metrics, the stratospheric AOA can be altered by the change of the residual mean circulation or isentropic mixing.

In response to a localized SST warming at the equator, the tropical upwelling branch of the Brewer-Dobson Circulation (BDC) is accelerated, and the isentropic mixing is enhanced in the lower troposphere near the subtropical jet. These changes are accompanied by a decrease of the AOA in the middle and high latitude stratosphere. When the latitude of the SST warming center is gradually shifted towards the subtropics, the responses of all the metrics are reduced in amplitude but with a similar structure. By contrast, when the zonal span of the equatorial SST heating is gradually broadened, the subtropical jet experiences a transition from a decelerated jet due to SST-forced tropical waves to an accelerated jet associated with increased meridional temperature gradient. Zonal wind in mid-latitude lower stratosphere varies similarly. These transitions of zonal flows alter the pattern of wave propagation in the lower stratosphere, leading to a dramatic change in the residual circulation from a shallow lower stratospheric response to a deep one. For all the experiments collectively, the AOA along the trajectory of BDC is correlated with the strength of the tropical upwelling, whereas the AOA near the subtropical jet is correlated with the strength of local isentropic mixing.