Understanding the Basin Asymmetry in Surface Response to Sudden Stratospheric Warmings from an Ocean-Atmosphere Coupled Perspective

The canonical tropospheric response to a weakening of the stratospheric vortex-an equatorward shift of the eddy-driven jet-is mostly limited to the North Atlantic following sudden stratospheric warmings (SSWs). A coherent change in the Pacific eddy-driven jet is notably absent. Why is this so? Using daily reanalysis data, we show that air-sea interactions over the North Pacific are responsible for the basin-asymmetric response to SSWs. Prior to the onset of some SSWs, their tropospheric precursors produce a dipolar SST pattern in the North Pacific, which then persists as the stratospheric polar vortex breaks down following the onset of the SSW. By reinforcing the lower-tropospheric baroclinicity, the dipolar SST pattern helps sustain the generation of baroclinic eddies, strengthening the near-surface Pacific eddy-driven jet and maintaining its near-climatological-mean state. This prevents the jet from being perturbed by the downward influence of the stratospheric anomalies. As a result, these SSWs exhibit a highly basin-asymmetric surface response with only the Atlantic eddy-driven jet shifted equatorward. For SSWs occurring without the atmospheric precursors in the North Pacific troposphere, the dipolar SST pattern is absent due to the lack of the atmospheric forcing. In the absence of the dipolar SST pattern and the resultant eddy-mean flow feedbacks, these SSWs exhibit a basin-symmetric surface response with both the Atlantic and the Pacific eddy-driven jets shifted equatorward. Our results provide an ocean-atmosphere coupled perspective on stratosphere-troposphere interaction following SSW events and have potential for improving subseasonal to seasonal forecasts for surface weather and climate.

Automated summary

The study shows that air-sea interactions over the North Pacific lead to the basin-asymmetric response to sudden stratospheric warmings (SSWs), while SSWs occurring without atmospheric precursors exhibit a basin-symmetric response.