Could stratospheric water vapor anomalies influence regional climate?

Variability in stratospheric water vapor (SWV) concentrations can arise from changes in the tropical cold-point temperature encountered by an air parcel as it ascends into the stratosphere. A uniform increase in SWV causes radiative cooling with a structure that enhances the equator-to-pole temperature gradient in the lower stratosphere. This presents a mechanism that may influence stratospheric variability on multi-annual timescales, the typical lifetime of a SWV anomaly. Changes in the stratospheric circulation have been shown to have the potential to dynamically influence the troposphere on timescales of up to several months. Therefore, a robust link between anomalies in SWV and changes in the tropospheric flow could present an atmospheric mechanism for a multi-annual signal in surface climate.

We investigate the changes in the stratospheric and tropospheric flow that result from a uniform change in SWV using an atmospheric general circulation model (AGCM). The circulation changes are led by the westerly zonal wind anomalies on the upper-poleward edge of the subtropical jets, which are driven in both hemispheres by the strong radiative cooling in the polar lower stratosphere.

We show that the tropospheric response to a uniform increase in SWV is consistent with a poleward shift in the eddy-driven jets. The surface response is largest in boreal winter in both hemispheres, and is focused in the North Atlantic region in the Northern hemisphere.

We compare the circulation changes that result from a uniform fixed SWV perturbation with those from more realistic anomalies in the AGCM. We assess whether changes in SWV similar to those seen in observations could influence the tropospheric circulation at a detectable level, and thus whether changes in SWV may be important for seasonal-to-decadal forecasting.