This event has been canceled.

Speaker – Peter Hitchcock, Cornell University, Dept. Earth and Atmospheric Sciences, Ithaca, NY (hosted by W. Robinson)

Seminar title – The Stratospheric Polar Vortex and its Influence on Surface Climate

Abstract – Each winter, sharp gradients in radiative heating around the polar night generate strong westerly winds in the Arctic stratosphere. This stratospheric polar vortex is subject to dramatic dynamical variability: roughly two out of every three winters, rapidly amplifying planetary-scale Rossby waves can entirely disrupt this vortex, reversing the westerly flow in events known as stratospheric sudden warmings. While the onset of these events occurs rapidly (over a week or so), the

anomalous stratospheric flow can persist for several months.

There is clear evidence that these stratospheric perturbations exert a significant downward influence on the near-surface flow. They produce an equatorward shift of the North Atlantic eddy-driven jet, a response that projects strongly on to the North Atlantic Oscillation. More recently they have also been implicated in producing strong cold-air outbreaks over North America.  Stratospheric sudden warmings provide considerable potential skill to subseasonal and seasonal forecasts, and are receiving
increasing attention from the operational community and from the public.

In this talk I will review the evidence for this downward influence and argue that while some progress has been made in understanding the mechanisms responsible, many open questions remain. I will also present some results on the value of these events from a forecasting perspective.