Stratospheric Sudden Warming (SSW) events in an idealized atmospheric
General Circulation Model (GCM), with a focus on the predictability of
the stratosphere's impact on the troposphere. Baldwin and Dunkerton
(2001) find a seemingly downward propagating signal in the Northern
Annular Mode following SSWs, in which the tropospheric jet shifts
equatorward a few days after the abrupt weakening of the stratospheric
polar vortex, and remains, on average, in this negative index state for
up to two months following the event. This behavior was captured in a
primitive equation GCM by Gerber and Polvani (2009). Here we use these
simulated warming events to examine and quantify the predictability of
the tropospheric signal. Given an SSW event in a control model
integration, we run a series of ensemble forecasts around the original
event to determine the sensitivity of the tropospheric response to
changes in the baroclinic and planetary scale eddies. We find that the
tropospheric response to the weakening of the stratospheric vortex is
robust, but that the recovery of the vortex itself is sensitive to the
tropospheric flow. Thus the predictability of the tropospheric
circulation depends on two way interactions between the stratosphere
and troposphere succeeding the SSW event.
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