J1.6 Stratosphere-Troposphere Dynamical Coupling and Tropospheric Predictability (2009

J1.6

Stratosphere-Troposphere Dynamical Coupling and Tropospheric Predictability

Clara Orbe, Columbia University, New York, NY; and E. P. Gerber and L. M. Polvani

We investigate stratosphere-troposphere coupling following

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.