Start Date/Time: Thursday, December 10, 2009, 3:30 PM
Ending Date/Time: Thursday, December 10, 2009, 4:20 AM
Location: ATG 310C
***Atmos. & Climate Dynamics (ATM S 521, Atmospheric Dynamics), Mike Wallace, Coordinator***
Speaker: Gang Chen, Asst. Prof., Earth & Space Sciences, Cornell University
Title: "Eddy Feedbacks and the Timescale of the Annular Modes"
I will present a framework to quantify the strength of the eddy feedback and the persistence of the zonal index. The framework is tested in an idealized model in which the zonal index is varied with different frictional damping rates but the climatological jet latitude and shape are kept unchanged with a constant external forcing.
The model can generate a nearly identical climatology and leading mode of the zonal mean zonal wind for different frictional damping rates, except when the jet undergoes a regime transition. For those experiments without a regime transition, as the surface friction is increased, the strength of eddy feedback is enhanced, but the zonal index becomes less persistent. A simple feedback model suggests that the e-folding decorrelation time scale of the zonal index can be determined by the frictional damping rate and the strength of eddy feedback. The strength of eddy feedback is found to be related to the instantaneous vertical wind shears near the surface controlled by the frictional damping. Furthermore, the climate response to an external zonal torque is proportional to the decorrelation time scale, although our simple prediction overestimates the climate response by a factor of two.