Seasonal variability of the polar stratospheric vortex in an idealized AGCM with varying tropospheric wave forcing

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Wednesday, 7 January 2015: 4:45 PM
212A West Building (Phoenix Convention Center - West and North Buildings)
Aditi Sheshadri, MIT, Cambridge, MA; and R. A. Plumb and E. P. Gerber

The seasonal variability of the polar stratospheric vortex is studied in a simplified AGCM driven by specified equilibrium temperature distributions. Seasonal variations in equilibrium temperature are imposed in the stratosphere only, enabling the study of stratosphere-troposphere coupling on seasonal timescales, without the complication of an internal tropospheric seasonal cycle. The model is forced with different shapes and amplitudes of simple bottom topography, resulting in a range of stratospheric climates. The effect of these different kinds of topography on the seasonal variability of the strength of the polar vortex, the average timing and variability in timing of the final breakup of the vortex (final warming events), the conditions of occurrence and frequency of midwinter warming events, and the impact of the stratospheric seasonal cycle on the troposphere are explored. The inclusion of wavenumber 1 and wavenumber 2 topographies results in very different stratospheric seasonal variability. Hemispheric differences in stratospheric seasonal variability are recovered in the model with appropriate choices of topography. In the model experiment with a realistic Northern Hemisphere-like frequency of midwinter warming events, the distribution of the intervals between these events suggest that the model has no year to year memory. There is, however, some evidence in the model for the persistence of weakened winds through the winter. Tropospheric annular mode timescales are large in the absence of topography and reduce when topography is included, leading to a large impact of the stratospheric seasonal cycle on the tropospheric jet in the absence of topography.