Steven M. Cavallo and Gregory J. Hakim
Department of Atmospheric Sciences, University of Washington,Seattle, WA
Monthly Weather Review, 139, submitted.
Tropopause polar vortices (TPVs) are a commonly
observed, coherent circulation feature of the Arctic with typical radii
as large as approximately 800 km. Intensification of cyclonic TPVs has
been shown to be dominated by infrared radiation. Here we test the
hypothesis that while radiation alone may not be essential for TPV
genesis, radiation has a substantial impact on the long-term population
characteristics of cyclonic TPVs.
A numerical model is used to derive two ten-year climatologies of
TPVs for both winter and summer: a control climatology with radiative
forcing and an experimental climatology with radiative forcing
withheld. Results from the control climatology are first compared to
those from the NCEP--NCAR Reanalysis Project (NNRP), which indicates
a sensitivity to both horizontal grid resolution and the use of polar
filtering in the NNRP. Smaller horizontal grid resolution of 60-km
in the current study yields sample-mean cyclonic TPV radii that are
smaller by a factor of ~2 compared to NNRP, and vortex track
densities in the vicinity of the North Pole are considerably higher
compared to NNRP.
The experimental climatologies show that winter (summer) vortex
maximum amplitude is reduced by 22.3% (38.0%), with a net tendency
to weaken without radiation. Moreover, while the number and lifetime
of cyclonic TPVs change little in winter without radiation, number
decreases 12% and mean lifetime decreases 19% during summer without
radiation. These results suggest that dynamical processes are
primarily responsible for the genesis of the vortices, and that
radiation controls their maximum intensity and duration during
summer, when the destructive effect of ambient shear is weaker.