Steven M. Cavallo and Gregory J. Hakim
Department of Atmospheric Sciences, University of Washington,Seattle, WA
Monthly Weather Review, 137, in press.
Long-lived coherent vortices based on the tropopause are often found
over polar regions, where potential vorticity gradients are weaker than
in midlatitudes. Although these vortices are a commonly observed
feature of the Arctic, and can have lifetimes longer than one month,
little is known about the mechanisms that control their evolution.
This paper examines mechanisms of intensity change for a cyclonic
tropopause polar vortex (TPV) using an Ertel potential vorticity (EPV)
diagnostic framework.
Results from a climatology of intensifying cyclonic TPVs suggest that
the essential dynamics are local to the vortex, rather than a
consequence of larger scale processes. This fact motivates a case
study using a numerical model to investigate the role of diabatic
mechanisms in the growth and decay of a particular cyclonic vortex. A
component-wise breakdown of EPV reveals that cloud-top radiational
cooling is the primary diabatic mechanism that intensifies the TPV
during the growth phase. Increasing amounts of moisture become
entrained into the vortex core at later times near Hudson Bay, allowing
the destruction of potential vorticity near the tropopause due to
latent heating to become comparable to the radiational tendency to
create potential vorticity.