A quasi-Lagrangian vorticity budget of composite cyclone- anticyclone couplets accompanying North American polar air outbreaks.

A composite cyclone-anticyclone couplet is constructed from 4 synoptically similar cases of polar air outbreaks into the Caribbean from off the N American continent. A quasi-Lagrangian vorticity budget is then computed from these data for 2 consecutive 12h time periods. The results show that the divergence and twisting terms in the lower troposphere, the horizontal advection term in the middle troposphere, and the horizontal, vertical and system advection in the upper troposphere are primary importance in generating negative vorticity tendencies in the area toward (from) which the surface anticyclone (cyclone) is moving. In contrast, only the divergence term in the lower troposphere and horizontal advection term in the mid and upper troposphere are primarily responsible for the intensification and movement of the downstream cyclone. Computations suggest an apparent anticyclone vorticity source in the mid and upper troposphere and sink in the lower troposphere for the large-scale motions over the anticyclone region with the reverse true the downstream cyclone region due to sub grid-scale processes. -Authors