Life history of mobile troughs in the upper westerlies

Abstract

Increasing evidence indicates that surface cyclogenesis is predominantly a response to the approach of a preexisting trough at upper levels. A question then arises about the origin of the upper-level predecessor. As an initial approach to this question, mobile troughs in the major band of westerlies were crudely tracked in daily Northern Hemispheric 500-mb analysis during nine recent cold seasons. Locations or origin and termination of individual troughs were distributed over all longitudes, but births greatly exceeded deaths over and east of the Rocky Mountains in North America and the highlands of central Asia. Trough terminations dominated over the eastern portions of the oceans. Within the quasi-steady planetary waves, origins and terminations of the smaller mobile troughs occurred preferentially in northwesterly and southwesterly flow, respectively. More detailed studies of the structure showed prominant vertical and lateral shear in the time-averaged 500-mb flow, rapid growth of the perturbations through the depth of the tropopause, with a vertical tilt upshear only in the lower half, pronounced maximum amplitude near the tropopause, and a variety of circumstances in which troughs became organized in the belt of major westerlies. -from Author