Abstract
Relatively little is known about coherent vortices near the extratropical tropopause, even with regard to basic facts about their frequency of occurrence, longevity, and structure. This study addresses these issues through an objective census of observed tropopause vortices. The authors test a hypothesis regarding vortex-merger asymmetry where cyclone pairs are repelled and anticyclone pairs are attracted by divergent flow due to frontogenesis. Emphasis is placed on arctic vortices, where jet stream influences are weaker, in order to facilitate comparisons with earlier idealized numerical simulations.
Results show that arctic cyclones are more numerous, persistent, and stronger than arctic anticyclones. An average of 15 cyclonic vortices and 11 anticyclonic vortices are observed per month, with maximum frequency of occurrence for cyclones (anticyclones) during winter (summer). There are are about 47% more cyclones than anticyclones that survive at least 4 days, and for longer lifetimes, 1-day survival probabilities are nearly constant at 65% for cyclones, and 55% for anticyclones. Mean tropopause potential-temperature amplitude is 13 K for cyclones and 11 K for anticyclones, with cyclones exhibiting a greater tail toward larger values.
An analysis of close-proximity vortex pairs reveals divergence between cyclones and convergence between anticyclones. This result agrees qualitatively with previous idealized numerical simulations, although it is unclear to what extent the divergent circulations regulate vortex asymmetries.
Corresponding author address: Gregory J. Hakim, Department of Atmospheric Sciences, University of Washington, Box 351640, Seattle, WA 98195-1640. Email: hakim@atmos.washington.edu
