Northern Hemisphere 500-hPa Trough Merger and Fracture: A Climatology and Case Study

Abstract

The results of an objective climatology of 500hPa trough merger (defined as the amalgamation of two initially separate vorticity maxima) and trough fracture (defined as the splitting of a single vorticity center into two separate vorticity centers) are presented for the Northern Hemisphere. The data source is the gridded National Meteorological Center (NMC; now the National Centers for Environmental Prediction) analyses available on CD-ROM on the NMC operational octagonal grid (381-km resolution at 60°N) for the period September 1957 to May 1989. A total of 2152 (3053) merger (fracture) events are identified. Merger and fracture events tend to occur preferentially in the band of middle-latitude westerlies. Merger events exhibit a tendency to avoid major mountainous regions and show a somewhat higher frequency of occurrence downstream of these regions. Fracture events cluster somewhat over eastern ocean basins, the southwestern United States, and northwestern Africa while also avoiding major mountainous regions. On average, trough genesis events exceed trough lysis events in northerly 500-hPa flow with the reverse being true for southerly flow. Genesis events maximize downstream of major mountain barriers such as the Rockies and the Stanovoi and Altai-Sayan Mountains of Mongolia, east of Greenland, over western Europe, and across much of the North Pacific and North Atlantic. An example of trough merger and associated cyclogenesis is presented to help offer a perspective on the climatological results and illustrate qualitatively the important role that large-scale confluent deformation frontogenesis plays in the merger and fracture process.