NORTHERN-HEMISPHERE 500-HPA TROUGH MERGER AND FRACTURE - A CLIMATOLOGY AND CASE-STUDY

The results of an objective climatology of 500-hPa trough merger (defi ned as the amalgamation of two initially separate vorticity maxima) an d trough fracture (defined as the splitting of a single vorticity cent er into two separate vorticity centers) are presented for the Northern Hemisphere. The data source is the gridded National Meteorological Ce nter (NMC; now the National Centers for Environmental Prediction) anal yses available on CD-ROM on the NMC operational octagonal grid (381-km resolution at 60 degrees N) for the period September 1957 to May 1989 . A total of 2152 (3053) merger (fracture) events are identified. Merg er and fracture events tend to occur preferentially in the band of mid dle-latitude westerlies. Merger events exhibit a tendency to avoid maj or mountainous regions and show a somewhat higher frequency of occurre nce downstream of these regions. Fracture events cluster somewhat over eastern ocean basins, the southwestern United States, and northwester n Africa while also avoiding major mountainous regions. On average, tr ough genesis events exceed trough lysis events in northerly 500-hPa Bo w with the reverse being true for southerly Bow. Genesis events maximi ze downstream of major mountain barriers such as the Rockies and the S tanovoi and Altai-Sayan Mountains of Mongolia, east of Greenland, over western Europe, and across much of the North Pacific and North Atlant ic. An example of trough merger and associated cyclogenesis is present ed to help offer a perspective on the climatological results and illus trate qualitatively the important role that large-scale confluent defo rmation frontogenesis plays in the merger and fracture process.