Objective Identification of Cyclones and Their Circulation Intensity, and Climatology

Abstract

An updated procedure for objective identification and tracking of surface cyclones from gridded analyses is described. Prior smoothing of the raw data with a constant radius spatial filter is used to remove distortions related to the particular grid configuration used and to consistently admit a known scale of disturbance over the domain. Pitfalls of using central pressure or vorticity to infer cyclone intensity are illustrated, and a procedure for obtaining a more realistic areal measure of circulation is described. An automated selection procedure for storms having specific properties is outlined. Case selection is by computer search of a database of cyclone tracks, obtained from an application of the cyclone finding and tracking procedure to an extended series of gridded mean sea level pressure analyses. A selection of winter season cyclone statistics for both hemispheres is obtained from European Centre for Medium-Range Weather Forecasts analyses. Discrepancies with and between earlier studies appear more related to differing cyclone detection and counting procedures than to any intrinsic variability in analysis quality or cyclone occurrence. Results are found in agreement with the widely accepted manually produced climatologies only when a similar cyclone counting procedure is used. As in previous studies, Northern Hemisphere cyclones form and intensify near the eastern seaboards of Asia and North America, with maximum activity near SST gradients. They move eastward and poleward during their lives before weakening in the Gulf of Alaska and near Iceland. Southern Hemisphere cyclones are more evenly distributed around the hemisphere. They tend to form and intensify in middle latitudes, near SST gradients over open oceans, and near the eastern coasts of South America and Australia, and decay at higher latitudes. There is some evidence that newly formed and intensifying cyclones in both hemispheres possess a tighter inner structure than mature and decaying systems.