INTENSE EXTRATROPICAL NORTHERN-HEMISPHERE WINTER CYCLONE EVENTS - 1899-1991

The climatology of intense northern hemisphere winter cyclone events i s extracted from a long series of daily mean sea level pressure analys es. An intense event is the occurrence of a low with a central pressur e less than or equal to 970 mbar. The majority of intense events are f ound near the ''semipermanent'' Aleutian and Icelandic Lows. Compared with the Pacific, intense events are more numerous in the Atlantic and occur over a larger geographical area. There is little or no trend in the number of intense events before about 1970. After 1970, there is a noticeable increase in the number of Pacific and Atlantic events. Th ere is a weak out-of-phase relationship between the Pacific and the At lantic events, with an indication of a 25-year periodicity. Temporal c orrelations between the number of intense Pacific events and sea surfa ce temperatures (SST) suggest a relationship between SST gradients ave raged between the equator and midlatitudes and cyclone behavior. Weak correlations suggest that a similar relationship does not exist for th e intense events in the Atlantic.