Intense extratropical northern hemisphere winter cyclone events: 1899–1991

The climatology of intense northern hemisphere winter cyclone events is extracted from a long series of daily mean sea level pressure analyses. An intense event is the occurrence of a low with a central pressure less than or equal to 970 mbar. The majority of intense events are found 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. There is a weak out‐of‐phase relationship between the Pacific and the Atlantic events, with an indication of a 25‐year periodicity. Temporal correlations between the number of intense Pacific events and sea surface temperatures (SST) suggest a relationship between SST gradients averaged between the equator and midlatitudes and cyclone behavior. Weak correlations suggest that a similar relationship does not exist for the intense events in the Atlantic.