A 15-Year Climatology of Northern Hemisphere 500 mb Closed Cyclone and Anticyclone Centers

Abstract

A 15-year (1963?77) Northern Hemisphere 2 ? 5 degree latitude-longitude gridded dataset of 500 mb geopotential heights has been used to construct a climatology of 500 mb closed circulation centers. These centers, defined by at least one closed 30 m contour around a central minimum or maximum geopotential height value, were identified objectively between 24° and 82°N from the twice-daily analysis grids. Tracks for all closed circulation centers were computed to establish genesis and lysis distributions, and to examine the monthly, seasonal and interannual variability characteristics of the closed circulation center distributions for specified regions. The occurrence of closed cyclone centers is maximized north of and within the main belt of westerlies extending from northeast Asia to the Gulf of Alaska near 50°N, and extending from eastern Canada and the extreme northeast United States to southeast of Greenland and west of the United Kingdom. Their occurrence is also maximized south of the main belt of westerlies in a band extending from the east-central Atlantic Ocean across southern Europe to the Caspian Sea and central Asia. Important regional features include cool season maxima over the Mediterranean basin and the southwestern United States, and year-round maxima south of the jet stream extending from the eastern Pacific into the southwestern United States. The occurrence of closed anticyclone centers is maximized over the subtropical oceans in all seasons and over the subtropical continents during the summer. These centers show a tendency to be displaced substantially northward along the west coasts of both North America and Europe. Closed anticyclone center maxima are also noted north of the main belt of westerlies in the North Pacific and Atlantic ocean basins, and in a band extending from northwestern Europe to central Asia. The anticyclone centers north of the jet stream coincide with the locations of some of the persistent geopotential height anomalies, low pass geopotential height anomalies and low pass geopotential height variance maxima reported elsewhere in the literature. Genesis and lysis regions tend to coincide for both cyclone and anticyclone centers south of the jet stream, indicative of quasi-stationary, equivalent barotropic disturbances. In some of the more baroclinically active regions of higher latitudes the closed cyclone center genesis regions tend to be found somewhat upstream of lysis regions, suggestive of propagating disturbances. Likewise, anticyclone genesis centers tend to be somewhat to the east of the anticyclone frequency maxima (especially over high latitude ocean basins), suggesting retrogression. Finally, there is considerable monthly, seasonal and interannual variability for both closed cyclone and anticyclone centers in our sample period.