A climatology of anticyclones and blocking for the Southern Hemisphere

Abstract

An automated procedure for locating and tracking high pressure centers is applied to 10 years of twice-daily ECMWF analyses during 1980-89 to survey the behavior of anticyclones in the Southern Hemisphere. Anticyclones are concentrated in the 25°-45°S band, with preferred regions consistent with previous studies except in the New Zealand (NZ)-South Pacific sector, where a double occurrence maximum noted in earlier studies was only weakly evident. Marked interannual variability was found in this region, with the poleward branch of maximum occurrence south of 50°S present in only 3 out of 10 years. Mean central pressure maximized about 10° south of the time-averaged ridge, with highest values in winter. Anticyclones poleward of 50°S involve an anomalous breakdown of the westerlies, with mean central pressures there 20-30 hPa higher than climatology. Highs tend to form and intensify in western regions of ocean basins and weaken and dissipate in the east of these oceans. Regions southeast of Australia, NZ, South America, and Africa are especially prone to rapid anticyclogenesis. Blocking episodes were objectively identified both from the anticyclone track data as persistent highs having central pressure exceeding the time-averaged mean sea level pressure (MSLP) by more than 20 hPa and as persistent positive MSLP anomalies. Weaker or more mobile blocks were found throughout the South Pacific east of NZ, with secondary maxima east of South America and southeast of Africa, as in previous studies. However, more persistent and intense blocks were largely confined to two regions of the South Pacific - southeast of NZ and near 55°S, 110°W, west of South America.