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An Objective Cyclone Climatology for the Southern Hemisphere

Mark R. SinclairNational Institute of Water and Atmospheric Research Ltd., Wellington, New Zealand

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Abstract

An objective method is developed and used to derive a climatology of centers of cyclonic vorticity for the Southern Hemisphere, based on twice-daily European Centre for Medium-Range Weather Forecasts (ECMWF) 1000-hPa analyses during 1980–86. These centers were computed as local minima of geostrophic relative vorticity ζg, extending previous studies based on pressure minima. This use of ζg, avoids a bias favoring slower and/or deeper cyclones that occurs when pressure is used and includes a large number of additional mobile vorticity centers in the 45°–55°S band that are missed where a local pressure minimum cannot be found. An automated tracking algorithm similar to that of Murray and Simmonds is used to match predicted location, pressure, and ζg from previous track history with available centers.

Large numbers of ζg centers were found south of 60°S and near the midlatitude continents, as in previous studies. These were a mixture of migratory centers and other fixed topographic features. The maxima around Antarctica may include many spurious centers that are artifacts of a fictitious surface anticyclone over the elevated continent. Maxima around East Antarctica were located near katabatic-prone stations. Large counts near the three midiatitude landmasses were mostly heat lows and lee troughs. These stationary orographic features were eliminated to retain just the traveling disturbances that dominate the weather and climate of the region. These mobile centers were distributed much more uniformly. When centers were counted just once per grid square per cyclone, the resulting “track density” maximized year-round in a belt near 50°S rather than within the cyclone “graveyard” of the circumpolar trough, in good agreement with baroclinic storm tracks obtained elsewhere from eddy statistics. A second maximum associated with the subtropical jet was found during winter and spring near 40°S in the New Zealand-Pacific sector. Intense cyclones, stratified by ζg, occurred most frequently in winter near New Zealand, east of South America, and in the southern Indian Ocean. An apparent increase in cyclone numbers in early 1933 coincided with the introduction of envelope orography at the ECMWF.

Abstract

An objective method is developed and used to derive a climatology of centers of cyclonic vorticity for the Southern Hemisphere, based on twice-daily European Centre for Medium-Range Weather Forecasts (ECMWF) 1000-hPa analyses during 1980–86. These centers were computed as local minima of geostrophic relative vorticity ζg, extending previous studies based on pressure minima. This use of ζg, avoids a bias favoring slower and/or deeper cyclones that occurs when pressure is used and includes a large number of additional mobile vorticity centers in the 45°–55°S band that are missed where a local pressure minimum cannot be found. An automated tracking algorithm similar to that of Murray and Simmonds is used to match predicted location, pressure, and ζg from previous track history with available centers.

Large numbers of ζg centers were found south of 60°S and near the midlatitude continents, as in previous studies. These were a mixture of migratory centers and other fixed topographic features. The maxima around Antarctica may include many spurious centers that are artifacts of a fictitious surface anticyclone over the elevated continent. Maxima around East Antarctica were located near katabatic-prone stations. Large counts near the three midiatitude landmasses were mostly heat lows and lee troughs. These stationary orographic features were eliminated to retain just the traveling disturbances that dominate the weather and climate of the region. These mobile centers were distributed much more uniformly. When centers were counted just once per grid square per cyclone, the resulting “track density” maximized year-round in a belt near 50°S rather than within the cyclone “graveyard” of the circumpolar trough, in good agreement with baroclinic storm tracks obtained elsewhere from eddy statistics. A second maximum associated with the subtropical jet was found during winter and spring near 40°S in the New Zealand-Pacific sector. Intense cyclones, stratified by ζg, occurred most frequently in winter near New Zealand, east of South America, and in the southern Indian Ocean. An apparent increase in cyclone numbers in early 1933 coincided with the introduction of envelope orography at the ECMWF.

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