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Synoptic Activity in the Seas around Antarctica

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  • 1 School of Earth Sciences, University of Melbourne, Victoria, Australia
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Abstract

The recent NCEP–Department of Energy (DOE) Reanalysis-2 update of the original NCEP–NCAR dataset provides what is arguably the highest quality analyses spanning two decades available for the high southern latitudes. It therefore offers an excellent starting point from which to assemble a modern, comprehensive, and reliable picture of synoptic activity in the subantarctic region. This set, covering the “modern satellite” era from January 1979 to February 2000, is used herein. In addition, the exploration in this study has been conducted with sophisticated feature-tracking and trajectory analysis software.

It is shown that the high southern latitude cyclone system density is greatest in the Indian Ocean and to the south of Australia near, or to the south of, 60°S. The numbers in winter exceed those in summer, except over a few, but important, regions such as the Bellingshausen Sea. The Antarctic coastal region is confirmed as one of high cyclonicity, as is that in the northern part of the Antarctic Peninsula and over and to the north of Drake Passage. Cyclolysis is much more confined to the near-coastal region. The mean intensity, radius, and depth of subantarctic cyclones assume their largest values near 60°S.

It is shown that the rate of change of cyclone central pressure is not a particularly useful gauge of intensification in the Southern Hemisphere, where large spatial variations of climatological pressure are found. When appropriate adjustments are made, it is found that the “corrected” central pressure of cyclones is seen to increase along the track for most systems found south of 45°S. The paper also documents the range of starting points of 4-day 500-hPa trajectories that reach points on the Antarctic coast. The broad frequency distribution reflects the very energetic nature of synoptic activity in the region. The counts of cyclones in the 21 yr of NCEP–DOE analyses show negative trends over most of the subantarctic region. At the same time, however, the annual mean cyclone intensity, radius, and depth all exhibit increases.

Finally, the frequency of occurrence of rapidly developing cyclones (or “bombs”) in the subantarctic environment is determined, and it is found that they are not uncommon features. Their number shows a maximum in winter but, unlike the Northern Hemisphere situation, many are also found in summer.

Corresponding author address: Dr. Ian Simmonds, School of Earth Sciences, University of Melbourne, Victoria 3010, Australia. Email: simmonds@unimelb.edu.au

Abstract

The recent NCEP–Department of Energy (DOE) Reanalysis-2 update of the original NCEP–NCAR dataset provides what is arguably the highest quality analyses spanning two decades available for the high southern latitudes. It therefore offers an excellent starting point from which to assemble a modern, comprehensive, and reliable picture of synoptic activity in the subantarctic region. This set, covering the “modern satellite” era from January 1979 to February 2000, is used herein. In addition, the exploration in this study has been conducted with sophisticated feature-tracking and trajectory analysis software.

It is shown that the high southern latitude cyclone system density is greatest in the Indian Ocean and to the south of Australia near, or to the south of, 60°S. The numbers in winter exceed those in summer, except over a few, but important, regions such as the Bellingshausen Sea. The Antarctic coastal region is confirmed as one of high cyclonicity, as is that in the northern part of the Antarctic Peninsula and over and to the north of Drake Passage. Cyclolysis is much more confined to the near-coastal region. The mean intensity, radius, and depth of subantarctic cyclones assume their largest values near 60°S.

It is shown that the rate of change of cyclone central pressure is not a particularly useful gauge of intensification in the Southern Hemisphere, where large spatial variations of climatological pressure are found. When appropriate adjustments are made, it is found that the “corrected” central pressure of cyclones is seen to increase along the track for most systems found south of 45°S. The paper also documents the range of starting points of 4-day 500-hPa trajectories that reach points on the Antarctic coast. The broad frequency distribution reflects the very energetic nature of synoptic activity in the region. The counts of cyclones in the 21 yr of NCEP–DOE analyses show negative trends over most of the subantarctic region. At the same time, however, the annual mean cyclone intensity, radius, and depth all exhibit increases.

Finally, the frequency of occurrence of rapidly developing cyclones (or “bombs”) in the subantarctic environment is determined, and it is found that they are not uncommon features. Their number shows a maximum in winter but, unlike the Northern Hemisphere situation, many are also found in summer.

Corresponding author address: Dr. Ian Simmonds, School of Earth Sciences, University of Melbourne, Victoria 3010, Australia. Email: simmonds@unimelb.edu.au

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