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Low-Frequency Variability of Southern Hemisphere Sea Level Pressure and Weather System Activity

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  • 1 National Institute of Water and Atmospheric Research, Ltd., Wellington, New Zealand
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Abstract

This study examines the month-to-month variations in the tracks of Southern Hemisphere weather systems and their relation to low-frequency circulation variability. Cyclones and anticyclones are identified and tracked from ECMWF analyses during 1980–94 via an automated method and the principal patterns of variation identified by EOF analysis of monthly track density anomaly fields. Only the first three EOFs of cyclone track density involving about one-third of the total track variance were distinguishable from noise. Spatial patterns derived from both unrotated and rotated EOF analysis were not reproducible on subsets of the data, pointing to secular changes in the variance structure of the cyclone dataset. An increase in cyclone numbers over the Southern Ocean during the 1980s suggested that detection of small-scale cyclones is sensitive to changes in data coverage and analysis procedure, as associated changes in the mean circulation were small during this period. EOFs of anticyclone track data were found to be more robust, indicating a variety of seesaw patterns across the hemisphere.

The leading modes of sea level pressure variability are found to be associated with regional variations in cyclone and anticyclone activity. The first pressure EOF, the so-called high-latitude mode, modulates cyclone activity between middle and high latitudes, with increased (decreased) westerlies near 55°–56°S accompanied by more (fewer) cyclones in the circumpolar regions and fewer (more) in middle latitudes. The second and third EOFs have centers of action near 60°S, 120°W and 55°S, 165°W, respectively, and are linked with blocking activity in these two regions. Blocks in the New Zealand sector occur in conjunction with a zonal wavenumber 3 pattern, while southeast Pacific blocks have no significant correlations outside the Pacific. A coherent cyclone response to ENSO was also found. During El Niño winters, increased cyclone activity occurs in a band spiraling southeastward from the subtropical Pacific toward South America, while fewer cyclones are found across the subtropical Indian Ocean, Australasia, and the southwest Pacific. During La Niñas, these patterns are almost exactly reversed, suggesting a predominantly linear cyclone response to ENSO.

Corresponding author address: Dr. Mark R. Sinclair, NIWA, P.O. Box 14-901, Kilbirnie, Wellington, New Zealand.

Email: msinclair@niwa.cri.nz

Abstract

This study examines the month-to-month variations in the tracks of Southern Hemisphere weather systems and their relation to low-frequency circulation variability. Cyclones and anticyclones are identified and tracked from ECMWF analyses during 1980–94 via an automated method and the principal patterns of variation identified by EOF analysis of monthly track density anomaly fields. Only the first three EOFs of cyclone track density involving about one-third of the total track variance were distinguishable from noise. Spatial patterns derived from both unrotated and rotated EOF analysis were not reproducible on subsets of the data, pointing to secular changes in the variance structure of the cyclone dataset. An increase in cyclone numbers over the Southern Ocean during the 1980s suggested that detection of small-scale cyclones is sensitive to changes in data coverage and analysis procedure, as associated changes in the mean circulation were small during this period. EOFs of anticyclone track data were found to be more robust, indicating a variety of seesaw patterns across the hemisphere.

The leading modes of sea level pressure variability are found to be associated with regional variations in cyclone and anticyclone activity. The first pressure EOF, the so-called high-latitude mode, modulates cyclone activity between middle and high latitudes, with increased (decreased) westerlies near 55°–56°S accompanied by more (fewer) cyclones in the circumpolar regions and fewer (more) in middle latitudes. The second and third EOFs have centers of action near 60°S, 120°W and 55°S, 165°W, respectively, and are linked with blocking activity in these two regions. Blocks in the New Zealand sector occur in conjunction with a zonal wavenumber 3 pattern, while southeast Pacific blocks have no significant correlations outside the Pacific. A coherent cyclone response to ENSO was also found. During El Niño winters, increased cyclone activity occurs in a band spiraling southeastward from the subtropical Pacific toward South America, while fewer cyclones are found across the subtropical Indian Ocean, Australasia, and the southwest Pacific. During La Niñas, these patterns are almost exactly reversed, suggesting a predominantly linear cyclone response to ENSO.

Corresponding author address: Dr. Mark R. Sinclair, NIWA, P.O. Box 14-901, Kilbirnie, Wellington, New Zealand.

Email: msinclair@niwa.cri.nz

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