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The South Pacific Pressure Trend Dipole and the Southern Blob

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  • 1 1 Department of Geophysics, Universidad de Chile, Santiago, Chile
  • | 2 2 School of Geography, Environment and Earth Sciences; Victoria University of Wellington, New Zealand
  • | 3 3 Climatología, Dirección Meteorológica de Chile, Santiago, Chile
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Abstract

During the last four decades, the sea level pressure has been decreasing over the Amundsen-Bellingshausen Sea (ABS) region and increasing between 30-40°S from New Zealand to Chile, thus forming a pressure trend dipole across the South Pacific. The trends are strongest in austral winter and have influenced the climate of West Antarctica and South America. The pressure trends have been attributed to decadal variability in the tropics, expansion of the Hadley cell and an associated positive trend of the Southern Annular Mode, but these mechanisms explain only about half of the pressure trend dipole intensity. Experiments conducted with two atmospheric models indicate that upper ocean warming over the subtropical southwest Pacific (SSWP), termed the Southern Blob, accounts for about half of the negative pressure trend in the ABS region and nearly all the ridging /drying over the eastern subtropical South Pacific, thus contributing to the central Chile megadrought. The SSWP warming intensifies the pressure trend dipole through warming the troposphere across the sub-tropical South Pacific and shifting the mid-latitude storm track poleward into the ABS. Multi-decadal periods of strong SSWP warming also appears in fully coupled pre-industrial simulations, associated with a pressure trend dipole and reduction in rainfall over the central tropical Pacific, thus suggesting a natural origin of the Southern Blob and its teleconnection. However, the current warming rate exceeds the range of natural variability, implying a likely additional anthropogenic contribution.

Corresponding author: René D. Garreaud, Department of Geophysics, Universidad de Chile, Blanco Encalada 2002, Santiago, CHILE. E-mail: rgarreau@dgf.uchile.cl

Abstract

During the last four decades, the sea level pressure has been decreasing over the Amundsen-Bellingshausen Sea (ABS) region and increasing between 30-40°S from New Zealand to Chile, thus forming a pressure trend dipole across the South Pacific. The trends are strongest in austral winter and have influenced the climate of West Antarctica and South America. The pressure trends have been attributed to decadal variability in the tropics, expansion of the Hadley cell and an associated positive trend of the Southern Annular Mode, but these mechanisms explain only about half of the pressure trend dipole intensity. Experiments conducted with two atmospheric models indicate that upper ocean warming over the subtropical southwest Pacific (SSWP), termed the Southern Blob, accounts for about half of the negative pressure trend in the ABS region and nearly all the ridging /drying over the eastern subtropical South Pacific, thus contributing to the central Chile megadrought. The SSWP warming intensifies the pressure trend dipole through warming the troposphere across the sub-tropical South Pacific and shifting the mid-latitude storm track poleward into the ABS. Multi-decadal periods of strong SSWP warming also appears in fully coupled pre-industrial simulations, associated with a pressure trend dipole and reduction in rainfall over the central tropical Pacific, thus suggesting a natural origin of the Southern Blob and its teleconnection. However, the current warming rate exceeds the range of natural variability, implying a likely additional anthropogenic contribution.

Corresponding author: René D. Garreaud, Department of Geophysics, Universidad de Chile, Blanco Encalada 2002, Santiago, CHILE. E-mail: rgarreau@dgf.uchile.cl
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