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An Objective Climatology of Tropical Plumes

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  • 1 Institute of Geophysics and Meteorology, University of Cologne, Cologne, Germany
  • | 2 School of Earth and Environment, University of Leeds, Leeds, United Kingdom
  • | 3 Institute of Geophysics and Meteorology, University of Cologne, Cologne, Germany
  • | 4 Institute of Flight Systems and Automatic Control, Technical University Darmstadt, Darmstadt, Germany
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Abstract

The first global objective climatology of tropical plumes (TPs), obtained from a novel algorithm based on gridded 10.8-μm brightness temperatures Tb, is presented for 1983–2006. TPs are defined as continuous cloud bands (>2000 km) crossing 15°N or 15°S with Tb anomalies of less than −20 K and a lifetime of at least 3 h. A minimum length-to-width ratio of 3 filters out elongated features. Numbers of identified TPs are sensitive to the chosen thresholds but not their geographical distribution and seasonal cycle.

TPs are an important indicator of tropical–extratropical interactions with impacts on radiation and moisture. TP occurrence during boreal winter is largely confined to oceanic regions with main maxima over the South Pacific and South Atlantic as well as the eastern North Atlantic and Pacific Oceans. The geographical distribution during boreal summer is similar, but with lower frequencies, except for monsoon-influenced regions. Interannual variations over the Indo-Pacific region are strongly related to El Niño. TPs often develop downstream of extratropical upper-level troughs propagating into low latitudes, particularly over the wintertime eastern North Pacific and North Atlantic, but also in regions where mean upper-level easterlies do not generally favor equatorward Rossby wave propagation. Synoptic-scale variations in the quasi-permanent cloud bands associated with the South Pacific and South Atlantic convergence zones frequently produce TP-like anomalies, which are climatologically associated with downstream upper-level troughs. Some regions also feature TPs associated with mesoscale tropical disturbances. The new TP algorithm will serve as a basis for more in-depth studies in the future.

Corresponding author address: Luise Fröhlich, Institut of Geophysics and Meteorology, University of Cologne, Kerpener Str. 13, 50937 Köln, Germany. E-mail: froehlil@uni-koeln.de

Abstract

The first global objective climatology of tropical plumes (TPs), obtained from a novel algorithm based on gridded 10.8-μm brightness temperatures Tb, is presented for 1983–2006. TPs are defined as continuous cloud bands (>2000 km) crossing 15°N or 15°S with Tb anomalies of less than −20 K and a lifetime of at least 3 h. A minimum length-to-width ratio of 3 filters out elongated features. Numbers of identified TPs are sensitive to the chosen thresholds but not their geographical distribution and seasonal cycle.

TPs are an important indicator of tropical–extratropical interactions with impacts on radiation and moisture. TP occurrence during boreal winter is largely confined to oceanic regions with main maxima over the South Pacific and South Atlantic as well as the eastern North Atlantic and Pacific Oceans. The geographical distribution during boreal summer is similar, but with lower frequencies, except for monsoon-influenced regions. Interannual variations over the Indo-Pacific region are strongly related to El Niño. TPs often develop downstream of extratropical upper-level troughs propagating into low latitudes, particularly over the wintertime eastern North Pacific and North Atlantic, but also in regions where mean upper-level easterlies do not generally favor equatorward Rossby wave propagation. Synoptic-scale variations in the quasi-permanent cloud bands associated with the South Pacific and South Atlantic convergence zones frequently produce TP-like anomalies, which are climatologically associated with downstream upper-level troughs. Some regions also feature TPs associated with mesoscale tropical disturbances. The new TP algorithm will serve as a basis for more in-depth studies in the future.

Corresponding author address: Luise Fröhlich, Institut of Geophysics and Meteorology, University of Cologne, Kerpener Str. 13, 50937 Köln, Germany. E-mail: froehlil@uni-koeln.de
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