Climatology and Interannual Variability of Boreal Spring Wet Season Precipitation in the Eastern Horn of Africa and Implications for Its Recent Decline

Brant Liebmann CIRES, University of Colorado Boulder, Boulder, Colorado
Physical Sciences Division, NOAA/Earth System Research Laboratory, Boulder, Colorado

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Ileana Bladé Grup de Meteorologia, Departament de Física Aplicada, Universitat de Barcelona, Barcelona, Spain

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Chris Funk U.S. Geological Survey, Earth Resources Observation and Science Center, Sioux Falls, South Dakota
Climate Hazards Group, Department of Geography, University of California, Santa Barbara, Santa Barbara, California

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Dave Allured CIRES, University of Colorado Boulder, Boulder, Colorado
Physical Sciences Division, NOAA/Earth System Research Laboratory, Boulder, Colorado

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Xiao-Wei Quan CIRES, University of Colorado Boulder, Boulder, Colorado
Physical Sciences Division, NOAA/Earth System Research Laboratory, Boulder, Colorado

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Martin Hoerling Physical Sciences Division, NOAA/Earth System Research Laboratory, Boulder, Colorado

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Andrew Hoell Physical Sciences Division, NOAA/Earth System Research Laboratory, Boulder, Colorado

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Pete Peterson Climate Hazards Group, Department of Geography, University of California, Santa Barbara, Santa Barbara, California

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Wassila M. Thiaw Climate Prediction Center, NOAA/National Centers for Environmental Prediction, Camp Springs, Maryland

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Abstract

The 1981–2014 climatology and variability of the March–May eastern Horn of Africa boreal spring wet season are examined using precipitation, upper- and lower-level winds, low-level specific humidity, and convective available potential energy (CAPE), with the aim of better understanding the establishment of the wet season and the cause of the recent observed decline. At 850 mb, the development of the wet season is characterized by increasing specific humidity and winds that veer from northeasterly in February to southerly in June and advect moisture into the region, in agreement with an earlier study. Equally important, however, is a substantial weakening of the 200-mb climatological easterly winds in March. Likewise, the shutdown of the wet season coincides with the return of strong easterly winds in June. Similar changes are seen in the daily evolution of specific humidity and 200-mb wind when composited relative to the interannual wet season onset and end, with the easterlies decreasing (increasing) several days prior to the start (end) of the wet season. The 1981–2014 decrease in March–May precipitation has also coincided with an increase in 200-mb easterly winds, with no attendant change in specific humidity, leading to the conclusion that, while high values of specific humidity are an important ingredient of the wet season, the recent observed precipitation decline has resulted mostly from a strengthening of the 200-mb easterlies. This change in the easterly winds appears to be related to an increase in convection over the Indonesian region and in the associated outflow from that enhanced heat source.

© 2017 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author e-mail: Ileana Bladé, ileanablade@ub.edu

Abstract

The 1981–2014 climatology and variability of the March–May eastern Horn of Africa boreal spring wet season are examined using precipitation, upper- and lower-level winds, low-level specific humidity, and convective available potential energy (CAPE), with the aim of better understanding the establishment of the wet season and the cause of the recent observed decline. At 850 mb, the development of the wet season is characterized by increasing specific humidity and winds that veer from northeasterly in February to southerly in June and advect moisture into the region, in agreement with an earlier study. Equally important, however, is a substantial weakening of the 200-mb climatological easterly winds in March. Likewise, the shutdown of the wet season coincides with the return of strong easterly winds in June. Similar changes are seen in the daily evolution of specific humidity and 200-mb wind when composited relative to the interannual wet season onset and end, with the easterlies decreasing (increasing) several days prior to the start (end) of the wet season. The 1981–2014 decrease in March–May precipitation has also coincided with an increase in 200-mb easterly winds, with no attendant change in specific humidity, leading to the conclusion that, while high values of specific humidity are an important ingredient of the wet season, the recent observed precipitation decline has resulted mostly from a strengthening of the 200-mb easterlies. This change in the easterly winds appears to be related to an increase in convection over the Indonesian region and in the associated outflow from that enhanced heat source.

© 2017 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author e-mail: Ileana Bladé, ileanablade@ub.edu
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