The onset and cessation of the “long rains” in eastern Africa and their interannual variability |
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Authors: | P Camberlin R E Okoola |
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Institution: | (1) UMR 5080 CNRS Centre de Recherches de Climatologie, Université de Bourgogne, Dijon, France, FR;(2) Department of Meteorology, University of Nairobi, Nairobi, Kenya, KE |
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Abstract: | Summary ?Thirty years (1958–1987) of daily rainfall data for Kenya and north eastern Tanzania are analysed with the aim to characterize
the interannual variability of the onset and cessation of the East African “long rains” (boreal spring). The leading principal
component (PC1) depicts consistent rainfall variations over much of the region. Cumulative PC1 scores for each year serve
to identify onset and cessation dates. The robustness of the dates derived from this method is demonstrated through the use
of an independent sample of stations. Their spatial representativity is assessed by daily rainfall composites. Average onset
occurs on March 25th, and cessation on May 21st. The interannual variability of the onset (standard deviation of 14.5 days) is larger than that of the withdrawal (10.3 days),
but the onset is also spatially much more consistent. Mean dates and dates in selected anomalous years agree well with previous
studies.
The relationship between onset time-series and large-scale atmospheric and oceanic fields is analysed. On a monthly time-scale,
interannual variations in “long rains” onset are associated with sea-surface temperature (SST) and sea-level pressure (SLP)
patterns that have a different sign for the Atlantic and Indian Oceans. A warm South Atlantic and a cool Indian Ocean are
associated with low and high SLP anomalies, respectively. These patterns are conducive to enhanced equatorial easterlies and
surface divergence over East Africa. This maintains the meridional branch (north–south orientated) of the Intertropical Convergence
Zone (ITCZ) further west, and the net result is a delayed onset of the “long rains”. Some of the South Atlantic features are
already present during January–February, suggesting some potential for monitoring interannual variations in the wet season
onset, based on SST and SLP patterns. Additional signals are found over Europe and the Mediterranean Sea in terms of the interaction
between the Northern Hemisphere extratropics and equatorial eastern Africa. A surge in the mid-tropospheric northerlies at
this time induces instability that may lead to an early onset event.
Received July 3, 2002; revised November 28, 2002; accepted December 7, 2002
Published online March 17, 2003 |
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