D. Shanko et P. Camberlin, THE EFFECTS OF THE SOUTHWEST INDIAN-OCEAN TROPICAL CYCLONES ON ETHIOPIAN DROUGHT, International journal of climatology, 18(12), 1998, pp. 1373-1388
Tropical cyclones are one of the prominent weather systems that are ge
nerated over the tropical oceans. The cyclones that develop in the Sou
thwest Indian Ocean (SWIO) usually travel west then southwest and fina
lly recurve to southeast, generally before reaching the East African c
oast. However, it is shown in this study how SWIO-tropical cyclones/de
pressions can indirectly affect Ethiopian weather. Using correlation a
nd composite analyses, interannual and intraseasonal rainfall variatio
ns in Ethiopia were compared with the frequency and time of occurrence
of the tropical cyclones. Years showing the consecutive occurrence of
several tropical depressions over the SWIO coincide with the drought
years of Ethiopia. Reciprocally, years of abnormally low frequency of
tropical cyclones are associated with heavy rainfall in Ethiopia. Belg
rainfall ('small rains', February-May) is much more influenced by the
cyclonic activity than Kiremt rainfall ('big rains', June-September),
which occurs outside the cyclonic season of the SWIO. On a daily basi
s, rainfall activity during the Belg period is significantly reduced w
hen a tropical depression is observed in the SWIO, before picking up a
gain a few days later. No systematic time-lag was found between the cy
clone occurrence and the decrease in daily rainfall amounts. However,
at interannual time-scales, a higher (lower) frequency of tropical dep
ressions during the months of November-January tends to be followed by
abnormally low (high) Belg rainfall. One of the mechanisms suggested
to explain this seasonal persistence in the frequency of tropical depr
essions involves sea-surface temperatures in the Southeast Indian Ocea
n, from where many of the depressions originate. The atmospheric proce
sses accounting for the teleconnection between Ethiopia rainfall and t
ropical depressions were examined. Monthly anomalies of upper air wind
s for years showing frequent occurrence of tropical depressions proved
significant not only over the SWIO, but also the Northern Hemisphere
above Northeast Africa. In addition to a slightly enhanced cross-equat
orial flow, diverting moisture advection towards the active centre of
the system, much stronger than usual equatorial easterlies in the uppe
r troposphere and a reduced southward excursion of the subtropical jet
stream result in weaker convective activity over Ethiopia. (C) 1998 R
oyal Meteorological Society.