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El Nino, Indian Ocean dynamics and
extremely rainy years in East Africa
Emily Black, Julia Slingo and Ken Sperber
[email protected]
Seasonal and interannual variability in East
African rainfall
Introduction
Rainfall in East Africa is, economically and socially the most
important part of the climate system. This study focuses on the
boreal autumn rainy season (the short rains) in equatorial and
southern tropical East Africa (see Figure 1).
Much has been published about the association between El
Nino and high rainfall in tropical, coastal East Africa. Figure 2,
however, shows that precipitation during some El Ninos (for
example 1986-1987) is average or low implying that the El Ninorainfall relationship is modulated by some other factor. Analysis
of time series and composites (Figures 2, 3 and 4) suggests
that this factor is the Indian Ocean dipole or zonal mode (IOZM).
Figure 1: The seasonal
cycle in East African
rainfall (mean rainfall and
one standard deviation
either side). Note that the
largest interannual
variability is experienced
during the short rains
Rainfall anomaly
Short
rains
We propose a dynamic scenario in which the wind anomalies
that characterise IOZM events drive above average rainfall in
East Africa (see Figures 5 and 6). We further suggest that the
IOZM can, in certain circumstances, be associated with El Nino
and this relationship explains the observed teleconnection
between East African rainfall and ENSO.
Figure 2: A time series of the
short rains in coastal, equatorial
East Africa highlighting IOZM
events and El Nino year zeros
400
200
0
El Nino year zero
IOZM year
-200
A paradigm for very strong short rains
In the following conditions, the short rains in coastal
tropical East Africa are likely to be above average:
SST anomaly patterns during rainy years
•El Nino is sufficiently strong during the boreal summer
to perturb the convection and circulation in the vicinity
of the Maritime Continent
Figure 3: SST
anomaly composite
for SON of very rainy
years in coastal
equatorial East Africa.
•The perturbation of the climate in this region is strong
enough to generate a persistent change in the Hadley
circulation with enhanced southerly winds in the
eastern Indian Ocean
Figure 4: Seasonal cycle in SST difference
between the tropical eastern and western Indian
Ocean during rainy years (dots). The solid lines
show the mean seasonal cycle and one standard
deviation above and below. Note that the reversal
of the climatological SST gradient during rainy
boreal autumns is characteristic of IOZM events.
•The enhanced southerlies are so long-lasting that the
cooling of the eastern Indian Ocean reverses the zonal
gradient in SST and triggers an IOZM
•The easterly wind anomalies resulting from the IOZM
extend over the Indian Ocean and significantly
suppress the mean westerly flow
A dynamic scenario for high autumn rainfall in East Africa
Climatology
El Nino
Average
vector wind
during
IOZM years
Average zonal wind
anomaly during
IOZM years
Figure 5: Wind during IOZM years
compared to climatology
Figure 6: The anomalous
winds shown in Figure 5
and the paradigm for
strong short rains
summarised above are
combined into a dynamic
scenario for strong East
African short rains
Cooler SST in
the eastern
Indian Ocean
Weakening of
the Indonesian
through flow
if the El Nino is very strong
or if the gradient in SST in
the Indian Ocean is
vulnerable to reversal…
Easterly zonal
wind anomalies
in the northern
central Indian
Ocean
Weakening of the
climatological
moderately strong
westerlies
Reversed SST
gradient for several
months
Less transport of
moisture away
from the African
continent
Anomalous
along shore
southerly
winds near
Sumatra
??? (some
other factor)
High short rains
in East Africa