Large Scale Influences on Interannual Variability Physical Mechanisms Potential Predictability and Prediction of Short Rains over East Africa
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Date
2014-11
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Addis Ababa University
Abstract
The climate of the East Africa exhibits marked interannual
uctuations that provoke
droughts or
ooding which lead to enormous impact on socio-economic activities over
the region. Therefore, understanding of the mechanisms that produce this variability
and developing both dynamical and statistical approaches for extended range forecast
and projected information on future climate is of great importance. In the rst part
of this study observational datasets and a series of Sea Surface Temperature (SST)
forced Atmospheric General Circulation Model (AGCM) ensemble simulations for the
whole 20th century are analyzed to investigate the physical mechanism and potential
predictability of East African short rains variability.
It is found that there is substantial skill in reproducing the East African short
rains variability given the SSTs are known. Consistent with recent previous studies it
is found that the Indian Ocean (IO) and in particular the western pole of the Indian
Ocean dipole (IOD) play a dominant role for the prediction skill, whereas SST outside
the IO play a minor role. The physical mechanism for the western IO in
uence on
East African rainfall in the model is consistent with previous ndings and consists of a
gill-type response to a warm (cold) anomaly that induces a westerly (easterly) low-level
ow anomaly over equatorial Africa and leads to moisture
ux convergence (divergence)
over East Africa. On the other hand a positive El Ni~no-Southern Oscillation (ENSO)
anomaly leads to a spatially non coherent reducing e ect over parts of East Africa,
but the relationship is not strong enough to provide any predictive skill in our model.
The East African short rains prediction skill is also analyzed within a model derived
potential predictability framework and it is shown that the actual prediction skill is
broadly consistent with the models potential prediction skill. Low frequency variations
of the prediction skill are mostly related to SSTs outside the IO region and likely due
to an increased interference of ENSO with the IO in
uence on East African short rains
after the mid-70s climate shift.
Based on results from a series of AGCM experiments, the performance of dynamical
seasonal forecast systems are evaluated for the prediction of SSTAs over tropical IO
and short rains anomalies over equatorial East Africa. The evaluation is based on observational
datasets and the Asia-Paci c Climate Center (APCC) Ocean-Atmosphere-
Land coupled Multi-Model Ensemble (MME) retrospective forecasts (hindcasts) using
common years for all models from 1982 to 2005.
The coupled climate models ensemble reproduces seasonal characteristics of low
level wind, the spatial distribution of SON mean rainfall and seasonal climate variations
over equatorial East Africa with further improvement in MME mean. Ensemble mean
of individual coupled models and MME mean also show statistically signi cant skill in
forecasting sea surface temperatures anomalies (SSTAs) over the western and eastern
parts of the tropical IO, giving signi cant correlation at 99% con dence level for IOD.
Moreover, ve out of ten coupled models and MME mean show statistically signi cant
skill in predicting equatorial East Africa short rains. The delity of hindcasts is further
measured by Anomaly Correlation Coe cient (ACC) and four models as well as MME
mean show signi cant skill over East Africa. It is shown that the reproduction of the
observed variability in the East African region is mainly due to a realistic relationship
of East African rainfall with the IOD. Overall, the skill of the dynamical models is
attributed to the fact that slowly evolving SSTs are the primary source of predictability,
and to the fact that coupled climate models produce skillful predictions of SON SST
anomalies over tropical IO.
This study therefore provides insight into interannual rainfall variability and predictability
over East Africa, in view of tropical Indian Ocean-Atmosphere climate
patterns and underlying mechanisms. In addition, the information on coupled forecast
systems will open the possibility of using readily available seasonal forecasts as skillful
predictions of equatorial East Africa short rains. On the whole, the results found in
this study will feed into real-time monitoring and forecasting at seasonal to interannual
timescales to enhance early warning and disaster preparedness activities and minimize
the impacts of climate-related catastrophes that are prevalent in the region
Description
Keywords
Physical Mechanisms