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Theoretical and Applied Climatology

, Volume 130, Issue 1–2, pp 91–106 | Cite as

Secular spring rainfall variability at local scale over Ethiopia: trend and associated dynamics

  • Gizaw Mengistu Tsidu
Original Paper
  • 205 Downloads

Abstract

Spring rainfall secular variability is studied using observations, reanalysis, and model simulations. The joint coherent spatio-temporal secular variability of gridded monthly gauge rainfall over Ethiopia, ERA-Interim atmospheric variables and sea surface temperature (SST) from Hadley Centre Sea Ice and SST (HadISST) data set is extracted using multi-taper method singular value decomposition (MTM-SVD). The contemporaneous associations are further examined using partial Granger causality to determine presence of causal linkage between any of the climate variables. This analysis reveals that only the northwestern Indian Ocean secular SST anomaly has direct causal links with spring rainfall over Ethiopia and mean sea level pressure (MSLP) over Africa inspite of the strong secular covariance of spring rainfall, SST in parts of subtropical Pacific, Atlantic, Indian Ocean and MSLP. High secular rainfall variance and statistically significant linear trend show consistently that there is a massive decline in spring rain over southern Ethiopia. This happened concurrently with significant buildup of MSLP over East Africa, northeastern Africa including parts of the Arabian Peninsula, some parts of central Africa and SST warming over all ocean basins with the exception of the ENSO regions. The east-west pressure gradient in response to the Indian Ocean warming led to secular southeasterly winds over the Arabian Sea, easterly over central Africa and equatorial Atlantic. These flows weakened climatological northeasterly flow over the Arabian Sea and southwesterly flow over equatorial Atlantic and Congo basins which supply moisture into the eastern Africa regions in spring. The secular divergent flow at low level is concurrent with upper level convergence due to the easterly secular anomalous flow. The mechanisms through which the northwestern Indian Ocean secular SST anomaly modulates rainfall are further explored in the context of East Africa using a simplified atmospheric general circulation model (AGCM) coupled to mixed-layer oceanic model. The rainfall anomaly (with respect to control simulation), forced by the northwestern Indian Ocean secular SST anomaly and averaged over the 30-year period, exhibits prevalence of dry conditions over East and equatorial Africa in agreement with observation. The atmospheric response to secular SST warming anomaly led to divergent flow at low levels and subsidence at the upper troposphere over regions north of 5° S on the continent and vice versa over the Indian Ocean. This surface difluence over East Africa, in addition to its role in suppressing convective activity, deprives the region of moisture supply from the Indian Ocean as well as the Atlantic and Congo basins.

Keywords

Indian Ocean Geopotential Height Granger Causality Atmospheric General Circulation Model Western Indian Ocean 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Notes

Acknowledgments

The author would like acknowledge greatly National Meteorological Services of Ethiopia, ECMWF and BADC for free access to raw gauge rainfall, ERA-Interim and HadSST1 datasets respectively. The author would also like to extend his gratitude to Alexander von Humboldt Foundation for the fellowship as well as the host-Karlsruhe Institute of Technology (KIT) during which this work is completed.

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Copyright information

© Springer-Verlag Wien 2016

Authors and Affiliations

  1. 1.Department of Earth and Environmental SciencesBotswana International University of Science and TechnologyPalapyeBotswana
  2. 2.Department of PhysicsAddis Ababa UniversityAddis AbabaEthiopia
  3. 3.Karlsruhe Institute of Technology (KIT)Institute of Meteorology and Climate ResearchLeopoldshafenGermany

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