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ABSTRACT

DECADAL TEMPERATURE, RAINFALL AND HYDROLOGICAL TRENDS OVER THE GREATER HORN OF AFRICA

Fredrick Semazzi, Jared Bowden, Richard Anyah, Baris Onol & Ahmed Tawfik
17th Conference on Climate Variations, June 13th, 2005

This study investigates decadal trends of temperature, rainfall and river discharge over the Greater Horn of Africa (GHA) through the diagnosis of observations and General Circulation Model (GCM) output. In the recent decades (1979-2001), observations suggest northern GHA is becoming increasingly wetter while southern GHA is becoming drier. Intraseasonal variability of the observed trend depicts a complex spatial distribution of the rainfall anomalies with the tendency for October to be the largest contributor to the rainfall anomaly trend. Model rainfall diagnostics from NCAR Community Atmosphere model version 2.0.1 suggests that the increasing trend in the recent decades may be representative of a quasi-decadal trend over GHA. Trends from the Blue Nile and White Nile Rivers also indicate decadal climate variability since the early 1910s. The projected rainfall from NCAR/NASA-CCM3 A2 scenario suggest a dipole like change in the rainfall anomalies with maximum precipitation change centered over Lake Victoria. As for temperature, observed and model seasonal mean surface temperatures generally show a warming trend since the 1960s. Projected surface temperatures suggest substantial temperature increase over GHA exceeding 3.5°C in higher elevations.

The objective of this study was to diagnose the rainfall variability over the Greater Horn of Africa (GHA) during the October, November, December season using Empirical Orthogonal Function (EOF) analysis. EOF analysis was performed on pentad and monthly stratified CPC Merged Analysis of Precipitation (CMAP) data for 23 years, 1979-2001. EOF analysis performed on both data sets revealed two dominant modes of variability: mixed El Nino Southern Oscillation / Indian Ocean Zonal Mode (ENSO-IOZM) and decadal trend, respectively.

The dominant mode of variability characteristically represents the interannual variability of ENSO with zonal SST anomalies over the Indian Ocean, commonly termed the Indian Ocean Zonal Mode (IOZM). The loading pattern and time series is consistent with previous studies in which the majority of the GHA experiences positive rainfall anomalies during warm ENSO events. Intraseasonal analysis reveals the non-linear nature of ENSO over the GHA. Warm ENSO events experience wet conditions throughout the entire season despite large deviations between pentads. Cold ENSO events are generally dry but the region can experience mild wet spells within the season. Simple investigation of Indian Ocean weekly SST anomalies and low-level (925 hPa) wind field in relation to the intraseasonal rainfall anomalies depicts several rainfall peaks during warm events occur in conjunction with onshore low-level flow and positive SST anomalies over the western Indian Ocean.

The second mode of variability is associated with a decadal trend. The decadal trend mode correlates well correlated with the Atlantic Multidecadal Oscillation Index of the North Atlantic Ocean and may have important predictive potential for the rainfall variability. The decadal trend mode is strongest during the month of October compared to the other two months (November and December) of the analysis. The decadal trend mode imply increasingly earlier onset of the rainy season over northern GHA where the EOF loadings are positive, and a late start over southern GHA where the loadings are of the opposite sign.

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