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August 6th, 2004
The objective of this study is to investigate the recent climate variability on intra-seasonal, interannual, and decadal time scales for the Greater Horn of Africa for the seasons of October, November, December (OND) and March, April, May (MAM). We use Empirical Orthogonal Functions (EOFs) to separate the variability. The observed climate variability is used to characterize the historical Parallel Climate Model variability. We then investigate the projected climate variability from the Parallel Climate Model business-as-usual run.
Specifically, we demonstrate the observed rainfall-circulation relationships for ENSO during the OND and MAM seasons in relation to the rainfall over GHA using EOFs and the weighted wind composites. We find that during the recent climate of the OND season ENSO and the Indian Ocean Zonal Mode (IOZM) occur simultaneously with strong anticyclonic flow near Sumatra. As for the MAM season, there is no sign of the IOZM. During the OND season, the historical EOFs separate ENSO and the IOZM, suggesting the two can behave independently. Overall, the GHA region experiences positive anomalous rainfall during the OND season with the positive phase of the IOZM. The MAM season has no IOZM and demonstrates a complicated spatial temporal pattern because ENSO is in a transition phase during this season.
We also demonstrate the significance of a trend mode for both OND and MAM seasons. The trend of the OND season is highly correlated to the tropical South Atlantic Index. The trend is seen in all months but strongest during October. The MAM characteristically favors the tropical South Atlantic Index, but the correlations are much lower. Overall, the combined affect of the MAM and OND season would generate an increasingly wetter northwest GHA and drier southern GHA. Somalia is the only region that compensates the increasing trend through an opposite loading relationship between the OND and MAM seasons.