Long-Lead Forecasts of Seasonal Precipitation in Africa Using CCA

Abstract

A potentially operational forecast system for 3-month total precipitation for three sections of the African continent has been developed at NOAA's Climate Prediction Center using the statistical method of canonical correlation analysis (CCA). The levels and sources of predictive skills have been explored at lead times of up to 1 year, using a cross-validation design. The predictor field is quasi-global sea surface temperature (SST). Four consecutive 3-month predictor periods are used to detect evolving as well as steady-state SST conditions. Low to modest forecast skills are found for most regions and seasons of the year. However, moderate skills (correlation > 0.5) are found for parts of northern tropical Africa (the Sahel) for lead times of up to several seasons in its July?August?September rainy season. CCA generally outperforms persistence, even at short lead times. The El Niño-Southern Oscillation (ENSO) phenomenon is found to play some role in the precipitation variability in all three African regions. Warm SO episodes tend to suppress northern summer rainfall in the Sahel and southern summer rainfall in portions of southern Africa, and possibly enhance rainfall at a very limited number of eastern African stations in their two rainy seasons (March?April?May and October?November?December). However, ENSO does not determine precipitation in any part of Africa nearly as strongly as it does for regions closer to the tropical Pacific. Other major sources of predictive skill include interdecadal regimes and still longer term trends. A decadal change toward Sahel dryness began in the 1970s in association with increased SST in the Indian, South Atlantic, and eastern tropical Pacific Oceans. A more gradual Sahelian drying has paralleled a warming (cooling) of the Southern (Northern) Hemisphere SST, particularly in the Atlantic. Parts of southern Africa's summer rainfall is enhanced by positive SST anomalies in the adjacent South Atlantic and southwest Indian Oceans but suppressed by generalized warm worldwide SST. Wet season rainfall at a small minority of stations in eastern Africa is increased by positive SST anomalies in the upwind western Indian Ocean. While African forecast skills are generally modest, operational issuance of forecasts and their expected skills could begin by 1997 for seasons and locations for which skill is usable.