An Investigation of Interannual Rainfall Variability in Africa

John E. Janowiak Climate Analysis Center, NMC/NWS/NOAA, Washington, DC

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Abstract

Interannual variations in African rainfall are examined using rotated principal component analysis (PCA) applied to anomalies from the annual mean as well as seasonal anomalies. The rotated PCA loading patterns suggest several “preferred” continental-scale rainfall anomaly patterns. The dominant features of year-to-year variations in African rainfall appear to be the high spatial coherence of rainfall anomalies over large portions of the continent. In addition, several dipole regions, that is, adjacent regions which tend to experience rainfall anomalies of opposite sign, are found and discussed. One dipole region, the sub-Saharan region, appears to have a relationship with a characteristic Atlantic sea surface temperature anomaly pattern during boreal summer. During austral summer, the tendency for a large-scale dipole pattern in southeast Africa is apparent, as is an association of this pattern with the warm and cold phases of the El Niñ/Southern Oscillation (ENSO) phenomenon.

Normalized rainfall departures over areas determined from the PCA loading patterns are used as an index from which composites are formed. Patterns of rainfall anomalies, based on composites of high and low values of these indices, are produced to quantify the year-to-year variation in rainfall in the areas of study. During the austral summer of warm episode ENSO years, rainfall tends to be 10%–25% above normal east of 20°E between the equator and 10°S and correspondingly lower than normal east of 20°E, between 15° and 30°S. The opposite anomaly pattern tends to be observed during cold phases of ENSO in this region. Anomaly dipoles of similar magnitude (when expressed as percentages) tend to be found in sub-Saharan Africa during boreal summer.

Abstract

Interannual variations in African rainfall are examined using rotated principal component analysis (PCA) applied to anomalies from the annual mean as well as seasonal anomalies. The rotated PCA loading patterns suggest several “preferred” continental-scale rainfall anomaly patterns. The dominant features of year-to-year variations in African rainfall appear to be the high spatial coherence of rainfall anomalies over large portions of the continent. In addition, several dipole regions, that is, adjacent regions which tend to experience rainfall anomalies of opposite sign, are found and discussed. One dipole region, the sub-Saharan region, appears to have a relationship with a characteristic Atlantic sea surface temperature anomaly pattern during boreal summer. During austral summer, the tendency for a large-scale dipole pattern in southeast Africa is apparent, as is an association of this pattern with the warm and cold phases of the El Niñ/Southern Oscillation (ENSO) phenomenon.

Normalized rainfall departures over areas determined from the PCA loading patterns are used as an index from which composites are formed. Patterns of rainfall anomalies, based on composites of high and low values of these indices, are produced to quantify the year-to-year variation in rainfall in the areas of study. During the austral summer of warm episode ENSO years, rainfall tends to be 10%–25% above normal east of 20°E between the equator and 10°S and correspondingly lower than normal east of 20°E, between 15° and 30°S. The opposite anomaly pattern tends to be observed during cold phases of ENSO in this region. Anomaly dipoles of similar magnitude (when expressed as percentages) tend to be found in sub-Saharan Africa during boreal summer.

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