Interannual Variability of South-eastern African Summer Rainfall. Part 1: Relationships with Air-sea Interaction Processes

This paper investigates the role that air–sea interaction processes may play in interannual variability of south-eastern African summer rainfall. The principal spatial modes of south-eastern African summer rainfall are first identified using principal component analysis. Four modes are retained. The most important mode of variability is found to represent rainfall variability over most of the domain, particularly in the regions to the south. The influence of ENSO (as measured by the SOI) on summer rainfall is investigated in detail for different SOI leads. The relationship is such that during the summer following the onset of an ENSO event, south-eastern Africa tends to experience dry conditions. Strongest relationships are found with the SOI leading rainfall by about 3 to 6 months. A second index, the Brandon–Marion Index (BMI) which is indicative of changes in the pressure field over the Indian Ocean correlates with rainfall better than the SOI. Strongest correlations are found when this index leads rainfall by about 1 to 3 months. More importantly, a partial correlation analysis reveals that the BMI influences rainfall independently of ENSO. Both the SOI and the BMI are potential predictors of summer rainfall. An investigation of rainfall associations with global SST anomalies reveals areas in the tropical Indian and Pacific Oceans that are linked with rainfall changes over the subcontinent. The relationship is such that warm anomalies tend to be followed by dry conditions over much of south-eastern Africa. Strongest relationships are found when SSTs lead the rainfall season by about 1 to 3 months. Well-defined atmospheric anomalies are identified during dry south-eastern African summers. These include, amongst others, anomalously warm tropospheric temperatures and marked low-level cyclonic circulation anomalies over the central Indian Ocean, which generate abnormally weak easterly winds along much of the south-eastern coast of Africa. These perturbations to the low-level flow divert moisture from the continent and result in precipitation decreases. An important and related finding is the fact that the SST–rainfall link over the Indian Ocean remains strong after the ENSO effects have been removed, suggesting that the atmospheric circulation anomalies observed over south-eastern Africa during dry summers, are linked mainly to SST anomalies over the Indian Ocean. This hypothesis will be tested in a companion paper through a series of GCM simulations.