Statistical indices of the northward seasonal rainfall progression over eastern Africa

Two statistical indices are developed to represent the mean northward progression of rainfall between late January and late July over eastern Africa, and examine perturbations from the climatological mean. The first index, a Seasonal Position Index, extracts the seasonal phase associated with a daily rainfall map using a principal component analysis. The second index, a Rainfall Regime Index, describes the seasonal cycle as a progression through a series of rainfall clusters. The indices are used to examine interannual variability in the timing and character of rainfall “jumps”, and to investigate circulation features associated with perturbations ahead of and behind the mean seasonal cycle. These indices are potentially useful for exploring the predictability of rainfall transitions at both short-range and seasonal timescales.