Statistical indices of the northward rainfall progression over eastern Africa

摘要

Two statistical indices are developed to represent the northward progression of rainfall between late January and late July over eastern Africa. The first, a Seasonal Location Index, extracts the seasonal phase associated with a daily rainfall map using a principal component analysis. The second, a Rainfall Cluster Index, describes the seasonal cycle as a progression through a series of rainfall patterns that have been defined using a cluster analysis. The indices are used to identify large seasonal shifts in the latitude of rainfall, to examine when these shifts occur, 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.