North Atlantic Oscillation Influence on Precipitation Variability around the Southeast African Convergence Zone

Abstract

The relationship between the North Atlantic oscillation (NAO) and austral summer (December?February) rainfall variability over southeastern Africa is described. Thirty-one stations in 0°?16°S and 25°?40°E have statistically significant correlations to the NAO index over varying periods of record starting since 1895 and form a regional normalized rainfall index of southeast African rainfall (SEAR) correlated to the NAO index (NAOI) at r = ?0.48 over 1894/95?1989/90, although the relationship is r = ?0.70 since 1958. The spectrum of the SEAR index has significant amplitude at 7.6 yr, a periodicity commonly associated with the NAO, and the NAOI/SEAR cospectrum has its largest power at this periodicity. NCEP?NCAR reanalysis data, extending from 1958/59 to 1995/96 are used to evaluate moisture and circulation field variations associated with both NAO and SEAR indices. Precipitable water over southeastern Africa varies significantly such that anomalously high (low) convective rainfall occurs over southeast Africa when the NAO is weak (strong). Unusually wet summers are associated with anomalous equatorial westerly flow originating in the subtropical Atlantic and traversing the continent. Relatively dry summers are associated with increased southeasterly monsoon flow originating over the subtropical Indian Ocean. The NAO linkage to southeastern African rainfall is especially pronounced in 300-hPa zonal winds where five highly significant elongated bands of alternating zonal wind anomalies extend from the Atlantic Arctic to equatorial Africa. The latter 300-hPa equatorial band exhibits westerly (easterly) flow during wet (dry) austral summers and undergoes regional divergence (convergence) over southeastern Africa. The westerly flow, along with orographic uplift, has an element of instability due to the vertical component of the Coriolis parameter that assists rain production during wet summer. Potential interactions between the NAO and ENSO in producing regional latitudinal ITCZ shifts are discussed.