Diagnostics of hydrometeorological anomalies in the Zaire (Congo) basin

Interannual variations in the large‐scale circulation over the tropical Atlantic are studied in relation to hydrometeorological anomalies in the Zairee (Congo) basin. Composite departure patterns of atmospheric and oceanic fields in the Atlantic are constructed for extremely dry and for extremely wet years in the Zaire river catchment. These maps are complemented by isocorrelate charts between river level and selected elements in 5° blocks of the Atlantic. Water levels of the Zaire river in the first and second halves of the year are positively related to sea surface temperature off Angola, and to humidity and to actual and convective instability at Luanda, approximately upwind from the Zaire catchment. However, lapse rates at the Angola coast are strongly controlled by upper air conditions: cold air aloft may enhance instability and thus rainfall. Unlike during the first semester, sea surface temperature anomalies during the second half of the year do not seem to be forced by prior behaviour of wind stress in the equatorial Atlantic, nor by local wind speed and cloudiness. Equatorial south‐westerlies flowing across the Angola—Gabon coast toward the Zaire catchment are strong/weak during second semesters of high/low water level. Rainfall over the Zaire basin appears to be modulated by a zonal circulation along the Atlantic equator.