Transition from the Southern to the Northern Hemisphere summer of zones of active convection over the Congo Basin

Summary

This study used monthly rainfall totals for the period 1961 to 1988 and pentad OLR values for the period 1974 to 1991 to study the structure and transition of active convention across the Congo Basin (10° S–5° N, 15° E–35° E) from the southern to the northern hemisphere summer. This involved the examination of map patterns and cross-sections of monthly rainfall and pentad OLR data.

The results from the study indicated that there were two seasons observed over the Congo Basin; one is the wet season lasting from September to April and the other a dry season covering the rest of the year. The onset of the wet season takes place rapidly with active convection spreading very quickly to the south near latitude 20° S. This is due to the formation of the meridional (north–south) branch of the ITCZ over this region.

This study has confirmed that the annual rainfall over the Congo Basin is reliable with the coefficient of variation of less than 30%. The wet seasons (e.g., SON and DJF) also show reliable rainfall occurrence but the dry season (e.g., JJA) has low reliability.

The anomalously wet seasons are characterised by a relatively slow transition rate (∼1° latitude per pentad) of zones of active convection resulting in a late withdrawal of the rainy season while the dry seasons show a rapid transition rate with an early withdrawal of zones of active convection.

High-rainfall months (>200 mm) are concentrated within the Southern Hemisphere summer months. These high-rainfall months progress from the equator to the southern latitude following generally the movement of the overhead sun.

The results further revealed that the years 1987/1984 had the lowest/highest mean OLR values over the Congo Basin within the period 1979 to 1991. The rates of transition of the zones of low OLR values were 0.9/5.0 degrees of latitude during 1987/1984, respectively.