Spatial and Temporal patterns of intra-seasonal oscillations over tropical Africa, their evolution and propagation

The spatial and temporal structure of Intra-seasonal oscillations over tropical Africa are investigated using 21 years (1980- 2000) of pentad CMAP rainfall and NCEP 700 hPa wind data.

Results show that the first three Principal Components account for about 71% and 37% of the total variance of CMAP rainfall and 700hPa zonal wind respectively. Convection reveals a larger intra-seasonal signal for standing modes, whilst the wind systems are less coherent and may propagate.

The dominant mode of the intra-seasonal rainfall is over the northern Congo reflecting low-pressure and uplift throughout the year. The second and third modes are over the southwest Congo / Angola and East African highlands respectively. Zonal wind modes are over the Gulf of Guinea, West Indian Ocean and Southeast Atlantic Ocean respectively. Cross correlation of rainfall and zonal wind modes at intra-seasonal scale reveals that wet spells over the Congo and eastern Africa are triggered by increased westerly flow over the Atlantic. These start to develop a few pentads before the rainfall event over equatorial Africa.

Intra-seasonal variability within the equatorial troposphere is characterized by eastward propagating, stationary and westward planetary-scale disturbances. Eastward propagation is more pronounced in the velocity potential anomalies than in the OLR anomalies.