The annual cycle of African climate and its variability

Abstract

A statistical analysis of African rainfall and temperature, and surface wind components and sea level pressure over the tropical Atlantic and Indian Oceans is done in the period 1965 to 1995. The annual cycle is found to be the dominant mode, exhibiting a subtropical north - south dipole with a cycle period of 12 months. The second mode is equatorial with 6-month oscillations prevalent. Annual cycle variance is of the order of 40%, compared with ~ 10% for the interannual residual. The north-south alternation of winds over the adjacent oceans accounts for 92% of north-south rainfall oscillations over the African continent. The annual cycle varies coherently over periods of 3 to 8 years. Years of high (low) annual cycle correspond with La Nina (El Nino) and increased (decreased) inter-annual fluctuations of the African climate. A composite analysis shows that the South Atlantic Hadley cell and standing waves in the subtropical jet may connect annual and interannual signals.

Key words: annual cycle, African monsoon, climate diagnostics

Water SA Vol.31(1) 2005: 1-8