The season cycle in Cameroon is essentially governed by the annual shifts of the intertropical convergence zone (ITCZ), between the convergence zones of the southern trade wind (monsoon, humid, coming from the St Helen anticyclone) and of the northern trade wind (harmattan, dry, coming from the Saharian anticyclone). In the Nyong basin, the climate is equatorial. For the Southern Cameroonian plateau, it is called four season-guinean climate; the short (March to May) and the long (September to November) rainy seasons are separated by a short dry season, whereas the long dry season goes from December to February. The annual rainfalls are between 1500 and 2000 mm, for a mean annual temperature of about 25°C with amplitude of 2.4°C. In the coastal basin, concerning the 80-100 km of the downstream course of the Nyong River, the climate is Cameroonian-type, with only one (relatively) dry season between December and February. The annual mean rainfall is 2000-3500 mm, the annual mean temperature is 26°C with a 2.8°C amplitude; in this very warm and humid zone, the annual mean hygrometry is more than 85%.

Figure 3. Interannual isohyets (mm) over the Nyong watershed
Figure 3. Interannual isohyets (mm) over the Nyong watershed

The table below displays some mean climatic data (rainfall, temperature and Piche evaporation) for the 1991-2000 decade, at the meteorological stations of Yaoundé, Sangmelima and Abong Mbang. At Nsimi, the mean rainfall is 1682 mm/yr between 1993 and 2001; for the 1996-2001 period, the evaporation measured using a semi-buried tank is 1336 mm/yr. Figure 4 shows the two humped variation graph obtained for the rainfall at Olama station in 1998/99.

mean rainfall (mm)mean temperature (°C)mean Piche evaporation (mm)
Stationsannualm. maxm. minannualm. maxm. minannualm. maxm.min
Abong Mbang1746,8350,94,124,726,422,9524,2*68,6*22,9*

Compared interannual climatic parameters at 3 stations of Southern Cameroon (period 1991-2000)

For each parameter, the following values are given: interannual mean, means for months corresponding to maximal and minimal values (*) At Abong Mbang station, data of 1995 and 1998 are incomplete.

Figure 4. Rainfall and Discharge at the Olama station over 1998/1999
Figure 4. Rainfall and Discharge at the Olama station over 1998/1999