Monthly maps of the mean 20¿C isotherm topography in the western Indian Ocean have been compiled from historical data. They reveal strong annual and semi-annual fluctuations. The yearly cycle has significant phase lags across the area investigated that reflects the different circulation patterns during and in between the two monsoon periods. In particular, the different development of the Somali Current north and south of 5¿N can be seen in these data. The flow in the southern part reverses 1 month prior to the large scale onset of the SW-monsoon but otherwise follows the monsoonal windfield. In the northern part southward flow is found only for 2 months during the height of the NE-monsoon. The Somali eddy spins up immediately after the SW-monsoon onset but remains strong for 2 months after the winds weaken. The semi-annual fluctuations are in phase from 10¿N to 10¿S and can be related to the zonal winds over the equator. Eastward surface currents cause thermocline upwelling during boreal spring and fall which has maxima along the coast and at 5¿ north and south of the equator. It can feed the currents at a rate of 15¿106 m3 s-1 during a 3-month period.