The interannual variations of the atmospheric circulation over the tropical oceans were investigated by using water vapor transport fields derived from SSM/I and the global reanalysis data such as the National Centers for Environmental Prediction (NCEP), the European Centre for Medium-Range Weather Forecasts (ECMWF) 40-year Reanalysis (ERA40), and Japanese 25-year Reanalysis Project (JRA25) data.

The EOF analysis of the monthly mean divergent component of water vapor transport derived from ERA40 for the period of 1979-2001 indicates that the dominant variability of divergent water vapor transports which represent the Hadley and Walker circulations is closely related to the ENSO for the last two decades. The variations of the Hadley and Walker circulations are then examined by using the water vapor transport potential function (Ö) to define the Hadley and Walker circulation indices.

The time series of the Hadley circulation index indicates intensifying trend in boreal winter from SSM/I and all reanalyses. However, there is no clear trend in all reanalyses while there is an increasing trend from SSM/I in boreal summer. On the other hand, the Walker circulation index shows a weakening trend in recent decades in SSM/I and all three reanalyses.