Abstract: In the 1st part of this paper an observational study is given. We choose 3～5 year low frequency (LF) period range for the band-pass filter and EOF analysis to the data set of SSTA over tropical Pacific, 1951～1992. We then apply the LF band-pass filter to the global belt 850 hPa wind variables and the link between the leading mode EOF-1 of LF oscillation of SSTA and circulation anomalies is investigated.In the 2nd part of this work a simple equatorial model for one baroclinic mode with explicit moisture evolution equation is developed. The interaction between convective condensational heating and large-scale flow as well as the feedback between evaporation, sensible heating and perturbation wind are included. Also, the realistic underlying surface temperature field and sea-land contrast are introduced.By introducing typical SST anomalies in El Nino or La Nina years, respectively, into the lower boundary, the anomalous tropical circulation is simulated. The model experiments show that during a warm episode, the convection center is situated further east, convective activities over eastern Pacific are strengthened and there is anomalous westerly over central Pacific Ocean. On the contrary, during a cold episode, the eastward propagation of the disturbances is much prohibited, the convective active region are confined to the west of the date line.