Abstract: It is suggested in this paper that thermal forcing of sea and land of the two hemispheres may be an important cause of the formation of the cross-equatorial flow. The numerical experiments show that thermal structure of longitudinal inhomogeneity is not only advantageous to the intensification of cross-equatorial flow, but also results in the inverse characters between high and low level. In this paper, the correlation between ageostrophic flow and middle-latitude westerlies distribution is studied. The effects on westerlies of seasonal variation of thermal forcing structure and discussed.