NUMERICAL STUDY OF TROPICS-EXTRATROPICS INTERACTION

A nonlinear steady-state baroclinic primitive-equation numerical model of atmospheric forced stationary waves is used to investigate the tropics-extratropics interactions.Newtonian cooling,Rayleigh friction and biharmonic horizontal diffusion are included in the model.The Eliassen-Palm (EP) cross-section and threedimensional wave activity flux,which was derived by Plumb (1985) for linear quasi-geostrophic stationary waves on a zonal flow,are used as diagnostics for the vertical and horizontal propagation of the waves.Results of the numerical experiments and diagnostics analyses suggest that the extratropical influence on the tropical large-scale motion is important.The mid-latitude orographic forcing,especially of the Qing-hai-Xizang Plateau,and the extratropical thermal forcing make substantial contribution to the mainenance of the cyclonic circulation over the eastern tropical and subtropical Pacific as well as the inverse circulation over the western Pacific in the upper troposphere.In addition,the longitudinal variation of diabatic heating in tropics has a significant influence on the wintertime stationary waves at higher latitudes.