The structures and mechanism of the northward- propagating boreal summer intraseasonal oscillation ( BSISO) in the southern Asian monsoon region are simulated and investigated in a three- dimensional intermediate model ( 3D model). The horizontal structure of the intraseasonal variability in the 3D model depicts the Kelvin - Rossby wave - type disturbance, which may or may not produce the northward- propagating disturbance in the Indian Ocean, depending on the seasonal- mean background winds. Two experiments are conducted in order to identify what characteristic of seasonal- mean background can induce the northwestward- tilted band in the Kelvin - Rossby wave, whose overall eastward movement gives the impression of the northward propagation at a given longitude. When the prescribed boreal summer mean winds are excluded in the first experiment, the phase difference between the barotropic divergence tendency and convection disappears. Consequently, the Rossby wave - type convection forms a zonally elongated band. As a result, the northward propagation of convection at a given longitude disappears. When the easterly vertical shear is introduced in the second experiment, the horizontal and the vertical structures of BSISO become similar to that of the northward- propagating one. The reoccurrence of the northwestward-directed convective band and the phase difference between the barotropic divergence tendency and the convection suggest that the summer mean zonal winds in the boreal Indian summer monsoon region are a critical condition that causes the horizontal and vertical structures of northward- propagating BSISO in the southern Asian monsoon region.
