Using a hybrid atmosphere-ocean coupled model, we have shown that northward-propagating, intraseasonal oscillations (NPISOs) during the boreal summer are strongly coupled to the underlying sea surface temperature (SST) in the Indian Ocean sector. On the one hand, the intraseasonal atmospheric convection changes the SST through the solar radiation, latent heat flux and mixed-layer base entrainment; on the other, the induced SST anomalies feedback to affect the intraseasonal convection. The preferential northward, rather than southward, propagation of the boreal summer ISOs in the Indian Ocean is due to interactions among the summer-mean climate state, the atmospheric disturbances and ocean surface temperature.

The atmosphere-only model forced with daily SST produces much stronger NPISOs than that with monthly mean SST as the boundary forcing (AMIP-type run). However, the atmosphere-only model, even forced with daily SST from the coupled model, is unable to reproduce the NPISOs in the coupled case. In the coupled system, intraseasonal SST anomalies are forced by intraseasonal atmospheric convection, and hence are in quadrature with the convection. In the stand-alone atmospheric model, however, SST acts only as a boundary forcing, and the resultant atmospheric convection has almost the same phase with the underlying SST. One result is that the intensity of the SST-forced intraseasonal convection in the stand-alone atmospheric model is much weaker than in the coupled model.

Finally, our solutions indicate that the movement of the off-equatorial convection in the northern Indian Ocean is more closely related to local SST perturbations rather than the equatorial eastward-moving Madden-Julian Oscillation. The positive/negative SST perturbations in the northern Indian Ocean lead the active/beak phases of the intraseasonal oscillation by about 2 pentads (10 days). Therefore, the SST perturbation on the intraseasonal time scale over the northern Indian Ocean is potentially a very useful index to forecast the active/break spells of the south Asian summer monsoon.