The northward-propagating monsoon intraseasonal oscillation (MISO) is the most pronounced variability over the tropical Indian Ocean during the Indian summer monsoon (June–September). MISO is accompanied by significant air-sea interactions, however, the mechanism of the oceanic feedback to MISO is still a great scientific challenge. In this study, the role of the intraseasonal sea surface temperature (SST) gradient in MISO is diagnosed using reanalysis products and model sensitivity experiments. It is well-known that an intraseasonal PBL convergence anticipates convection for approximately 3 days, which benefits the new convection buildup and enhances the MISO. This study finds that the positive meridional gradient of intraseasonal SST induces positive wind convergence in the planetary boundary layer (PBL) and leads convection by about 1-2 days. The meridional SST gradient at the intraseasonal timescale can effectively intensify the intraseasonal PBL convergence by the vertical momentum mixing. Before the deep convection center, the warm SST anomalies at intraseasonal timescale destabilizes the PBL and thus enhance downward transport of momentum from aloft. This accelerates the intraseasonal northerly wind in the PBL. By contrast, the cold SST anomalies around deep convection weaken the downward vertical momentum transport, thereby inducing a deceleration in the intraseasonal northerly. Consequently, changes in the speed of intraseasonal northerly along the meridional direction strengthen the PBL wind convergence ahead of deep convection. The model sensitivity experiments suggest that this oceanic feedback accounts for approximately half of the total wind convergence in the PBL to the north of convection during MISO events. Moreover, it is found that the SST gradient can significantly affects the intensification of intraseasonal rainfall associated with MISO over the summer monsoon region. In addition, historical runs from the CMIP6 models reveal that a strong (weak) SST meridional gradient at an intraseasonal timescale amplifies (lessens) the MISO intensity in the model simulation. Therefore, this study highlights the role of SST meridional gradient in the feedback to MISO, which differs from the weak contribution of the warm SST itself to the wind convergence mentioned in previous studies. Collectively, these findings indicate that consideration of oceanic feedback is necessary to improve the understanding and simulation of MISO.

SST,MJO,Indian Ocean,Indian summer monsoon