A diagnostic study of the space–time characteristics of the summer monsoon intraseasonal oscillations (MISO) during strong monsoon (SM) and weak monsoon (WM) years is carried out to gain insight on the role of the seasonal mean states on the MISOs using long observational and reanalysis datasets. Prominent asymmetry is noted in the duration and magnitude of active-break spells associated with MISO during SM and WM years. Such difference is also reflected in the northward propagation of MISOs over the Indian subcontinent that show fast (slow) propagation during active phase of WM (SM), while the situation is just opposite during breaks. We hypothesize that considerable differences in the seasonal mean states during these extreme monsoon years may be responsible for these asymmetries. In support of this hypothesis, it is found that during WM (SM) years, the seasonal mean zonal winds associated with the convective region have anomalous barotropic (baroclinic) vertical structure, while a baroclinic (barotropic) structure dominates over region of suppressed convection. Evolution of meridional–vertical structure of the MISO anomalies during SM and WM also highlights the role of barotropic zonal wind anomalies in the northward propagation. From the barotropic zonal momentum budget analysis, it is demonstrated that meridional and vertical advection of the seasonal mean zonal wind anomaly may be largely responsible for such asymmetry in intensity, life span and propagation characteristics of the MISOs during SM and WM years.