The boreal summer Intraseasonal Oscillation and the South Asian monsoon

The South Asian monsoon exhibits pronounced intraseasonal variability on timescales greater than three weeks. The principal purpose of this study is to provide a comprehensive analysis of the low-frequency monsoon intraseasonal vaiability and to determine its structure in both space and time. Large scale active and break periods of rainfall are associated with an apparent northward propagation of convection emanating from the central equatorial Indian Ocean. A cross-correlation and lagged regression analysis is utilized to investigate the temporal and spatial evolution of convection and large-scale circulation patterns associated with the slowly varying Intraseasonal Oscillations (ISOs). Outgoing longwave radiation (OLR) filtered to eastward wavenumbers 1-3 and periods 25 to 80 days is used as the predictor for two independent regression analyses which focus on the northern winter (DJFM) and northern summer (JJAS) periods, respectively. The evolution of ISO convection and the large-scale circulation in northern summer appears similar in many respects to that of northern winter and can be thought of in terms of propagating equatorial modes. The primary difference between the seasons is that the northern summer ISO exhibits an off-equatorial convection part, forced by surface frictional convergence into the low pressure center of the Rossby cell that is excited by equatorial ISO convection. The result is a summertime ISO convection signal that appears as a band tilting northwestward with latitude and stretching from the equator to about 20°N. Viewed along any meridian the mode appears to propagate northward while the equatorial convection propagates to the east.

The summer ISO is examined in further detail and is found to contain two predominant modes of evolution that occur with approximately the same frequency. The first mode exhibits a stationary component in low-frequency equatorial Indian Ocean convection (23 events) while the second mode exhibits primarily an eastward component (27 events) in equatorial Indian Ocean convection. Both modes are characterized by northward propagation onto the Indian subcontinent. In all cases, northward propagation does not occur in the absence of eventual eastward propagation of convection along the equator. The primary difference between the two modes, apart from the stationary versus eastward propagating development of convection in the central equatorial Indian Ocean, is that the eastward propagating events typically occur later in the monsoon season, when the sea surface temperature (SST) across the Indian Ocean basin has cooled relative to the early portion of the monsoon season.