Similarities and Differences in BSISO over the Indian Ocean and the West Pacific Ocean

Northward propagating Boreal Summer Intraseasonal Oscillations (BSISO) are identified using Multiple Object tracking algorithm over the Indian and the west Pacific Ocean from Outgoing Longwave Radiation (OLR) over the period 1979 -2011 (32 years). There are 61 BSISO events identified in the Indian Ocean sector, while 41 BSISO events identified in the west Pacific Ocean sector. In both the ocean basins, average latitudinal displacement rates are similar (~1.3 m/s) and life span of the events are also similar (~22 days). In the Indian Ocean, BSISO either propagates strictly northward or first moves eastward and then turns northward. In the west Pacific Ocean, BSISO propagates in northwestward direction. A Lagrangian composite analysis with respect to the center of mass of each convective cloud system suggests that the average size of northward propagating events is 10⁶ km² and the OLR anomaly at the center of convection is -45 w/m². The spatial asymmetries in the mean background state, composite moisture, moist static energy, moisture convergence, and vertical velocity suggest that the development phase of convection lies east of the convection center. A cyclonic shear line in the lower tropospheric mean circulation is a common feature in both the ocean basins. One contrast between northward propagating events in the Indian and Western Pacific Ocean is that the directions of the mean zonal wind at the southern and northern edges of the convection center are opposite in direction, with westerly (easterly) to the north (south) of convection in the Indian Ocean, and easterly (westerly) to the north (south) of convection in the western Pacific Ocean, while the free tropospheric averaged zonal wind anomalies are the same: westerly (easterly) to the south (north) of the convection center. The zonal wind anomalies in the Indian Ocean tend to slow down the mean circulation, while the zonal wind anomalies in the western Pacific Ocean tend to speed up the mean circulation.