Seasonality of the Tropical Intraseasonal Oscillation

The tropical intraseasonal oscillation (TISO) is a quasi-regular variation of convection and related atmospheric circulation, operating at a time scale of 20-100 days, that significantly affects tropical precipitation. However the TISO varies seasonally in terms of direction of propagation and the geographical location of its occurrence. Minima in the Lanczos bandpass filtered (20-100 days) daily NOAA Outgoing Longwave Radiation (OLR) from 1979-2011, which represent maxima in precipitation rate, are tracked by using newly developed tracking algorithm based on a multiple-object tracking method. A total of 192 events were identified over the period, which were then classified into six categories on the basis of their propagation direction and geographical location. The TISO remains active throughout the year in one or another of the classes. Eastward propagation south of the equator occurs mostly in boreal winter. Northward propagation in the Pacific and Indian Ocean sectors occurs in boreal summer months. The seasonality of TISO is related to the intra-hemispheric migration of the mean background states of sea surface temperature (SST), zonal wind and specific humidity. All events prefer warm SST, westerly or weak easterly wind and high tropospheric specific humidity. The intra-hemispheric migration of the mean state may play important role in the seasonality of TISO and its characteristics.