Annual changes of tropical convective activities as revealed from equatorially symmetric OLR data

How and why equatorial convections are activated in certain preferred regio ns during specific times of the calendar year, are investigated utilizing e quatorially symmetric OLR data. In the equatorial African and American cont inents, semiannual variability is predominant with two peaks of convective activities in boreal spring and fall due to the in-situ radiational heating . Over the oceanic regions, the role of in-situ surface heating becomes insig nificant and gives place to remote forcings in excitement of equatorial sym metric convections and associated Rossby-Kelvin wind responses within the e quatorial duct between about 15 degreesN and 15 degreesS, that are determin ed by the Rossby deformation radius at the equator. The active convective p hase in the equatorial western Pacific (EWP) lasts about five months from N ovember to March in association with a systematic southward migration of th e surface pressure trough. When the trough arrives at the equator in Novemb er, the zonal as well as meridional down-pressure gradient winds cause sign ificant low-level convergence to enhance convections in EWP. Here, January is the month of most active convections, since southward down-pressure grad ient winds become strongest due to equatorward penetration of the winter-ti me North Pacific high. During boreal winter, EWP corresponds to the updraft leg of the equatorial E-W overturning with wavenumbers 1 to 2. There exist s a gigantic season-fixed clockwise phase rotation of low surface pressure across the Indian Ocean and western Pacific, namely, northward along 75 deg reesE in spring to summer, eastward at 10 degreesN from summer to fall, sou thward along 155 degreesE in fall to winter, and westward at 10 degreesS fr om winter to spring, thus completing an annual journey. As such, equatorial convections in EWP are activated during the fall-winter phase of southward migration. In the equatorial Indian Ocean (EIO), convections are not really activated before and during the South and Southeast Asian summer monsoon (SEAM), sinc e it persistently induces divergent northward down-pressure gradient winds in EIO. Here, the preferred period of active convections differs significan tly with different longitudes. Between about 80 degrees to 100 degreesE (EI O1), October of the post SEAM season is the month of intensified convection s due primarily to the convergence via the so-called beta -effect. The wint er-time Indian Ocean high, which penetrates equatorward along the Kenya coa st, is responsible for causing a substantial west-to-east pressure gradient and convergent equatorial westerlies in EIO1. Between about 100 degrees an d 120 degreesE (EIO2), December represents the peak convective phase under the influence of the northern hemisphere winter monsoon bursting out of Sib eria. The role of this winter monsoon system is two fold, i.e.; first, acce lerating equartorward down-pressure gradient winds which meridionally conve rge into regions of heavy convections near Sumatra and Borneo, and second, enhancing the convergence due to the beta -effect in association with an in creased west-to-east pressure gradient and intensified westerlies along the equator. Equatorial convections in EIO remain inactive during the northwar d propagation phase of low surface pressure in spring.