Annual Changes of Tropical Convective Activities as Revealed from Equatorially Symmetric OLR Data

Abstract

How and why equatorial convections are activated in certain preferred regions during specific times of the calendar year, are investigated utilizing equatorially symmetric OLR data. In the equatorial African and American continents, 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 insignificant and gives place to remote forcings in excitement of equatorial symmetric convections and associated Rossby-Kelvin wind responses within the equatorial duct between about 15°N and 15°S, that are determined by the Rossby deformation radius at the equator. The active convective phase in the equatorial western Pacific (EWP) lasts about five months from November to March in association with a systematic southward migration of the surface pressure trough. When the trough arrives at the equator in November, the zonal as well as meridional down-pressure gradient winds cause significant low-level convergence to enhance convections in EWP. Here, January is the month of most active convections, since southward down-pressure gradient winds become strongest due to equatorward penetration of the winter-time North Pacific high. During boreal winter, EWP corresponds to the updraft leg of the equatorial E-W overturning with wavenumbers 1 to 2. There exists a gigantic season-fixed clockwise phase rotation of low surface pressure across the Indian Ocean and western Pacific, namely, northward along 75°E in spring to summer, eastward at 10°N from summer to fall, southward along 155°E in fall to winter, and westward at 10°S from 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), since it persistently induces divergent northward down-pressure gradient winds in EIO. Here, the preferred period of active convections differs significantly with different longitudes. Between about 80° to 100°E (EIO₁), October of the post SEAM season is the month of intensified convections due primarily to the convergence via the so-called β-effect. The winter-time Indian Ocean high, which penetrates equatorward along the Kenya coast, is responsible for causing a substantial west-to-east pressure gradient and convergent equatorial westerlies in EIO₁. Between about 100° and 120°E (EIO₂), December represents the peak convective phase under the influence of the northern hemisphere winter monsoon bursting out of Siberia. The role of this winter monsoon system is two fold, i. e.; first, accelerating equartorward down-pressure gradient winds which meridionally converge into regions of heavy convections near Sumatra and Borneo, and second, enhancing the convergence due to the β-effectin association with an increased west-to-east pressure gradient and intensified westerlies along the equator. Equatorial convections in EIO remain inactive during the northward propagation phase of low surface pressure in spring.