Seasonal Migration of Monsoons between the Northern and Southern Hemisphere as Revealed from Equatorially Symmetric and Asymmetric OLR Data

抄録

The climatological pentad mean OLR data are partitioned into symmetric component OLR´, and asymmetric component OLR´´, with reference to the equator. Objective criteria are then introduced to define the intensity, center and areal extent of strong convections with OLR´ (OLR´´) of less than 220 (-20) Wm^-2 over the equatorial (subtropical) domains, and seasonal migration of monsoons between the two hemispheres is investigated. Over the equatorial continents, such as Africa and South America, OLR´ exhibits a common feature of distinct semi-annual standing oscillations, while OLR´´ is of annual standing oscillation character that is peculiar to the monsoon system. Coexistent of two standing oscillations of different periodicity implies occurrence of a systematic cross-equatorial propagation of strong convections twice a year, northward during the spring transition, and southward in fall. The manner in which the monsoon transits from one hemisphere to another is distinctly different between the Indian Ocean and Western Pacific. This is associated with difference in the systematic equatorial basic flow which is, to a large extent, regulated by equatorial convections occurring over the maritime continent. Eastward (westward) of the maritime continent around 125°E, is a Kelvin (Rossby)-type basic flow regime; that is subject to significant annual variations.
During late fall to early winter over the equatorial Western Pacific is a very pronounced Kelvin-type basic flow enhanced by northerly surges emanating out of the Siberian and North Pacific high pressure systems. The symmetric Kelvin flow acts as a bridge through which convections migrate southward from the WNPM (Western North Pacific Monsoon) to WSPM (Western South Pacific Monsoon) domain by crossing the equator in November and December. Gradual weakening of the Kelvin-type flow regime after winter inhibits northward return journey of convections from WSPM to WNPM in April and May. During spring and early summer over the eastern equatorial Indian Ocean is a prominent symmetric Rossby-type basic flow, which serves as a medium for convections to propagate northward, although not well organized, from SIOM (South Indian Ocean Monsoon) to SEAM (Southeast Asian Monsoon). The fall transition is of different character. The asymmetric convection center established in SEAM migrates southward, until reaching the equator by October. Instead of penetrating into the South Indian Ocean, the convective center suddenly changes its direction and propagates eastward along the equator, reaching near Borneo by the end of the calender year.