LOW-FREQUENCY VARIATION OF TROPICAL CONVERGENCE ZONES

Intraseasonal variation of tropical convergence zones (TCZ) is studied focussing on the three major features of the TCZ over the Indian longitudes during the summer monsoon viz. (i) the oscillation between active and weak spells, (ii) the occurrence of two favourable zones-one over the equatorial oceans and another over the heated continent and (iii) poleward propagations of the oceanic TCZ onto the heated continent. An observational study of the intraseasonal variation over different parts of the tropics has shown that the first feature may be an ubiquitous feature of the TCZ variations, the second occurs only over the Asian summer and winter monsoon zones, and the third only over the Asian summer monsoon. Analysis of a simple monsoon model has revealed that poleward propagation occurs in the presence of a meridional surface temperature gradient because the convective heating is asymmetric, with more heating on the poleward side. Preliminary analysis of the T-21 version of the ECMWF model has shown that it is capable of simulating the three major features of the intraseasonal variation of the TCZ over the Indian longitudes during the summer monsoon.