Overview of the meteorological conditions and atmospheric transport processes during INDOEX 1999

During February the northern zone was more active and the cross-equatorial flow (N→S) was weak, both suggesting that less pollution was transported to the southern hemisphere. In February it was occasionally possible to sample a southern hemispheric air mass on the southern side of the INDOEX domain. During March 1999 the southern convergence zone became dominant and moved to a more southern position (near 5°-10°S).

It is shown that four channels carry pollution into the INDOEX domain: (1) NE trades over the western Arabian Sea, (2) NW-NE flow along the west coast of India, (3) NE trades over the west Bay of Bengal, and (4) NE flow from SE Asia. The strength of each channel is modulated by transients moving across Pakistan and northern India (western disturbances). The heating of the Indian subcontinent in March resulted in a eastern shift of the subtropical high from central India to the Bay of Bengal, which also affected channels 2, 3, and 4.

Episodes of high and low carbon monoxide concentrations as measured in Kaashidhoo (Maldives) during the 1999 IFP can qualitatively be explained by the operationality of the flow channels, determined through trajectory analyses, in combination with the intensity of the northern convergence zone.