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

We give an overview of the regional meteorological situation during the Ind ian Ocean Experiment INDOEX intensive field phase (IFP) in February and Mar ch 1999. The INDOEX domain, reaching from 30 degreesN to 30 degreesS and fr om 50 degreesE to 100 degreesE, was chosen because the low-level outflow of pollution from India is carried by the northeasterly trades into the tropi cal convergence zone, where cloud processing modifies the properties of the aerosols. In contrast, there is also an inflow of pristine southern hemisp heric air by the southeasterly trades into the convergence zone. However, d uring the 1999 IFP some deviations from the climatological mean were observ ed. In 1999 the Intertropical Convergence Zone (ITCZ) was broken into a nor thern convergence zone and a southern convergence zone. During February the northern zone was more active and the cross-equatorial flow (N -->S) was w eak, both suggesting that less pollution was transported to the southern he misphere. In February it was occasionally possible to sample a southern hem ispheric air mass on the southern side of the INDOEX domain. During March 1 999 the southern convergence zone became dominant and moved to a more south ern position (near 5 degrees -10 degreesS). 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 acr oss 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 me asured in Kaashidhoo (Maldives) during the 1999 IFP can qualitatively be ex plained by the operationality of the flow channels, determined through traj ectory analyses, in combination with the intensity of the northern converge nce zone.