Spatial variation of aerosol spectral optical depth and columnar water vapour content over the Arabian Sea and Indian Ocean during the IFP of INDOEX

Abstract

Extensive solar spectral extinction measurements are made over the Arabian Sea and Indian Ocean (between 60°E and 78°E longitude and 15°N and 20°S latitude) using a ten-channel multi wavelength solar radiometer (MWR) and a 4-channel EKO Sun photometer on-board the cruise #141 of ORV Sagar Kanya during the Intense Field Phase (IFP) of the Indian Ocean Experiment (INDOEX) from 20 January to 12 March 1999. From these measurements, the columnar aerosol optical depths and columnar content of water vapour are estimated; and their spatial variations examined. The results show occurrence of fairly high values of aerosol optical depths along the West Coast of India. The optical depths decrease gradually as one moves down south, with an e–1 scaling distance of ~ 1200 to 1300 km. Extremely low (near zero) values are encountered in mid Indian Ocean due south of 10° latitude (i.e. on the south of the ITCZ) followed by a weak increase close to Mauritius. Presence of an extensive region of enhanced aerosol optical depths (which we call the West Asian High) is seen in the mid-Arabian Sea between 5° and 10°N westward of 65°E longitude which is stronger and wider than those seen along the coastal regions of India. This effect is seen at the longest wavelength (1025 nm) also. Variation of columnar water vapour content (W g cm^–2) shows highest values(> 3 g cm^–2) around the equator (~ 3°N to 5°S extendingover the entire longitude region) possibly associatedwith the ITCZ. Moderately high values are encountered along the coastal regions of India and also close to Mauritius. Compared to these, the northwest Arabian Sea (between 5 to 15°N and 60° to 70°E) shows rather dry atmosphere with W < 2 g cm^–2, indicatingprevalence of drier continental conditions. Theimplications of the findings are discussed.