Spatial and temporal heterogeneity in aerosol properties and radiative forcing over Bay of Bengal: sources and role of aerosol transport

Abstract

The Bay of Bengal (BoB) region is a small oceanic region surrounded by landmasses with distinct natural and anthropogenic aerosol sources. Despite this and the significant influence of BoB on the Indian monsoon and weather, information on aerosols and its spatial and temporal heterogeneity is sparse. The observations onboard several cruises and on an island location have been used in this paper along with satellite (MODIS onboard TERRA satellite) data (2000-2004) to study the spatial and temporal heterogeneity in aerosol properties. Our studies show that seasonal variation in aerosol optical depths over northern BoB are similar to that of east coast of India and west China, whereas variations over the southern BoB region are similar to that of Arabian Sea. The aerosol optical depths (AODs) and black carbon (BC) mass fraction (MF) over northern BoB reach their maximum value during April/May (AOD ~0.48 ± 0.06; BCMF ~6%) and minimum during October/November (AOD ~0.19 ± 0.02; BCMF ~3%) in contrast to the seasonal pattern reported over Arabian Sea by earlier investigations. Over equatorial Indian Ocean south of BoB, AODs were low (~0.11 ± 0.03) and seasonal variations were not very significant. The whole-sky (including clouds) aerosol surface radiative forcing (0.2 to 40 μm) over northern BoB was in the range of -9 to -30 W m^-2, whereas that over southern BoB was in the range of -3 to -12 W m^-2. The corresponding atmospheric forcing was in the range of +6 to +20 W m^-2 and +1 to +6 W m^-2. The atmospheric absorption translates to a heating rate of 0.5 to 1.0 K/day.