Spatial heterogeneity in spectral variability of aerosol optical depth and its implications to aerosol radiative forcing in the tropical Indian Ocean and in the Indian Ocean sector of southern ocean

Abstract

The aerosol optical depths (AODs) in the wavelength range 380–875 nm and black carbon (BC) mass concentrations were estimated over the tropical Indian Ocean and in the Indian Ocean sector of Southern Ocean, between 14°N and 53°S, during December 2011–February 2012, onboard the Ocean Research Vessel (ORV) Sagar Nidhi. The data were analysed to understand the spectral variability, micro-physical characteristics of aerosols and the associated radiative forcing. Concurrent MODIS-derived chlorophyll a (Chl-a) and sea-surface temperature (SST) provided ancillary data used to understand the variability of biomass in association with fronts and the possible role of phytoplankton as a source of aerosols. AODs and their spectral dependencies were distinctly different north and south of the Inter-Tropical Convergence Zone (ITCZ). North of 11°S (the northern limit of ITCZ), the spectral distribution of AOD followed Ängstrom turbidity formule (Junge power law function), while it deviated from such a distribution south of 16°S (southern boundary of ITCZ). At the southern limit of the ITCZ and beyond, the spectral variation of AOD showed a peak around 440 nm, the amplitude of which was highest at ~43°S, the axis of the subtropical front (STF) with the highest Chl-a concentration (0.35 ;µg ;l⁻¹) in the region. To understand the role of Chl-a in increasing AOD at 440 nm, AOD at this wavelength was estimated using Optical properties of Aerosols and Clouds (OPAC) model. The anomalies between the measured and model-estimated (difference between the measured and estimated AOD values at 440 nm) AOD₄₄₀ were correlated with Chl-a concentrations. A very high and significant association with coefficient of determination (R²=0.80) indicates the contribution of Chl-a as a source of aerosols in this part of the ocean. On the basis of the measured aerosol properties, the study area was divided into three zones; Zone 1 comprising of the area between 10°N and 11°S; Zone 2 from 16°S to 53°S; and Zone 3 from 52°S to 24°S during the return leg. BC mass concentration was in the range 520 ng m⁻³ to 2535 ng m⁻³ in Zone 1, while it was extremely low in the other zones (ranging from 49.3 to 264.4 ng m⁻³ in Zone 2 and from 61.6 ng m⁻³ to 303.3 ng m⁻³ in Zone 3). The atmospheric direct-short wave radiative forcing (DRSF), estimated using a radiative transfer model (Santa Barbara DISORT Atmospheric Radiative Transfer – SBDART), was in the range 4.72–27.62 wm⁻² north of 16°S, and 4.80–6.25 wm⁻² south of 16°S.