Study on variability of sea salt aerosol in the North Indian ocean

Abstract

Aerosols are the severe threats to human life as well as ecosystem. Highest amount of natural and anthropogenic aerosols are observed over the South Asian region. In the total aerosols, sea salt aerosol (SSA) have significant concentrations which are analyzed using the Monitoring Atmospheric Composition & Climate (MACC) reanalysis data collected from European Centre for Medium-Range Weather Forecasts (ECMWF) dataset for 10 years from 2003 to 2012. Horizontal resolution of this data is ~80 km. Daily average data of aerosol optical depth (AOD) at 550 nm is used in this study. Two regions are considered to study SSA which are the North Indian Ocean (NIO) and Bangladesh (BD). Again, NIO is divided into two parts:- Bay of Bengal (BoB) and Arabian Sea (AS). SSA is found approximately 18% of the total aerosols in the NIO. The annual SSA is increased significantly with an approximate value of 18% for the NIO. Seasonal distributions of SSA in the BoB provide a highest concentration of 0.098 AOD during the monsoon season whereas in the winter, pre-monsoon and post-monsoon seasons have the values of 0.039, 0.043, and 0.044, respectively. In the AS, it is observed that the highest concentration of SSA (0.126) is found also in the monsoon. However, 25% more SSA is estimated in the AS than that in the BoB in the monsoon. Average monthly distribution of SSA is periodic in nature with a minimum value of 0.033 for the BoB and 0.022 for the AS in April and a maximum value of 0.110 for the BoB and 0.163 for the AS in July. In this study, Evaporation-Precipitation (salinity change) and wind velocity are examined over the NIO and compare with the SSA to find out the reason of formation of SSA. Correlation coefficient between E-P with SSA over the NIO is found -0.70, whereas, the average wind velocity over the NIO has strong positive correlation with SSA (r = ~0.92), which plays the major role for the production of SSA as well as the seasonal and monthly variation of SSA distribution. In this work, correlation between SSA and temperature at two levels i.e.1000-850 hPa and 700-500 hPa are studied. The correlation coefficients of low-level and mid-level temperature with SSA are found 0.45 and 0.85, respectively in the NIO. SSA is highly correlated with mid-level temperature which indicates that SSA helps to increase the mid-level temperature by scattering the insolation. The monthly average SSA has high positive correlation with precipitation with the value of 0.82. The correlation between average surface relative humidity and SSA is 0.70 for the NIO. Therefore, SSA in the NIO indirectly increases the amount of precipitation by allowing more cloud condensation nuclei and relative humidity by absorbing water vapor in the atmosphere About 9% SSA is measured in Bangladesh with respect to its total aerosol contain. SSA in BD is mainly advected from the NIO, especially from the BoB. Because SSA and wind velocity in BD and in the BoB are highly correlated which are 0.72 and 0.90, respectively. In BD, positive correlation indicates that the wind comes from BoB to BD and carries the SSA from the BoB. Within the 10 years, SSA has been increased 28% in BD. The correlation (r) of SSA with the parameters of low-level temperature, mid-level temperature, precipitation and relative humidity are 0.53, 0.72, 0.79, and 0.74, respectively in BD which are highly significant except low-level temperature.