Climatology, trend of aerosol-cloud parameters and their correlation over the Northern Indian Ocean

Aerosols are one of the important atmospheric constituents and exert indirect impact on climate through the modification of microphysical and radiative properties of clouds that in turn perturb the precipitation pattern. Thus, the long term quantification of changes in aerosol and cloud characteristics and their interactions on both temporal as well as spatial scale will provide a crucial information for the better assessment of future climate change. In present study, 18 years (2003-2020) MODerate Resolution Imaging Spectro-radiometer (MODIS) derived aerosol-cloud dataset over the Northern Indian Ocean (NIO) were analysed to assess climatology and trend of aerosol, cloud characteristics and their correlation. We found a strong heterogeneity in spatio-temporal variation of aerosol and cloud parameters over the NIO that are more prominent for the coastal region. The climatological mean of aerosol loading is found high (AOD ≥ 0.5) over the outflow region along the Indian sub-continent and low (AOD ≤ 0.2) over the northern equatorial open ocean. The climatological mean of cloud properties shows dominance of optically thicker deep convective (CTP < 600 hPa and CTT < 260 K) clouds over the southern Bay of Bengal (BoB) and thinner shallow (CTP > 700 hPa and CTT > 273 K) over the northwestern Arabian Sea (AS). Similarly, bigger effective radii (>17 µm) observed along the equatorial open ocean whereas smaller CER (<17 µm) were found over Indian sub-continental coastline and western AS. Further, trend analysis reveals an increasing pattern in AOD (0.002 yr^-1), CER (0.051 µm yr^-1), LWP (0.033 gm^-2 yr^-1) and CF (0.002 yr^-1) while COD, CTT and CTP show negative trend in order of -0.005 yr^-1, -0.094 K yr^-1 and -1.160 hPa yr^-1, respectively. We also perform similar analysis for seven sub-region of interest (R1 to R7) across the NIO and results show a decreasing pattern in AOD (-0.001 yr^-1) at R4 against maximum mean AOD (0.44 ± 0.03). However, coastal sub-regions R1 and R5 illustrate maximum increase in aerosol loading (>0.003 yr^-1) suggesting a significant impact of sub-continental outflow over the regions. The spatial correlation of cloud properties with respect to AOD shows a positive slope for CER (0.14) and CF (0.48) and a negative for COD (-0.19), LWP (-0.18), CTT (-0.37), CTP (-0.41). The present study provides in-depth information about the aerosol-cloud characteristics for a long term scale over NIO and could be useful in regional aerosol-cloud interaction induced climate forcing estimation.