Variability, trend and controlling factors of Ocean acidification over Western Arabian Sea upwelling region

Western Arabian Sea (WAS) is subjected to strong bio-physical forcings which results in significant variability in biological productivity and intense CO2 emission at different timescales. The study aims to understand the major physical factors controlling seasonal variability and trend in pH over WAS. Two sets of sensitivity experiments are carried out in order to delineate the individual factors such as Sea Surface Temperature (SST), Dissolved Inorganic Carbon (DIC), Alkalinity (ALK) and Salinity (S) which control the pH seasonal variability and its trend. The first set of sensitivity experiments extract the major controlling factors of pH seasonal variability and the second set identifies the major contributors to the trend in pH over WAS. The results show that DIC and SST are the principal drivers of seasonal variability of pH and they act in opposite directions. Seasonal warming and cooling have significant effect in pH variability during pre-monsoon and post-monsoon period, respectively. DIC plays a major role in pH seasonality during the monsoon period. WAS has been acidified from a pH of 8.12 (in 1960) to a pH of 8.05 (in 2010). The trend in pH over WAS is due to contributions from DIC and SST of 109% and 16%, respectively. The effect of ALK is to buffer the existing trend in pH by −36% while S contribution is only 7%. Collectively, DIC and ALK contribute up to 73% to the net pH trend. SST warming alone contributes another 16%. pH shows a decreasing trend of −0.0091 for 50 years for October to December and − 0.0039 for July to August with a correlation of 0.374 and 0.244 with increasing trend in SST for the corresponding period, respectively. The study raises a red flag since this region is warming rapidly which is only exacerbating acidification.