Understanding the seasonality, trends and controlling factors of Indian
Ocean acidification over distinctive bio-provinces
Abstract
The Indian Ocean (IO) is witnessing acidification as a direct
consequence of the continuous rising of atmospheric CO2 concentration
and indirectly due to the rapid ocean warming, which disrupts the pH of
the surface waters. This study investigates the pH seasonality and
trends over various bio-provinces of the IO and regionally assesses the
contribution of each of its controlling factors. Simulations from a
global and a regional ocean model coupled with biogeochemical modules
were validated with pH measurements over the basin, and used to discern
the regional response of pH seasonality (1990-2010) and trend
(1961-2010) in response to changes in Sea Surface Temperature (SST),
Dissolved Inorganic Carbon (DIC), Total Alkalinity (ALK) and Salinity
(S). DIC and SST are significant contributors to the seasonal
variability of pH in almost all bio-provinces. Total acidification in
the IO basin was 0.0675 units from 1961 to 2010, with 69.3%
contribution from DIC followed by 13.8% contribution from SST. For most
of the bio-provinces, DIC remains a dominant contributor to changing
trends in pH except for the Northern Bay of Bengal and Around India
(NBoB-AI) region, wherein the pH trend is dominated by ALK (55.6%) and
SST (16.8%). Interdependence of SST and S over ALK is significant in
modifying the carbonate chemistry and biogeochemical dynamics of NBoB-AI
and a part of tropical, subtropical IO bio-provinces. A strong
correlation between SST and pH trends infers an increasing risk of
acidification in the bio-provinces with rising SST and points out the
need for sustained monitoring of IO pH in such hotspots.