Role of river discharge and warming on ocean acidification and pCO₂ levels in the Bay of Bengal

Abstract

Shifts in surface ocean pCO 2 and pH are important controls governing global climate. Based on the linear relationship of observed surface pH and pCO 2 with sea surface temperature (SST), sea surface salinity (SSS) and Chlorophyll-a (Chl-a) multiple linear regression equations were developed. Based on remote sensing SST, Chl-a and model-derived SSS, pH and pCO 2 data were derived from 1998 to 2015. Overall warming of BoB is noticed at the rate of 0.004 to 0.03 C/y whereas cooling is found in the northwestern BoB during winter and spring seasons associated with an increase in atmospheric dust. Decrease in SSS is noticed during all seasons due to melting of Himalayan ice cover associated with increase in fresh water flux due to increase in atmospheric temperature. Increase in pH is observed in the eastern and southern Bay during all seasons associating with warming and decrease in salinity. In contrast, decrease in pH (À0.001 y À1 ) and pCO 2 increase (þ0.1 to þ0.7 matm y À1 ) is noticed in the western and head Bay during winter and spring seasons due to deposition of atmospheric pollutants. This study suggests that increase in freshwater input due to melting of Himalayan ice cover and deposition of atmospheric pollutants are dominant controlling factors on surface ocean pH and pCO 2 in the BoB between 1998 and 2015 and this region is acting as a stronger sink for the atmospheric CO 2 in the present than that in the past two decades. The global coastal regions are significantly influenced by river discharge and atmospheric deposition of pollutants and they are not part of the global models leading to ill-reproduction of seasonal variability in pH and pCO 2 . Inclusion of these processes may improve prediction of pH and pCO 2 in the regions heavily influenced by discharge/deposition from land and atmosphere.

Variability in Sea Surface pCO2 and Controlling Factors in the Bay of Bengal Based on buoy Observations at 15°N, 90°E

“…The residual term had some impacts on diurnal pCO 2sea variability at 12:17-18:17 (Figure 10a). This might be due to the nonlinear relationship between pCO 2sea and SST or DIC (Xue et al, 2016) and factors controlling pCO 2sea , such as ubiquitous eddies (Cheng et al, 2018;Sarma et al, 2019Sarma et al, , 2020 and deposition from land and atmosphere (Sridevi and Sarma., 2021), in the BoB. Similar results were found by using Taylor-series deconvolution (Figure S13 in Supporting Information S1).…”

“…The residual term had a contribution of 2 μatm (8%) on the seasonal pCO 2sea variability, with a slightly higher contribution from March-June. This might be due to the nonlinear relationship between pCO 2sea and SST or DIC, and other factors such as calcium carbonate production/dissociation produced by coccolithophorids and foraminifera (Boehme et al, 1998) and deposition from land and atmosphere (Sridevi & Sarma, 2021). The influence of cyclonic and anticyclonic eddies on pCO 2sea cannot be ignored (Sarma et al, 2019(Sarma et al, , 2020.…”

“…This high freshwater influx and the seasonally reversal monsoon winds affect the physical characteristics and consequently the carbonate dynamics (Chakraborty et al., 2021; Joshi et al., 2021). This leads to high spatial and temporal variabilities in sea surface salinity (SSS) (Joshi et al., 2021) and brings in nutrients which leads to challenges in modeling carbonate system parameters [pCO 2sea , dissolved inorganic carbon (DIC), total alkalinity (TA), and pH] for the bay (Joshi et al., 2020; Sarma et al., 2013; Sridevi and Sarma, 2021; Valsala & Maksyutov, 2010). Both observation and model based studies have shown low pCO 2sea in the northeastern BoB and high pCO 2sea in the southern region (Joshi & Warrior, 2022; Joshi et al., 2021; Sarma et al., 2018).…”

“…The wind speed during the pre‐monsoon season (March–May) and post‐monsoon season (October–November) is relatively weak. It also receives a high influx of freshwater from rivers and precipitation (Sengupta et al., 2006; Sridevi and Sarma., 2021). This high freshwater influx and the seasonally reversal monsoon winds affect the physical characteristics and consequently the carbonate dynamics (Chakraborty et al., 2021; Joshi et al., 2021).…”

Air‐Sea Fluxes of CO2 in the Indian Ocean Between 1985 and 2018: A Synthesis Based on Observation‐Based Surface CO2, Hindcast and Atmospheric Inversion Models

“…Kumar et al (1996) suggested that the BoB is a sink for atmospheric CO 2 in the 1990s whereas Sarma et al (2015; found that it is a mild source of the atmosphere due to the deposition of atmospheric pollutants. More recently Sridevi and Sarma (2021) indicated that salinity in the surface waters of the BoB is decreasing over the past two decades due to an increase in the warming of Himalayan glaciers (Goes et al, 2020). Since the pH of the Ganges and Brahmaputra River waters are relatively basic (Sarma et al, 2012), an increase in pH and a decrease in pCO 2 was noticed in the past two decades in the central and eastern BoB (Sridevi and Sarma, 2021).…”