Southern Bay of Bengal: A possible hotspot for CO₂ emission during the summer monsoon

Highlights

• Monsoon Current brings high salinity water to the east of Sri Lanka Dome in Summer.

• This water mass is rich in CO₂ and reaches very close to the sea surface.

• CO₂ disequilibrium in top 45 m reaches + 404 µatm and ventilate with atmosphere.

• A 2° × 2° area release ~0.78 Gg C day⁻¹, much higher than photosynthetic production.

Abstract

During the summer monsoon (June–September), the influence of cyclonic curl of local winds causes the formation of a thermocline dome called the Sri Lankan Dome (SLD) in the southern Bay of Bengal (BoB). In addition, the subsurface flow associated with the Summer Monsoon Current (SMC) brings Arabian Sea-High Salinity Water mass to the southern BoB. We show that both these oceanographic features are enriched in dissolved CO₂ with the upper boundary shoaling very close to the surface. We observed episodic deep mixing events leading to entrainment of CO₂-rich subsurface water with the mixed layer. CO₂ disequilibrium within the top 45 m reached as high as + 404 µatm. Our estimated mixed layer ventilation rates ranged between 2.4 and 4.6 days between sampling stations suggesting equilibration with upwelled waters were still evolving. We also encountered a patch of Arabian Sea-High Salinity Water mass with low aqueous pCO₂ suggesting past ventilation. Our modest estimates suggest a grid area of 2° latitude × 2° longitude can trigger a mean release of 0.78 Gg C day⁻¹, which is significantly higher than the estimated new production rates due to upwelled nutrients. Our study illustrates that upwelled water associated with the SLD in conjunction with the barrier layer erosion accompanied with the flow of SMC has the potential to occasionally ventilate in southern BoB. We believe that these processes can negate the region's benefits by acting as a CO₂ source which underscores the need for detailed investigation.

Introduction

The seasonal reversal of monsoonal winds and heavy freshwater discharge into the Bay of Bengal (BoB) (Shetye et al., 1996) creates a unique environment to study biogeochemical processes in monsoon regimes (Hood et al., 2015). Reports on the distribution of carbon parameters from oceanic waters of BoB are limited, whereas the coastal areas have been modestly investigated (Kumar et al., 1996, Sarma et al., 2012). Takahashi et al. (2009) reported that BoB is a perennial sink for atmospheric CO₂. Kumar et al. (1996) measured surface partial pressure of CO₂ in equilibrium with water (pCO₂) levels in the western BoB during spring intermonsoon (March–April) and northeast monsoon (NEM) (December 1991). They found that pCO₂ levels were always less than the atmospheric values, sometime exceeding 100 µatm. More recent measurements (Sarma et al., 2012) suggest that freshwater discharge exerts dominant control on the inorganic carbon components in surface waters. Lower than present atmospheric pCO₂ levels were found in the northwestern (NW) than southwestern (SW) coastal BoB. However, the pCO₂ levels in the peninsular rivers were an order of magnitude higher (5000–17,000 µatm) than atmospheric levels and glacial river Ganges (∼ 500 µatm). Most of the studies are from the coastal regions in the NW and northern BoB, whereas southern BoB remains poorly quantified.

The circulation in the southern Bay of Bengal to the east of Sri Lanka during summer monsoon consists of the Summer monsoon current (SMC, Schott et al., 1994, Schott and McCreary, 2001, Shankar et al., 2002, Rath et al., 2019) that intrudes into the BoB (Vinayachandran et al., 1999, Webber et al., 2018) and the Sri Lanka Dome (SLD, Vinayachandran and Yamagata, 1998) which is a cyclonic circulation feature associated with a doming of the thermocline. Arabian Sea High Salinity Water (ASHSW) enters into the southern BoB via the SMC (Murty et al., 1992) as a prominent subsurface High Salinity Core (HSC, Vinayachandran et al., 2013, Vinayachandran et al., 2018), allowing the exchange of water with the Arabian Sea and the Equatorial Indian Ocean (Jensen, 2001). The SLD is caused by the cyclonic curl of the local wind forcing. The upward Ekman pumping induced by this cyclonic curl brings cooler water to the near-surface layers (Vinayachandran and Yamagata, 1998). The upwelling accompanying SLD influences the local environment by modulating the water column properties by cooling the sea surface temperature. It also enhances the biological production and air-sea interaction (Vinayachandran et al., 2004, De Vos et al., 2014, Thushara et al., 2019). To the east of SLD, the intrusion of the SMC takes place between May and September. The SMC flows eastward to the south of India, turns around Sri Lanka, and enters the BoB, carrying high salinity water (>35.0 psu) from the Arabian Sea along its path (Murty et al., 1992, Vinayachandran et al., 1999, Webber et al., 2018). On encountering the lighter water of lower salinity BoB, the Arabian Sea water subducts beneath the former (Vinayachandran et al., 2013). The intrusion of high-salinity (35.0–35.6 psu) water occurs below the mixed layer, to a maximum depth of about 150 m (Vinayachandran et al., 2018, George et al., 2019). The biogeochemical manifestation of these complex circulation patterns associated with the SLD and SMC in the southern BoB is not well understood. One recent modelling study suggests biological control of pCO₂ in the southern BoB could be more dominant than upwelling (Chakraborty et al., 2018). They hypothesized that, in the BoB, upwelling might decrease pCO₂ due to the low salinity and pCO₂-poor in the surface layer. They, however, neither did examine the carbon dynamics associated with upwelled waters around SLD nor illustrated the biogeochemical properties which SMC brings along with ASHSW to southern BoB between May and September.

Using an in situ observational data set, here we show that waters in the vicinity of SLD and SMC are rich in dissolved CO₂ and are episodically exchanged with the surface mixed layer during the summer monsoon. We also calculate the CO₂ fluxes for the southern BoB and evaluate the potential impacts of such exchanges. Results presented in this study assume global significance, as we show that southern BoB can act as a hotspot for CO₂ emission during the summer monsoon under the influence of intrusion and spreading of SMC and the development of SLD. This has large implications to the ocean–atmosphere gas exchange inventory and in biogeochemical models for a region that is otherwise known to act as a potential sink until now (Takahashi et al., 2014).