Comment on bg-2020-486

However, the Biogeochemistry about the carbon cycle in the upwelling zones is somewhat found less emphasized despite a considerable number of studies and research has gone into it. These studies are also useful to highlight potential gap areas in the observations of the Indian Ocean carbon cycle, pCO2, and acidification. A few are mentioned below, kindly incorporate them also in this review and synthesis effort.

However, the Biogeochemistry about the carbon cycle in the upwelling zones is somewhat found less emphasized despite a considerable number of studies and research has gone into it. These studies are also useful to highlight potential gap areas in the observations of the Indian Ocean carbon cycle, pCO2, and acidification. A few are mentioned below, kindly incorporate them also in this review and synthesis effort.

Lines-1034:1035: Being 1034 limited with very few studies on carbon dynamics over both east and west coasts, the temporal evolution of surface ocean acidification is still not clear.
Takahahsi et al, (2014) compilation show the clear seasonal cycle of pH in the western Arabian Sea. Further, modeling studies show that the western Arabian Sea has been acidified from a pH of 8.12 (in 1960) to a pH of 8.05 (in 2010). The trend in pH over the western Arabian Sea is due to contributions from dissolved inorganic carbon (DIC) and SST at a value of 109% and 16%, respectively. The effect of alkalinity (ALK) is to buffer the trend in pH by -36% while salinity contribution is only +7%. Collectively, DIC and ALK contribute up to 73% to the net pH trend. SST warming alone contributes another 16%, which is quite alarming considering the intense warming of the western Indian Ocean (Roxy et al., 2016). This calls for the sustained observational efforts required for the Indian Ocean upwelling zones to monitor and model ocean acidification.

Lines-1401-1403: Efforts to develop and improve biogeochemical models of the 1401 upwelling systems are also in progress (e.g., Sreeush et al., 2018).
In addition, Sreeush et al., (2020) showed improving biogeochemical models in upwelling zones using inversion of surface observation such as pCO2 and imposed constraints that can cascade through solubility and the biological pump in the upwelling zones to retrieve valuable subsurface ocean parameters such as community compensation depth in models.

Lines-1673:1674: Finally, there is still considerable uncertainty in whether the Indian Ocean is a net source or sink of carbon to the atmosphere because the variability in pCO2 fluxes across the air-sea interface is poorly constrained by existing observations, particularly in active upwelling zones like the SCTR
Other studies also highlighted the variability of seasonal and interannual cycles of sea-toair CO2 fluxes, pCO2 in the upwelling regions of the Indian Ocean. Valsala and Maksyutov (2013) identified that the interannual variability of western Arabian Sea sea-to-air CO2 fluxes and pCO2 are complementarily controlled by ENSO and IOD-related forcing and dynamics. In the south of Sri-Lanka, the interannual variability of the carbon cycle is controlled by variability in wind-induced upwelling dynamics of the dissolved inorganic carbon (Valsala and Maksytov, 2013). The western Arabian Sea is also home to intraseasonal variability in sea-to-air CO2 fluxes and pCO2 due to the eddy dynamics associated with Great Whirl and Southern Gyre (Valsala and Murtugudde, 2015) as verifiable with limited observations of surface ocean pCO2. More observational efforts are required to understand such fine-scale variability of pCO2 in the Indian Ocean.
Recent studies pointed out that the south Java-Sumatra coast also exhibits interannual variability in sea-to-air CO2 fluxes and pCO2 due to upwelling variability linked to IOD, as identifiable from gap-filled observations using neural networks , Lanschutzer al., 2016. The sea-to-air CO 2 fluxes, surface ocean partial pressure of CO 2 (pCO 2 ), the concentration of dissolved inorganic carbon (DIC), and ocean alkalinity (ALK) range as much as ±1.0 mole m -2 yr -1 , ±20 μatm, ± 35 μmole kg -1 , and ± 22 μmole kg -1 within 80 o E-105 o E, 0-10 o S due to IOD. The DIC and ALK are significant drivers of pCO 2 variability associated with IOD. The roles of temperature (T) and biology are found negligible. A relatively warm T and extremely high freshwater forcing make the southeastern tropical Indian Ocean carbon cycle variability submissive to DIC and ALK evolutions in contrast to the tropical eastern Pacific where changes in DIC and T dominate the pCO 2 interannual variability .