Impact of eddies on dissolved inorganic carbon components in the Bay of Bengal

Abstract

To examine the influence of anticyclonic (ACE) and cyclonic (CE) eddies on variations in hydrographic properties and total inorganic carbon system (CT), a study was conducted in the western Bay of Bengal (BoB). The depth of seasonal thermocline, defined as depth of 20 °C isotherm (D20), shoaled (∼80 m) in the CE but deepened to ∼180 m in the ACE relative to non-eddy (NE) conditions (∼110 m) due to convergence and divergence associated with ACE and CE, respectively. Both surface and depth integrated (0–100 m) chlorophyll-a concentrations were higher in CE than in ACE and NE regions suggesting higher primary production in the CE region. Upward displacement of 2100 μM of CT and 800 μatm of pCO2 isosurfaces from a depth of 175 to 50 m was observed in the CE region resulting in an increase in mixed layer CT and pCO2 compared to that in NE region. Low CT (∼1850 μM) and high pH (∼8.15) isolines deepened to 150–175 m in ACE region. Strong positive relations were observed for CT and pCO2 with salinity whereas weak inverse correlations were found for chlorophyll-a and dissolved oxygen saturation with salinity normalized CT. This suggests that both biological and physical processes control CT in the BoB. Both biological and mixing effects contributed significantly to pCO2 variability in the CE region whereas mixing and thermal effects contributed in the ACE region. The sea-to-air fluxes of CO2 varied between −0.21 and 0.31 molC m−2 month−1. The CE and south ACE (ACEs) regions were found to act as sink for the atmospheric CO2 whereas northern ACE (ACEn) and NE regions were source. The mean CO2 flux suggests that the study region was a moderate source (0.05 ± 0.01 molC m−2 month−1) to the atmosphere. The mean sinking flux of CT to the deeper layer in ACEn and ACEs was ∼0.07 ± 0.02 molC m−2 month−1 whereas it was ∼0.02 ± 0.006 C m−2 month−1 in CE and NE regions. Assuming that the estimated flux at the eddies sampled here is applicable to all eddies in the Bay, the vertical DIC flux to the deep BoB by all eddies amounted to 3.29 ± 0.9 TgC y−1 (1 TgC = 1012 gC). Nevertheless this study suggests that ACEs transports CT to deeper waters, besides being source to the atmospheric CO2.