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

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AbstractDiurnal variability of the CO2 partial pressure (pCO2) at the sea surface (pCO2sea) and its impact on air‐sea CO2 flux estimations are currently poorly understood. In this work, diurnal variations in pCO2sea were examined in the Bay of Bengal (BoB) based on 3‐hourly observations collected at the BoB Ocean Acidification moored buoy between November 2013 and November 2018 by a moored autonomous pCO2 system. Significant mean diurnal pCO2sea cycles and a mean diurnal amplitude of 11 ± 9 μatm were observed based on 1215 complete diurnal cycles. One‐dimensional mass budget model calculations suggest that temperature was responsible for these mean diurnal variations in pCO2sea and that these variations were intensified by biological activity. The effects of air‐sea gas fluxes and mixing on the mean diurnal pCO2sea variability were very low. A weak annual net source of 12.1 mmol m−2 year−1 was estimated at this site. The sampling frequency greatly impacted the annual air‐sea CO2 flux estimations. Compared to the 3‐hr sampling frequency, random daily sampling resulted in a bias of ±7.7 mmol m−2 year−1 or 64%, and regular daily sampling resulted in a maximum bias of 400%. Factorial experiments showed that wind speed had the largest impact on the air‐sea CO2 flux estimations, followed by pCO2sea and atmospheric pCO2 (pCO2air). These findings indicate the importance of obtaining continuous pCO2 measurements to accurately estimate air‐sea CO2 flux; additionally, pCO2 and wind speed measurements should be obtained at frequencies higher than 6 hr.