The Link between CO2 Flux and the Upwelling–Downwelling System of a Tropical Coastal Sea

Abstract

This work presents the relationship between the monsoons and the upwelling–downwelling processes of the tropical coastal sea of the Sunda Shelf on CO2 flux measured using the eddy covariance method. The understanding on the co-impact of upwelling–downwelling processes to physical–biological parameters that affect the CO2 flux in the region is limited. This study aims to determine the factors and trends influencing the flux. We found that upwelling (downwelling) overlapped with the Northeast (Southwest) Monsoon as evidenced by the high (low) daily averaged chlorophyll-α concentrations. Increased chlorophyll-α with a mean of 1.70 mg m−3 was observed in the upwelling period, whereas in the downwelling period, it decreased to 0.49 mg m−3. The CO2 flux correlated with the seawater temperature and wind speed. The CO2 flux was negative, −2.33 mmol m−2 day−1 (positive, 0.54 mmol m−2 day−1), in the upwelling-Northeast Monsoon (downwelling-Southwest Monsoon) season. On the climatological hourly timescale, this pattern showed that during the downwelling-Southwest Monsoon season, there were durations of increased, positive pulses of CO2 flux, possibly because of higher temperatures, lower wind speeds, and low levels of chlorophyll-α. Conversely, in the upwelling-Northeast Monsoon season, there were periods of decreased flux, which occurred briefly during the day because of increasing concentrations of chlorophyll-α and at night due to lower temperatures and higher wind speeds.