Effects of Typhoons on Surface Seawater pCO₂ and Air-Sea CO₂ Fluxes in the Northern South China Sea

This study assessed the effects of typhoons on sea surface pCO(2) and CO2 flux in the northern South China Sea (SCS). During the passage of three major typhoons from May to August 2013, sea surface pCO(2), surface seawater temperature (SST), and other meteorological parameters were continuously measured on a moored buoy. Surface water in the region was a source of CO2 to the atmosphere with large variations ranging from hours to months. SST was the primary factor controlling the variation of surface pCO(2) through most of the time period. Typhoons are seen to impact surface pCO(2) in three steps: first by cooling, thus decreasing surface pCO(2), and then by causing vertical mixing that brings up deep, high-CO2 water, and lastly triggering net uptake of CO2 due to the nutrients brought up in this deep water. The typhoons of this study primarily impacted air-sea CO2 flux via increasing wind speeds. The mean CO2 flux during a typhoon ranged from 3.6 to 5.4 times the pretyphoon mean flux. The magnitude of the CO2 flux during typhoons was strongly inversely correlated with the typhoon center distance. The effect of typhoons accounted for 22% of the total CO2 flux in the study period, during which typhoons occurred only 9% of the time. It was estimated that typhoons enhanced annual CO2 efflux by 23-56% in the northern SCS during the last decade. As such, tropical cyclones may play a large and increasingly important role in controlling CO2 fluxes in a warmer and stormier ocean of the future. Plain Language Summary The global ocean absorbs about 25% of anthropogenic carbon dioxide emissions from the atmosphere and plays an important role in regulating global climate. One important question regarding estimates of carbon dioxide flux at the ocean surface is how episodic events such as typhoons affect surface seawater carbon dioxide and air-sea carbon dioxide exchange. In this study, seawater temperature generally played a primary role in controlling surface seawater carbon dioxide under nontyphoon conditions in the northern South China Sea. We found that typhoons seem to have a three-step effect on surface carbon dioxide: first cooling, which decreases carbon dioxide content, followed by vertical mixing that can bring up deep water with high concentrations of carbon dioxide and nutrients, and lastly the triggering of net uptake of carbon dioxide due to phytoplankton growth fueled by those nutrients. Increased air-sea carbon dioxide flux during a typhoon is caused primarily by high wind speed. Typhoons enhanced carbon dioxide flux to the atmosphere by 23-56% annually in the study region. As such, typhoons may play a more important role in the control of carbon dioxide fluxes in a warmer and stormier ocean of the future.