Enhanced sea‐air CO2 exchange influenced by a tropical depression in the S outh C hina S ea

Abstract Ship measurements made 2 days after the passage of a tropical depression (TD) in the South China Sea (SCS, April 2011) showed two contrasted responses of the partial pressure of CO2 at sea surface (pCO2,sw). In low sea‐surface salinity (SSS) water, pCO2,sw was low (349 ± 7 μatm), and the area was a carbon sink (−4.7 ± 1.8 mmol CO2 m−2 d−1), whereas in water with high SSS and chlorophyll a and low dissolved oxygen and sea surface temperature, pCO2,sw was higher than for normal SCS water (376 ± 8 versus 362 ± 4 μatm) and the area was a carbon source (1.2 ± 3.1 mmol CO2 m−2 d−1). Satellite data showed two large areas of low SSS before the TD, which were likely influenced by rainfall, and these areas were considered to have low pCO2,sw because of their low SSS. The high pCO2,sw after the TD is explained by the uplifting to the surface of deeper and CO2‐rich water, due to winds accompanied by the TD. The difference in sea‐air CO2 flux between the TD‐affected area and the lower‐SSS water was 1.99 + 4.70 = 6.7 mmol CO2 m−2 d−1, indicating a 100% change caused by the TD compared to the average seasonal value in spring in southern SCS (3.3 ± 0.3 mmol CO2 m−2 d−1). Undersaturation of CO2 prior to the TD due to dilution by freshwater and the preexisting cold eddy, and slow translation speed of the TD, are considered to account for the CO2 flux change.