Enhanced sea‐air CO 2 exchange influenced by a tropical depression in the S outh C hina S ea

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 CO 2 at sea surface ( p CO 2,sw ). In low sea‐surface salinity (SSS) water, p CO 2,sw was low (349 ± 7 μatm), and the area was a carbon sink (−4.7 ± 1.8 mmol CO 2 m −2 d −1 ), whereas in water with high SSS and chlorophyll a and low dissolved oxygen and sea surface temperature, p CO 2,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 CO 2 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 p CO 2,sw because of their low SSS. The high p CO 2,sw after the TD is explained by the uplifting to the surface of deeper and CO 2 ‐rich water, due to winds accompanied by the TD. The difference in sea‐air CO 2 flux between the TD‐affected area and the lower‐SSS water was 1.99 + 4.70 = 6.7 mmol CO 2 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 CO 2 m −2 d −1 ). Undersaturation of CO 2 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 CO 2 flux change.