Enhanced sea‐air CO₂ exchange influenced by a tropical depression in the South China Sea

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 CO 2 at sea surface (pCO 2,sw ). In low sea-surface salinity (SSS) water, pCO 2,sw was low (349 6 7 latm), and the area was a carbon sink (24.7 6 1.8 mmol CO 2 m 22 d 21 ), whereas in water with high SSS and chlorophyll a and low dissolved oxygen and sea surface temperature, pCO 2,sw was higher than for normal SCS water (376 6 8 versus 362 6 4 latm) and the area was a carbon source (1.2 6 3.1 mmol CO 2 m 22 d 21 ). 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 pCO 2,sw because of their low SSS. The high pCO 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 1 4.70 5 6.7 mmol CO 2 m 22 d 21 , indicating a 100% change caused by the TD compared to the average seasonal value in spring in southern SCS (3.3 6 0.3 mmol CO 2 m 22 d 21 ). 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.

Storm‐induced changes in pCO2 at the sea surface over the northern South China Sea during Typhoon Wutip

“…The temperature was found to be the decisive factor in decreasing pCO 2 in the case of Hurricanes Felix and Frances in the North Atlantic Ocean [ Bates et al ., ; Huang and Imberger , ]. By comparison, DIC is the most important factor in increasing pCO 2 in the case of tropical depression in the SCS [ Sun et al ., ]. In this study, the increased pCO 2sea in the CEIN and OUT was due to the fact that the CO 2 ‐rich subsurface waters were mixed and upwelled to the surface, while the decreased pCO 2sea in the AEIN was due mainly to SST cooling.…”

“…Previous studies also suggested that the typhoon‐induced SST cooling was the dominant reason for the decrease of pCO 2sea [ Bates et al ., ; Huang and Imberger , ]. The typhoon‐uplifted deeper CO 2 ‐rich water, however, could increase pCO 2sea [ Sun et al ., ]. An increase in the CO 2 efflux from the ocean to atmosphere caused by typhoons was observed [ Bates et al ., ; Perrie et al ., ; Nemoto et al ., ].…”

“…An increase in the CO 2 efflux from the ocean to atmosphere caused by typhoons was observed [ Bates et al ., ; Perrie et al ., ; Nemoto et al ., ]. A tropical depression was found to transform a carbon sink area to carbon source in the SCS [ Sun et al ., ]. In this study, we documented two different responses after the passage of Typhoon Wutip: high pCO 2sea with a high efflux of CO 2 and low pCO 2sea with a low influx of CO 2 .…”

“…A typhoon typically accompanies large precipitations. It was found that heavy rain can induce significant chemical dilution and reduce the pCO 2sea by more than 30 μatm in the low‐wind and high‐precipitation regions [ Turk et al ., ; Sun et al ., ]. Furthermore, the presence of cyclonic eddies was found to either decrease or increase pCO 2sea [ Chen et al ., ], while anticyclonic eddies were found to increase pCO 2sea .…”

“…Only a few studies, however, were made on the combined effects of a cyclonic or anticyclonic eddy and rainfall under the impact of a typhoon on the pCO 2sea and F CO2 . The passage of a tropical depression was found to change the SCS from influx into efflux under the combined effect of rainfall and a preexisted cold eddy [ Sun et al ., ]. The present study aims to investigate the important effects of a typhoon, rainfall, preexisted cold, and warm eddies on the pCO 2sea and local F CO2 over the northern South China Sea (NSCS).…”