Surface Water PCO₂ And Air-Sea CO₂ Fluxes In The South China Sea In Summer-Estimations By Using Remote Sensing Data

Determining the air-sea CO₂ flux is essential in studying the global carbon cycling and the climate change.Up to date,the air-sea CO₂ fluxes are primarily estimated by shipboard measurements of the air and sea surface pCO₂ on research cruises,which obviously have limitations of spatial and temporal coverage.An empirical approach is presented for the estimation of partial pressure of carbon dioxide(pCO₂) and air-sea CO₂ fluxes in the northern South China Sea(SCS) in summer using satellite-derived sea surface temperature(SST), chlorophyll-a(Chl a) concentration,and wind field.Two algorithms were attempted.The first one used an SST dependent equation,and the other having Chl a introduced.Regression equations were developed for summer based on in situ data obtained in July,2004.Using the monthly average SST and Chl a fields derived respectively from the AVHRR(Advanced Very High Resolution Radiometer) and SeaWiFS(Sea-viewing Wide Field of View Sensor),monthly pCO₂ fields were computed.The derived pCO₂ were compared with the shipboard pCO₂ observations conducted in July,2000.That one derived from the SST dependent algorithm resulted in a root mean square error(RMS) of 25.1μatm,while the other derived from the SST-Chl a dependent algorithm resulted in a RMS of 4.6μatm.This suggests that the satellite-derived pCO₂ was in general agreement with the in situ observations when the SST-Chl a algorithm is applied.The pCO₂ distribution in the northern South China Sea in summer from 1998 to 2005 was then estimated by applying the SST-Chl a dependent algorithm.It is shown that pCO₂ in 1998 was 413.8μatm,which is the highest among the 8 years and is probably associated with the significant warm event occurring in the SCS in response to the 1997-1998 El Nino.On the other hand, pCO₂ in coastal areas is lower than in the basin water,for example,the averaged pCO₂ in the Gulf of Tongkin was approximately 380μatm. The air-sea CO₂ flux was further computed with the aid of monthly mean QuikSCAT wind speed.The CO₂ flux was lower in the western region than in the eastern region,and also was lower in coastal areas than in the basin water. For example,the averaged sea to air CO₂ flux off Vietnam and in the Gulf of Tongkin is approximately 3.5 mmol CO₂ m^-2 day^-1,while it ranges between 7～9 mmol CO₂ m^-2 day^-1 in the northeastern SCS.Upwelling frequently occurs in the SCS,which may change the surface pCO₂ distribution from the normal state.Based on a dataset obtained from a cruise survey conducted in the water offshore Vietnam(where upwelling occurs) in August 2007,we attempted to develop a SST-dependent empirical function specific to the upwelling water.By applying this model and the above SST-Chl a dependent algorithm to upwelling and non-upwelling waters, respectively,pCO₂ distribution for the whole basin in August 2007 was then estimated.The results were compared with in situ measured data,which were not used to derive algorithms and were thus an independent dataset,resulting in a RMS of 11.3μatm.The air-sea CO₂ flux was also caculated,indicating that the upwelling event we detected might give a rise of CO₂ flux by 1.2%for the whole SCS basin,when the potential subsequent biological drawdown had not been considered.The results presented here are nevertheless preliminary.We contend that more shipboard data are necessary in refining the empirical algorithms and reducing the uncertainty.