Investigation Of Air-sea Gas Transfer Velocity Using In-situ Measurements And Remote Sensing Technique

Adequately accounting for sinks and sources of CO₂is needed to properlypredict the climate evolution. The global ocean plays a key role in cycling CO₂as itabsorbs a large portion of the anthropogenic CO₂from the atmosphere, accurateestimates of the air-sea CO₂flux is essential in understanding the feedback betweenthe air-sea gas exchange and climate change.Typically, estimates of the air-sea CO₂flux are performed using the combinationof gas transfer velocity （kw） and the air-sea CO₂partial pressure difference. Largeuncertainties were suggested to be induced by the gas transfer velocity formular. Gastransfer velocity is commonly parameterized as a function of wind speed （U10）.However, great discrepancies existed among the proposed formulas, especially at highwinds, which is largely due to the improper consideration of the wave breaking effect.The sea surface turbulence is the controlling factor for gas transfer velocity, andthe turbulence intensity is typically expressed in terms of turbulent kinetic energy（TKE） dissipation rate. Based on the energy conservation, we related the TKEdissipation rate to the ratio of wind-wave energy （WWE） dissipation rate to mixinglayer depth. The WWE dissipation rate is computed from the Phillips model using theJONSWAP spectrum and buoys’ directional spectrum respectively. It is found that thedissipation rate scales linearly with the breaking-wave parameter:R_B=u_*²/ν_aω_p.Measurements of TKE dissipation rate were performed on an offshore fixed platform,it was observed enhanced dissipations exceeding the wall layer values with1²ordersof magnitude near the sea surface. The vertical distribution of TKE dissipation ratefollows the breaking-wave scaling proposed by Terray et al.（1996）, which decays asz^-2. Giving the mixing layer depth as0.25H_s, the sea surface TKE dissipation rate isinferred from the above two scalings, it was found that the dissipation rate scaleslinearly with the friction wind speed. Based on the small-eddy model, we proposed a gas transfer velocity formula,which could be expressed as a function of friction wind speed and wind-waveparameters, it predicts an increasing gas transfer rate toward a fully developedwind-wave at a given wind speed. The new formula is validated against observationsand shows improved results. As inferred from the formula, the gas exchange inbreaking-area is dominant at high winds.In order to properly associate the proposed formula with remote sensingtechniques, we studied the wind-wave property of the sea surface mean square slope（MSS） measured by the altimeter. It was shown that the MSS not only depends onwind speed, but also closely correlated with wave age, which roughly scales withwave age to the3/2, while poor correlation was found between swell and MSS. Wecalculated the MSS using a wind-wave spectrum model, it has a good agreementswith the observations in terms of both wind and wave age variations. The long wavetitling is shown to account for the wave state effect.Based on the above information, we retrieved the U10and wave age from thealtimeter dual-frequency normalized radar cross sections, which could be used toestimate kwfrom the proposed gas transfer velocity formula. This approach wasapplied to estimate the global distribution of kwusing the Topex/Poseidonmeasurements during2002-2004. The global average kwwas computed to be about18.8cm/h. The spatial distribution of kwis mainly controlled by the wind speed, whilethe wave state has an impact on the regional characters. The seasonal variations of areinvestigated, the northern hemisphere was found to have a strong seasonal cycle,which is also the case for southern hemisphere at latitudes lower than50oS, at higherlatitudes, the kwis persistent high all year round.