A STABILITY DEPENDENT THEORY FOR AIR-SEA GAS-EXCHANGE

The influence of thermal stability at the air-sea interface on compute d values of the transfer velocities of trace gases is examined. The no vel ''whitecap'' model for air-sea gas exchange of Monahan and Spillan e (1984), extended here to include thermal stability effects, is linke d with an atmospheric general circulation model to compute global tran sfer velocity patterns of a climate reactive gas, CO2. The important t erms in the model equations such as the whitecap coverage, friction ve locity, neutral and local drag coefficients and the stability paramete r PSI(m)(Z/L) are discussed and analyzed. The atmospheric surface leve l air temperature, relative humidity, wind speed and sea surface tempe rature, obtained from the National Center for Atmospheric Research Com munity Climate Model 1 (CCM1) are used to drive algorithms describing the air-sea transfer velocity of trace gases. The transfer velocity fo r CO2 (k(CO2)) is then computed for each 2.8-degrees x 2.8-degrees lat itudinal-longitudinal area every 24 hours for 5 years of the seasonal- hydro runs of the CCM1. The new model results are compared to previous ly proposed formulations using the identical CCM1 forcing terms, Air-s ea thermal stability effects on the transfer velocity for CO2 are most important at mid-high wind speeds. Where cold air from continental in teriors is transported over relatively warm oceanic waters, the transf er velocities are enhanced over neutral stability values. The depressi on of computed k(CO2) values when warm air resides over cold water is especially important, due to asymmetry in the stability dependence of the drag coefficient. The stability influence is 20% to 50% of k(CO2) for modest air-sea temperature differences and up to 100% for extreme cases of stability or instability. The stability dependent ''whitecap' ' model, using the transfer velocity coefficients for whitecap and non whitecap areas suggested by Monahan and Spillane (1984), produces CO2 transfer velocities that range from 13 to 50 cm h-1 for a monthly mean . High-latitude regions of both hemispheres experience winter season m eans of 40 to 50 cm h-1. The global area-weighted mean CO2 transfer ve locity is 19.2 cm h-1, in reasonable agreement with the C-14 estimate of Broecker and Peng (1974). Although consistent with global C-14 esti mates, the initial version of the model predicts a factor of 2 to 3 hi gher CO2 transfer velocities over areas with low wind speeds relative to the parameterizations of Liss and Merlivat (1986) and Tans et al. ( 1990). New transfer velocity coefficients for whitecap and nonwhitecap areas are suggested that bring the low wind speed results into better agreement with observations and other models. The calculations descri bed here suggests that oceanic gas exchange with the atmosphere is sen sitive to thermal stability at the air-sea interface. This specific, t urbulence-related geophysical forcing may account for a portion of the observed scatter in previously obtained experimental data that has be en correlated with wind speed alone.