Study on air-sea CO2 gas exchange with wave breaking
Document TypePhD thesis
Access StatusOpen Access
© 2020 Shuo Li
Atmospheric CO2 is accumulating in recent decades due to excessive anthropogenic activities such as fossil fuel combustion, which intensifies global warming and affects the balance of carbon cycle. With more CO2 in the air, ocean which acts as one of the largest carbon reservoirs absorbs about 26% of the human emissions of CO2 to the atmosphere. The increased ocean inventory of anthropogenic carbon has caused the acidification of ocean water, making it critical to assess the CO2 gas transfer process at air-sea interface. CO2 gas transfer velocity (KCO2) is the main subject in the estimation of gas flux. To date, KCO2 is traditionally parameterized with wind speed, but it is the waves which produce the turbulence and bubbles that enhance the CO2 intake. In our work, the new parameterization of KCO2 with wave parameters is proposed through laboratory experiments, validated by field observation and applied to the estimation of the air-sea CO2 flux over global ocean. To investigate the direct relationship between CO2 gas transfer and waves, laboratory experiments are conducted in a wind-wave flume. Three kind of waves are forced in the flume: monochromatic waves generated by a wavemaker, mechanically-generated monochromatic waves with superimposed wind forcing, and pure wind waves with 10-meter wind speed ranging from 4.5 m/s to 15.5 m/s. The wave parameters are found to be well correlated with KCO2 while wind speed alone can not adequately describe KCO2. To reconcile the data sets, non-dimensional empirical formulae are established in which KCO2 is expressed as a function of wave parameters as the dominant term and an enhancement factor to account for additional influence of the wind. The parameterization is further validated by using field campaign data from different locations of open ocean and improved including considering the importance of bubble-mediated gas transfer at sea. The verified formula is able to collapse the results of both laboratory and field with reduced uncertainties. The net air-sea CO2 flux of global ocean is evaluated in a 33-year period from 1985 to 2017 by using our gas transfer parameterization. The results are in high agreement with previous studies, which is another evidence of the validity of the parameterization. A general increasing trend of global ocean net uptake of CO2 is observed. The CO2 flux in period 2017-2100 is also projected by using different CMIP6 forecast of future scenarios.
KeywordsCO2 gas exchange; ocean waves; wave breaking; air-sea interaction
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