Collaborative Research: Measuring Air-Sea Gas Exchange in High Winds for Improvement of Physics-Based Air-Sea Transfer Parameterizations

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Predicting future climate requires, among other factors, accurate estimates of the atmospheric concentration of carbon dioxide and other greenhouse gases and an understanding of the feedbacks between atmospheric radiative processes and surface ocean biogeochemistry. In this context, of particular interest to climate modeling is an understanding of the exchange of gases (like carbon dioxide and dimethylsufide, DMS) between the atmosphere and ocean surface. DMS is a contributor to secondary aerosol and cloud condensation nuclei production. Numerous physical processes are responsible for gas exchange, including momentum transfer to the ocean surface from wind shear, transfer of momentum directly to waves via pressure differential across the wave face (form drag), breaking waves which generate bubbles of air forced into the water column, near-surface biogeochemical processes which modify surface tension and water column thermal stability, gas species chemical properties, etc. In order to parameterize air-sea gas exchange for application in climate models, it is crucial to understand which of these processes are important under specific environmental conditions. Recent experimental work has shown that simple parameterizations based solely on wind speed are not adequate to describe the gas transfer, and this is particularly true at the higher wind regimes where breaking wave processes dominate the gas exchange. For example, observations have shown that gas transfer from bubbles is enhanced for less soluble gases, and theory suggests that form drag becomes increasingly important at high winds; this effect will modulate the water-side mixing from tangential shear transfer. This realization has led to the development of a new air-sea gas transfer parameterization which expresses the exchange in terms of the forcing physical processes. However, the parameterization has not been tested at higher wind regimes where the bulk of the globally-integrated transfer occurs. A four-year effort will be undertaken which includes two cruises on ships-of-opportunity and one dedicated cruise with the intention of investigating air-sea gas transfer across a range of wind speeds (the dedicated cruise will focus on high winds), a range of gas solubility (e.g., DMS, carbon dioxide, acetone, carbon monoxide, sulfur dioxide), and a range of wave states (control on fetch and distribution of wind stress between tangential and form drag components) and wave breaking (bubble processes).The results from the experimental work will be used to further develop the new gas transfer parameterization which is applicable in larger scale climate models that include the effects of air-sea gas transfer. This potentially leads to better predictions of future climate, and these projections will be of significant value to society and policy decision makers. The project will include scientist participation in workshops focused on providing local earth science teachers with tools necessary to understand and communicate climate science to their elementary and high school students. Scientist lectures will be teleconferenced to remote sites, and the project contribution to the workshops will include imparting knowledge about connections between observations and models, as well as assisting teachers in understanding the limitations and strengths of models in predicting climate.