A sea state dependent gas transfer formulation

This project aims to produce a mechanistic formulation for air-sea gas fluxes, more accurate than empirical relationships currently used. It will develop and test a novel parameterization that fully accounts for the effects of sea-state, wind, and key physicochemical variables such as diffusivity, solubility and temperature on the bubble mediated gas exchange.

This is novel as it spans all the scales relevant to the gas transfer problem, from the bubble scale, to the wave statistics at the ocean surface. The flux will be modelled through a sea-state dependent gas transfer velocity, within a unified framework for all gas species. The impact of key variables which are known to influence bubble mediated gas transfer, such as the bubble size distribution, bubble residence time, its dependence on salinity, viscosity and temperature, as well as the diffusivity and solubility of different gases are directly incorporated in the formulation.

Better understanding and improved parameterizations of the gas transfer are necessary to better predict the associated global biogeochemical cycles of carbon dioxide, oxygen or dimethyl sulfide. Understanding how the wave field modulates the fluxes of these climate-relevant gases will lead to general improvements in climate and weather models and forecast.

Since increased CO2 causes ocean acidification impacting shell-forming marine animals, and limitations in oxygen have broad ecological effects, improved parameterization of the exchange of these gases can help interpret existing observations, and might improve our understanding of local processes and their impact on ecosystems. The general framework to account for sea-state dependence is not limited to gas transfer but could be generalized to other type of fluxes as well.

This project will expose undergraduate and graduate students at Princeton to these critical environmental challenges that require research on fundamental multi-phase flows, and promote the use of open-source methods through workshops and teaching activities.

This research promotes a general theoretical framework to account for the complex nature of wave breaking and air entrainment, a two-phase turbulent process, and the very large range of scales involved in the process, from wave statistics scales of order of km, O(1km), to wave breaking dynamics, O(1-10m), air entrainment, bubble generation and dissolution O(cm to m). Leveraging recent progress in wave modeling, a state-of-the-art wave model will be used to directly compute the breaking statistics, which will help investigate the role of the full wave complexity on the gas flux at high temporal and spatial resolution.

Bubble contribution to air-sea gas exchange will be evaluated regionally and globally for various gases like carbon dioxide and dimethyl sulfide, and the formulation will be extended to low solubility gases such as oxygen by considering the bubble asymmetric contribution. Regions and seasons will be identified where capturing the wave field and associated storms is critical to represent field observations.

Systematical comparisons of this modeling approach to recent and historical data sets will leverage the large effort by the air-sea interaction community in producing high quality field measurements. A consistent data set of global and regional gas transfer velocity will be produced, that will be used to develop a unified parameterization for the transfer of any gas, and which will be made available to the ocean and climate community, to be used in coupled wave-ocean-atmosphere and climate models.

This should significantly reduce the uncertainties of air-sea gas exchange at moderate to high wind speeds in biogeochemical cycles.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.