Wave Breaking in High Winds and its Effects on the Air-Sea Exchange of Gasses of Varying Solubility

Lead PI: Dr. Christopher J. Zappa

Unit Affiliation: Ocean and Climate Physics, Lamont-Doherty Earth Observatory (LDEO)

September 2015 - January 2018
Arctic Ocean ; Greenland
Project Type: Research

DESCRIPTION: The ocean is a major player in the storage and transfer of many greenhouse gases. Understanding the amount of these gases going through the interface between ocean and atmosphere, known as the air-sea flux, is of upmost importance to accurate biogeochemical and climate predictions. The mechanisms which move the gases through the interface are too small to be resolved by most coupled ocean atmosphere models, and are therefore parameterized to account for the effect of the small scale processes. Air-sea fluxes are largely dictated by wind speed, but are sensitive to many other environmental factors such as surfaces waves and surfactants. In situ observations have shown large variability between observed and parameterized air-sea fluxes, especially under high wind speed conditions, suggesting wind speed alone cannot capture all of the factors that influence air-sea gas exchange. This project will analyze a dataset collected during a NSF-funded research project near the southern tip Greenland. This dataset is unique and may offer insight into some previously unmeasured mechanisms with control on air-sea gas flux such as breaking waves and bubble formation under high winds. The findings could help to refine gas transfer parameterizations which, in turn, would help to constrain regional and global estimates of climate sensitive gases. The project will support the training of a PhD student and the development of stimulating teaching materials about research into gas exchange, storms, breaking waves and climate change, available to K-12 teachers.  Global air-sea gas flux estimates are based on parameterizations of the gas transfer velocity k. To first order, k is dictated by wind speed (U) and is typically parameterized as a non-linear function of U. There is, however, a large spread in k predicted by the traditional parameterizations, especially at high wind speed. This is due to a large variety of environmental forcings and processes that actually influence k, suggesting wind speed alone cannot capture the variability of air-water gas exchange. At high wind speed, breaking waves become a key factor to take into account when estimating gas fluxes. Wave breaking results in additional upper ocean turbulence and generation of bubble clouds. Here, we propose to analyze the diverse data set collected during the High Wind Gas exchange Study experiment in 2013 to understand and quantify the control of breaking waves on gas transfer velocities. This will be a first study linking turbulent kinetic energy disSchool of International and Public Relationstion rates resulting from wave breaking to gas transfer velocities. Insights gained from observation will be incorporated into physical gas transfer models. Whitecap coverage and breaking wave statistics will be determined from visible imagery acquired from the port and starboard side of the flying bridge of the Research Vessel Knorr. Both very soluble (Methanol and Acetone) and less soluble (Carbon Dioxide, Dimethyl Sulfide) gases will be considered, allowing to contrast the degree of wave breaking mediated transfer. Sea state conditions will be computed from laser altimeter and wave rider buoy measurements. Eddy covariance fluxes and sea water concentration of Carbon Dioxide, Dimethyl Sulfide, Methanol and Acetone allow for direct calculation of transfer velocities.