Influence of wind and bottom generated turbulence on air-sea gas exchange in shallow water environments

Researchers: F. Veron (U Delaware), D. Ho (U Hawaii), G. Pawlak (MAE/SIO UCSD)

Funding: National Science Foundation

A series of experiments are being carried out on the barrier reef of Kane‘ohe Bay, Hawaii to determine factors that control gas exchange in shallow ocean environments. During the experiment, gas transfer velocities will be measured with tracers (N2O, SF6, Rhodamine WT) and eddy covariance of CO2, along with concurrent measurements of processes such as wind, current, waves and turbulence. With the data collected in Kane‘ohe Bay, we are aiming to:

1. Assess when or whether it is valid to apply open ocean wind speed/gas exchange parameterizations to shallow water environments

2. Improve our understanding of how physical processes such as wind, currents, waves interact with the shallow bottom to produce surface-, and bottom-driven turbulence

3. Examine how surface and bottom generated turbulence control air-sea gas exchange in shallow coastal waters

4. Parameterize gas exchange in shallow water environments in terms of easily measured environmental variables such as wind speeds, current velocities, and water depth.

Air-sea transfers of energy and mass (gas) are critically important for predictions of short term weather, as well as long term climate trends. Over the past decades, considerable effort has been devoted to parameterizing these fluxes in terms of widely measured variables such as wind speed. However, these parameterizations are inadequate in shallow coastal regions where fetch, waves, and bottom-generated turbulence influence the physical mechanisms responsible for the air-sea fluxes, which have implications for our ability to understand important environmental processes. For example, uncertainty in how to parameterize air-sea gas exchange impedes progress in understanding and addressing issues such as coral reef metabolism or carbon cycling. Furthermore, shallow coastal water ecosystems are susceptible to anthropogenic pollution, leading to hypoxia or anoxia. In order to understand the future states of these ecosystems that are exposed to continued anthropogenic perturbations, biogeochemical budgets and fluxes have to be established, including exchanges across the air-water interface, i.e., they require quantitative knowledge of factors controlling gas exchange. It is anticipated that the results from this study will yield improved understanding of the influence of physical processes such as wind, currents, waves on producing surface-, and bottom-driven turbulence, and how they control air-sea gas exchange in shallow coastal waters. They should also allow these processes to be parameterized in terms of easily measured environmental variables such as wind speeds, current velocities, and water depth.