Momentum flux budget across air-sea interface in high wind conditions

Momentum flux from the air, also known as the wind stress, is a drag or tangential force imposed on Earth's surface by the motion of an adjacent body of air. How to get the right wind stress under high wind conditions has been a study topic for over 50 years. Every body knows that wind stress is essential for oceanic and atmospheric modeling. But, knowing the accurate wind stress is not enough for oceanic model.

Figure 1. Kinematic relation between wind stress () and downward momentum flux into subsurface current ().

When wind blow over the ocean surface, we can see both surface gravity waves and wind driven currents. Especially during high wind conditions, the surface gravity waves are quiet large, and the wind driven currents are very strong. This tells us that there's momentum going into the surface gravity waves as well as the subsurface currents. So far, the stress calculations in existing ocean models assume the momentum flux into subsurface currents () exactly equals to the momentum flux from the air (). This leaves us a question that how accurate this assumption is. When surface gravity wave propagating on a horizontal surface, it will transport momentum horizontally. When wave grow, it will extract momentum from the air, and when it decays, it will give up momentum to the subsurface currents. All these processes need to be taken into account in order to get an accurate estimate of the air-sea momentum flux budget (Figure 1).

The purpose of our study is to investigate the affect of surface gravity waves on air-sea momentum transfer in high wind conditions. Modeling experiments were done on the wave response to homogenous wind field, stationary hurricane, and moving hurricane conditions using a high-resolution wave model (WAVEWATCH III). A wave boundary layer model was coupled with WAVEWATCH III to estimate the momentum flux from air and momentum flux into subsurface currents (Figure 2).

Figure 2. Coupled model system

Publications

Fan, Y., I. Ginis, T. Hara, and I. J. Moon, 2009: Kinetic Energy and Momentum Flux Budget Across Air-sea Interface: Part I: Steady Uniform Wind. J. Geophy. Res., in review.    [PDF]

Fan, Y., I. Ginis, T. Hara, and I. J. Moon, 2009: Kinetic Energy and Momentum Flux Budget Across Air-sea Interface: Part II: Tropical Cyclones. J. Geophy. Res., in review.    [PDF]