Abstract
The sea surface drag coefficient plays an important role in momentum transmission between the atmosphere and the ocean, which is affected by ocean waves. The total air–sea momentum flux consists of effective momentum flux and sea spray momentum flux. Sea spray momentum flux involves sea surface drag, which is largely affected by the ocean wave state. Under strong winds, the sea surface drag coefficient (CD) does not increase linearly with the increasing wind speed, namely, the increase of CD is inhibited by strong winds. In this study, a sea surface drag coefficient is constructed that can be applied to the calculation of the air–sea momentum flux under high wind speed. The sea surface drag coefficient also considers the influence of wave state and sea spray droplets generated by wave breaking. Specially, the wave-dependent sea spray generation function is employed to calculate sea spray momentum flux. This facilitates the analysis not only on the sensitivity of the sea spray momentum flux to wave age, but also on the effect of wave state on the effective CD (CD, eff) under strong winds. Our results indicate that wave age plays an important role in determining CD. When the wave age is >0.4, CD decreases with the wave age. However, when the wave age is ≤0.4, CD increases with the wave age at low and moderate wind speeds but tends to decrease with the wave age at high wind speeds.
Highlights
Sea surface wind stress is the main factor that drives upper-ocean circulation and wind waves
This is because the momentum flux of sea spray droplets keeps increasing with the increase of wave age, i.e., the momentum flux acquired from the air increases with the increase of wind speed at high wind speeds
When the wave age is greater than 0.4 but the wind speed is changing from low-to-medium to high, CD,e f f decreases with the increase of wave age from low to high wind speeds
Summary
Sea surface wind stress is the main factor that drives upper-ocean circulation and wind waves. An accurate estimate of sea surface wind stress is important for modelling and predicting the dynamic processes involving ocean and atmosphere interactions. Sea surface wind stress has been widely observed and studied in the past decades, which is usually parameterized by a sea surface drag coefficient (CD ) or sea surface aerodynamic roughness [1,2,3,4]. The study of sea surface wind stress is the study of CD or sea surface aerodynamic roughness. Researchers tended to parameterize CD only by wind speed [5,6,7,8]
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