Sea-Surface Drift Currents
Authors: Wu Jin 
Published: October 2008

ABSTRACT
Wind and wave-induced surface drift currents at various fetches were separately estimated on the basis of the existing wind and wave data and the respective scaling laws. The ratio between the wind-induced drift and the wind velocity was found to decrease with the increasing fetch, while the ratio between the wave-induced drift and the wind velocity found to increase with the fetch. The total surface drift, however, is almost independent of the wind fetch and is about 3.5 percent of the wind velocity at long fetches. The estimated surface current compares very favorably with the measured value.

INTRODUCTION
Recent intensive studies of air-sea interaction seem to be concentrating on wind and wave measurements. So far, the surface drift currents have been rather inadequately investigated. There are also uncertainties as to whether the sea-surface drift consists mainly of a wind- or a wave-induced component component. The drift current affects momentum, mass, and heat transfer processes across the air-water interface. These dynamical processes are directly related to global weather and ocean circulation. Furthermore, the surface currents influence to a large extent the diffusion dispersion (Wu, 1969a), and drift of foreign masses such as pollutants and oil spills in natural water bodies. On the basis of earlier studies on wind-wave interaction (Wu, 1968), scaling law of wind stress (Wu 1969b), wave data at various fetches (Weigel, 1962), and measurements of drift currents (Wu, 1973b), an estimation is presented of the surface drifts at all fetches to meet needs for engineering and oceanographic practice. The estimated value was found to agree well with the oceanic data (Hughes, 1956). Further comparisons were made between the results obtained from the present study and those from other investigations (Keulegan, 1951; van Dorn, 1953; Bye, 1967; Kenyon, 1969; Shemdin, 1972), and a number of features of these results are explained.

LABORATORY RESULTS
Wind- and wave-induced currents were studied earlier (WU, 1973b); portions of the results on surface currents are abstracted here.

Current Measurements
The drift current immediately below the water surface was measured by timing floats passing two stations along the tank. Spherical particles of different sizes and a thin circular disc were used as the surface floats. The specific gravity of the float is 0.95. The velocity of the float was considered as the drift current at the depth of the centroid of the longitudinally projected area of the submerged portion of the float. The surface drift current was determined by extrapolating the current-distribution curve to the water surface. This technique (Wu, 1968) not only provides a more accurate measurement of the surface drift than the use of a single float with a finite size, but is also convenient at high wind velocities where breaking waves tend to sink small, thin floats. The ratio between the surface drift current V and the free-stream wind velocity U is plotted in Figure 1a. As reported earlier (Keulegan, 1951; Wu, 1968), a trend with the surface drift current approaching an equilibrium fraction of the wind velocity is seen, as the wind blows harder and waves start to break.