Momentum flux and wave spectra observations from an ocean tower

Abstract

Three hundred seventy-seven eddy correlation estimates of momentum flux were obtained with a ‘flux meter’ system and cross-spectrum analysis at an elevation of 7.5 meters above mean sea level at Argus Island Tower during 1964, 1967, and 1969. Argus Island, a former research tower which stands in 60 meters of water, is located 45 km southwest of Bermuda with an unrestricted fetch in most directions. The influence of the pre-existing ocean waves in the open ocean (January to May) is manifest in a decreasing drag coefficient as mean horizontal wind velocities increase up to 6.5 m sec−1. As the wind velocity increases up to 14.5 m sec−1, the differential interplay of wave background and atmospheric stability is associated with a nearly linear increase of drag coefficient under unstable conditions, and stability effects appear to be more dominant than the surface waves. However, the influence of the ocean-surface waves is clearly evident when drag coefficients are observed at neutral and stable conditions. Under these conditions the observations indicated an oscillation of the drag coefficient as function of mean horizontal wind velocities between 7 and 12 m sec−1. There is some evidence of the dependence of the drag coefficient on dimensionless wave-phase velocity and dimensionless wave height. Wave-induced peaks in vertical velocity and quadrature spectra, corresponding to ocean-wave spectral peaks, are illustrated for light winds (4–6 m sec−1) and moderate winds (9.5–10.5 m sec−1). The ratio of the standard deviation of the vertical wind velocity to the friction velocity increased linearly as a function of the ratio of wave-phase velocity of the spectral peak to the friction velocity at all observed stabilities.