Inner Shelf Current Structure Under Different Forcing Mechanisms— Combining ADCP and HF Radar Measurements

Abstract:

Our understanding of wind and wave driven currents in the coastal ocean has been limited by our inability to measure currents within a few meters of the surface. To improve our understanding of current dynamics in the upper few meters of the ocean we combine 8 months of ADCP measurements from the Martha’s Vineyard Coastal Observatory with high-frequency (HF) radar measurements of surface currents. These techniques compliment each other, as the HF radar can only observe the surface velocities, and the ADCP cannot take accurate measurements in the upper few meters of the water column. We find that currents can vary significantly over the upper two meters of the inner shelf. Extrapolations to the surface based on the ADCP data are inconsistent with that observed by the HF radar. When different types of forcing conditions were examined, it was found that this difference is most evident in cases when wind stresses are weak (<less than 0.05 Pa) and waves are large (greater than 0.6m). In this case, the cross-shore HF radar observations were onshore, in the opposite direction of the near-surface ADCP observations. Models indicate this difference can be attributed in part to Stokes’ drift, measured by the HF radar but not the ADCP. When the ADCP velocities are corrected for Stokes’ drift, the difference between the HF radar surface current and the top ADCP current is consistent with a simple model of near-surface wind-driven current shear. This suggests future attempts to extrapolate ADCP currents toward the surface should consider wind-driven shear. This result can be used to more accurately estimate onshore transport, which has applications in calculating nutrient and pollutant transport.