Observations of Superinertial and Near-Inertial Wind-driven Flow

Abstract

The superinertial and near-inertial wind-driven flow in the western North Atlantic is examined using data from two recent experiments. The Frontal Air?Sea Interaction Experiment (FASINEX) took place at 27°N, 70°W during 1986. The Long-Term Upper-Ocean Study (LOTUS) took place at 34°N, 70°W during 1982. Each experiment included moored measurements of meteorological variables that allowed estimation of the wind stress and oceanic currents. The directly wind-driven flow is isolated from other sources of variability, such as internal waves and mooring motion, using a transfer function between geocentric acceleration and wind stress. The transfer function is examined in rotary spectral bands bounded by periods of 36 and 12 h, and 12 and 2 h. For surface-moored observations, wind-driven mooring motion is found to cause a response that extends at least to 1000 m (much deeper than the frictional layer of direct wind forcing). Once this artifact is removed, the directly wind-driven flow is identified. This response is found to rotate to the left (right) for clockwise (counterclockwise) rotating superinertial wind stress, in agreement with the solution of the time-dependent Ekman spiral. When vertically integrated the Ekman transport relation is satisfied, indicating that all of the wind-driven flow has been isolated.