Observations of Seasonal Variation in the Ekman Layer

Abstract

The seasonal variation in the Ekman layer is examined using observations from the Long-Term Upper Ocean Study (LOTUS), including surface meteorology, current, and temperature. The near-surface current is found to be coherent with the wind at low frequencies (periods greater than one day). Using a wind-relative averaging method, the authors find the observed near-surface volume transport to be consistent with Ekman transport. The mean current profile has a spiral shape that is flat compared to the classic Ekman spiral in that it rotates less with depth than does the Ekman spiral. The mean current e-folds over 12 m in the summer and 25 m in the winter, with a decay in amplitude that implies eddy viscosities of 60 ? 10?4 and 250 ? 10?4 m2 s?1, respectively. Diurnal cycling is the dominant mode of variability in the summer and it determines the vertical structure of the spiral. In the winter, diurnal cycling is almost nonexistent due to greatly reduced solar insolation and stronger winds. A persistent downwind shear exists in the upper 15 m during the winter, which is comparable in magnitude to both a logarithmic ?wall-layer? shear and a bias induced by surface wave motion. Given the winter dataset alone, it would be hard to exclude either of these possibilities. However, the nearly complete absence of this downwind shear in summer (fair weather) conditions is more consistent with the source being a wave-induced bias, or possibly Langmuir circulations.