Drifter observations of coastal surface currents during CODE: The statistical and dynamical views

Abstract

Observations of near‐surface coastal currents were made off the Northern California coast during CODE by using 164 current‐following drifters. These observations are used to describe the two‐dimensional structure of the mean surface flow and the scales of its variability. The mean flow is a broad equatorward current, strongly sheared only within 10 km of the shore, and a mean offshore flow producing an average divergence ∇H·u ≈ 3 × 10−6 s−1. Divergence is uniformly distributed across the shelf, but variation of the alongshore flow causes an upwelling center near Point Arena. The spatial correlation scale is less than 40 km in both the alongshore and across‐shelf directions, even though 55% of the surface kinetic energy is described by a single mode with gradual across‐shelf variation and an alongshore wavelength of the order 200 km. The surface flow is well correlated with flow at 30‐m depth. The Lagrangian time scale (≈1.5 day) is significantly shorter than the Eulerian time scale (≈5 days), indicating that the flow is dominated by highly nonlinear quasi‐stationary eddies. Drifter displacements indicate that the mean lateral eddy transport of passive scalars can be described by an anisotropic and inhomogeneous eddy diffusivity, but this diffusivity cannot be used to relate eddy Reynolds stresses and the mean shear. Analysis of two‐particle separations, which determine the size of dispersing property clouds, shows that dispersal cannot be described by a scale‐dependent diffusivity and indicates the importance of small‐scale convergences in retarding dispersal. Over the entire 100‐km by 50‐km region the surface layer heat budget is dominated by upwelling cooling and surface heating, with onshore eddy heat flux playing a smaller role. Substantial convergence of the alongshore eddy heat flux is apparently required to balance upwelling cooling in the upwelling center. Drifters are found to have a Lagrangian mean acceleration caused by eddy processes. Analysis of this acceleration and of the horizontal flow contributions to the eddy Reynolds stress allows examination of the importance of eddy processes in the mean momentum budget. While the alongshore flow must be in approximate geostrophic balance, there is a clear pattern to the eddy forcing, which appears to be important in the alongshore momentum equation.