Elliptical modulation and temporal decays of near-inertial surface currents under submesoscale horizontal shear and strain components in a coastal region

Abstract

We investigate elliptical modulations and modified temporal decays of near-inertial coastal surface currents in the observations of coastalradar-derived submesoscale surface currents and satellite-tracked surface drifters as well as forward realistic coastal circulation simulations off the Oregon coast in terms of the influence of horizontal shear and strain components. The elliptical near-inertial surface currents appear with dominant clockwise and non-negligible counter-clockwise near-inertial variance, and their temporal decays are enhanced in fall and winter compared with those in spring and summer, especially near the coast (25 to 50 km from the coast), with seasonal variability. We formulate a slab layer model to explain the potential contribution of submesoscale horizontal shear and strain components to the near-inertial motions, along with anisotropic constant frictional coefficients under wind-driven coastal dynamics. The horizontal shear components, including the background vorticity associated with submesoscale eddies, contribute to the elliptical near-inertial currents, reduce the regionally dominant polarization, and correct the effective CoriolisfrequencyuptoahalfofthelocalCoriolisfrequencycompared with that in the modulation of near-inertial currents due to mesoscale geostrophic currents. The horizontal strain components related to submesoscale ageostrophic currents (e.g., vertical currents due to direct wind-driven upwelling and down-welling, indirect wind stress curldriven Ekman pumping, and frontal-scale vertical circulation) result in modification of the temporal decay time scales of the near-inertial surface currents, which is an uncounted dynamic component under the non-divergent mesoscale geostrophic currents.