A data-driven wind-to-current response function and application to ocean surface current estimates

Abstract

This study investigates the reconstruction of wind-driven currents based on an empirical impulse response function. Surface current observations derived from drifting buoy data and wind-stress from the ERA5 reanalyses are used to derive the response function. The function is expected to be sensitive to the ocean mixed-layer depth and more generally the turbulent viscosity profile which can display strong spatio-temporal variability. In this work, however, only seasonal and meridional variations are considered. Despite this crude approximation, the simplified response function can explain a significant portion of the current variability in independent observations. A practical application is the release of a new total surface current product (denoted WOC). Compared to existing products based on the same input datasets, such as the CMEMS MOB-TAC surface current product, the WOC estimates are designed to include higher frequency content, in particular in the inertial band. Beyond successful validation, the characteristics of the response function (amplitudes and phases) reveal interesting properties of the upper-ocean variability. The function shows some similarities to one derived theoretically from a simple 1-layer (slab) model, but also differences that highlight the value of fitting the function to the data without the use of an explicit dynamical model. These results open perspectives for studying some dependencies between subsurface variables and the response function, particularly interesting in the context of future spaceborne Doppler scatterometers such as ODYSEA , expected to provide simultaneous wind and current observations. This instrument could indirectly probe subsurface properties through the synoptically-observed response function.