A new high-resolution ocean wind forcing product for the Copernicus Marine Service

As more than 70% of the earth surface is covered by water, exchanges of heat, gases and momentum at the air-sea interface are a key part of the dynamical earth system and its evolution. The ocean surface wind plays an essential role in the exchange at the atmosphere-ocean interface. It is therefore crucial to accurately represent the wind forcing in physical ocean model simulations. Scatterometers provide high-resolution ocean surface wind observations, but have limited spatial and temporal coverage. On the other hand, numerical weather prediction (NWP) model wind fields have better coverage in time and space, but do not resolve the small-scale variability in the air-sea fluxes. In addition, Belmonte Rivas and Stoffelen (2019) documented substantial systematic errors in global NWP fields on both small and large scales, using scatterometer observations as a reference. Trindade et al. (2020) combined the strong points of scatterometer observations and atmospheric model wind fields into ERA*, a new ocean wind forcing product. ERA* uses temporally-averaged differences between geolocated scatterometer wind data and European Centre for Medium-range Weather Forecasts (ECMWF) reanalysis fields (ERA-Interim) to correct for persistent local NWP wind vector biases. Verified against independent observations, ERA* reduced the variance of differences by 20% with respect to the uncorrected NWP fields. We present a new hourly ocean wind forcing product that will be included in the Copernicus Marine Service (CMEMS) catalogue in 2022. To best serve the ocean modelling community, this Level-4 product will include global bias-corrected 10-m stress-equivalent wind (De Kloe et al., 2017) and surface wind stress fields at 0.125o horizontal spatial resolution. The near real-time (NRT) version of the product is based on the ECMWF operational model (OPS*) and the reprocessed (REP) version on the ERA5 re-analysis (ERA5*). Ocean surface winds from the existing 6-hourly CMEMS L4 wind product and ERA5* were validated against observations from an independent scatterometer (Haiyang-2B). ERA5* winds show better correspondence to Haiyang-2B winds, particularly outside the tropics, where the 6-hourly product is not able to resolve the fast-moving atmospheric systems. Like any CMEMS product, the new wind product will be freely and openly available for all operational, commercial and research applications.