Multi-decadal variability in ocean surface wind differences between scatterometer observations and reanalysis model fields

The ocean surface wind plays a key role in the exchange of heat, gases and momentum at the atmosphere-ocean interface. High-quality wind records from scatterometers are available from 1991 onwards, with care taken to account for changes in scatterometer instrument types and spatial coverage over time. On the other hand, modelled surface winds from global climate reanalyses suffer from changes in the density and coverage of observational timeseries used in the data assimilation process. Still, global numerical weather prediction (NWP) model wind fields are widely used in ocean research.

A comparison of scatterometer observations and global NWP model wind fields reveals substantial, persistent local systematic errors in wind vector components and spatial derivatives. Temporally-averaged gridded differences between geolocated scatterometer wind data and NWP wind fields can be used to correct for persistent local model wind vector biases. By combining these bias corrections with global, hourly NWP wind fields, high-resolution wind forcing products can be created for the ocean modelling community and other users.

In 2022, new hourly and monthly Level-4 (L4) surface wind products were introduced in the Copernicus Marine Service catalogue. These products include global bias-corrected 10-m stress-equivalent wind, surface wind stress fields and spatial derivatives. The bias corrections are calculated from Copernicus Marine Service Level-3 wind products for a combination of scatterometers and their collocated European Centre for Medium-range Weather Forecasts (ECMWF) ERA5 reanalysis model winds.

We used the multi-year L4 products to identify long-term changes in ocean surface wind differences over the period 1995-2024. The spatial distribution of differences between scatterometer observations and collocated ECMWF ERA5 reanalysis are found to be highly consistent between different scatterometers and over time. Bias corrections for a single instrument display long-term variations of comparable magnitude to the scatterometer-model differences, pointing to artificial changes in the ERA5 winds over time. Furthermore, regional local bias anomalies are found for climate phenomena like the El Niño Southern Oscillation. These artificial features should be taken into account in multi-decadal ocean studies.