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 ocean surface wind records from scatterometers are available from 1991 onwards. Care has been taken to account for changes in scatterometer instrument types and spatial coverage over time, such that these records can be used to assess changes in the ocean surface wind over the past 30 years. On the other hand, modelled surface winds from global climate reanalyses (e.g. ERA) 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 the computation of ocean surface processes to study climate trends and variability in ocean variables.

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 ERA/NWP wind fields can be used to correct for persistent local model wind vector biases. By combining these scatterometer-based bias corrections with global, hourly ERA/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) model winds.

We used the monthly multi-year L4 product 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. Remaining small differences between individual scatterometers could be caused by different instrument characteristics, sampling, coverage and processing and may be further reduced by continued intercalibration efforts. Bias corrections for a single instrument display long-term variations of comparable magnitude to the scatterometer-model differences, which point 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 any long-term reanalysis of ocean surface wind fields.