Abstract

A first attempt was made to quantify the impact of the assimilation of Surface Water Ocean Topography (SWOT) swath altimeter data in a global 1/12° high resolution analysis and forecasting system through a series of Observing System Simulation Experiments (OSSEs). The OSSE framework (Nature Run and Free Run) and data assimilation scheme have been described in detail in a companion article (Benkiran et al., 2021). The impact of assimilating data from SWOT and three nadir altimeters was quantified by estimating analysis and forecast error variances for sea surface height (SSH), temperature, salinity, zonal, and meridional velocities. Wave-number spectra and coherence analyses of SSH errors were also computed. SWOT data will significantly improve the quality of ocean analyses and forecasts. Adding SWOT observations to those of three nadir altimeters globally reduces the variance of SSH and surface velocities in analyses and forecasts by about 30 and 20%, respectively. Improvements are greater in high-latitude regions where space/time coverage of SWOT is much denser. The combination of SWOT data with data from three nadir altimeters provides a better resolution of wavelengths between 50 and 200 km with a more than 40% improvement outside tropical regions with respect to data from three nadir altimeters alone. The study has also highlighted that the impact of using SWOT data is likely to be very different depending on geographical areas. Constraining smaller spatial scales (wavelengths below 100 km) remains challenging as they are also associated with small time scales. Although this is only a first step, the study has demonstrated that SWOT data could be readily assimilated in a global high-resolution analysis and forecasting system with a positive impact at all latitudes and outstanding performances.

Highlights

  • Sea surface height (SSH) altimeter measurements are essential for ocean prediction

  • A common 3-month spin up was performed for all Observing System Simulation Experiments (OSSEs) from October 1 to December 31, 2014

  • The impact of data from Surface Water Ocean Topography (SWOT) and from nadir altimeters on SSH is first discussed in section “Impact on Sea Surface Height.”

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Summary

Introduction

Sea surface height (SSH) altimeter measurements are essential for ocean prediction. For nearly three decades, the TOPEX/Poseidon, Jason 1/2 and 3 reference altimeter missions, complemented by missions such as ERS-1/2, ENVISAT, Alti-Ka, Cryosat-2, and Sentinel-3A&B, have been providing SSH measurements that have been used to constrain ocean models through data assimilationImpact of the SWOT Data (see Le Traon et al, 2017 for a review). This significant improvement of OSSE3 with respect to OSSE1 is evidenced in the zonally averaged variance of SSH ocean analysis error (Figure 6E) in the high-latitude regions, even though SWOT assimilation alone is less good than the assimilation of nadir altimeters at mid-latitudes where western boundary currents are located.

Results
Conclusion

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