Gap filling of missing and invalid values in SWOT wide-swath sea surface height data using Attention U-Net

High-resolution sea surface height (SSH) data are essential for studying mesoscale and submesoscale ocean dynamics. The Surface Water and Ocean Topography (SWOT) satellite, launched on 16 December 2022, provides high-resolution SSH observations over a 120-km swath. However, the 20-km nadir gap and invalid data regions in SWOT Level-3 products create spatial discontinuities, limiting continuous observations of fine-scale ocean processes. To address this problem, an Attention U-Net-based gap-filling model is proposed to fill both the nadir gap and invalid data regions in SWOT wide-swath SSH data. Simulated SWOT SSH data were generated from GLORYS12V1 reanalysis data along SWOT Level-3 ground tracks and used to train the model. The trained model was then applied to global SWOT wide-swath observations. Results show that the proposed method outperforms inverse distance weighting, cubic spline interpolation, optimal interpolation and U-Net. Validation against SWOT nadir and Sentinel-6A altimeter data shows strong agreement, with RMSE of 2.3–3.2 cm, R2 of 0.889–0.945, and bias of 0.28 cm. Submesoscale spatial features and spectral consistency are well preserved in energetic regions, including the Kuroshio Extension and Gulf Stream, supporting SWOT applications.