Impact of SWOT KaRIn observations on Level-4 sea level products along European coasts

The Ka-band Radar Interferometer (KaRIn) on the Surface Water and Ocean Topography (SWOT) mission provides two-dimensional sea surface height observations at kilometric resolution, beyond the along-track sampling of nadir altimeters. We assess how merging these observations into a delayed-time Level-4 (L4) sea level product changes its coastal accuracy relative to the nadir-only multi-year product, using more than 400 European tide gauges over SWOT's 1-day repeat Calibration and Validation (CalVal) phase (March-July 2023) and its 21-day repeat Science phase (July 2023-July 2025). Varying the extraction radius from 10 to 100km shows that the improvement beneath the swath is largest at short radii, with a median of 15% at 10-20km, and that the Science-phase network median improvement decreases from 1.9% at 10km to 0.2% at 100km. The gain is concentrated in the weakly tidal straits of the transition zone between the North Sea and the Baltic Sea, where the mean root-mean-square difference (RMSD) decreases by 0.9cm during CalVal and 0.5cm during Science and stations beneath the swath improve by a median of 22%. No improvement is found where the nadir-only product is already accurate (microtidal basins) or where the error is dominated by tides (macrotidal shelves). At stations beneath the swath, the L4 product that includes KaRIn agrees better with the tide gauges than the Level-3 KaRIn observations (median RMSD of 1.9--2.5cm against 2.6-2.7cm), which shows the benefit of joint mapping with the nadir constellation for daily gridded products