Internal Tides From SWOT: A 75-Day Instantaneous Mode-1 M 2 ${\mathrm{M}}_{2}$ Internal Tide Model

Abstract

Seventy-five days of sea surface height measurements made by the Surface Water and Ocean Topography (SWOT) mission from 7 September to 21 November 2023 are used to explore SWOT's capability of observing internal tides. Mode-1 $M_2$ internal tides are mapped by our updated mapping technique. SWOT-75d represents a 75-day instantaneous model. Nadir-30y is constructed using 30 years of nadir altimetry data from 1993 to 2022 and represents a climate normal. The nadir altimetry data in 2023 are used for model evaluation. Despite its large errors, SWOT-75d reveals the basic features of the global mode-1 $M_2$ internal tide field, and causes positive variance reduction in regions of strong internal tides. Nadir-30y performs better overall, but SWOT-75d performs better in the tropical South Atlantic Ocean, the central North Pacific Ocean, and the Melanesian region. Evaluation using seasonally subsetted altimetry data reveals that $M_2$ internal tides have significant temporal variations. SWOT-75d performs the best in fall, because the model is constructed using data largely in fall. SWOT-75d has large phase anomalies, which are spatially smoothed and used to adjust the phases in Nadir-30y. The phase-adjusted model can better make internal tide correction for SWOT and its performance is improved by 20%. Our results demonstrate that (a) mode-1 $M_2$ internal tides can be extracted from 75 days of SWOT data by our mapping technique, and (b) the instantaneous internal tide model can be used to improve internal tide correction for SWOT.