James Girton (University of Washington, United States)

We have recently developed a 3-step mapping technique that consists of two rounds of plane wave analysis with spatial bandpass filtering in between. Our technique is to construct and decompose the internal tide field from satellite altimetry, which samples the global sea surface topography irregularly with insufficient spatial and temporal resolution. Our technique extracts internal tides in three rounds of temporal and spatial filtering and thus can significantly suppress model errors. On a global average, model errors are as low as <0.5 mm in ZHAO30yr, our latest model based on 30 years of nadir altimetry data. The low model errors also allow us to extract minor constituents K2, P1 and Q1, which have not been tackled in previous empirical internal tide models.

We map internal tides from SWOT by our improved mapping technique. The new SWOT mission samples sea surface topography along a 120-km swath with a 2-km spatial resolution. The combination of our improved technique and SWOT data yields unprecedented details of the global internal tide field. In our 2024 JGR-Oceans paper, we demonstrated that we can extract M2 internal tides using as short as 75 days of SWOT data. In some regions (the tropical South Atlantic Ocean, the central North Pacific Ocean, and the Melanesian region), the 75-day model performs better than our 30-year model. Evaluation using seasonally subsetted altimetry data reveals that internal tides have significant temporal variations. The 75-day model performs the best in fall, because the model is constructed using SWOT data largely in fall. We suggested that the large phase anomalies in the 75-day model can be used to better make internal tide correction to SWOT. In 2025, we further applied our mapping technique to two years of SWOT data. We mapped instantaneous M2 and K1 internal tides using 3-cycle subsetted SWOT data (~63 days). The resulting time series of internal tide fields demonstrate significant seasonal and year-to-year variations. We separated a few long-range internal tidal beams and quantified their along-beam spatiotemporal variations. The coherence of internal tides is studied using the time series instantaneous internal tide fields.