Mapping Internal Tides using Synthetic SWOT Measurements

The upcoming altimeter mission SWOT (Surface Water and Ocean Topography) will measure sea surface height with 10 km-scale spatial resolution along a wide swath. Its temporal sampling rate is still low, with a repeat cycle of 21 days. Here we evaluate SWOT's ability to detect internal tide signals using one-year of synthetic measurements obtained by resampling a high-resolution global ocean simulation using the SWOT simulator. The global ocean simulation has nominal horizontal grid spacing of 1/48 degree and was produced using the MITgcm driven by ECMWF atmospheric forcing and realistic tidal potential, so that it contains mesoscale and submesoscale eddies as well as internal tides. We focus on the SWOT calibration and validation candidate region off California. The two-dimensional plane wave fit method (Zhao et al. 2016) is implemented to extract M2 and S2 internal tides using the synthetic SWOT measurements. We find that the mode-1 M2 and S2 internal tides can be clearly detected, as evidenced by the facts that (1) internal tides are collocated with remarkable topographic features such as Mendocino fracture zone and (2) well-defined internal tidal beams travel over long distances. In particular, relatively shorter and narrower mode-2 M2 internal tidal beams can also be detected. The results agree to a large extent with the true internal tide field in the model, which is obtained from a direct harmonic analysis of the hourly model output. We are currently investigating the error sources in the internal tide reconstruction and the separation of eddy and internal tide signals.