What will be the finest resolved scale of SWOT SSH data under different dynamical conditions?

Abstract

One of the aims of the SWOT satellite mission is to resolve sea surface height (SSH) at wavelengths down to 15km in the global oceans. In practice, whether SSH signals can be observed at a given wavelength will depend on the relative amplitude of the level of SSH variance and the level of instrumental and geophysical noise at that scale. Several methods have been proposed for mitigating the impact of instrumental noise in the SWOT swath but large uncertainties remain as to what will be the finest scales that SWOT SSH data will resolve in different regions and seasons. This is particularly due to the uncertainty as to how fast unbalanced motions (as for instance internal tides and inertia gravity waves) will affect SSH signals at different scales and times, and therefore affect the performance of existing denoising algorithms. Strong seasonal variations of the finest resolvable scale are also expected because of the strong but opposite seasonality of balanced submesoscale dynamics and fast unbalanced motions. In this study we use a collection of ocean model simulations and advanced denoising algorithms to document what scales SWOT SSH data will resolve in the global oceans. We use pseudo-SWOT SSH data drawn from hourly output of three high-resolution, tide-resolving, basin-to-global scale simulations based respectively on the MITgcm, HYCOM and NEMO ocean models. Our study focuses on different regions and seasons in order to document the variety of regimes that SWOT will actually sample. Our results show that existing denoising algorithms perform well even in the presence of intense unbalanced motions. We show that using denoising algorithms therefore systematically increase the largest resolvable wavenumber. Advanced denoising algorithms also allow to reliably reconstruct SSH gradients (related to geostrophic velocities) and second order derivatives (related to geostrophic vorticity) on the SWOT swath down to the finest resolvable scale. Our results also show that a significant uncertainty remains as to what will be SWOT’s finest resolved scale in a given region and season because of the large spread in the level of variance predicted among our high-resolution ocean model simulations.