The spatial scales resolved by SWOT (down to 7-20 km depending on sea state) have relatively short lifetimes (days to weeks) but crucially affect ocean physics and ecology up to the climate scale, due to the strong gradients created by their energetic dynamics. These gradients are associated with strong vertical transport of energy, matter and nutrients, connecting the ocean’s upper layer to its interior. The horizontal and vertical dynamics of fine scale processes modulate the energy cascade – i.e. the transfer of energy from large to small spatial scales – as well as ice-sea and air-sea interactions. The range of temporal scales associated with these horizontal and vertical fine scales is the same as the one observed in many important biogeochemical and ecological processes, including phytoplankton demography and competition, and the duration of foraging trips for many marine top predators. This temporal resonance is one of the reasons behind the finescale variability appearing in many features of marine ecosystems and their services, including the spatiotemporal patterns in biogeochemical cycles, biodiversity, and even in the foraging strategies of the megafauna.
On the modeling side, in the past few decades great progress has been made in characterizing this regime but a troubling gap has formed between models and observations. Field campaigns can target individual features, but they represent a tiny fraction of the possible ocean conditions. Moreover, most in situ studies are biased by choice of the stronger and longer-lived fine scales, which are the only ones that can be reliably tracked today with remote sensing tools. SWOT is expected to yield a major contribution to this gap between models and observations, providing synoptic images of fine scale features over large portions of the ocean surface and greatly enhancing in situ sampling strategies.
SWOT observations will shed new light on the way in which fine scales are associated with the conversion of potential energy to kinetic energy and eventually down to dissipation. As recently recognised by the CLIVAR Ocean Modeling Developing Panel, precise spatial and temporal representation of fine scale processes is needed for a correct estimation of the ocean energy budget and for designing optimal parameterizations for high resolution, as well as climate resolving, numerical models. For example, the intensity of fine scale density gradients and vertical velocities in numerical models strongly depends on the rate of kinetic energy accumulation along the direct cascade, before being dissipated by microscale nonlinear processes.
In terms of biogeochemistry and marine ecology, SWOT is expected first of all to help to constrain observation-based methods to estimate ocean vertical velocities and associated fluxes of nutrient and organic matter. Direct determinations of vertical velocity remain in general technologically out of reach, and possible only in particular situations of very strong frontal systems. In order to circumvent this problem, several indirect methods have been proposed, based on the derivation of vertical velocities from other observations. SWOT will not directly observe vertical velocities, but the combination of SWOT observations with in situ data is expected to improve indirect methods, which in turn will provide accurate estimations of vertical fluxes in a much broader range of conditions than today. A more precise estimation of horizontal velocities will also open the way to accurate estimations of the redistribution patterns of surface water masses and in general to Lagrangian applications, permitting to explore the role of mesoscale stirring in creating biodiversity hotspots and in constraining the behavior of marine organisms all along the trophic chain.
The SWOT Adopt-A-Crossover Consortium aims to facilitate these synergies between SWOT and in situ observations.