The Ocean Fine-scale Circulation as Revealed by High-resolution Field Observations and SWOT Altimetry

Abstract

The surface of the ocean is filled with small vortices, fronts, meanders and filaments embedded within the larger-scale circulation. Described as the ocean ’fine-scales’, they are associated with scales ranging from the submesoscale (0.1–10 km and Rossby numbers of 𝒪(1)) to the mesoscale (10–100 km) and a temporal variability from days to months (intra-seasonal to seasonal). Recent science advances suggest that these fine-scales affect the ocean circulation and the marine biome up to the climate scale by redistributing energy between scales and providing dynamical conduits connecting the ocean mixed layer to the interior. Their enhanced vertical velocities make them a major component of how the ocean and atmosphere exchange climate-properties like heat and carbon. Yet, a fuller contemporary understanding of these processes, including accurate assessments of their impacts on the vertical transport and transfer of energy, has been lacking due to the scarcity of observations made at the required time-space scales. While numerical models can help upscaling the detailed view of in situ measurements, this lack of observations has become a major barrier to the validation of high-resolution models which are now resolving processes that were parameterized in coarse-grid simulations. In this work, we focus on the observation and characterization of ocean dynamics at high spatio-temporal scales in two different dynamical regions of the world ocean. This is undertaken using novel high-resolution observations from two different disconnected research experiments, including both in situ data collected by autonomous platforms and sea surface height measurements from the novel Surface Water and Ocean Topography (SWOT) satellite altimeter. In the first region of the northwestern tropical Atlantic, Saildrone observations reveal the prevalence of salinity-dominated fronts with partial temperature compensation observed at the scale of frontal restratification. This can hinder the cascade of energy to smaller scales where dissipation occurs. While these in situ observations suggest freshwater fluxes are a first‐order parameter driving the formation of fronts, numerical simulations were investigated to gain insight into the driving mechanisms on a seasonal timescale. We found that in the vicinity of the continental shelf, up to 9°N, the fronts seasonally evolve between temperature-salinity dominance, as a result between the North Brazil Current rings, these large anticyclonic eddies, and the Amazon seasonal runoff. Further offshore, salinity dominates density gradients throughout the year. The second region of focus is the Agulhas Retroflection region, which was visited twice a day by the SWOT satellite altimeter during the three month calibration-validation period of the instrument. The SWOT observations are validated against both high-resolution in situ and auxiliary satellite data. Comparing this new dataset with the lower-resolution gridded altimetry product reveals sharper sea surface height gradients on the edges of the standing Agulhas current retroflection, in particular smaller-scale gradients. In situ horizontal velocity observations co-located with SWOT from an underwater glider and surface drifters, confirm the unparalleled capability of SWOT to capture the strongest velocities and velocity gradients. It is hypothesized that SWOT’s improved resolution of the sea surface height anisotropy, curvture, and frontal gradients enables a more accurate reconstruction of vertical velocities in the upper ocean. Using 2 years of SWOT observations, we examine the spatiotemporal variability of vertical velocities (𝑤) in the Agulhas Retroflection region. The 𝑤 field is estimated from the sea surface height applying the effective surface Quasi-Geostrophic framework. We show that 𝑤 can be reconstructed over the upper kilometer of the ocean with spatial coherency assessed against 𝑤 fields from a high resolution model of the region. Estimates of 𝑤 from daily SWOT observations during the calibration-validation period of the satellite show high frequency temporal variability of days to a week with 𝑤 reaching values of 𝒪(100) m day−1, including in the less energetic areas. Additionally, the region hosts strong spatial variability and extending the reconstruction to the 2-year long SWOT observations reveals the area under the influence of the highly energetic Agulhas Retroflection Current as a ’hotspot’ for strong 𝑤. Overall, this work makes progress in understanding fine-scale ocean variability using three types of dataset: in situ observations, satellite data and numerical simulations.