Fine-Scale Ocean Currents Derived From in situ Observations in Anticipation of the Upcoming SWOT Altimetric Mission

Abstract

After the launch of the Surface Water and Ocean Topography (SWOT) satellite planned for 2022, the region around the Balearic Islands (western Mediterranean Sea) will be the target of several in situ sampling campaigns aimed at validating the first available tranche of SWOT data. In preparation for this validation, the PRE-SWOT cruise in 2018 was conceived to explore the three-dimensional (3D) circulation at scales of 20 km that SWOT aims to resolve, included in the fine-scale range (1–100 km) as defined by the altimetric community. These scales and associated variability are not captured by contemporary nadir altimeters. Temperature and salinity observations reveal a front that separates local Atlantic Water in the northeast from recent Atlantic Water in the southeast, and extends from the surface to ~150 m depth with maximum geostrophic velocities of the order of 0.20 m s−1 and a geostrophic Rossby number that ranges between −0.24 and 0.32. This front is associated with a 3D vertical velocity field characterized by an upwelling cell surrounded by two downwelling cells, one to the east and the other to the west. The upwelling cell is located near an area with high nitrate concentrations, possibly indicating a recent inflow of nutrients. Meanwhile, subduction of chlorophyll-a in the western downwelling cell is detected in glider observations. The comparison of the altimetric geostrophic velocity with the CTD-derived geostrophic velocity, the ADCP horizontal velocity, and drifter trajectories, shows that the present-day resolution of altimetric products precludes the representation of the currents that drive the drifter displacement. The Lagrangian analysis based on these velocities demonstrates that the study region has frontogenetic dynamics not detected by altimetry. Our results suggest that the horizontal component of the flow is mainly geostrophic down to scales of 20 km in the study region and during the period analyzed, and should therefore be resolvable by SWOT and other future satellite-borne altimeters with higher resolutions. In addition, fine-scale features have an impact on the physical and biochemical spatial variability, and multi-platform in situ sampling with a resolution similar to that expected from SWOT can capture this variability.

Drifting Speed of Lagrangian Fronts and Oil Spill Dispersal at the Ocean Surface

“…This prediction can work on short-term studies (2-3 days) and can support management and prediction tools for oil spills and other floating contaminants. Our approach will undoubtedly benefit from future satellite altimetry missions, such as SWOT [56,69,70], in terms of their observations with increased resolution and reliability close to the coast, and may be extended to other environmental applications, such as conservation [52].…”

“…In this coastal region, altimetry seemed insufficient for the detection of near-shore fronts where the spilled oil was observed (the red slicks in Figure 9c). This is not surprising, as one limitation for satellite altimeters is their inaccuracy near the land shore (at least 5-10 km from the coast) that increases in regions of small Rossby radius such as in the Mediterranean [55][56][57][58][59]. For a more reliable near-shore analysis in the southeastern Mediterranean basin, we chose the data assimilating model analysis from the Mediterranean Forecast System (MFS).…”

Surface circulation properties in the eastern Mediterranean emphasized using machine learning methods

“…Aside from this, the surface circulation could be less accurate closer to the coast, where previous studies have shown a declining altimetry accuracy in the Levantine basin . Accordingly, the use of higher spatiotemporal resolution products, such as accurate models or upcoming altimetry missions like SWOT that will provide more reliability close to the coast (Morrow et al, 2019;Barceló-Llull et al, 2021), will improve the decomposition of the surface circulation by our method without changing the main conclusions.…”

Analysis of the OSSEs with multi-platform in situ data and impact on fine-scale structures

“…To anticipate the daily high-resolution two-dimensional (2D) SSH fields that SWOT will provide, the PRE-SWOT multi-platform experiment was conducted in the southern region of the Balearic Islands (western Mediterranean Sea) in May 2018 (Barceló-Llull et al, 2018, Barceló-Llull et al, 2021. In situ observations from gliders, drifters, a Conductivity Temperature Depth probe (CTD), and a hull-mounted Acoustic Doppler Current Profiler (ADCP), were collected to evaluate the horizontal and vertical velocities associated with the scales that SWOT will resolve (defined as fine scales).…”