In situ observations of a 10-km-scale elliptical cyclonic eddy reveal vertical subduction of chlorophyll-rich surface waters around the edges of the eddy where the flow curvature changes abruptly. The process is explained by relating flow curvature to horizontal divergence and vortex stretching using a theoretical framework that we develop in a local flow-following reference frame. The observed signatures of subduction, which extend from the surface to depths O(100) m, coincide with estimates from the variations of curvature number around the eddy, defined as Cu ; V/(fR), where V is the speed, R is the radius of curvature, and f is the Coriolis parameter. An idealized model successfully simulates the observed subduction process, uncovering a three-dimensional pathway where surface waters within the cyclone subduct along the domed isopycnal surfaces and ultimately detach from the eddy as filaments. By decomposing the frontogenetic Q vector in the omega equation for vertical velocity, we identify the role of curvature-induced frontogenesis in driving subduction. Submesoscale flows exhibit highly curved flow paths, and curvature-induced frontogenesis, which differs from the traditionally emphasized confluence-induced frontogenesis, can play a significant role in modifying vertical transport.

Earth’s oceans contain large numbers of cyclonic eddies, 10 to 25 kilometers in diameter and unresolved in climate simulations. However, we lack observations of these features due to their small size and fast evolution. Here, we present in situ observations of one such cyclonic eddy with intense chlorophyll at its center as it spontaneously splits into two smaller cyclonic eddies over a few days. This splitting rapidly transports surface waters to depth, with sustained vertical velocities of 60 meters per day, primarily from the center of the eddy where carbon concentrations are largest, facilitating efficient transfer of phytoplankton carbon to depth, below the well-mixed, sunlit surface layer. We reproduce the splitting process in an idealized ocean model and find that splitting is controlled by the initial elliptical eddy shape, size, and intensity. Our observations uncover a mechanism for subduction in the upper ocean and highlight the need for quantifying its global prevalence.

Ocean currents are crucial in regulating Earth's climate, with a significant impact in the distribution of ocean properties. During the Calibration/Validation phase of the Surface Water and Ocean Topography (SWOT) satellite mission, we performed a high-resolution, multi-platform experiment to evaluate SWOT's ability to resolve small-scale features, focusing on a similar to 25 km-radius anticyclonic eddy in the Western Mediterranean Sea. Acoustic Doppler Current Profiler (ADCP) recorded maximum velocities of 30 cm/s at 155 m depth and underwater glider data identified biconvex isopycnals, classifying the eddy as intrathermocline. SWOT successfully captured the sea level signal and surface geostrophic currents of the eddy, showing notable error reduction over conventional altimetry: 24% in sea level representation compared to glider observations, and 35% and 31% in horizontal velocity magnitude compared to Acoustic Doppler Current Profiler and drifter measurements, respectively. This study highlights SWOT's potential in resolving small-scale ocean dynamics.

The FaSt-SWOT experiments aim to integrate SWOT measurements with in-situ observations from two high-resolution multi-platform ocean campaigns and advanced data-assimilative models. The goal is to evaluate the performance of the satellite, characterize fine-scale (10-100 km) dynamics and quantify the associated horizontal and vertical transports. The FaSt-SWOT experiments were specifically designed to collect multi-platform in-situ observations within the swath of the SWOT satellite for validation, focusing on the area around the Balearic Sea. The campaigns took place in April and May 2023 and involved the simultaneous use of various ship-based instruments (CTD, Moving Vessel Profiler (MVP), thermosalinograph, ADCP, GoPros), autonomous platforms (surface drifters and gliders), and satellite observations (SST, ocean color, altimetry). In this presentation, we will focus on the analysis and processing of the multi-platform data collected during FaSt-SWOT. The sampling location was defined a few days before the first experiment based on the presence of a remarkable small-scale anticyclonic eddy (~20 km-diameter) detected in SST imagery and in the trajectory of a drifter within one of the SWOT swaths north of the Ibiza Island. Subsequent SST and ocean color maps showed the temporal evolution of the sampled eddy, which evolved into finer-scale features. Therefore, the context was ideal for analyzing SWOT's capability to detect this type of small structures. Indeed, the signature of the eddy was detected in the first sea level maps provided by SWOT and unresolved by conventional altimetry. The analyzed data consist of ~1000 MVP profiles of temperature and salinity from the surface down to a depth of 200 m along the ship trajectory (100 m during the first phase), combined with CTD rosette casts at fixed stations down to 700 m depth. Additionally, we will present data from two gliders that conducted 2 back-and-forth transects along the satellite swath with a 1-day delay between them, and the data from the 45 surface drifters deployed during both phases of the experiment. We will discuss the processing of the MVP observations, as well as the cross-calibration performed between CTD, MVP, and gliders measurements. This is a really important step to ensure that we are providing accurate and reliable quality-controlled observations, as well as a necessary process since these data will be available. Finally, we will provide a comparison between in-situ observations and SWOT data, and an analysis of the dynamics in the sampled area.

The FaSt-SWOT sea trial experiments, conducted in the Balearic Sea (Western Mediterranean Sea) between 25-28 April and 7-10 May 2023, aimed at collecting multi-platform in-situ observations of meso- and submesoscale ocean structures in the area covered by the SWOT satellite during its initial fast-sampling phase. The general objectives of the FaSt-SWOT project are twofold: 1) participate with these data to the satellite cal/val activities, and 2) improve the characterization and understanding of the fine-scale dynamics by combining in-situ multi-platform and satellite data with high-resolution numerical models and machine-learning-based computational techniques. The experiments consisted in 2 phases both using multi-scale ship-based instruments (CTD, Moving Vessel Profiler, thermosalinograph, ADCP and GoPros), autonomous platforms (surface drifters and gliders), and satellite observations (SST, ocean color and altimetry). In addition, 2km-resolution data-assimilative modelling simulations were produced to provide a complementary view of the fine-scale ocean variability. Finally, machine-learning-based optimization algorithms were also tested to define adaptive sampling strategies during the experiment. The sampling first focused on a 20km-diameter anticyclonic eddy detected under the swath of the satellite thanks to satellite imagery and drifter trajectories. Several cross-sections of the Moving Vessel Profiler and underwater gliders provided insights into the vertical structure of temperature and salinity fields and the associated signals in chlorophyll and dissolved oxygen. Two gliders were programmed to perform back-and-forth sections during a 3-week time with a 1-day delay between them, allowing to evaluate the temporal variability of the ocean fields at the period of repetitivity of the satellite. The second phase started 9 days after the end of the first one. A 48-hour dense radiator-like pattern was performed by R/V SOCIB, allowing to characterize the evolution of the small eddy observed during the first leg. A total of 45 surface drifters were deployed during the two phases to evaluate in-situ surface currents and their associated convergence and divergence in the vicinity of the eddy. While conventional altimetry was not able to properly represent the sea level signature of the observed eddy, initial SWOT measurements indicate an improved detection capability by the new satellite. In addition, high-resolution numerical simulations reproduce a small anticyclonic eddy with similar characteristics as that of the observed eddy. These simulations are used to provide a more general understanding of the situation, indicate the origin of the eddy in the frontal area between recent and modified Atlantic waters, and provide insights into the vertical extension of the small mesoscale structure. We provide here an overview of the whole FaSt-SWOT dataset, including both observing and modelling components. A more detailed analysis of the measurements is provided in a companion presentation.

Vertical transport pathways in the ocean are still only partially understood despite their importance for biogeochemical, pollutant, and climate applications. Detailed measurements of a submesoscale frontal jet in the Alboran Sea (Mediterranean Sea) during a period of highly variable winds were made using cross-frontal velocity, density sections and dense arrays of surface drifters deployed across the front. The measurements show divergences as large as +/-(SIC) implying vertical velocities of order 100 m/day for a asymptotic to 20 m thick surface layer. Over the 20 hr of measurement, the divergences made nearly one complete oscillation, suggesting an important role for near-inertial oscillations. A wind-forced slab model modified by the observed background frontal structure and with initial conditions matched to the data produces divergence oscillations and pattern compatible with that observed. Significant differences, though, are found in terms of mean divergence, with the data showing a prevalence of negative, convergent values. Despite the limitations in data sampling and model uncertainties, this suggests the contribution of other dynamical processes. Turbulent boundary layer processes are discussed, as a contributor to enhance the observed convergent phase. Water mass properties suggest that symmetric instabilities might also be present but do not play a crucial role, while downward stirring along displaced isopycnals is observed.

This repository holds the data used in the study entitled "Inertial oscillations and frontal processes in an Alboran Sea jet: Effects on divergence and vertical transport" which is being submitted to the Journal of Geophysical Research: Oceans (first submission: August 2022). This study reports on the wind response interaction with an ocean current jet in geostrophic balance. The primary objective of the study is to better understand the potential role played by Near Inertial Oscilations (NIOs) in generating vertical transport in the upper ocean. This dataset includes near-surface drifters' tracks (CARTHE and SVP), vessel mounted ADCP (Acoustic Doppler Current Profiler) and underway CTD (Conductivity, Temperature and Depth) as well as data generated by an idealised numerical model (slab-layer type). Details of the files are provided in a README.txt and details on the data processing and analysis are provided in the manuscript to be published.

Measuring vertical motions represent a challenge as they are typically 3-4 orders of magnitude smaller than the horizontal velocities. Here, we show that surface vertical velocities are intensified at submesoscales and are dominated by high frequency variability. We use drifter observations to calculate divergence and vertical velocities in the upper 15 m of the water column at two different horizontal scales. The drifters, deployed at the edge of a mesoscale eddy in the Alboran Sea, show an area of strong convergence (O $\mathcal{O}$(f)) associated with vertical velocities of -100 m day(-1). This study shows that a multilayered-drifter array can be an effective tool for estimating vertical velocity near the ocean surface.

Horizontal and vertical motions associated with mesoscale (10-100 km) and submesoscale (1-10 km) features, such as fronts, meanders, eddies, and filaments, play a critical role in redistributing physical and biogeochemical properties in the ocean. This study makes use of a multiplatform data set of 82 drifters, a Lagrangian float, and profile timeseries of temperature and salinity, obtained in a similar to 1-m/s semipermanent frontal jet in the Alboran Sea as part of CALYPSO (Coherent Lagrangian Pathways from the Surface Ocean to Interior). Drifters drogued at similar to 1-m and 15-m depth capture the mesoscale and submesoscale circulation aligning along the perimeter of fronts due to horizontal shear. Clusters of drifters are used to estimate the kinematic properties, such as vorticity and divergence, of the flow by fitting a bivariate plane to the horizontal drifter velocities. Clusters with submesoscale length scales indicate normalized vorticity zeta/f > 1 with Coriolis frequency f and normalized divergence of delta/f similar to O(1) occurring in patches along the front, with error variance around 10%. By computing divergence from drifter clusters at two different depths, we estimate minimum vertical velocity of O(-100 m day(-1)) in the upper 10 m of the water column. These results are at least twice as large as previous estimates of vertical velocity in the region. Location, magnitude, and timing of the convergence are consistent with behavior of a Lagrangian float subducting in the center of a drifter cluster. These results improve our understanding of frontal subduction and quantify convergence and vertical velocity using Lagrangian tools.

An Algerian Eddy, anticyclonic vortex generated by the instability of the Algerian Current in the southwestern Mediterranean Sea, is studied using data provided by drifters (surface currents), Argo floats (temperature and salinity profiles), environmental satellites (absolute dynamic topography maps and ocean color images) and operational oceanography products. The eddy was generated in May 2018 and lasted as an isolated vortex until November 2018. Its morphology and kinematics are described in June–July 2018 when drifters were trapped in its core. During that period, the eddy was slowly moving to the NE (~2 km/day), with an overall diameter of about 200 km (slowly growing with time) and maximal surface swirl velocity of ~50 cm/s at a radius of ~50 km. Geostrophic currents derived from satellite altimetry data compare well with low-pass filtered drifter velocities, with only a slight overestimation, which is expected as its maximum vorticity corresponds to a small Rossby number of ~0.6. Satellite ocean color images and some drifters show that the eddy has an elliptical spiral structure. The looping tracks of the drifters trapped in the eddy were analyzed using two statistical methods: least-squares ellipse fitting and wavelet ridge analysis, revealing a typical eccentricity of about 0.5, a wide range of inclination and a rotation period between 3 and 10 days. Clusters of drifters on the northeastern limb of the eddy were also considered to estimate divergence and vorticity. The results indicate convergence (divergence) and downwelling (upwelling) at scales of 20–50 km near the northeastern (northwestern) edge of the eddy, in agreement with the quasi-geostrophic theory. Vertically, the eddy extends mostly down to 250 m depth, with a warm, low-salinity and low-density signature and with geostrophic currents near 50 cm/s in the top layer (down to ~80 m) reducing to less than 10 cm/s near 250 m. Near the surface, colder water is advected into it.

Trabajo presentado en la Ocean Surface Topography Science Team Meeting (OSTST), celebrada online del 19 al 23 de octubre de 2020.

The cruise was organized into 2 legs, the first without the equipment in the container and the second, with the equipment after the container was picked up in Almería.

Trabajo presentado en la Ocean Sciences Meeting, celebrada en San Diego del 16 al 21 de febrero de 2020.

Trabajo presentado en la Ocean Sciences Meeting, celebrada en San Diego del 16 al 21 de febrero de 2020.