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The SWOT satellite mission (Surface Water and Ocean Topography), launched in December 2022, is measuring sea surface height with a resolution an order of magnitude higher than conventional altimeters, providing an unprecedented view of the variability of the sea surface topography. This document reports the activities carried out at sea during the two campaigns of the FaSt-SWOT project (PID2021-122417NB-I00 funded by MCIN/AEI/10.13039/501100011033/ FEDER,UE). These campaigns had two main objectives: 1) participate to the SWOT satellite cal/val activities by collecting in-situ observations of fine-scale structures in the area covered by the satellite during its initial fast-sampling phase, and 2) improve the characterization and understanding of these structures by combining in-situ multi-platform and satellite data with numerical models and other computational techniques. The FaSt-SWOT experiments were conducted in the Balearic Sea (Western Mediterranean Sea) between 25-28 April and 7-10 May 2023, using R/V SOCIB. The experiments consisted in 2 legs both using multi-scale ship-based instruments (CTD, Moving Vessel Profiler, thermosalinograph, ADCP and GoPro action cameras), 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 representation 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 small anticyclonic eddy, with a diameter around 20-25km. 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 wit

Surface Water & Ocean Topography (SWOT) is the first satellite altimetry mission to measure the elevation of nearly all water on Earth’s surface. SWOT will provide high-resolution maps of the water surface topography over the open and coastal oceans, lakes rivers and reservoirs. After two decades of preparation , SWOT was launched on December 2022. For approximately the first 180 days of the mission, SWOT is in a 1-day repeat orbit for cal/val. This document represents the cruise plan of the FaSt-SWOT field campaigns co-led by IMEDEA(CSIC-UIB) and SOCIB as a contribution to the SWOT Science Team and SWOT-AdAC consortium. FaSt-SWOT multi-platform experiments took place in the Balearic Sea in April-May 2023 during the SWOT satellite mission fast-sampling phase. The generals objectives of these experiments were (1) to validate the first observations provided by the new altimeter mission during the calibration/validation phase and (2) to improve the characterization of oceanic fine scales through the combined use of in-situ multi-platform and satellite data in synergy with numerical models and innovative computational techniques. The campaigns have assessed the actual capability to map SSH variability over a range of scales (30-100 km) traditionally not resolved by conventional altimeters. A unique aspect of the FaSt-SWOT experiments is the combination of concurrent multi-scale ship-based instruments, autonomous platforms, and satellite observations with ad hoc modeling simulations, enabling the evaluation of underlying mechanisms. FaSt-SWOT federates two multiplatform in situ experiments, combining glider, drifter, ship observations together with satellite observations and high-resolution data-assimilative numerical simulations. Advanced Observing System Simulation Experiments (OSSEs) and new tools of artificial intelligence have been performed to optimize the sampling strategies. This document includes the plans for the cruises and it will be complemented with the cru

“Del espacio al Mediterráneo: Persiguiendo corrientes marinas” es un corto documental que recoge la experiencia de las oceanógrafas y oceanógrafos del IMEDEA (CSIC-UIB) y de la ICTS SOCIB durante las dos campañas del proyecto FaSt-SWOT, a bordo del Buque Oceanográfico B/O SOCIB. El objetivo de estas campañas consistió en tomar mediciones de variables y corrientes oceánicas a través de diferentes instrumentos incluyendo plataformas autónomas, como son los planeadores submarinos (gliders) o las boyas de deriva superficiales, para apoyar el análisis y la explotación de las primeras mediciones de alta resolución de la misión satélite SWOT (Surface Water and Ocean Topography o Topografía oceánica y de las aguas superficiales). El satélite SWOT, desarrollado por la NASA y el Centro National de Études Spatiales (CNES), con la contribución de la Agencia Espacial Canadiense y la Agencia Espacial del Reino Unido, fue lanzado al espacio en diciembre de 2022 y orbitará alrededor de la Tierra hasta 2026 para examinar el agua superficial del planeta y la elevación de la superficie marina con alta resolución.

"¡Boya al agua! Del espacio exterior al Mediterráneo", es una iniciativa del Instituto Mediterráneo de Estudios Avanzados (IMEDEA, CSIC-UIB) y del Sistema de Observación y Predicción Costero de las Illes Balears (ICTS SOCIB) para dar a conocer a la comunidad educativa el proyecto científico FaSt-SWOT y la importancia de las corrientes marinas. Este proyecto está destinado a mejorar nuestra comprensión de las corrientes oceánicas, combinando datos del nuevo satélite de la NASA (SWOT) con información recogida, in situ, en campañas alrededor de las Illes Balears. "¡Boya al agua!" pone a disposición del profesorado material para explicar en el aula la importancia del océano y el papel de las corrientes marinas en la regulación del clima del planeta, en los procesos esenciales para el desarrollo de la vida marina y en las cuestiones de seguridad marítima. Además, la iniciativa invitó al profesorado y su alumnado a participar en el proyecto FaSt-SWOT a través del amadrinamiento de hasta 20 boyas superficiales de deriva o drifters, que el equipo científico utilizó en una de sus campañas oceanográficas a bordo del Buque Oceanográfico SOCIB, el mes de mayo de 2024. Las boyas de deriva son instrumentos oceanográficos esenciales para conocer las corrientes marinas, que flotan y se desplazan con las corrientes midiendo variables como la temperatura, la presión atmosférica y la posición. El objetivo de estas campañas consistió en tomar mediciones de variables y corrientes oceánicas a través de diferentes instrumentos incluyendo plataformas autónomas, como son los planeadores submarinos (gliders) o las boyas de deriva superficiales, para apoyar el análisis y la explotación de las primeras mediciones de alta resolución de la misión satélite SWOT (Surface Water and Ocean Topography o Topografía oceánica y de las aguas superficiales). El satélite SWOT, desarrollado por la NASA y el Centro National de Études Spatiales (CNES), con la contribución de la Agencia Espacial Canadiense y la

Advancing the retrieval of accurate upper-layer ocean circulation products responding to the needs of the different key players engaged in the transition toward a clean, safe, sustainable and productive environment is the driving element of the World Ocean Circulation project. To achieve the objective, the common driver of the project to enhance the understanding, the monitoring and the phasing forecast of upper ocean currents, with a particular focus on the most intense ocean small-scale structures. In other word, our challenge is to retrieve the ocean velocities at the right place at the right time. An overview of the main results obtained during the first phase of the project will be presented. The project is organised around 4 themes. The theme 1 is focused Sea-state current interactions for Safe Navigation whereas theme 2 deals with 3D currents and vertical motion for Sustainable Fisheries. In theme 3 we address the Surface Lagrangian drift for a Clean Ocean. Theme 4 is dedicated to high resolution wave and current model assessment for a Productive Ocean. The main outcome of the first year will be presented. This first phase 1 was dedicated to the development of new algorithms where one year of products have been generated and validated for each theme. We will also present preliminary results of the impact assessment studies done by the WOC users. Visit https://www.worldoceancirculation.org/ to follow the main steps of the project

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 SWO

Sea Surface Height (SSH) observations describing scales in the range 10 - 100 km are crucial to betterunderstand energy transfers across scales in the open ocean and to quantify vertical exchanges of heatand biogeochemical tracers. The Surface Water Ocean Topography (SWOT) mission is a new wide-swathaltimetric satellite which is planned to be launched in 2022. SWOT will provide information on SSH at akilometric resolution, but uncertainties due to various sources of errors will challenge our capacity to extractthe physical signal of structures below a few tens of kilometers. Filtering SWOT noise and errors is a keystep towards an optimal interpretation of the data.The aim of this study is to explore image de-noising techniques to assess the capabilities of the future SWOTdata to resolve the oceanic fine scales. Pseudo-SWOT data are generated with the SWOT simulator forOcean Science, which uses as input the SSH outputs from high-resolution Ocean General Circulation Models(OGCMs). Several de-noising techniques are tested, to find the one that renders the most accurate SSHand its derivatives fields while preserving the magnitude and shape of the oceanic features present. Thetechniques are evaluated based on the root mean square error, spectra and other diagnostics.In Chapter 3, the pseudo-SWOT data for the Science phase is analyzed to assess the capabilities of SWOT toresolve the meso- and submesoscale in the western Mediterranean. A Laplacian diffusion de-noising techniqueis implemented allowing to recover SSH, geostrophic velocity and relative vorticity down to 40 - 60 km. Thisfirst step allowed to adequately observe the mesoscale, but space is left for improvement at the submesoscale,specially in better preserving the intensity of the SSH signal.In Chapter 4, another de-noising technique is explored and implemented in the same region for the satellite’sfast-sampling phase. This technique is motivated by recent advances in data assimilation techniques to removespatia, In Chapter 5, we investigate the finest spatial scale that SWOT could resolve after de-noising in severalregions, seasons and using different OGCMs. Although a method is found to mitigate the impact of SWOTinstrumental noise and errors, large uncertainties remain due to the incomplete understanding of the impactof fast unbalanced motions (e.g. internal gravity waves) on SSH signals, and therefore the impact on theperformance of the de-noising algorithm. Our study focuses on different regions and seasons in order todocument the variety of regimes that SWOT will sample. The de-noising algorithm performs well even inthe presence of intense unbalanced motions, and it systematically reduces the smallest resolvable wavelength.Advanced de-noising algorithms also allow to reliably reconstruct SSH gradients (related to geostrophicvelocities) and second order derivatives (related to geostrophic vorticity). Our results also show that asignificant uncertainty remains about SWOT’s finest resolved scale in a given region and season because ofthe large spread in the level of variance predicted among our high-resolution ocean model simulations.The de-noising technique developed, implemented and tested in this doctoral thesis allows to recover, insome cases, SWOT spatial scales as low as 15 km. This method is a very useful contribution to achieving theobjectives of the SWOT mission. The results found will help better understand the ocean’s dynamics andoceanic features and their role in the climate system.

In the near future, the Surface Water Ocean Topography (SWOT) mission will provide images of altimetric data at kilometric resolution. This unprecedented 2-dimensional data structure will allow the estimation of geostrophy-related quantities that are essential for studying the ocean surface dynamics and for data assimilation uses. To estimate these quantities, i.e., to compute spatial derivatives of the Sea Surface Height (SSH) measurements, the uncorrelated, small-scale noise and errors expected to affect the SWOT data must be smoothed out while minimizing the loss of relevant, physical SSH information. This paper introduces a new technique for de-noising the future SWOT SSH images. The de-noising model is formulated as a regularized least-square problem with a Tikhonov regularization based on the first-, second-, and third-order derivatives of SSH. The method is implemented and compared to other, convolution-based filtering methods with boxcar and Gaussian kernels. This is performed using a large set of pseudo-SWOT data generated in the western Mediterranean Sea from a 1/60 ∘ simulation and the SWOT simulator. Based on root mean square error and spectral diagnostics, our de-noising method shows a better performance than the convolution-based methods. We find the optimal parametrization to be when only the second-order SSH derivative is penalized. This de-noising reduces the spatial scale resolved by SWOT by a factor of 2, and at 10 km wavelengths, the noise level is reduced by factors of 104 and 103 for summer and winter, respectively. This is encouraging for the processing of the future SWOT data.

The SurfaceWater and Ocean Topography (SWOT) mission is a next generation satellite mission expected to provide a 2 km-resolution observation of the sea surface height (SSH) on a two-dimensional swath. Processing SWOT data will be challenging because of the large amount of data, the mismatch between a high spatial resolution and a low temporal resolution, and the observation errors. The present paper focuses on the reduction of the spatially structured errors of SWOT SSH data. It investigates a new error reduction method and assesses its performance in an observing system simulation experiment. The proposed error-reduction method first projects the SWOT SSH onto a subspace spanned by the SWOT spatially structured errors. This projection is removed from the SWOT SSH to obtain a detrended SSH. The detrended SSH is then processed within an ensemble data assimilation analysis to retrieve a full SSH field. In the latter step, the detrending is applied to both the SWOT data and an ensemble of model-simulated SSH fields. Numerical experiments are performed with synthetic SWOT observations and an ensemble from a North Atlantic, 1/60° simulation of the ocean circulation (NATL60). The data assimilation analysis is carried out with an ensemble Kalman filter. The results are assessed with root mean square errors, power spectrum density, and spatial coherence. They show that a significant part of the large scale SWOT errors is reduced. The filter analysis also reduces the small scale errors and allows for an accurate recovery of the energy of the signal down to 25 km scales. In addition, using the SWOT nadir data to adjust the SSH detrending further reduces the errors.

The growing availability of ocean data brought forth by recent advancements in remote sensing, in situ measurements, and numerical models supports the development of data-driven strategies as a powerful, computationally efficient alternative to model-based approaches for the interpolation of high-resolution, gap-free, regularly gridded sea surface geophysical fields from partial satellite-derived observations. In this paper, we investigate such data-driven strategies for the spatio-Temporal interpolation of sea level anomaly (SLA) fields in the Western Mediterranean Sea from satellite-derived altimetry data. We introduce and evaluate the analog data assimilation (AnDA) framework, which exploits patch-based analog forecasting operators within a classic Kalman-based data assimilation scheme. With a view toward the upcoming wide-swath surface water and ocean topography (SWOT) mission, two different types of altimetry data are assimilated: Along-Track nadir data and wide-swath SWOT altimetry data. Using an observing system simulation experiment, we demonstrate the relevance of AnDA as an improved interpolation method, particularly for mesoscale features in the 20-to 100-km horizontal scale range. Results report an SLA reconstruction RMSE (correlation) improvement of 42% (14%) with respect to optimal interpolation, and show a clear gain when the joint assimilation of SWOT and along-Track nadir observations are considered.

Spatio-temporal variability of surface geostrophic mesoscale currents in the Balearic Sea (western Mediterranean) is characterized from satellite altimetry in combination with in-situ velocity measurements collected, among others, by drifting buoys, gliders and high-frequency radar. Here, we explore the use of tracking data from living organisms in the Balearic Sea as an alternative way to acquire in-situ velocity measurements. Specifically, we use GPS-tracks of resting Scopoli¿s shearwaters Calonectris diomedea, that act as passive drifters, and compare them with satellite-derived velocity patterns. Results suggest that animal-borne GPS data can be used to identify rafting behaviour outside of the breeding colonies and, furthermore, as a proxy to describe local sea surface currents. Four rafting patterns were identified according to the prevailing driving forces responsible for the observed trajectories. We find that 76% of the bird trajectories are associated with the combined effects of slippage and Ekman drift and/or surface drag; 59% are directly driven by the sea surface currents. Shearwaters are therefore likely to be passively transported by these driving forces while resting. The tracks are generally consistent with the mesoscale features observed in satellite data and identified with eddy-tracking software.

The PRE-SWOT experiment was conducted onboard R/V García del Cid between 5 and 17 May 2018 in the southern region of the Balearic Islands (western Mediterranean Sea). PRE-SWOT aimed at anticipating the daily high-resolution 2D SSH fields that Surface Water & Ocean Topography (SWOT) will provide during the fast sampling phase after launch in selected areas of the global ocean. This experiment is a contribution to the preparatory SWOT cal/val activities and was coordinated with the PROTEUS-SWOT cruise (R/V Beautemps-Beauprè). The PRE-SWOT project (CTM2016-78607-P) is funded by the Spanish Research Agency and the European Regional Development Fund (AEI/FEDER, UE).

The PRE-SWOT experiment was conducted onboard R/V García del Cid between 5 and 17 May 2018 in the southern region of the Balearic Islands (western Mediterranean Sea). PRE-SWOT aimed at anticipating the daily high-resolution 2D SSH fields that Surface Water & Ocean Topography (SWOT) will provide during the fast sampling phase after launch in selected areas of the global ocean; one of them being the region around the Balearic Islands. In situ systems, including 2 gliders, 12 drifters, CTD, ADCP, and more than 2000 water samples, were collected in synergy with satellite data and modeling simulations to determine physical and biochemical ocean variability. This experiment will contribute to the preparatory SWOT cal/val activities and was coordinated with the PROTEUS-SWOT cruise (R/V Beautemps-Beauprè).