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We investigate ocean dynamics at different scales in the Agulhas current system, a region of important interocean exchange of heat and energy. While ocean observations and some of the most advanced climate models capture the larger mesoscale dynamics (>100 km), the smaller-scale fronts and eddies are underrepresented. The recently launched NASA-CNES Surface Water and Ocean Topography (SWOT) wide-swath altimeter mission observes the smaller ocean geostrophic scales down to 15 km in wavelength globally. Here we will analyze different eddy diagnostics in the Agulhas Current region and quantify the contributions from the larger mesoscales observable today, and the smaller scales to be observed with SWOT. Surface geostrophic diagnostics of eddy kinetic energy, strain, and energy cascades are estimated from modelled sea surface height fields of the MITGCM / LLC4320 simulation subsampled at 1/10°. In this region, the smaller scales (

Mesoscale eddies are ubiquitous structures in the ocean. As mesoscale eddies can trap and transport water within their cores over long distances, they have been investigated globally since the availability of altimetry maps. Mesoscale eddies are first detected in the sea surface, and then tracked in time and space. Several methods of detection and tracking have been developed, most of them propose to describe the evolution of mesoscale eddies with an association in individual trajectories, with a beginning and an end. Few methods are able to take into account the interactions between trajectories, and when merging and splitting events are recorded, it is necessary to change the semantics and the metrics to describe the behaviour of mesoscale eddies. Here we present a new mesoscale eddies dataset, where the structures are gathered in networks. Eddies are detected on daily absolute topography maps with the Py-Eddy-Tracker algorithm (PET, Mason et al., 2014, https://github.com/AntSimi/py-eddy-tracker). Following Pegliasco et al. (2015), successive eddies with overlapping contours are associated in the same network if the overlap ratio, defined as the intersection of their area divided by the union of their areas, is more than 5 %. Within networks, segments represent the temporal evolution of individual eddies and nodes between segments correspond to merging or splitting events. Segments are what was previously called trajectories, but the individuality (no interactions) is not assumed. During merging and splitting events, more than two eddies present an overlap. The highest overlap ratio is used to determine which segment stops in a merging event and which segment starts in a splitting event. We developed simple functions to manipulate and visualize this new type of dataset. To assess the networks’ coherence, we use a lagrangian perspective. A coherence level is obtained by advecting for 14 days particles injected within the eddy’s contours of maximum averaged speed, both backward and forward in time, with the surface currents derived from absolute dynamic topography. At the end of the advection, the number of particles still within the eddy’s contours is normalized by the initial number of particles. A coherence level is associated to each segment and each interaction and can be used for selection by the users. The META-Networks (Mesoscale Eddy Trajectories Atlas – Networks) can be used for any interdisciplinary research topic, for example by coupling the mesoscale eddies’ contours with in situ data, or describe the displacement of tracers along eddies’ paths, at a regional or global scale.

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Ocean mesoscale eddies are the most energetic component of the ocean dynamics, important for the horizontal fluxes of energy, momentum, and tracers such as heat, carbon and nutrients. Today, observations of smaller scales are limited, and we might be missing 75% of the mesoscale eddies at high latitudes. The future SWOT SAR-interferometry wide-swath altimeter mission aims to provide 2D swath observations of ocean surface topography to characterize the ocean mesoscale and submesoscale circulation from the large scale down to around 15 km wavelength. This research focuses on the understanding of this smaller mesoscale field in the Southern Ocean, with a focus on the Agulhas current. Our main aim is to investigate whether small-scale eddy fluxes reinforce or compensate for the large mesoscale fluxes that we can observe today, and where and when this is occurring. The study of mesoscale eddies and their interaction with the larger scale structures is achieved using various eddy diagnostics: the eddy kinetic energy (eke), strain rate, barotropic energy transfer, anisotropy of eddy variability, and energy flux between scales with a coarse graining approach. We compare the dynamics in key regions of the Agulhas boundary current, the Agulhas Retroflection and Cape Cauldron region, and the meandering Agulhas Current Extension. First, the dataset generated with the hourly MITgcm LLC4320 on a native grid at 1/48-degree is used. A comparison is made with today's observability using reconstructed DUACS maps, based on the optimal interpolation of the along-track sampling of the MITgcm at a coarser 1/10-degree resolution. For the observability with SWOT, diagnostics are finally computed on the simulated SWOT swaths, taking a sea surface height (ssh) based on the 1/48° MITgcm simulation and the SWOT ocean simulator, with and without the SWOT errors. The observable scales for these different diagnostics, and error reduction techniques for SWOT, will be discussed.

We use three years of Sea Surface Height (SSH) observations over the World Ocean to analyze its spectral characteristics: spectral slope and noise levels. Our analyses are based on previous works that focused on the spectral slope characterization through a linear regression over a geographically dependent wavelength range, corresponding to the mesoscale wavelength range. In our current analysis, we adopt a methodology that allows for the characterization of the spectral slope by optimal fit of a linear function, considering the statistical uncertainty of the observed spectra. This is particularly important in the wavelength range of the instrumental noise (15-30 km wavelength): the noise level influence the spectral shape over the geophysical spatial scales. The observed values of spectral slope are within the range of previous works however we observe a higher spatial variability in regions where the eddy kinetic energy varies sharply, with spectral slope values that drop significantly in low energy regions. Noise level distribution is also within the range of previous results, although we observe higher values in the mid latitudes (10 to 15%) and intertropical band. In addition, our current approach allows us to analyze (when present) the break in the mesoscale spectral slope that occurs at wavelengths shorter than 100 km, partly due to the internal tides and their energy cascade to smaller scales, but also other dynamical signal and instrumental errors. Preliminary results on this topic are also presented.

Wavelength spectral analysis of alongtrack altimetric data helps to characterize the seasonal and spatial variability of the sea level anomaly (SLA) signal. It allows us to infer the nature of the surface dynamics (balanced vs unbalanced motion, internal waves). For such analyses, low resolution mode data sets (1 Hz, 6-7 km sampling) are classically used. At this stage there is a strong gap in terms of effective resolution between the 1 Hz along track products (40-70 km resolution) and what is expected from the data of the future SWOT mission. However, the finest spatial sampling is 20 Hz for Jason, and 40 Hz for SARAL/AltiKa. For these products, the main limitation to observe small mesoscale is the existence of a spectral “hump” and a larger noise level due to surface waves and other surface inhomogeneities within the radar footprint. The new concept of SAR mode altimeter as used in Sentinel 3 provides better precision and resolution capabilities. In the prospect of the SWOT mission, this paper focuses on high resolution (HR) datasets from the recent satellite missions based on regional analysis including Jason-2/3, SARAL/AltiKa, and Sentinel-3. At the present time, there is no HR-SLA datasets for all these missions and over the global ocean that are easily usable. In this poster we review different methods and options in the data processing to get HR-SLA. The ability of the different datasets and processing to provide information on the dynamics is illustrated in two contrasted regions: the Bay of Biscay located in the mid-latitudes NE Atlantic and the subtropical region south of New Caledonia in the tropics. The benefit of HR data is evident for scales up to 100 km. It illustrates both the gain brought by the Ka-band of Saral and the SAR mode of S3A over the Ku band of Jason and the benefit of the long time series of Jason data for the studies of internal tides. We emphasize the need to take into account the anisotropy and the very local nature of the ocean dynamics properties in order to assess regional dynamics.

Status of the SWOT mission

Mesoscale eddies are ubiquitous in the ocean. When they rotate faster than they travel, they can trap water into their cores and transport it along their paths, redistributing heat, salt, chemical and biological components from their generation to their dissipation areas. The new versions of the global Mesoscale Eddy Trajectory Atlas (META) 3.2 Delayed Time all-satellites and two-satellites are produced by SSALTO/DUACS and distributed by AVISO+ (https://aviso.altimetry.fr) with support from CNES. Significant changes from the historical 2.0 version previously distributed and regularly updated were made to provide an improved representation of the mesoscale eddies, illustrated here. The META3.2 detection scheme is based on the Py-Eddy-Tracker algorithm (Mason et al., 2014, https://github.com/AntSimi/py-eddy-tracker), applied on Absolute Dynamic Topography (ADT, DUACS DT2021 reprocessing) maps filtered to remove large-scale patterns. ADT-detection better represents quasi-stationary and recurrent structures than sea level anomaly based detection. The amplitude threshold is reduced to 0.4 cm, increasing the number of small eddies, in particular during the growing and decay phases of the trajectories, or close to coasts and islands. Only one local extremum is tolerated within an eddy contour, with a better agreement with ocean color data tan the multiple-extrema structures authorized before. The tracking scheme is based on the contours’ overlap (following Pegliasco et al., 2015) instead of the research of eddy candidates in a restricted area, reducing the wrong temporal associations. To provide maximal information, META3.1exp products make available the structures detected only one day, the trajectories lasting less than 10 days and the trajectories lasting at least 10 days in different files. The products also include new information such as the mesoscale eddy shapes with the eddy edges and the contour associated with the maximum averaged-speed, and the eddy speed profiles from the center to the edge. Thus, META3.2 products allow easier collocation of data for transdisciplinary research. A future evolution of the product is to include the eddies’ interactions (merging and splitting events) with a network representation (META-Networks). This evolution will be discussed with the users to best meet their needs.

AlTiS (Altimetry Time Series) is a software to visualize and process radar altimetry data. It was developed to analyse radar altimetry data over small areas like river, lakes and wetlands, located in the altimetry groundtracks. Its major goal is the creation of time-series of water levels from the height derive from the altimeter measurement but it can also be used to generate time series of any altimetry-based parameters (e.g., corrections applied to the range, backscattering coefficients, or brithness temperatures). Through a Graphical User Interface (GUI), without any skills in data processing, the user can handle altimetry data in order to : • Display several parameters of altimetric measurement like surface height, altimetric range, atmospheric corrections applied to the range to take into account the propagation delay of the electromagnetic wave through the earth atmosphere (ionospheric correction and wet and dry tropospheric corrections) and also to display some characteristic parameters of the waveform like the backscatter coefficient, and peakiness. • Graphically select altimetric measurements to remove outliers and easily done owing to Landsat background image. • Generate water height time series from the valid altimetry data previously selected AlTiS accepts CTOH altimetry products (Level 2 GDR supplied by the CTOH). CTOH GDR data have been specifically conditioned to optimize the data size by making a geographical selection and includes the right altimetry parameters for hydrological studies. AlTiS can process several altimetric data products from followed missions : Jason-1/2/3, ERS-2, ENVISAT, SARAL, Sentinel-3A/B, and soon, Sentinel-6/Jason-CS and the nadir altimeter onboard SWOT. They are supplied for free through a web request form on the CTOH website (http://ctoh.legos.obs-mip.fr/applications/land_surfaces/altimetric_data/altis). AlTiS is mainly employed for hydrological applications and can be used for training courses on radar altimetry at bachelor or master levels. It is also a very convenient tool to analyse the radar altimetry data contained in the GDR over any type of land surfaces. AlTiS is a software developed by CTOH as part of its activities as a National Observation Service. AlTiS is a free software and it is released as an open source under the CeCill License. Altis is working under python3 environment and tested for GNU/Linux, Windows 10. AlTiS is available on GitLab : https://gitlab.com/ctoh/altis

The Center for Topography of the Oceans and Hydrosphere (CTOH) is a French Observation Service created in 1989 and dedicated to satellite altimetry studies. It focuses on the development and promotion of new processing approaches of the altimetric data for emerging research domains (coastal ocean, oceanic sub-mesoscale phenomenons, continental surface water, sea ice, polar caps). It works in close relationship with space agencies (mainly CNES and ESA) at different levels for satellite altimetry missions: preparation of new missions, definition of the user’s needs, CAL/VAL studies, signal analysis & data reprocessing, development of thematic products, teaching and outreach. In terms of data distribution, the CTOH maintains a global GDR data base for almost all altimetry missions since Topex/Poseidon. All the products are made homogeneous (addition of the most recent parameters / corrections for the old missions) and provided in netcdf format. A new version of ERS-2 data, reprocessed by the CTOH for hydrological applications is also available (it includes ICE-1 and ICE-2 retrackers). Both 1Hz and 10/20/40Hz data are available globally. For some products, the CTOH database contains also L1 products (waveforms). In addition, the CTOH develops new altimetry products: • Fine-resolution ocean products: position of the main Southern Ocean polar fronts and global climatology of near-inertial current characteristics. • Coastal products : X-TRACK along-track SLA time series and tidal harmonic constants, reprocessed with a software designed for coastal altimetry, but also the recent high-resolution (20-Hz) X-TRACK/ALES sea level product, now reaching a distance of 3-4 km from the coast on average (see Leger et al., OSTST 2022). These products are distributed by AVISO+ and by ESA, respectively. • Continental hydrology products : including Topex/Poseidon reprocessed by the CASH project and water level maps developed for 5 rivers: the Amazon, Orenoque, Gange-Bramhapoutre, Congo and Mekong. It also contributes to the development of sea-ice products distributed by AVISO+ (Arctic Sea Ice Thickness and Snow depth monthly maps over sea ice) and to the “Hydroweb” data base for monitoring river and lake levels.