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Over the last 3 decades, satellite altimetry has observed sea surface height variations, providing a regular monitoring of the surface ocean circulation. Altimetry measurements have an intrinsic signal-to-noise ratio that limits the spatial scales of the currents that can be captured. However, the recent progress made on both altimetry sensors and data processing allows us to observe smaller geophysical signals, offering new perspectives in coastal areas where these structures are important. In this methodological study, we assess the ability of three altimeter missions with three different technologies to capture the Northern Current (northwestern Mediterranean Sea) and its variability, namely Jason-2 (Ku-band low-resolution-mode altimeter, launched in 2008), SARAL/AltiKa (Ka-band low-resolution-mode altimeter, launched in 2013) and Sentinel-3A (synthetic aperture radar altimeter, launched in 2016). Therefore, we use a high-resolution regional model as a reference. We focus along the French coast of Provence, where we first show that the model is very close to the observations of high-frequency radars and gliders in terms of surface current estimates. In the model, the Northern Current is observed 15–20 km from the coast on average, with a mean core velocity of 0.39 m s−1. Its signature in terms of sea level consists of a drop whose mean value at 6.14∘ E is 6.9 cm, extending over 20 km. These variations show a clear seasonal pattern, but high-frequency signals are also present most of the time. In comparison, in 1 Hz altimetry data, the mean sea level drop associated with the Northern Current is overestimated by 3.0 cm for Jason-2, but this overestimation is significantly less with SARAL/AltiKa and Sentinel-3A (0.3 and 1.4 cm respectively). In terms of corresponding sea level variability, Jason-2 and SARAL altimetry estimates are larger than the model reference (+1.3 and +1 cm respectively), whereas Sentinel-3A shows closer values (−0.4 cm). When we derive geostrophic surface currents from the satellite sea level variations without any data filtering, in comparison to the model, the standard deviations of the velocity values are also very different from one mission to the other (3.7 times too large for Jason-2 but 2.4 and 2.9 times too large for SARAL and Sentinel-3A respectively). When low-pass filtering altimetry sea level data with different cutoff wavelengths, the best agreement between the model and the altimetry distributions of velocity values are obtained with a 60, 30 and 40–50 km cutoff wavelength for Jason-2, SARAL and Sentinel-3A data respectively. This study shows that using a high-resolution model as a reference for altimetry data allows us not only to illustrate how the advances in the performances of altimeters and in the data processing improve the observation of coastal currents but also to quantify the corresponding gain.

In the context of the ESA CCI Coastal Sea Level project, a complete reprocessing of high resolution along-track altimetry data of the Jason-1, Jason-2 and Jason-3 missions over January 2002 to January 2020 was performed along the world coastal zones. This reprocessing has provided valid sea level data in the 0-15 km band from the coast. A total of 750+ altimetry-based virtual coastal stations have been selected and sea level anomalies time series together with associated coastal sea level trends have been computed over the 2002-2020 time span. These virtual stations offer a unique tool for estimating sea level change close to the coast (typically up to 3 km to the coast but in many instances up to 1 km or even closer), especially in the coastal regions devoid from tide gauges. Results show that at about 90% of the virtual stations, the rate of sea level rise at the coast is similar to the rate offshore (15+ km away from the coast). In the remaining 10%, the sea level rate within 4-5 km from the coast is either faster or slower than offshore. In this presentation, we focus on a few coastal sites where the rate of sea level rise differs from that offshore. We discuss potential small-scale coastal processes (e.g., changes in shelf currents, waves, fresh water input from river runoff in deltas and estuaries, etc.) able to explain the observations., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

Multidecadal efforts have made possible the current record of ocean sea surface height variations, 30-year long, provided by high-precision altimeter satellites. These observations have greatly improved our knowledge of the open ocean and are now an essential component of many operational marine systems and climate studies. It is not the case in the coastal ocean, where satellite altimetry encounters different issues that make it more difficult to derive accurate geophysical data. However, monitoring coastal sea level changes at global scale is now a critical need that the too rare in-situ data cannot fulfill. This has motivated many efforts to bring the sea level retrieval by satellite altimetry as close as possible to the shore. The result of these efforts is now the availability of many new algorithms for retracking radar altimeter data, correcting sea surface heights and finally deriving sea level variations. The main objective of this study, initiated and funded by the French space agency (CNES), is to objectively define the best set of altimetry algorithms for computing sea level anomalies (SLA) in coastal zones. We focus on Low Resolution Mode altimetry and aim for the creation of long-term time series spanning different missions. For the different processing components (retrackers, geophysical corrections, and auxiliary parameters, for a total of 21 algorithms), the relative quality of the different solutions is assessed with clearly defined metrics for variability, data availability, and impact on SLA as a function of distance to the nearest coast. The analysis is made at both global and regional scales for 3 specific zones: the Mediterranean Sea, the North East Atlantic and the East Australian coasts. In the regions considered, a comparison against available tide gauge data is also performed. This methodology is applied to Jason-2 and Jason-3 missions, and we determine which algorithms are the most limiting, and how results vary from one mission to the other. The set of best performing algorithms near the coast will finally be presented, with the criteria used for the selection. This set will constitute the baseline algorithms for the generation of a new Level-3 SLA product.

Until recently, classical radar altimetry could not provide reliable sea level data within 10 km to the coast. However dedicated reprocessing of radar waveforms together with geophysical corrections adapted for the coastal regions now allows to fill this gap at a large number of coastal sites. In the context of the ESA Climate Change Initiative (CCI) Sea Level project, we have recently performed a complete reprocessing of along-track, high resolution (20 Hz, i.e. 300m resolution along-track) altimetry data of the Jason-1, Jason-2 and Jason-3 missions over 2002-2020 along the coastal zones of Northeast Atlantic, Mediterranean Sea, Africa, North Indian Ocean, Southeast Asia, Australia, and America. This reprocessing has provided valid sea level data in the 0-15 km band from the coast. A total of 773 altimetry-based virtual coastal stations (distance

Through different projects, the Center for Topographic studies for the Ocean and Hydrosphere (CTOH) contributes largely to advances in coastal altimetry and to its use in coastal applications. It has developed the X-TRACK software dedicated to the reprocessing of coastal altimetry data. X-TRACK is now a mature sea level product distributed worldwide by AVISO+ (https://www.aviso.altimetry.fr), cited in many scientific publications. It consists in long time-series of SLA from most altimetry missions, processed homogeneously, but also as long-track empirical tidal constants. The latter provides independent synoptic data on the coastal ocean for tidal studies, validation of tidal models or assimilation into tidal models. In order to continue provide the most complete sea level datasets to coastal users, the CNES L2P Product (https://www.aviso.altimetry.fr/en/data/products/sea-surface-height-products/global/along-track-sea-level-anomalies-l2p.html) will now be integrated into the X-TRACK processing chain: 12 altimetry missions, covering the 1992-2021 time period. The CTOH contributes also, trought the high performance of X-TRACK software, to the development of new coastal sea level products specifically designed for climate studies, in close collaboration with ESA, TUM, CLS, NOC and SKYMAT. As part of the ESA Climate Change Initiative project, the Adaptive Leading Edge Subwaveform (ALES) Retracker (Passaro et al., 2014) and the X-TRACK software (Birol et al., 2017), previously validated and successfully applied to coastal sea level research, have been combined for the first time, in order to reprocess 18 years of sea level anomaly data (Jan. 2002 to Jan 2020) at a high frequency level (20 Hz) based on Jason missions. This new coastal sea level product called X-TRACK/ALES (https://climate.esa.int/en/projects/sea-level/data/) significantly extends the spatial coverage of sea level altimetry data in the coastal direction, now reaching a distance of 1.2-4 km from the coast on average (Birol et al., 2021). This is an unprecedented coastal coverage for any long-term altimetry data set, and then a significant contribution to the coastal research community. In addition, a new network of virtual altimetry-based stations in coastal areas of the world has also been derived from this dataset, providing new information on sea level change in the world coastal zones (see Cazenave et al., OSTST 2022).

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

Since 2009, the Meridional Overturning Circulation (MOC) in the South Atlantic has been observed with an array of in situ moorings on each side of the basin at 34.5°S, the South Atlantic MOC Basin-wide Array (SAMBA). To date, the component of the meridional transport inshore of the shallowest moorings on either side (about 1300 m depth) has been estimated using a time-mean from an ocean model simulation due to lack of better observations. However, because of their position offshore on the shelf break, the transport that is not directly observed by the SAMBA moorings is expected to be significant. We use along-track coastal altimetry, combined with existing in situ data, to estimate the unobserved inshore component of the MOC transport at 34.5S. This requires designing a method to estimate the geostrophic transport based on surface altimetry data. The analysis of the vertical ocean structure on each side of the SAMBA array, based on available in situ observations, suggests that the currents are mostly barotropic or equivalent barotropic. This property allows for the estimation of the meridional geostrophic transport offshore of the SAMBA array using along-track altimetry. It is found that the northward transport inshore of SAMBA on the Eastern side of the array and the southward transport on the Western side tend to compensate each other on average. However, the variability of the total inshore component (~4 Sv) is not negligible compared to the ~17 Sv total MOC transport at 34.5°S and should be monitored.

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.

International audience; Twenty-year coastal altimetry data set (X-TRACK) is used, for the first time, to gain insight into the long-term inter-annual variations of the surface circulation in the Sicily Channel. Firstly, a spectral along with a time/space diagram analysis are applied to the monthly means of the X-TRACK geostrophic currents over the period 1993-2013. They reveal a regionally coherent current patterns from track to track with a marked inter-annual variability that is unequally shared between the Atlantic Tunisian Current and Atlantic Ionian Stream inflows in the Sicily Channel and the Bifurcation Tyrrhenian Current outflow northeast of Sicily. Secondly, an empirical altimetry-based transport-like technique is proposed to quantify volume budgets inside the closed boxes formed by the crossing of the altimetry tracks and coastlines over the study area. A set of hydrographic measurements is used to validate the method. The inferred altimetry transports give a well-balanced mean eastward Atlantic Waters baroclinic flow of 0.4 Sv and standard deviations of 0.2 Sv on a yearly basis throughout the Sicily Channel and toward the Ionian Sea, which is fairly coherent with those found in the literature. Furthermore, the analysis allows to quantify the intrusions of Atlantic Waters over the Tunisian Shelf (0.12 +/- 0.1 Sv) and highlights two main modes of variability of the main surface waters path over the Sicily Channel through the Bifurcation Atlantic Tunisian Current and Atlantic Ionian Stream systems. Some physical mechanisms are finally discussed with regards to changes in the observed currents and transports.

International audience; Technological advances in the recent satellite al-timeter missions of Jason-2, SARAL/AltiKa and CryoSat-2 have improved their signal-to-noise ratio, allowing us to observe finer-scale ocean processes with along-track data. Here, we analyse the noise levels and observable ocean scales in the northwestern Mediterranean Sea, using spectral analyses of along-track sea surface height from the three missions. Jason-2 has a higher mean noise level with strong seasonal variations, with higher noise in winter due to the rougher sea state. SARAL/AltiKa has the lowest noise, again with strong seasonal variations. CryoSat-2 is in synthetic aperture radar (SAR) mode in the Mediterranean Sea but with lower-resolution ocean corrections; its statistical noise level is moderate with little seasonal variation. These noise levels impact on the ocean scales we can observe. In winter, when the mixed layers are deepest and the submesoscale is energetic, all of the altimeter missions can observe wavelengths down to 40–50 km (individual feature diameters of 20–25 km). In summer when the submesoscales are weaker, SARAL can detect ocean scales down to 35 km wavelength, whereas the higher noise from Jason-2 and CryoSat-2 blocks the observation of scales less than 50–55 km wavelength. This statistical analysis is completed by individual case studies, where filtered along-track altimeter data are compared with co-located glider and high-frequency (HF) radar data. The glider comparisons work well for larger ocean structures, but observations of the smaller, rapidly moving dynamics are difficult to co-locate in space and time (gliders cover 200 km in a few days, altimetry in 30 s). HF radar surface currents at Toulon measure the meandering Northern Current, and their good temporal sampling shows promising results in comparison to co-located SARAL altimetric currents. Techniques to separate the geostrophic component from the wind-driven ageostrophic flow need further development in this coastal band.