Abstract. The mean dynamic topography (MDT) is a key reference surface for altimetry. It is needed for the calculation of the ocean absolute dynamic topography, and under the geostrophic approximation, the estimation of surface currents. CNES-CLS mean dynamic topography (MDT) solutions are calculated by merging information from altimeter data, GRACE, and GOCE gravity field and oceanographic in situ measurements (drifting buoy velocities, High Frequency radar velocities, hydrological profiles). The objective of this paper is to present the newly updated CNES-CLS22 MDT. The main improvement of this new CNES-CLS22 MDT over the previous CNES-CLS18 MDT is in the Arctic, with better coverage, no artifacts and a more realistic solution. This is due to the use of a new first guess estimated with the CNES-CLS22 MSS and the GOCO06s geoid to which optimal filtering has been applied, as well as Lagrangian filtering at the coast to reduce the intensity of normal currents at the coast. Improvements also include updating the drifting buoy and T/S profile databases, as well as processing to obtain synthetic mean geostrophic velocities and synthetic mean heights. In addition, a new data type, HF radar data, was processed to extract physical content consistent with MDT in the Mid Atlantic Bight region. The study of this region in particular has shown the improvements of the CNES-CLS22 MDT, but that there is still work to be done to obtain a more physical solution over the continental shelf. The CNES-CLS22 MDT has been evaluated against independent height and velocity data in comparison with the previous version, the CNES-CLS18. The new solution presents slightly better results, although not identical in all regions of the globe.

We assess the accuracy and spatial resolution of the Surface Water and Ocean Topography (SWOT) swath altimeter for measuring marine gravity anomalies. The analysis is performed at the Foundation Seamounts in the South Pacific where we developed a highly accurate gravity field by combining the long-wavelength (> 40 km) gravity field derived from previous nadir altimeters with the shorter wavelength gravity field from the seafloor topography as constrained by the ship gravity. In this region, the slope of the ocean variability is 50-100 times smaller than the gravity/slope signal of the seamounts so can be ignored in the analysis. Each SWOT cycle can deliver gravity anomaly/SSS with an accuracy of 2.6 mGal/μrad and a spatial resolution of 14 km, with accuracy diminishing when significant wave height (SWH) exceeds ~6 meters. Averaging repeated SWOT measurements improves the accuracy and resolution. For example, we expect that averaging just 10 repeats (7 months) results in accuracy/resolution that matches the best marine gravity maps based on 230 months of nadir altimetry. With a mission lasting over a year, SWOT promises a substantial leap in marine gravity accuracy and resolution, uncovering previously uncharted details of the seafloor, including thousands of uncharted seamounts.

Two methods for the mapping of ocean surface currents from satellite measurements of sea level and future current vectors are presented and contrasted. Both methods rely on the linear and Gaussian analysis frameworkwith different levels of covariance definitions. The first method separately maps sea level and currents with single-scale covariance functions and leads to estimates of the geostrophic and ageostrophic circulations. The second maps both measurements simultaneously and projects the circulation onto 4 contributions: geostrophic, ageostrophic rotary, ageostrophic divergent and inertial. When compared to the first method, the second mapping moderately improves the resolution of geostrophic currents but significantly improves estimates of the ageostrophic circulation, in particular near-inertial oscillations. This method offers promising perspectives for reconstructions of the ocean surface circulation. Even the hourly dynamics can be reconstructed from measurements made locally every few days because nearby measurements are coherent enough to help fill the gaps. Based on numerical simulation of ocean surface currents, the proposed SKIM mission that combines a nadir altimeter and a Doppler scatterometer with a 300 km wide swath (with a mean revisit time of 3 days) would allow the reconstruction of 50% of the near-inertial variance around an 18 hour period of oscillation.

The originality of this study is to propose a new calibration method based on two calibration phases between Jason-3 and Sentinel-6A (S6A) to better estimate the relative global and regional mean sea level drifts between the two missions. To date, a first calibration phase of approximately 12 months is planned from January 15, 2021, to December 31, 2021, when both satellites will be on the same orbit spaced out by approximately 30 seconds. This calibration will allow for a very accurate assessment of the GMSL bias between Jason-3 and S6A (less than 0.5 mm, see ​ Zawadzki and Ablain, 2016​). A second calibration phase after a few years would reduce the uncertainty levels of the GMSL (global mean seal level) drift estimate. The uncertainty would be low enough to detect any drift detrimental to the stability of the current GMSL record. It would indeed be possible to evaluate the stability between the two satellites with an accuracy at least 3 times better at the global scale than with the most accurate method to date. At regional scales, the second calibration phases would provide regional MSL drift estimates with very good precision. This study also shows that the time spent between the two calibration phases is significantly more sensitive than the length of the second calibration phase for the reduction in uncertainties. Finally, a possible scenario proposed by this study would consist of carrying out the beginning of the second calibration phase approximately 1.5-2 years after the first and for a duration of 3-4 months. This calibration would allow the detection of a relative GMSL drift of approximately 0.15 mm/yr and 0.4-0.5 mm/yr at oceanic basin scales (2000-4000 km).

The Geodetic Mission (GM) of Jason-2 was planned to provide ground-tracks with a systematic spacing of 4 km after 2 years and 2 km after 4 years to increase the spatial resolution of global altimetric gravity fields. Jason-2 ceased operation after 2 years of GM but provided a fantastic dataset. We highlight and evaluate the improvement to the gravity field which has been derived from the GM. The ageing Jason-2 suffered from several safe-holds and instrument outages. Here, we try to quantify the effect of safe-holds on marine gravity and discuss suitable approaches advising future GM like Jason-3. We evaluate the importance of attempting to “rewind” the mission to recover missing tracks as well as the possibility to continue an existing GM by using the same orbital plane. The latter idea would allow bisecting the already 2-years Jason-2 GM creating a 2 km grid after 2 years of Jason-3 GM.

The ocean’s sea surface height (SSH) field is a complex mix of motions in geostrophic balance and high-frequency tides, internal tides and internal gravity waves. The transition scale Lt, at which unbalanced ageostrophic motions dominate balanced geostrophic motions, is documented for the first-time using satellite altimetry. Lt is critical to define the scales where surface geostrophic currents can be inferred from SSH. We use a statistical approach based on the analysis of SSH wavenumber spectra to obtain Lt. Small values of Lt are observed in regions of energetic mesoscale activity, while large values are observed in tropical latitudes and regions of low mesoscale activity. Seasonally, larger Lt values are observed during summer than during winter, following the seasonality of upper ocean dynamics. These results are consistent with recent analyses of in situ observations and high-resolution models. Limitations of our results and implications for nadir and swath altimetric missions are discussed.