New global Mean Dynamic Topography CNES-CLS-22 combining drifters, hydrological profiles and High Frequency radar data

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. The MDT is the missing component for the optimal assimilation of altimeter data into operational ocean system. However, in coastal areas, where in-situ measurements are sparse, mainly on the shelf, the global MDT solutions are often less accurate than in the open ocean. Considering the availability of long time-series of high-quality HF-Radar velocity measurements in the Mid Atlantic Bight, the main objective is to include these data to calculate an improved MDT is this area. The prerequisites for the computation of this new Mean Dynamic Topography are to remove the non-geostrophic signal from the HF radar measured velocities. The first step is thus to pre-process these data. We used average currents from December 2006 through November 2016, estimated from HF radars and processed by Rutgers University (https://tds.marine.rutgers.edu/thredds/cool/codar/cat_totals.html ; Roarty et al, 2021). Then the mean wind-driven currents modelized as in Mulet et al (2021), were removed. Besides, the first guess MDT computed from altimetry and gravimetry was improved by using a dedicated filter along the coast. Finally, the improved MDT was estimated from the first guess MDT and the processed in-situ data (HF-radars, drifters and T/S profiles) through a multivariate objective analysis. This new regional MDT defines better currents near the coast. In particular, the MDT shows more organized across-shelf gradient following the shelf-break, and a more continuous mean flow on the shelf. Mulet, Sandrine, Marie-Hélène Rio, Hélène Etienne, Camilia Artana, Mathilde Cancet, Gérald Dibarboure, Hui Feng, et al. « The New CNES-CLS18 Global Mean Dynamic Topography ». Ocean Science 17, nᵒ 3 (17 juin 2021): 789‑808. https://doi.org/10.5194/os-17-789-2021. Roarty, Hugh, Scott Glenn, Joseph Brodie, Laura Nazzaro, Michael Smith, Ethan Handel, Josh Kohut, et al. « Annual and Seasonal Surface Circulation Over the Mid-Atlantic Bight Continental Shelf Derived From a Decade of High Frequency Radar Observations ». Journal of Geophysical Research: Oceans 125, nᵒ 11 (2020): e2020JC016368. https://doi.org/10.1029/2020JC016368.