Search

This report presents the results of Task 4.4: Improving the use of in situ observations for the long-term validation of satellite observations

This report includes recommendations for the planification of in situ experiments aimed to reconstruct fine-scale ocean currents (~20 km), such as those that will be conducted to validate SWOT satellite observations.

The maritime traffic around the Loop current of the Gulf of Mexico is large enough to help retrieving the oceanic surface current. Using Automatic Identification System (AIS) data streams for the period 2019-2020, a distance-weighted least-squares approach [Le Goff et al 2021] is used to estimate ocean surface currents at a depth corresponding to the mean draught of the commercial vessels (about 7m). The calculated gridded data product has spatial resolution of 0.125⁰ × 0.125⁰ and daily temporal resolution. These estimations provide a valuable resource to investigate the ageostrophic processes that altimeter-based data are not sufficient to resolve. However, the limited coverage of the AIS Data (limited to the main maritime routes), prevents studies from reaching regional basins and thus limits potential applications of the collected data at larger scales where altimetry data is sufficient. As such, synergy between AIS-derived sea surface currents and altimetry data needs to be considered to increase the accuracy of sea surface currents in the Gulf of Mexico. The merging is done through the Multiscale Inversion for Ocean Surface Topography (MIOST) [Ubelmann et al., 2020] variational tool to retrieve both geostrophic and ageostrophic current. This tool allows the decomposition of the signal into different components representing different time and space scales (i.e., mesoscale to large-scale) and different physical signals (i.e., geostrophic, internal waves, near-inertial oscillations, Ekman current…). The resulting improved ocean surface current estimates will be compared to the intensity of surface currents estimated from the AOML drifters. Comparisons with high-resolution Sea Surface Temperature (SST) images from infrared instruments will be focused on the eddy separation events for the Gulf of Mexico loop current and on the associated eddy paths. Identification of warm rings with an eddy tracking algorithm (e.g., AMEDA [Le Vu et al 2018]) will allow the determination of the characteristics and life cycle of these eddies. [Le Goff et al 2021] : Le Goff, C., Boussidi, B., Mironov, A., Guichoux, Y., Zhen, Y., Tandeo, P., et al. (2021). Monitoring the greater Agulhas Current with AIS data information. Journal of Geophysical Research: Oceans, 126, e2021JC017228. https://doi.org/10.1029/2021JC017228 [Ubelmann et al 2021]: Ubelmann, C., Dibarboure, G., Gaultier, L., Ponte, A., Ardhuin, F., Ballarotta, M., & Faugère, Y. (2021). Reconstructing ocean surface current combining altimetry and future spaceborne Doppler data. Journal of Geophysical Research: Oceans, 126, e2020JC016560. https://doi.org/10.1029/2020JC016560 [Le Vu et al 2018]: Le Vu, B., Stegner, A., & Arsouze, T. (2018). Angular Momentum Eddy Detection and tracking Algorithm (AMEDA) and its application to coastal eddy formation. Journal of Atmospheric and Oceanic Technology, 35(4), 739-762.

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.

The Stokes drift is the main contribution from ocean surface waves to particles drift in the ocean's upper layer. At the moment, estimates of the Stokes drift at the global scale rely on numerical models, due to the unavailability of such measurements from space. CFOSAT is a new mission concept, carrying SWIM, a rotating scatterometer, operating both at nadir and off-nadir. It aims at measuring the ocean surface waves slopes spectrum, from which the Stokes drift can be estimated. Potential for Stokes drift estimation from SWIM is illustrated using statistical diagnostics on SWIM ocean waves spectra. Although the Stokes drift is carried in majority by waves which are not resolved by SWIM, it is shown that an algorithm based on the extrapolation of SWIM waves spectra could be thought of. Such potential combined with the characteristics of SWIM make the effort to design and qualify a Stokes drift product interesting. Such a product would be of particular interest in areas such as the Southern Ocean, where Stokes drift is significant and poorly known, and where the observational conditions are favorable to SWIM (resolved wind sea, favorable wave direction)

Trabajo presentado en la AGU Fall Meeting (2020), celebrada online del 1 al 17 de diciembre de 2020., Funded by the European Commission, the H2020 EuroSea project has the objective to improve the European ocean observing system as an integrated entity within a global context, delivering ocean observations and forecasts to advance scientific knowledge about ocean climate, marine ecosystems, and their vulnerability to human impacts and to demonstrate the importance of the ocean to an economically viable and healthy society. In the framework of this project, our goal is to improve the design of multi-platform in situ experiments for validation of high-resolution SWOT observations with the aim of optimizing the utility of these observing platforms. To achieve this goal, a set of Observing System Simulation Experiments (OSSEs) will be developed to evaluate different sampling strategies and their impact on the reconstruction of fine-scale sea surface height fields and currents. Observations from CTD, ADCP, gliders, and altimetry will be simulated from three nature run models to study the sensitivity of the results to the model used. Different sampling strategies will be evaluated to analyze the impact of the spatial and temporal resolution of the observations, the depth of the measurements, the season of the multi-platform experiment, and the impact of changing rosette CTD casts for a continuous underway CTD, and adding gliders. After generating the simulated observations in different scenarios, three methods of reconstruction will be tested: multivariate reconstruction analysis, machine-learning techniques, and modelling data assimilation. To assess the best sampling strategies to validate SWOT observations during the fast-sampling phase, the reconstructed fields will be compared to (i) the ocean “truth” from the nature run models, (ii) simulated SWOT observations, and (iii) simulated observations of drifters, Argo buoys and moorings. The regions of study are the western Mediterranean Sea and the northwestern Atlantic Ocean.

The MESCLA project, funded in the framework of the MyOcean 20009 R&D Call, is focused on the estimation and analysis of the vertical exchanges associated with mesoscale dynamics and of their interannual variability. The rationale for this project comes, on one hand, from the fundamental role played by mesoscale in modulating ocean circulation and the fluxes of heat, freshwater and biogeochemical tracers between the surface and the deeper layers, and, on the other hand, from the high level of uncertainty related to the possible feedback of mesoscale variability on both Earth climate and marine ecosystem functioning. MESCLA presents several original and innovative aspects, related to the proposed methodologies to analyze the physical data, i.e. (1) applying quasi-geostrophic diagnostics to different MyOcean products, both model and observation based, (2) comparing its results with corresponding primitive equation solutions, and (3) testing alternative multivariate techniques to interpolate and combine satellite and in situ observations. A description of the MESCLA approach, activities and results achieved during the first year will be presented.

Vertical motion associated with mesoscale and sub-mesoscale oceanic features is of fundamental importance for the exchanges of heat, fresh water and biogeochemical tracers between the surface and the ocean interior. Unfortunately, direct measurements of the vertical velocity are difficult to obtain for usual values (order 10’s m/day). Various indirect methodologies have thus been proposed to estimate vertical velocity from observed density and geostrophic velocity fields. The most used technique is based on the solution of the quasi-geostrophic (QG) Omega equation.

The Copernicus Marine Environment Monitoring Service (CMEMS) Ocean State Report (OSR) provides an annual report of the state of the global ocean and European regional seas for policy and decision-makers with the additional aim of increasing general public awareness about the status of, and changes in, the marine environment. The CMEMS OSR draws on expert analysis and provides a 3-D view (through reanalysis systems), a view from above (through remote-sensing data) and a direct view of the interior (through in situ measurements) of the global ocean and the European regional seas. The report is based on the unique CMEMS monitoring capabilities of the blue (hydrography, currents), white (sea ice) and green (e.g. Chlorophyll) marine environment. This first issue of the CMEMS OSR provides guidance on Essential Variables, large-scale changes and specific events related to the physical ocean state over the period 1993–2015. Principal findings of this first CMEMS OSR show a significant increase in global and regional sea levels, thermosteric expansion, ocean heat content, sea surface temperature and Antarctic sea ice extent and conversely a decrease in Arctic sea ice extent during the 1993–2015 period. During the year 2015 exceptionally strong large-scale changes were monitored such as, for example, a strong El Niño Southern Oscillation, a high frequency of extreme storms and sea level events in specific regions in addition to areas of high sea level and harmful algae blooms. At the same time, some areas in the Arctic Ocean experienced exceptionally low sea ice extent and temperatures below average were observed in the North Atlantic Ocean.