the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Regional mapping of energetic short mesoscale ocean dynamics from altimetry: performances from real observations.
Florian Le Guillou
Lucile Gaultier
Maxime Ballarotta
Sammy Metref
Clément Ubelmann
Emmanuel Cosme
Marie-Helène Rio
Abstract. For over 25 years, satellite altimetry has provided invaluable information about the ocean dynamics at many scales. In particular, gridded Sea Surface Height (SSH) maps allow to estimate the mesoscale geostrophic circulation in the ocean. However, conventional interpolation techniques rely on static optimal interpolation schemes, hence limiting the estimation of non linear dynamics at scales not well sampled by altimetry (i.e. below 150–200 km at mid latitudes). To overcome this limitation in the resolution of small-scale SSH structures (and thus small-scale geostrophic currents), a Back and Forth Nudging algorithm combined with a Quasi-Geostrophic model, a technique called BFN-QG, has been successfully applied on simulated SSH data in Observing System Simulation Experiments (OSSEs), showing a significant reduction in interpolation error and an improvement of space-time resolutions of the experimental gridded product compared to operational products. In this study, we propose to apply the BFN-QG to real altimetric SSH data in a highly turbulent region spanning a part of the Agulhas current. The performances are evaluated within Observing System Experiments (OSEs) that use independent data (such as independent SSH, Sea Surface Temperature and drifter data) as ground-truth. By comparing the mapping performances to the ones obtained by operational products, we show that the BFN-QG improves the mapping of short, energetic mesoscale structures and associated geostrophic currents both in space and time. In particular, the BFN-QG improves (i) the spatial effective resolution of the SSH maps by a factor of 20 %, (ii) the zonal and (especially) the meridional geostrophic currents and (iii) the prediction of Lagrangian transport for lead times up to 10 days. Unlike the results obtained in the OSSEs, the OSEs reveal more contrasting performances in low variability regions that are discussed in the paper.
Florian Le Guillou et al.
Status: open (extended)
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RC1: 'Comment on egusphere-2023-509', Anonymous Referee #1, 26 Apr 2023
reply
The study is novel and could be worth publication after minor-to-major revisions.
General comments:
The presentation of the major findings and results as associated with the key figures should be more adequately outlined and addressed. Now it often appears too brief as it rushes from one figure and relative short explanation and discussion to the next. This should be improved.
Does the choice of the 1.5 layer QG model impact the results in any way? In other words, are there other modes than the first baroclinic mode that might be influecing the dynamics and variability in the greater Agulhas Current region? Is it possible to find discussion of this in the literature? Need some more consideration.
Line 210 comments on the Agulhas Current area and claim that it is characterized by weak advection of the frontal structures. However, in view of the area (Figure 9) the presence of Natal pulses can propagate southwestward on the nearshore side of the Agulhas Current core while highly rich and complicated dynamics occur in the retroflection area where eddies are often shed and also sometimes reintegrated into the retroflection. These areas are characterized by advection and sometimes rapid occurrances and closing of frontal structures. So the statement must be refined to better balance these conditions.
In Figure 10: The black line in the 3 panels seems to be contours of the land-ocean boundary. Must be explained. A bit strange to see values within boxes that are completely inland.
In the discussion and conclusion it would be worthwhile to comment on possible use of SWOT and Sentinel-1 image data to further strengthen this study approach.
Specific editorial comments:
The submitted paper is attached with red-markings suggesting update of the text.
Additional edisotrial comments:
Agulhas Current should always be with captial C.
Be consistent in use of reference to figures (Fig., Figure, figure, I prefer Figure).
Equation 10: The expressions in the exponent should have larger font. Difficult to see properly.
The space scale symbol D (in equation 10) must for consistency also be used in Table 1 (nudging space scale ).
Florian Le Guillou et al.
Florian Le Guillou et al.
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