Global observations of fine-scale ocean surface topography with the surface water and ocean topography (SWOT) mission

© The Author(s), 2019. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in [citation], doi:[doi]. Morrow, R., Fu, L., Ardhuin, F., Benkiran, M., Chapron, B., Cosme, E., d'Ovidio, F., Farrar, J. T., Gille, S. T., Lapeyre, G., Le Traon, P., Pascual, A., Ponte, A., Qiu, B., Rascle, N., Ubelmann, C., Wang, J., & Zaron, E. D. Global observations of fine-scale ocean surface topography with the surface water and ocean topography (SWOT) mission. Frontiers in Marine Science, 6(232),(2019), doi:10.3389/fmars.2019.00232.The future international Surface Water and Ocean Topography (SWOT) Mission, planned for launch in 2021, will make high-resolution 2D observations of sea-surface height using SAR radar interferometric techniques. SWOT will map the global and coastal oceans up to 77.6∘ latitude every 21 days over a swath of 120 km (20 km nadir gap). Today’s 2D mapped altimeter data can resolve ocean scales of 150 km wavelength whereas the SWOT measurement will extend our 2D observations down to 15–30 km, depending on sea state. SWOT will offer new opportunities to observe the oceanic dynamic processes at scales that are important in the generation and dissipation of kinetic energy in the ocean, and that facilitate the exchange of energy between the ocean interior and the upper layer. The active vertical exchanges linked to these scales have impacts on the local and global budgets of heat and carbon, and on nutrients for biogeochemical cycles. This review paper highlights the issues being addressed by the SWOT science community to understand SWOT’s very precise sea surface height (SSH)/surface pressure observations, and it explores how SWOT data will be combined with other satellite and in situ data and models to better understand the upper ocean 4D circulation (x, y, z, t) over the next decade. SWOT will provide unprecedented 2D ocean SSH observations down to 15–30 km in wavelength, which encompasses the scales of “balanced” geostrophic eddy motions, high-frequency internal tides and internal waves. This presents both a challenge in reconstructing the 4D upper ocean circulation, or in the assimilation of SSH in models, but also an opportunity to have global observations of the 2D structure of these phenomena, and to learn more about their interactions. At these small scales, ocean dynamics evolve rapidly, and combining SWOT 2D SSH data with other satellite or in situ data with different space-time coverage is also a challenge. SWOT’s new technology will be a forerunner for the future altimetric observing system, and so advancing on these issues today will pave the way for our future.The authors were mostly funded through the NASA Physical Oceanography Program and the CNES/TOSCA programs for the SWOT and OSTST Science teams. AnP acknowledges support from the Spanish Research Agency and the European Regional Development Fund (Award No. CTM2016-78607-P). AuP acknowledges support from the ANR EQUINOx (ANR-17-CE01-0006-01)

Global Observations of Fine-Scale Ocean Surface Topography With the Surface Water and Ocean Topography (SWOT) Mission

The future international Surface Water and Ocean Topography (SWOT) Mission, planned for launch in 2021, will make high-resolution 2D observations of sea-surface height using SAR radar interferometric techniques. SWOT will map the global and coastal oceans up to 77.6 latitude every 21 days over a swath of 120 km (20 km nadir gap). Today’s 2D mapped altimeter data can resolve ocean scales of 150 km wavelength whereas the SWOT measurement will extend our 2D observations down to 15–30 km, depending on sea state. SWOT will offer new opportunities to observe the oceanic dynamic processes at scales that are important in the generation and dissipation of kinetic energy in the ocean, and that facilitate the exchange of energy between the ocean interior and the upper layer. The active vertical exchanges linked to these scales have impacts on the local and global budgets of heat and carbon, and on nutrients for biogeochemical cycles. This review paper highlights the issues being addressed by the SWOT science community to understand SWOT’s very precise sea surface height (SSH)/surface pressure observations, and it explores how SWOT data will be combined with other satellite and in situ data and models to better understand the upper ocean 4D circulation (x, y, z, t) over the next decade. SWOT will provide unprecedented 2D ocean SSH observations down to 15–30 km in wavelength, which encompasses the scales of “balanced” geostrophic eddy motions, high-frequency internal tides and internal waves. Frontiers in Marine Science | www.frontiersin.org 1 May 2019 | Volume 6 | Article 232 Morrow et al. SWOT Fine-Scale Global Ocean Topography This presents both a challenge in reconstructing the 4D upper ocean circulation, or in the assimilation of SSH in models, but also an opportunity to have global observations of the 2D structure of these phenomena, and to learn more about their interactions. At these small scales, ocean dynamics evolve rapidly, and combining SWOT 2D SSH data with other satellite or in situ data with different space-time coverage is also a challenge. SWOT’s new technology will be a forerunner for the future altimetric observing system, and so advancing on these issues today will pave the way for our future

What Vertical Mode Does the Altimeter Reflect? On the Decomposition in Baroclinic Modes and on a Surface-Trapped Mode

International audienc

Topologie du mélange dans un fluide turbulent géophysique

Geophysical turbulent fluids are characterized by the presence of organized energetic structures which control tracer mixing and develop a tracer cascade down to small scales. We study the physical mechanisms of this cascade in two-dimensional turbulence, both theoretically and numerically.The properties of this turbulent tracer cascade are first reviewed and show that we need to understand the dynamics of tracer gradients as they are associated to this cascade. We show that this dynamics is mainly governed by the competition between strain and effective rotation (due to both vorticity and rotation of strain axes). Tracer gradients should align with directions that depend only on velocity and acceleration gradient tensors. This is confirmed in numerical simulations and this allows a better understanding of the cascade in physical space than previous studies.Then, we compare the cascades of passive scalar and vorticity as vorticity is an active one. We review geophysical observations and theoretical results concerning these cascades and we compare the dynamics of gradients of passive scalar and vorticity in numerical simulations. We observe that the strongest tracer gradients behave identically whereas the weakest ones behave differently.Finally, we establish a parallel between our results concerning the two-dimensional tracer gradients and the results of the literature concerning the vorticity vector in three-dimensional turbulence.Les fluides géophysiques se caractérisent par la présence de structures énergétiques organisées qui contrôlent le mélange de traceur et permettent une cascade de traceur vers les petites échelles. Nous étudions théoriquement et numériquement les mécanismes physiques de cette cascade en turbulence bidimensionnelle.Après avoir rappelé les propriétés connues de la cascade turbulente de traceur, nous étudions la dynamique des gradients de traceur. Nous montrons que cette dynamique est régie principalement par la compétition entre la déformation et la rotation effective (due à la vorticité et à la rotation des axes de déformation). Les gradients de traceur s'alignent alors avec des directions qui ne dépendent que des tenseurs de gradient de vitesse et d'accélération. Ceci est confirmé dans des simulations numériques et cette étude permet de mieux comprendre la cascade dans l'espace physique.Ensuite, nous comparons les cascades de scalaire passif et de vorticité afin d'examiner la nature active de la vorticité. Nous passons en revue les observations géophysiques et les résultats théoriques sur ces cascades; puis, nous comparons la dynamique des gradients de scalaire passif et de vorticité dans des simulations numériques. Nous observons que les forts gradients des deux traceurs ont des comportements identiques alors que les faibles gradients ont des comportements différents.Enfin, nous mettons en parallèle nos résultats sur les gradients bidimensionnels de traceur avec les résultats connus sur le vecteur vorticité en turbulence tridimensionnelle

Surface Quasi-Geostrophy

Oceanic and atmospheric dynamics are often interpreted through potential vorticity, as this quantity is conserved along the geostrophic flow. However, in addition to potential vorticity, surface buoyancy is a conserved quantity, and this also affects the dynamics. Buoyancy at the ocean surface or at the atmospheric tropopause plays the same role of an active tracer as potential vorticity does since the velocity field can be deduced from these quantities. The surface quasi-geostrophic model has been proposed to explain the dynamics associated with surface buoyancy conservation and seems appealing for both the ocean and the atmosphere. In this review, we present its main characteristics in terms of coherent structures, instabilities and turbulent cascades. Furthermore, this model is mathematically studied for the possible formation of singularities, as it presents some analogies with three-dimensional Euler equations. Finally, we discuss its relevance for the ocean and the atmosphere