Study of the stability of a large realistic cyclonic eddy

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L’Hégaret

et al. 2020

Sci Rep

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Oceanic vortices are ubiquitous in the ocean. They dominate the sub-inertial energy spectrum, and their dynamics is key for the evolution of the water column properties. The merger of two like-signed coherent vortices, which ultimately results in the formation of a larger vortex, provides an efficient mechanism for the lateral mixing of water masses in the ocean. Understanding the conditions of such interaction in the ocean is thus essential. Here, we use a merger detection algorithm to draw a global picture of this process in the ocean. We show that vortex mergers are not isolated, contrary to the hypothesis made in most earlier studies. Paradoxically, the merging distance is well reproduced by isolated vortex merger numerical simulations, but it is imperative to consider both the β-effect and the presence of neighbouring eddies to fully understand the physics of oceanic vortex merger.Oceanic vortices, named eddies, impact biological activities 1 , tracer transport 2 , and properties of the water column 3 . It has become clear that the mesoscale (10-100 km) eddy field, is at least as energetic as the large scale circulation 2 , essential for the air-sea interactions 4 , and thus for the evolution of climate 5 . Eddy-eddy interactions therefore play a central role in the evolution of the ocean/atmosphere physics and biology. In particular, vortex merger -the physical process in which two like-signed coherent vortices collapse, ultimately resulting in a larger vortex-is of key importance for the distribution and the transfers of energy across scales in the ocean 6-11 . Since the 80's, an important effort has been undertaken to understand the physics of oceanic vortex merger, including in situ 12 , laboratory 13,14 , and numerical 15 observations, associated with intense theoretical debates [16][17][18] . Most fundamental studies addressed the 'isolated vortex merger' problem, omitting the influence of neighbouring eddies, large scale currents, or boundary, topographic, and planetary effects [19][20][21][22][23][24][25] . Efforts to include more physical effects in studies of vortex merger [26][27][28][29] were often impaired by the lack of general observations in the global ocean, or by the complexity of the resulting dynamical system. Despite recent trials 30 and pointwise observations 12,31-33 , a global description of vortex merger in the ocean is lacking.In this paper, we present an analysis of merging events in the global ocean, using a global 1/12° re-analysis dataset. This study was conducted over a five-year period, in five domains in the middle of each major oceanic basins -these domains a priori respect the hypothesis of isolated vortex merger studies: they are far away from the coastlines, topographic features, and strong currents, and are located at mid-latitudes (see Fig. 1). From 5,867 detected merging events, we infer the actual characteristics of mesoscale eddies that merge, and test the isolated vortex merger problem hypotheses. Further, we compare this analysis with the outcome of 1,600 idealized...

Section: Numerical Simulations Of Vortex Merger Numerical Setupmentioning

confidence: 99%

Oceanic vortex mergers are not isolated but influenced by the β-effect and surrounding eddies

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L’Hégaret

et al. 2020

Sci Rep

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“…To study the wave propagation through a realistic eddy, we use the outputs of the simulation performed by de Marez et al (2020). In this study, authors performed spindown idealized simulations, using the Coastal and Regional Ocean COmmunity model, CROCO (Shchepetkin and McWilliams, 2005), that solves the hydrostatic primitive equations (PE) for the velocity u = (u, v, w), temperature T , and salinity S, using a full equation of state for seawater (Shchepetkin and McWilliams, 2011).…”

Section: The Realistic Cyclonic Eddymentioning

confidence: 99%

“…The manuscript is organised as follows. In section 2, we introduced the eddy structure used in the study, based on the work of de Marez et al (2020), and the numerical model built from Ardhuin et al (2017) without source terms. In section 3, we present the results of the numerical experiments.…”

Section: Introductionmentioning

confidence: 99%

Variability of wind wave field by realistic mesoscale and submesoscale eddy field

Marechal

2021

Preprint

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Abstract. Recent altimeters and numerical studies have shown that wind waves interact strongly with small scale open ocean currents, and subsequently modify their amplitude, frequency, and direction. In the present paper we investigate the interactions of wind waves with a large realistic cyclonic eddy. This eddy is subject to instabilities leading to the generation of specific features both at mesoscale and submesoscale. We use the WAVEWATCH III framework to force wind waves in the eddy before and after instabilities occurred. Our findings show that the spatial variability of wave direction frequency and amplitude is very sensitive to the presence of underlying submesoscale structures resulting from the eddy destabilisation. As the surface current vorticity, the intrinsic frequency of incident waves is key in the wave response of the current modulation. Our findings also suggest that surface current gradients can be retrieved thanks to wave height gradients at scale where traditional altimeter measurements fail.

“…Submesoscale oceanic structures can be generated by the interaction of regional currents with the topography, or by the interaction of mesoscale structures, or by the instability of mesoscale currents. External forcings—for example, outflows, extreme wind events, and thermal vents—can also yield submesoscale structures (D'Asaro, 1988; de Marez et al, 2020; McWilliams, 2019). These submesoscale features, among which vortices and filaments (with a typical size on the order of 10 km), are more arduous to detect than mesoscale eddies, all the more so as they lie below the ocean surface.…”

Section: Introductionmentioning

confidence: 99%

Observations of a Deep Submesoscale Cyclonic Vortex in the Arabian Sea

Geophysical Research Letters

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Corréard³

et al. 2020

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Submesoscale coherent vortices (SCVs) are numerous in high‐resolution numerical simulations, but their observations are scarce. Among the few in situ available measurements of SCVs, a vast majority concern anticyclones. No cyclonic SCV with large dynamical Rossby number (|ζ/f| > 1) has ever been sampled. This suggested that such small cyclones may lack robustness. Here, we present in situ measurements of an intense cyclonic SCV in the Arabian Sea. This eddy lay at 600 m depth, with a Rossby number Ro=scriptOfalse(1false) and a dynamical Rossby number |ζ/f| > 1.5. This cyclone was most likely generated at the mouth of the Gulf of Aden. It trapped and advected Red Sea Water, from there on. This highlights the role of deep SCVs in the spreading of salty waters across the Arabian Sea.