Coherent Lagrangian swirls among submesoscale motions

Abstract

The emergence of coherent Lagrangian swirls (CLSs) among submesoscale motions in the ocean is illustrated. This is done by applying recent nonlinear dynamics tools for Lagrangian coherence detection on a surface flow realization produced by a dataassimilative submesoscale-permitting ocean general circulation model simulation of the Gulf of Mexico. Both mesoscale and submesoscale CLSs are extracted. These extractions prove the relevance of coherent Lagrangian eddies detected in satellitealtimetry-based geostrophic flow data for the arguably more realistic ageostrophic multiscale flow.Lagrangian coherence | mesoscale eddies | submesoscale eddies | satellite altimetry | high-resolution ocean models A dvances in nonlinear dynamical systems theory over the past few years (1-5) have enabled the discovery of unexpectedly resilient mesoscale material eddies in ocean velocity fields derived from satellite-sensed sea-surface height (SSH) fields (6). Ranging between 50 km and 250 km in diameter, these supercoherent Lagrangian eddies show no breakup or disintegration over a period of several months, in some cases up to 2 y (3, 7-10). Detected in a fashion fully independent from the observer, such Lagrangian eddies are bounded by uniformly stretching fluid (i.e., material) loops that stationarize the material-line-averaged tangential strain functional (3, 4). As a result, these material boundaries or elliptic Lagrangian coherent structures (11) will nearly exactly reassume their initial arc length at the end of the coherence assessment interval. Coupled with area preservation in the incompressible case, the preservation of boundary length renders these special Lagrangian eddies coherent to a previously undocumented and unexpected degree.The presence of one such supercoherent Lagrangian eddy in the Gulf of Mexico has recently been confirmed by independent measurements of satellite-derived chlorophyll and satellitetracked drifter paths (12). On the other hand, while satellitebased measurements are widely used for monitoring mesoscale variability in the ocean (13), they are incapable of resolving submesoscales, ranging from 100 m to a few tens of kilometers (14). There is, therefore, reason to believe (15, 16) that unresolved submesoscale motions, if sufficiently energetic, will erode the boundaries of supercoherent Lagrangian eddies inferred from satellite data. For this reason, the ubiquitous presence of submesoscale turbulence in the ocean (17, 18) has casted some doubt on the practical relevance of supercoherent mesoscale eddies inferred from satellite altimetry.The goal of this paper is to demonstrate that, even in the presence of active submesoscale motions, mesoscale swirling structures will exist, even though their boundaries will show increased filamentation relative to the supercoherent boundaries inferred from satellite altimetry. Such material interfaces will constrain mixing in the presence of multiscale ocean motions, ranging from largely geostrophic (balanced, equilibrated) mesoscale motions to likely ageostrophi...

Modulation of Lateral Transport by Submesoscale Flows and Inertia‐Gravity Waves

“…We extract the velocity field from such a model, apply different levels of filtering, and calculate a range of Lagrangian mixing and transport diagnostics. This study can thus Journal of Advances in Modeling Earth Systems 10.1029/2018MS001508 be seen as an update to an extension of prior work by Beron-Vera (2010), Rypina et al (2012), Haza et al (2016), and Beron-Vera, Hadjighasem, et al (2018).…”

“…The authors speculated that this failure could be an artifact of the data assimilation process, related to more general challenges of ocean data assimilation in the submesoscale regime (Sandery & Sakov, 2017). Studying the same eddy, however, Beron-Vera, Hadjighasem, et al (2018) reached a slightly different conclusion: While the geodesic eddy method was too strict to find any Lagrangian coherent structure, the LAVD method was more successful. With a high enough choice of CD, mesoscale eddies were shown to persist, even though their boundaries exhibited increased filamentation relative to the extremely coherent boundaries inferred from satellite altimetry.…”

Observation and quantification of inertial effects on the drift of floating objects at the ocean surface

“…Examples of relevant aspects include clustering at the center of the subtropical gyres 18,19 , phenomenon supported on measurements of plastic debris concentration 7 and the analysis of undrogued drifter trajectories 18,19 , or the role of mesoscale eddies as attractors or repellers of inertial particles depending on the polarity of the eddies and the buoyancy of the particles 19,27,28 despite the Lagrangian resilience of their boundaries [48][49][50][51] , which is also backed on observations 52 . The cited phenomena, which act on quite different timescales, all require both O(1) and O(τ ) terms in (12) for their description 18,19,27,28 consistent with the slow manifold M τ in (14), rather than the critical M 0 , controlling the time-asymptotic dynamics of the τ → 0 limit of the Maxey-Riley set (5).…”

Building a Maxey–Riley framework for surface ocean inertial particle dynamics

“…also Gatignol 9 ), yet with a form of the added mass term different than (14), which corresponds to the correction due to Auton, Hunt, and Prud'homme 10 . A Maxey-Riley model with lift force, which has not been so far considered in ocean dynamics despite its relevance in the presence of unbalanced (submesoscale) motions (e.g., Beron-Vera et al 42 ), can be found in Montabone 43 , Chapter 4 (cf. similar forms in Henderson, Gwynllyw, and Barenghi 44 , Sapsis et al 45 ).…”