Anisotropy and Inhomogeneity in Drifter Dispersion

Huntley, Helga S.; Lipphardt, B. L.; Kirwan, A. D.

doi:10.1029/2019jc015179

Cited by 5 publications

(4 citation statements)

References 43 publications

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“…Equally important for the transport of surface trapped material is the distinction between horizontally divergence‐free quasi‐geostrophic turbulence and

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Rossby number submesoscale turbulence where surface convergence rates stronger than Coriolis, f , exist locally. Advection by submesoscale surface turbulence is highly anisotropic (Huntley et al, 2019), acting not only to spread, but also to trap tracers in surface convergence zones (D'Asaro et al, 2018).…”

Section: Introductionmentioning

confidence: 99%

Submesoscale Kinematic Properties in Summer and Winter Surface Flows in the Northern Gulf of Mexico

et al. 2020

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Statistical properties of near-surface horizontal velocity gradients are obtained from four drifter experiments conducted in the Gulf of Mexico during Summer 2012 and Winter 2016. The data density provided by the near-simultaneous deployments of 90-326 surface drifters in each allows direct, drifter-based estimates of the scale dependence of velocity gradients at separation scales ranging from 200 m to 5 km. The robustness of these estimates, derived from uniquley sampled, nearly equilateral triplets, is confirmed by comparisons with estimates produced from larger drifter clusters, and with estimates based on concurrent Eulerian X-band radar observations. The winter launches were deployed above a ∼80 m deep mixed layer, one in a region with nearly homogeneous horizontal density structure, the other in a region of strong surface density gradients associated with filaments of fresh Mississippi River water. The summer launches occurred in a shallow (10m) mixed layer, one launched across a mesoscale frontal jet separating regions of horizontally homogeneous density and the other, similar to the corresponding winter launch, also in a region filamented by shallow lenses of cold, fresh water. Seasonal differences are observed, with larger velocity fluctuations and greater variance in divergence and vorticity, especially at the smallest scales, in winter. Differences between same-season launches are, however, as large as seasonal differences. In both seasons, observations sampling regions directly impacted by fresh water fluxes show strongly skewed vorticity distributions, with cyclonic vorticity dominating strain. For the other launches, one in each season, strain dominated minimally skewed vorticity. Plain Language Summary Variations in the near-surface currents act to redistribute floating material in the ocean. While much is known about variations at large scales, 50-100 km, the statistics of surface currents at smaller scales, 100m-10km, are not nearly as well documented. Here observations provided by hundreds of satellite-tracked drifting instruments (drifters) are used to measure such statistics in four distinct experiments, two each in summer and winter. The large number of drifters in each case allows us to determine, with a new level of statistical certainty, how surface current properties vary on scales ranging from 200 m to 5 km and provides benchmark observations of seasonal and spatial variability. Supporting observations of the upper-ocean density structure allows investigation of this impact on small-scale currents. Higher variability occurs in the more energetic winter flows, as expected. However, differences between two launches within a single season are equally large. Similarities between two flows, occurring in different seasons, can be traced to the presence of fresh, Mississippi River water in each. The drifter estimates were found to be consistent with measurements derived from a completely independent set of observations (surface radar images) indicating the potential for measuring near-surface c...

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“…Equally important for the transport of surface trapped material is the distinction between horizontally divergence‐free quasi‐geostrophic turbulence and

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Section: Introductionmentioning

confidence: 99%

Submesoscale Kinematic Properties in Summer and Winter Surface Flows in the Northern Gulf of Mexico

et al. 2020

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“…These processes are generated in presence of surface fronts and filaments and represent the transition zone between mesoscale and three-dimensional turbulence (McWilliams, 2016;Klein et al, 2019). They play an important role in the ocean energetics, since they provide a route to dissipation for mesoscale flows, and strongly influence horizontal dispersion and vertical transport in the upper ocean (Mahadevan and Tandon, 2006;Poje et al, 2014;Huntley et al, 2019) with potential significant impact on the distribution of pollutants and biogeochemical quantities (Lévy et al, 2001;Lévy, 2003).…”

Section: Introductionmentioning

confidence: 99%

Submesoscale Vorticity and Divergence in the Alboran Sea: Scale and Depth Dependence

et al. 2021

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The statistics of submesoscale divergence and vorticity (kinematic properties, KPs) in the Alboran Sea (Mediterranean Sea) are investigated, using data from drifters released during two experiments in June 2018 and April 2019 in the framework of the Coherent Lagrangian Pathways from the Surface Ocean to Interior (CALYPSO) project. Surface drifters sampling the first meter of water (CARTHE and CODE) and 15 m drifters (SVP) are considered. The area of interest is dominated by processes of strong frontogenesis and eddy formation as well as mixing, related to the high lateral gradients between Mediterranean and Atlantic waters. Drifter coverage and distribution allow to investigate the dependence of KPs on horizontal scales in a range between 1 and 16 km, that effectively bridges submesoscale and mesoscale processes, and at two depths, of 1 and 15 m. For both experiments, the surface flow is highly ageostrophic at 1 km scale, with positive vorticity skewness indicating the presence of submesoscale features. Surface divergence quickly decreases at increasing scales with a slope compatible with a turbulent process with broadband wavenumber spectrum, suggesting the influence of surface boundary layer processes such as wind effects, waves and Langmuir cells at the smaller scales. Vorticity, on the other hand, has a significantly slower decay, suggesting interaction between submesoscale and mesoscale dynamics. Results at 15 m are characterized by reduced ageostrophic dynamics with respect to the surface, especially for divergence. Submesoscale processes are present but appear attenuated in terms of KP magnitude and skewness. The results are generally consistent for the two experiments, despite the observed differences in the mixed layer stratification, suggesting that submesoscale instabilities occur mostly at surface fronts associated with filaments of Atlantic and Mediterranean waters that are present in both cases. The results are compared with previous literature results in other parts of the world ocean and a synthesis is provided. Good agreement with previous surface results is found, suggesting some general properties for divergence and vorticity scale dependence. The importance of further investigating very high resolution frontal processes at scales of tens of meters, as well as processes of interaction with high wind effects is highlighted.

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“…In presence of daily fluctuations in vertical mixing due to varying winds or to diurnal heating and cooling, these processes can lead to significant fluctuations in divergence and w (Qu et al, 2022;Sun et al, 2020), as described by the so-called Transient Turbulent Thermal Wind balance (TTTW, Dauhajre and McWilliams, 2018). Also submesoscale instability processes at fronts can provide a mechanism for the occurrence of high divergence and w. Submesoscale instabilities occur in flows with high Rossby and Richardson numbers of 𝐴𝐴 (1) , with typical horizontal space scales 1-10 km and time scales of 1 day (McWilliams, 2016), and can be seen as an intermediate regime between the mesoscale and three-dimensional turbulence (Klein et al, 2019;McWilliams, 2016) playing a significant role in the horizontal and vertical transport in the upper ocean (Huntley et al, 2019;Mahadevan & Tandon, 2006;Poje et al, 2014). Areas with high horizontal gradients of buoyancy are especially conducive to submesoscale instabilities, leading to ageostrophic secondary circulations and overturning cells.…”

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confidence: 99%

Inertial Oscillations and Frontal Processes in an Alboran Sea Jet: Effects on Divergence and Vertical Transport

et al. 2023

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Understanding vertical transport between the ocean surface and the interior is still an open and challenging question, despite its importance for several ecological and climatic issues such as exchange of heat, biogeochemical tracers and pollutants (Lévy, 2003;Lévy et al., 2001). Vertical velocity (w) is typically several order of magnitude smaller than horizontal velocity in the ocean, reaching significant values up to 𝐴𝐴 (1 cm∕s) only during sporadic events occurring at short time and space scales (typically of the order of a day or hours and less than 10 km [Capó et al., 2021]). From an observational point of view, identifying and measuring these processes is therefore quite complex. Frontal regions are especially prone to the occurrence of large w (D'Asaro et al., 2018;Tarry et al., 2021), even though the transport is often oscillatory and confined to near surface layers. Irreversible subduction, which implies crossing the base of the mixed layer along outcropping isopycnals is a rare event

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Anisotropy and Inhomogeneity in Drifter Dispersion

Cited by 5 publications

References 43 publications

Submesoscale Kinematic Properties in Summer and Winter Surface Flows in the Northern Gulf of Mexico

Submesoscale Kinematic Properties in Summer and Winter Surface Flows in the Northern Gulf of Mexico

Submesoscale Vorticity and Divergence in the Alboran Sea: Scale and Depth Dependence

Inertial Oscillations and Frontal Processes in an Alboran Sea Jet: Effects on Divergence and Vertical Transport

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