Mechanism of eddy separation from coastal currents

A series of multi-layer numerical experiments show that classical finite amplitude instabilities in boundary currents are not sufficient to account for the pinched-off eddies observed in the ocean and in laboratory experiments. These instabilities (barotropic or baroclinic) are shown to lead to an entrainment of offshore fluid into the boundary currents. Eddy separation, on the other hand, requires an additional process, such as a larger scale of motion containing a downstream velocity convergence of finite amplitude; this might be produced by long period fluctuations in the discharge from an upstream source region which controls the boundary current, or by topographic features. In our spatially idealized model, we numerically computed the temporal evolution of an assumed initial state consisting of a fast moving upstream region separated by a potential vorticity front from a slow moving downstream region. We verify long-wave theories which show that this initial state indeed leads to frontal steepening and to a blocking wave. This eventually produces large transverse velocities followed by complete detrainment of eddies without any entrainment into the residual boundary current

Vorticity dynamics near sharp topographic features

In ocean models, the interaction with boundaries is often parameterized as it involves small-scale processes that are usually hard to capture in a large-scale model. However, such interactions can play important roles in the model dynamics. For example, the choice of boundary conditions (free-slip vs. no-slip) has a direct impact on the vorticity (enstrophy) budget: with no-slip boundary conditions, vorticity is injected into the system, whereas with free-slip boundary conditions, there should be no vorticity injection as long as the coastline is smooth. However, we show here that at boundary singularities (e.g., corners), vorticity is injected into the domain even for free-slip boundary conditions. In this article, we use North Brazil Current rings to better understand the dynamics of eddy-topography interaction. This complex interaction is first analyzed in terms of a point vortex interacting with a wall. Within this simplified framework, we can describe the vorticity generation mechanism as a pseudoinviscid process. To quantify this vorticity injection, we first consider the inviscid limit for which we can derive an analytical formula. This theoretical prediction is then evaluated in conventional gridded ocean models. In such models, the representation of such a viscous boundary interaction may be affected by the grid representation and the discretization of the advection and viscous operators

Circulation and cross-shelf transport in the Florida Big Bend

The Florida Big Bend region in the northeastern Gulf of Mexico contains both spawning sites and nursery habitats for a variety of economically valuable marine species. One species, the gag grouper (Mycteroperca microlepis), relies on the shelf circulation to distribute larvae from shelf-break spawning grounds to coastal sea-grass nurseries each spring. Therefore, identifying the dominant circulation features and physical mechanisms that contribute to cross-shelf transport during the springtime is a necessary step in understanding the variation of the abundance of this reef fish. The mean circulation features and onshore transport pathways are investigated using a numerical ocean model with very high horizontal resolution (800–900 m) over the period 2004–2010. The model simulation demonstrates that the mean springtime shelf circulation patterns are set primarily by flow during periods of southeastward or northwestward wind stress, and that significant cross-shelf flow is generated during southeastward winds. Lagrangian particle tracking experiments demonstrate that a primary pathway exists south of Apalachicola Bay by which particles are able to reach inshore, and that significantly more particles arrive inshore when they originate from an area adjacent to a known gag spawning aggregation site. The results provide, for the first time, a description of the pathways by which onshore transport is possible from gag spawning sites at the shelf break to sea-grass nurseries at the coast in the Florida Big Bend

Lagrangian spin parameter and coherent structures from trajectories released in a high-resolution ocean model

A study of the mesoscale eddy field in the presence of coherent vortices, by means of Lagrangian trajectories released in a high-resolution ocean model, is presented in this paper. The investigation confirms previous results drawn from real float data statistics (Veneziani et al., 2004) that the eddy field characteristics are due to the superposition of two distinct regimes associated with strong coherent vortices and with a typically more quiescent background eddy flow. The former gives rise to looping trajectories characterized by subdiffusivity properties due to the trapping effect of the vortices, while the latter produces nonlooping floats characterized by simple diffusivity features. Moreover, the present work completes the study by Veneziani et al. (2004) in regard to the nature of the spin parameter Ω, which was used in the Lagrangian stochastic model that best described the observed eddy statistics.The main result is that the spin obtained from the looping trajectories not only represents a good estimate of the relative vorticity of the vortex core in which the loopers are embedded, but it is also able to follow the vortex temporal evolution. The Lagrangian parameter Ω is then directly connected to the underlying Eulerian structure and could be used as a proxy for the relative vorticity field of coherent vortices

Hydraulic adjustment to an obstacle in a rotating channel

Author Posting. © Cambridge University Press, 2000. This article is posted here by permission of Cambridge University Press for personal use, not for redistribution. The definitive version was published in Journal of Fluid Mechanics 404 (2000):117-149, doi:10.1017/S0022112099007065.In order to gain insight into the hydraulics of rotating-channel flow, a set of initial-value problems analogous to Long's towing experiments is considered. Specifically, we calculate the adjustment caused by the introduction of a stationary obstacle into a steady, single-layer flow in a rotating channel of infinite length. Using the semigeostrophic approximation and the assumption of uniform potential vorticity, we predict the critical obstacle height above which upstream influence occurs. This height is a function of the initial Froude number, the ratio of the channel width to an appropriately defined Rossby radius of deformation, and a third parameter governing how the initial volume flux in sidewall boundary layers is partitioned. (In all cases, the latter is held to a fixed value specifying zero flow in the right-hand (facing downstream) boundary layer.) The temporal development of the flow according to the full, two-dimensional shallow water equations is calculated numerically, revealing numerous interesting features such as upstream-propagating shocks and separated rarefying intrusions, downstream hydraulic jumps in both depth and stream width, flow separation, and two types of recirculations. The semigeostrophic prediction of the critical obstacle height proves accurate for relatively narrow channels and moderately accurate for wide channels. Significantly, we find that contact with the left-hand wall (facing downstream) is crucial to most of the interesting and important features. For example, no instances are found of hydraulic control of flow that is separated from the left-hand wall at the sill, despite the fact that such states have been predicted by previous semigeostrophic theories. The calculations result in a series of regime diagrams that should be very helpful for investigators who wish to gain insight into rotating, hydraulically driven flow.The authors have been supported by the National Science Foundation through Grants (OCE-9810599 for L.J.P. and K.R.H. and OCE-9711186 for EPC). L.J.P. also received support from the Office of Naval Research under Grant N00014-95-1-0456 and K.R.H. under grant N00014-93-1-0263

Three-Dimensional Model-Observation Comparison in the Loop Current Region

Accurate high-resolution ocean models are required for hurricane and oil spill pathway predictions, and to enhance the dynamical understanding of circulation dynamics. Output from the 1/25° data-assimilating Gulf of Mexico HYbrid Coordinate Ocean Model (HYCOM31.0) is compared to daily full water column observations from a moored array, with a focus on Loop Current path variability and upper-deep layer coupling during eddy separation. Array-mean correlation was 0.93 for sea surface height, and 0.93, 0.63, and 0.75 in the thermocline for temperature, zonal, and meridional velocity, respectively. Peaks in modeled eddy kinetic energy were consistent with observations during Loop Current eddy separation, but with modeled deep eddy kinetic energy at half the observed amplitude. Modeled and observed LC meander phase speeds agreed within 8% and 2% of each other within the 100-40 and 40-20 day bands, respectively. The model reproduced observed patterns indicative of baroclinic instability, that is, a vertical offset with deep stream function leading upper stream function in the along-stream direction. While modeled deep eddies differed slightly spatially and temporally, the joint development of an upper-ocean meander along the eastern side of the LC and the successive propagation of upper-deep cyclone/anticylone pairs that preceded separation were contained within the model solution. Overall, model-observation comparison indicated that HYCOM31.0 could provide insight into processes within the 100-20 day band, offering a larger spatial and temporal window than observational arrays

Variability of the Iceland‐Scotland overflow water transport through the Charlie‐Gibbs fracture zone : results from an eddying simulation and observations

Author Posting. © American Geophysical Union, 2018. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 123 (2018): 5808-5823, doi:10.1029/2018JC013895.Observations show that the westward transport of the Iceland‐Scotland overflow water (ISOW) through the Charlie‐Gibbs Fracture Zone (CGFZ) is highly variable. This study examines (a) where this variability comes from and (b) how it is related to the variability of ISOW transport at upstream locations in the Iceland Basin and other ISOW flow pathways. The analyses are based on a 35‐year 1/12° eddying Atlantic simulation that represents well the main features of the observed ISOW in the area of interest, in particular, the transport variability through the CGFZ. The results show that (a) the variability of the ISOW transport is closely correlated with that of the barotropic transports in the CGFZ associated with the meridional displacement of the North Atlantic Current front and is possibly induced by fluctuations of large‐scale zonal wind stress in the Western European Basin east of the CGFZ; (b) the variability of the ISOW transport is increased by a factor of 3 from the northern part of the Iceland Basin to the CGFZ region and transport time series at these two locations are not correlated, further suggesting that the variability at the CGFZ does not come from the upstream source; and (c) the variability of the ISOW transport at the CGFZ is strongly anticorrelated to that of the southward ISOW transport along the eastern flank of the Mid‐Atlantic Ridge, suggesting an out‐of‐phase covarying transport between these two ISOW pathways.Woods Hole Oceanographic Institution; National Oceanic and Atmospheric Administration Grant Number: NA15OAR4310088; U.S. National Science Foundation Grant Numbers: 1537136, OCE‐09266562019-02-2

Inertial gyre solutions from a primitive equation ocean model

A numerical exploration of inertial equilibrium states obtained with a primitive equation ocean model suggests that they can be described using statistical mechanics theory developed in the framework of quasi-geostrophy. The performance of the numerical model is first assessed with respect to the quasi-geostrophic model considering a series of experiments in the quasi-geostrophic range, in a closed basin with flat bottom and varying Rossby numbers. The results show that our model is consistent with the quasi-geostrophic model even in terms of dependence from boundary conditions and eddy viscosity values, and that the free surface contribution is negligible. As in the quasi-geostrophic experiments, a tendency toward Fofonoff flows is observed. This tendency remains in a second series of experiments performed outside the quasi-geostrophic range, namely with flows with higher Rossby numbers and with steep topography, characterized by sloping boundaries with an order one fractional change in the depth. It is only close to the boundaries that ageostrophic effects modify the flows. In conclusion, the fact that statistical mechanics theory, initially developed in the framework of quasi-geostrophy, holds for more realistic flows with steep topography supports development of subgrid scale parameterizations based on statistical mechanics theory, to be used in realistic general circulation models

Spreading of Denmark Strait overflow water in the western subpolar North Atlantic : insights from eddy-resolving simulations with a passive tracer

Author Posting. © American Meteorological Society, 2015. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 45 (2015): 2913–2932, doi:10.1175/JPO-D-14-0179.1.The oceanic deep circulation is shared between concentrated deep western boundary currents (DWBCs) and broader interior pathways, a process that is sensitive to seafloor topography. This study investigates the spreading and deepening of Denmark Strait overflow water (DSOW) in the western subpolar North Atlantic using two ° eddy-resolving Atlantic simulations, including a passive tracer injected into the DSOW. The deepest layers of DSOW transit from a narrow DWBC in the southern Irminger Sea into widespread westward flow across the central Labrador Sea, which remerges along the Labrador coast. This abyssal circulation, in contrast to the upper levels of overflow water that remain as a boundary current, blankets the deep Labrador Sea with DSOW. Farther downstream after being steered around the abrupt topography of Orphan Knoll, DSOW again leaves the boundary, forming cyclonic recirculation cells in the deep Newfoundland basin. The deep recirculation, mostly driven by the meandering pathway of the upper North Atlantic Current, leads to accumulation of tracer offshore of Orphan Knoll, precisely where a local maximum of chlorofluorocarbon (CFC) inventory is observed. At Flemish Cap, eddy fluxes carry ~20% of the tracer transport from the boundary current into the interior. Potential vorticity is conserved as the flow of DSOW broadens at the transition from steep to less steep continental rise into the Labrador Sea, while around the abrupt topography of Orphan Knoll, potential vorticity is not conserved and the DSOW deepens significantly.This work is supported by ONR Award N00014-09-1-0587, the NSF Physical Oceanography Program, and NASA Ocean Surface Topography Science Team Program.2016-06-0

2013 program of study : buoyancy-driven flows

The 2013 Geophysical Fluid Dynamics Summer Study Program theme was Buoyancy- Driven Flows. Professor Paul Linden of the University of Cambridge was the principal lecturer. He ably introduced the topic from simple beginnings to sophisticated models and observations, guiding the audience in the cottage and on the porch through fundamental theory and applications. A number of topics from the lectures resurfaced in the fellows' projects. The first ten chapters of this volume document these lectures, each prepared by pairs of the summer's GFD fellows. Following the principal lecture notes are the written reports of the fellows' own research projects.Funding was provided by the Office of Naval Research under Contract No. N00014-09-1-0844 and the National Science Foundation Grant No. OCE-082463