Copepod Aggregations: Influences of Physics and Collective Behavior

Dense copepod aggregations form in Massachusetts Bay and provide an important resource for right whales. We re-examine the processes which might account for the high concentrations, investigating both horizontally convergent flow, which can increase the density of depth-keeping organisms, and social behavior. We argue that the two act in concert: social behavior creates small dense patches (on the scale of a few sensing radii); physical stirring brings them together so that they merge into aggregations with larger scales; it also moves them into areas of physical convergence which retain the increasingly large patch. But the turbulence can also break this apart, suggesting that the overall high density in the convergence zone will not be uniform but will instead be composed of multiple transient patches (which are still much larger than the sensing scale)

Quantifying and Interpreting Striations in a Subtropical Gyre: A Spectral Perspective

The amplitude, origin, and direction of striations in the subtropical gyre are investigated using simulated and analytical multidimensional spectra. Striations, defined as banded structures in the low-frequency motions, account for a noticeable percentage of zonal velocity variability in the east North Pacific (ENP: 25°–42°N, 150°–130°W) and central North Pacific (CNP: 10°–22°N, 132°E–162°W) regions in an eddying global ocean model. Thus, they likely are nonnegligible in mixing and transport processes. Striations in the ENP region are nonzonal and are embedded in the nonzonal gyre flow, whereas striations in the CNP region are more zonal, as are the mean gyre flows. An idealized 1.5-layer model shows the gyre flow partially determines their directions, which qualitatively resemble those in the global eddying model. In the linear limit, structures are quasi-stationary (frequency ω → 0) linear Rossby waves and the gyre flow influences the direction by influencing the nature of the zero Rossby wave frequency curve. In the nonlinear regime, striations are consistent with the nondispersively propagating eddies, whose low-frequency component has banded structures. The gyre flow influences the striation direction by changing the eddy propagation direction. Their origin in the nonlinear regime is consistent with the existence of a nondispersive line in the frequency–wavenumber spectra. This study does not exclude other striation mechanisms from literature, considering that the interpretations here are based on an idealized model and only from a spectral perspective.United States. National Aeronautics and Space Administration (NNX09AI87G)United States. National Aeronautics and Space Administration (NNX08AR33G)United States. National Aeronautics and Space Administration (NNX11AQ12G

The role of mixed-layer instabilities in submesoscale turbulence

Upper-ocean turbulence at scales smaller than the mesoscale is believed to exchange surface and thermocline waters, which plays an important role in both physical and biogeochemical budgets. But what energizes this submesoscale turbulence remains a topic of debate. Two mechanisms have been proposed: mesoscale-driven surface frontogenesis and baroclinic mixed-layer instabilities. The goal here is to understand the differences between the dynamics of these two mechanisms, using a simple quasi-geostrophic model. The essence of mesoscale-driven surface frontogenesis is captured by the well-known surface quasi-geostrophic model, which describes the sharpening of surface buoyancy gradients and the subsequent breakup in secondary roll-up instabilities. We formulate a similarly archetypical Eady-like model of submesoscale turbulence induced by mixed-layer instabilities. The model captures the scale and structure of this baroclinic instability in the mixed layer. A wide range of scales are energized through a turbulent inverse cascade of kinetic energy that is fuelled by the submesoscale mixed-layer instability. Major differences to mesoscale-driven surface frontogenesis are that mixed-layer instabilities energize the entire depth of the mixed layer and produce larger vertical velocities. The distribution of energy across scales and in the vertical produced by our simple model of mixed-layer instabilities compares favourably to observations of energetic wintertime submesoscale flows, suggesting that it captures the leading-order balanced dynamics of these flows. The dynamics described here in an oceanographic context have potential applications to other geophysical fluids with layers of different stratifications

Transformation of an Agulhas eddy near the continental slope

The transformation of Agulhas eddies near the continental slope of southern Africa and their subsequent self-propagation are analyzed in both observational data and numerical simulations. Self-propagation results from a net dipole moment of a generalized heton structure consisting of a surface-intensified anticyclonic eddy and deep cyclonic pattern. Such Agulhas vortical structures can form near the retroflection region and further north along the western coast of southern Africa. We analyze nonlinear topographic wave generation, vortex deformations, and filament production as an important part in water mass exchange. Self-propagating structures provide a conduit for exchange between the deeNational Science Foundation (U.S.) (Grant OCE-0752346

Interdisciplinary study of warm core ring physics, chemistry, and biology

We are conducting an interdisciplinary study of the structure and dynamics of Gulf Stream \Warm Core Rings by a time series investigation of selected rings. This program consists of highly integrated components which include physical, chemical, and biological investigation and modeling studies. These components are designed to provide information on the structure of rings and exchange mechanisms at ring boundaries, on their marine chemistry, and on the environmental controls of biological activity of selected constituents associated with Warm Core Rings. This research is being conducted by approximately two dozen investigators from thirteen marine institutions. An interdisciplinary program of the scope proposed is required in order to understand the interdependence among biological, chemical, and physical processes in the ocean. This study of the structure and evolution of Warm Core Rings will enhance the understanding of fundamental oceanic processes and the role of rings in the region where they occur

The Contribution of Striations to the Eddy Energy Budget and Mixing: Diagnostic Frameworks and Results in a Quasigeostrophic Barotropic System with Mean Flow

Low-frequency oceanic motions have banded structures termed “striations.” Since these striations embedded in large-scale gyre flows can have large amplitudes, the authors investigated the effect of mean flow on their directions as well as their contribution to energetics and mixing using a β-plane, barotropic, quasigeostrophic ocean model. In spite of the model simplicity, striations are always found to exist regardless of the imposed barotropic mean flow. However, their properties are sensitive to the mean flow. Rhines jets move with the mean flow and are not necessarily striations. If the meridional component of the mean flow is large, Rhines jets become high-frequency motions; low-frequency striations still exist, but they are nonzonal, have small magnitudes, and contribute little to energetics and mixing. Otherwise, striations are zonal, dominated by Rhines jets, and contribute significantly to energetics and mixing. This study extends the theory of β-plane, barotropic turbulence, driven by white noise forcing at small scales, to include the effect of a constant mean flow. Theories developed in this study, based upon the Galilean invariance property, illustrate that the barotropic mean flow has no effect on total mixing rates, but does affect the energy cascades in the frequency domain. Diagnostic frameworks developed here can be useful to quantify the striations’ contribution to energetics and mixing in the ocean and more realistic models. A novel diagnostic formula is applied to estimating eddy diffusivities.United States. National Aeronautics and Space Administration (Grant NNX09AI87G)United States. National Aeronautics and Space Administration (Grant NNX08AR33G)United States. National Aeronautics and Space Administration (Grant NNX11AQ12G)National Science Foundation (U.S.) (Grant OCE-1459702

Squeeze dispersion and the effective diapycnal diffusivity of oceanic tracers

We describe a process called “squeeze dispersion” in which the squeezing of oceanic tracer gradients by waves, eddies, and bathymetric flow modulates diapycnal diffusion by centimeter to meter-scale turbulence. Due to squeeze dispersion, the effective diapycnal diffusivity of oceanic tracers is different and typically greater than the average “local” diffusivity, especially when local diffusivity correlates with squeezing. We develop a theory to quantify the effects of squeeze dispersion on diapycnal oceanic transport, finding formulas that connect density-averaged tracer flux, locally measured diffusivity, large-scale oceanic strain, the thickness-weighted average buoyancy gradient, and the effective diffusivity of oceanic tracers. We use this effective diffusivity to interpret observations of abyssal flow through the Samoan Passage reported by Alford et al. (2013, https://doi.org/10.1002/grl.50684) and find that squeezing modulates diapycnal tracer dispersion by factors between 0.5 and 3.National Science Foundation (U.S.) (Grants OCE-1029268, OCE-1029483 and OCE-1657795