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  • Articles  (8)
  • American Meteorological Society  (7)
  • American Physical Society  (1)
  • 2015-2019  (8)
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  • Articles  (8)
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  • 1
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    Time-Dependent Eddy-Mean Energy Diagrams and Their Application to the Ocean (2016)
    American Meteorological Society
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    Publication Date: 2016-09-01
    Description: Insight into the global ocean energy cycle and its relationship to climate variability can be gained by examining the temporal variability of eddy–mean flow interactions. A time-dependent version of the Lorenz energy diagram is formulated and applied to energetic ocean regions from a global, eddying state estimate. The total energy in each snapshot is partitioned into three components: energy in the mean flow, energy in eddies, and energy temporal anomaly residual, whose time mean is zero. These three terms represent, respectively, correlations between mean quantities, correlations between eddy quantities, and eddy-mean correlations. Eddy–mean flow interactions involve energy exchange among these three components. The temporal coherence about energy exchange during eddy–mean flow interactions is assessed. In the Kuroshio and Gulf Stream Extension regions, a suppression relation is manifested by a reduction in the baroclinic energy pathway to the eddy kinetic energy (EKE) reservoir following a strengthening of the barotropic energy pathway to EKE; the baroclinic pathway strengthens when the barotropic pathway weakens. In the subtropical gyre and Southern Ocean, a delay in energy transfer between different reservoirs occurs during baroclinic instability. The delay mechanism is identified using a quasigeostrophic, two-layer model; part of the potential energy in large-scale eddies, gained from the mean flow, cascades to smaller scales through eddy stirring before converting to EKE. The delay time is related to this forward cascade and scales linearly with the eddy turnover time. The relation between temporal variations in wind power input and eddy–mean flow interactions is also assessed.
    Print ISSN: 0022-3670
    Electronic ISSN: 1520-0485
    Topics: Geosciences , Physics
    Published by American Meteorological Society
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  • 2
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    Weak Jets and Strong Cyclones: Shallow-Water Modeling of Giant Planet Polar Caps (2016)
    American Meteorological Society
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    Publication Date: 2016-04-01
    Description: Giant planet tropospheres lack a solid, frictional bottom boundary. The troposphere instead smoothly transitions to a denser fluid interior below. However, Saturn exhibits a hot, symmetric cyclone centered directly on each pole, bearing many similarities to terrestrial hurricanes. Transient cyclonic features are observed at Neptune’s South Pole as well. The wind-induced surface heat exchange mechanism for tropical cyclones on Earth requires energy flux from a surface, so another mechanism must be responsible for the polar accumulation of cyclonic vorticity on giant planets. Here it is argued that the vortical hot tower mechanism, claimed by Montgomery et al. and others to be essential for tropical cyclone formation, is the key ingredient responsible for Saturn’s polar vortices. A 2.5-layer polar shallow-water model, introduced by O’Neill et al., is employed and described in detail. The authors first explore freely evolving behavior and then forced-dissipative behavior. It is demonstrated that local, intense vertical mass fluxes, representing baroclinic moist convective thunderstorms, can become vertically aligned and accumulate cyclonic vorticity at the pole. A scaling is found for the energy density of the model as a function of control parameters. Here it is shown that, for a fixed planetary radius and deformation radius, total energy density is the primary predictor of whether a strong polar vortex forms. Further, multiple very weak jets are formed in simulations that are not conducive to polar cyclones.
    Print ISSN: 0022-4928
    Electronic ISSN: 1520-0469
    Topics: Geography , Geosciences , Physics
    Published by American Meteorological Society
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  • 3
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    Resonant activation of population extinctions (2017)
    American Physical Society
    Details
    Publication Date: 2017-10-30
    Print ISSN: 2470-0045
    Electronic ISSN: 2470-0053
    Topics: Physics
    Published by American Physical Society
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  • 4
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    Quantifying and Interpreting Striations in a Subtropical Gyre: A Spectral Perspective (2015)
    American Meteorological Society
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    Publication Date: 2015-02-01
    Description: 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.
    Print ISSN: 0022-3670
    Electronic ISSN: 1520-0485
    Topics: Geosciences , Physics
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  • 5
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    The Contribution of Striations to the Eddy Energy Budget and Mixing: Diagnostic Frameworks and Results in a Quasigeostrophic Barotropic System with Mean Flow (2015)
    American Meteorological Society
    Details
    Publication Date: 2015-08-01
    Description: 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.
    Print ISSN: 0022-3670
    Electronic ISSN: 1520-0485
    Topics: Geosciences , Physics
    Published by American Meteorological Society
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  • 6
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    A Multiwavenumber Theory for Eddy Diffusivities and Its Application to the Southeast Pacific (DIMES) Region (2015)
    American Meteorological Society
    Details
    Publication Date: 2015-07-01
    Description: A multiwavenumber theory is formulated to represent eddy diffusivities. It expands on earlier single-wavenumber theories and includes the wide range of wavenumbers encompassed in eddy motions. In the limiting case in which ocean eddies are only composed of a single wavenumber, the multiwavenumber theory is equivalent to the single-wavenumber theory and both show mixing suppression by the eddy propagation relative to the mean flow. The multiwavenumber theory was tested in a region of the Southern Ocean (70°–45°S, 110°–20°W) that covers the Drake Passage and includes the tracer/float release locations during the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES). Cross-stream eddy diffusivities and mixing lengths were estimated in this region from the single-wavenumber theory, from the multiwavenumber theory, and from floats deployed in a global ° Parallel Ocean Program (POP) simulation. Compared to the single-wavenumber theory, the horizontal structures of cross-stream mixing lengths from the multiwavenumber theory agree better with the simulated float-based estimates at almost all depth levels. The multiwavenumber theory better represents the vertical structure of cross-stream mixing lengths both inside and outside the Antarctica Circumpolar Current (ACC). Both the single-wavenumber and multiwavenumber theories represent the horizontal structures of cross-stream diffusivities, which resemble the eddy kinetic energy patterns.
    Print ISSN: 0022-3670
    Electronic ISSN: 1520-0485
    Topics: Geosciences , Physics
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  • 7
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    On the Steadiness and Instability of the Intermediate Western Boundary Current between 24° and 18°S (2019)
    American Meteorological Society
    Details
    Publication Date: 2019-12-01
    Description: The Intermediate Western Boundary Current (IWBC) transports Antarctic Intermediate Water across the Vitória–Trindade Ridge (VTR), a seamount chain at ~20°S off Brazil. Recent studies suggest that the IWBC develops a strong cyclonic recirculation in Tubarão Bight, upstream of the VTR, with weak time dependency. We herein use new quasi-synoptic observations, data from the Argo array, and a regional numerical model to describe the structure and variability of the IWBC and to investigate its dynamics. Both shipboard acoustic Doppler current profiler (ADCP) data and trajectories of Argo floats confirm the existence of the IWBC recirculation, which is also captured by our Regional Oceanic Modeling System (ROMS) simulation. An “intermediate-layer” quasigeostrophic (QG) model indicates that the ROMS time-mean flow is a good proxy for the IWBC steady state, as revealed by largely parallel isolines of streamfunction [Formula: see text] and potential vorticity [Formula: see text]; a [Formula: see text] scatter diagram also shows that the IWBC is potentially unstable. Further analysis of the ROMS simulation reveals that remotely generated, westward-propagating nonlinear eddies are the main source of variability in the region. These eddies enter the domain through the Tubarão Bight eastern edge and strongly interact with the IWBC. As they are advected downstream and negotiate the local topography, the eddies grow explosively through horizontal shear production.
    Print ISSN: 0022-3670
    Electronic ISSN: 1520-0485
    Topics: Geosciences , Physics
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  • 8
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    Role of Residual Overturning for the Sensitivity of Southern Ocean Isopycnal Slopes to Changes in Wind Forcing (2019)
    American Meteorological Society
    Details
    Publication Date: 2019-10-30
    Description: The Antarctic Circumpolar Current plays a central role in the ventilation of heat and carbon in the global ocean. In particular, the isopycnal slopes determine where each water mass outcrops and thus how the ocean interacts with the atmosphere. The region-integrated isopycnal slopes have been suggested to be eddy saturated, that is, stay relatively constant as the wind forcing changes, but whether or not the flow is saturated in realistic present day and future parameter regimes is unknown. This study analyzes an idealized two-layer quasigeostrophic channel model forced by a wind stress and a residual overturning generated by a mass flux across the interface between the two layers, with and without a blocking ridge. The sign and strength of the residual overturning set which way the isopycnal slopes change with the wind forcing, leading to an increase in slope with an increase in wind forcing for a positive overturning and a decrease in slope for a negative overturning, following the usual conventions; this behavior is caused by the dominant standing meander weakening as the wind stress weakens causing the isopycnal slopes to become more sensitive to changes in the wind stress and converge with the slopes of a flat-bottomed simulation. Eddy saturation only appears once the wind forcing passes a critical level. These results show that theories for saturation must have both topography and residual overturning in order to be complete and provide a framework for understanding how the isopycnal slopes in the Southern Ocean may change in response to future changes in wind forcing.
    Print ISSN: 0022-3670
    Electronic ISSN: 1520-0485
    Topics: Geosciences , Physics
    Published by American Meteorological Society
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