Momentum, mass, heat, and vorticity balances from oceanic measurements of current and temperature

Bryden, Harry L.

doi:10.1575/1912/1266

Cited by 5 publications

(3 citation statements)

References 22 publications

(34 reference statements)

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“…The mean currents (the vectors in Figure 1) at sites 1 and 3 in 3000 m water depth are veered roughly 45 ø to the right of the mean surface path, whereas at site 2 in 3700 m water depth the mean current is approximately colinear with the surface mean path. This veering to the right of the surface current may be associated with an inflow from the slope water, or it may be a manifestation of the vertical veering of current associated with advection of heat [Bryden, 1975]. For the observed weekly-to-monthly time scales of motion (shown below) one may assume that measurements separated by 1 month are statistically independent, and therefore a rough estimate of the number of degrees of freedom (M) associated with each estimate of the mean current can be obtained by dividing the respective record length by 1 month.…”

Section: Resultssupporting

confidence: 79%

Gulf Stream meanders: Observations on the deep currents

Johns¹,

Watts²

1985

J. Geophys. Res.

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During 1979–1980, an array of inverted echo sounders (IES) and three deep current meter moorings were deployed on the continental slope 100–200 km northeast of Cape Hatteras, North Carolina. This array continuously monitored the path of the Gulf Stream and the deep currents under it. The mean currents at two sites 1000 m off the bottom near the northern edge of the stream were veered to the right of the mean surface path, indicating a deep inflow to the stream. Mean currents 500 and 1000 m off the bottom 50 km farther offshore were northeastward, nearly colinear with the surface Gulf Stream path. The deep velocity fluctuations are characterized by a transition from transverse flow aligned with the local bathymetry for periods longer than about 12 days to fluctuations with a cross‐stream orientation for shorter periods. For periods between 4 days and 1 month, cross‐stream movements of the Gulf Stream temperature front are vertically coherent and nearly barotropic, based on correlations between the IES‐measured stream path and deep temperature fluctuations. Temperature fluctuations at the current meter sites lead cross‐stream (positive onshore) velocity fluctuations by approximately 90°. Consideration of the nondiffusive fluctuating heat equation for deep layers suggests a three‐term balance between local rate of change, cross‐stream horizontal advection, and vertical advection of temperature, with the first two being of like sign. Kinematically, this requires |wTz| > |υTy|, so that parcel trajectories in the cross‐stream plane are inclined at angles steeper than the mean cross‐stream slope of the isotherms. Eddy energy conversion between the fluctuations and the mean field in deep layers is predominantly baroclinic, with e‐folding growth time scales of approximately 10 days.

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Section: Resultssupporting

confidence: 79%

Gulf Stream meanders: Observations on the deep currents

Johns¹,

Watts²

1985

J. Geophys. Res.

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“…This formulation incorporates all the essential physical and environmental factors mentioned in the previous section and, fortunately, accords well with observations made in the mid-ocean (Gould, Schmitz, and Wunsch, 1974;Bryden, 1975).…”

supporting

confidence: 79%

“…To lowest order the fields are geostrophic and hydrostatic, an approximation consistent with the results of the MODE-I field experiment (Bryden, 1975 …”

supporting

confidence: 65%

The sensitivity and predictability of mesoscale eddies in an idealized model ocean

Haidvogel¹

1976

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Two numerical applications of two-level quasigeostrophic theory are used to investigate the interrelationships of the mean and mesoscale eddy fields in a closed-basin ocean model. The resulting techniques provide a more accurate description of the local dynamics, origins, and parametric dependences of the eddies than that available in previous modelling studies.First, we propose a novel and highly efficient quasigeostrophic closed-domain model which has among its advantages a heightened resolution in the boundary layer regions. The pseudospectral method, employing an orthogonal expansion in Fourier and Chebyshev functions, relies upon a discrete Green's function technique capable of satisfying to spectral accuracy rather arbitrary boundary conditions on the eastern and western (continental) walls. Using this formulation, a series of four primary numerical experiments tests the sensitivity of wind-driven single and double-gyred eddying circulations to a transition from free-slip to no-slip boundary conditions. These comparisons indicate that, in the absence of topography, no-slip boundaries act primarily to diffuse vorticity more efficiently. The interior transport fields are thus reduced by as much as 50%, but left qualitatively unchanged. In effect, once having separated from the western wall, the internal jet has no knowledge, apart from its characteristic flow speed, of the details of the boundary layer structure.Next, we develop a linearized stability theory to analyze the local dynamic processes responsible for the eddy fields observed in these idealized models. Given two-dimensional (x, z) velocity profiles of arbitrary horizontal orientation, the resulting eigenfunction problems are solved to predict a variety of eddy properties: growth rate, length and time scales, spatial distribution, and energy fluxes. This simple methodology accurately reproduces many of the eddy statistics of the fully nonlinear fields; for instance, growth rates of 10-100 days predicted for the growing waves by the stability analysis are consistent with observed model behavior and have been confirmed independently by a perturbation growth test. Local energetic considerations indicate that the eddy motions arise in distinct and recognizable regions of barotropic and baroclinic activity. The baroclinic instabilities de end sensitively on the vertical shear which must exceed 0(5 cm sec~ ) across the thermocline to induce eddy growth.As little as a 10% reduction in luz1, however, severely suppresses the cascade of mean potential energy to the eddy field. In comparison, the barotropic energy conversion process scales with the horizontal velocity shear, Ju |, whose threshold values for instability, 0(2 x 10-6 sec-1), is undoubtedly geophysically realizable. A simple scatter diagram of Iu I versus |u I for all the unstable modes studied shows a clear separation between Ehe regions of barotropic and baroclinic instability. While the existence of baroclinic modes can be deduced from either time mean or instantaneous flow profiles, b...

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A study of MODE dynamics

Leetmaa

1977

Deep Sea Research

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Momentum, mass, heat, and vorticity balances from oceanic measurements of current and temperature

Cited by 5 publications

References 22 publications

Gulf Stream meanders: Observations on the deep currents

Gulf Stream meanders: Observations on the deep currents

The sensitivity and predictability of mesoscale eddies in an idealized model ocean

A study of MODE dynamics

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