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J. R. Luyten
and
A. R. Robinson

Abstract

Simultaneous path and bottom velocity measurements made during the Transient Meander Experiment, reported in Part I, are analyzed in terms of a quasi-geostrophic thin jet model of the meandering Gulf Stream. The theory gives an explicit representation of the velocity field which may be used to decompose the observed velocities. This representation is shown to be consistent with the observations. The dynamics of this model provides an equation of the path of the Stream, a cross-sectional average of the vorticity equation. A linearized form of this equation is used to examine the relations between the space and time scales of the variability. The historical data on the space and time scales of the meandering are shown to be consistent with those implicit in the linearized form of the path equation. The contributions to the local vorticity balance are estimated from the observations reported in Part I. The data, although complicated by observational errors, suggest a balance between the local rate of change of vorticity and the advection of vorticity. The contributions from vortex stretching due to variable topography appear to be unimportant for the scales of the meandering. The local dynamics appears to be fully time-dependent.

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A. R. Robinson
and
D. B. Haidvogel

Abstract

The initial/boundary value problem for the barotropic version of a quasi-geostrophic open ocean model which requires normal flow everywhere on the boundary and vorticity on the inflow is studied. Parameter dependencies and sensitivities are determined for dynamical forecast experiments carried out over a 500 square kilometer domain with data simulated to represent the mid-ocean eddy field at 1500 m. The computational rms forecast error due to open boundary conditions is kept to 5% after one year of integration. Errors, gaps and noise are then introduced into the boundary and initial condition data. Objective analysis is introduced for mapping coarsely-distributed data onto the computational grid, and vorticity is derived from the streamfunction by several methods. Forecast error is sensitive to the frequency of updating of boundary data, but generally insensitive to vorticity errors. A simulated forecast experiment with composite error sources representative of feasible oceanic conditions is carried out for four months duration with rms error maintained to about 10%.

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A. R. Robinson
and
J. C. McWilliams

Abstract

Baroclinic instability is examined in a two-layer, quasi-geostrophic model for linearized mesoscale waves (i.e., with periods of a few months and length scales near the internal deformation radius). The mid-ocean wave environment includes the, β-effect, bottom topography and mean currents, all presumed to vary only on scales much greater than those of the wave. An optimization of the local rate of unstable growth shows the process to be potentially important for mesoscale generation: typically, a vertical velocity shear of 5 cm sec−1 permits an e-folding time of two months. The many processes included in the model allow a great variety of behavior; for example, although both β and topography are generally stabilizing by themselves, their combination can be destabilizing.

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Michael A. Spall
and
Allan R. Robinson

Abstract

Primitive equation and quasi-geostrophic eddy resolving, open ocean models are used for hindcast studies in the Gulf Stream meander and ring formation region. A feature model approach is used to initialize the models, based on one month of observations during November to December 1984. Flat bottom and topographic calculations are carried out using an initial Gulf Stream velocity profile based on the Pegasus dataset. All of the major events observed in the upper thermocline are reproduced by both numerical models. The addition of bottom topography is shown to significantly alter the character of the deep velocity fields. Large, basin scale circulations found near the bottom in both flat bottom calculations were replaced by energetic jets and eddies associated with the dominant spatial scales of the bottom topography. Use of the quasi-geostrophic model to dynamically adjust the initial conditions for the primitive equation model is shown to reduce the growth of large scale meanders on time scales of one month. A local primitive equation energy and vorticity analysis (PRE-EVA) routine is used to determine the dominant processes of simulated warm and cold ring formation events. The warm ring formation is achieved by differential horizontal advection of a developed meander system. The cold ring formation involves geostrophic and ageostrophic horizontal advection, vertical advection, and baroclinic conversion. Ageostrophic horizontal and vertical advections and stronger baroclinic conversion are believed to be responsible for the more realistic structure of the rings produced by the primitive equation model.

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D. E. Harrison
and
A. R. Robinson

Abstract

A simple linear model of the barotropic basin response to forcing imposed along the northern boundary is described. The dependence on latitude of the response may include oscillatory behavior or not, depending on whether the forcing frequency is smaller or greater than the fundamental free basin mode frequency. When oscillatory behavior is found, the forced solution may resemble oceanic mesoscale eddies. The relevance of this simple model to a description of the eddy fields of several mesoscale resolution general ocean circulation numerical experiments is examined. It is found that a single term of the analytical solution can very well describe the numerically produced eddy fields, away from the regions of strong currents. The possibility that this general mechanism might account for the existence of mesoscale eddies in the ocean is briefly discussed.

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A. R. Robinson
,
J. R. Luyten
, and
F. C. Fuglister

Abstract

The results from an observational experiment on the mesoscale space-time variability of the Gulf Stream are reported. Various techniques, including aerial surveys, ship trackings of the 15C isotherm at 200 m, drogues and moored current meters were used and are compared, to give estimates of the variability of the motion over a wide range of scales. A two-week time series of daily tracks of the Stream near 70W are used to interpolate instantaneous paths over 2° of longitude. These paths provide the first detailed information on the small-scale variability of the path indicator of the Gulf Stream northeast of Cape Hatteras. Similarly, the long time series of triweekly aerial surveys provides a detailed picture of the evolution of a large-scale meander.

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B. A. Taft
,
A. R. Robinson
, and
W. J. Schmitz Jr.

Abstract

No abstract available.

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Allan R. Robinson
,
Michael A. Spall
, and
Nadia Pinardi

Abstract

We present here a regional, eddy resolving, numerical study of the dynamics of Gulf Stream Meander and Ring (GSMR) interaction processes. We initialize the Harvard quasi-geostrophic open-boundary model with realistic meander and ring locations as indicated by remotely sensed sea surface temperature (SST) data and predict the flow evolution for the period 23 November to 19 December 1984. The methodology of Feature-Model initialization is introduced to extend the surface information to the thermocline and deep levels in terms of climatological structures, which are then dynamically adjusted by the model. Six numerical simulators are carried out to explore the influence of initial and boundary conditions on the flow evolution. All of the major events observed in the SST data are simulated, including the birth of new warm and cold core rings. The results show the relevance of quasi-geostrophic dynamics for the GSMR region on these time scales in the thermocline. A set of parameter and sensitivity experiments then elucidate the dependence on physical parameters; ring births are nonlinear baroclinic processes. The dynamics of these realistic cold and warm core formation events are quantified via local energy and vorticity budget analyses (EVA). The cold core case involves a process of nonlinear baroclinic cascades that convert available gravitational energy to kinetic energy and vice versa. The warm core case involves a differential horizontal advection process.

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A. R. Robinson
,
D. E. Harrison
,
Y. Mintz
, and
A. J. Semtner

Abstract

We present the results of a multi-level, constant depth, primitive equation general ocean numerical circulation simulation with mesoscale resolution. A single mid-latitude model gyre is driven by wind and heating. After 30 years of spin-up with a relatively coarse grid and large diffusion coefficients, the grid size and diffusion coefficients are reduced. The circulation then adjusts into a nonlinear and time-dependent flow with periods of tens of days and space scales of hundreds of kilometers. After a quasi-equilibrium state is achieved, two years of data are obtained which are separated into time-mean and time-dependent fluctuations, and analyzed. Dynamically distinct regions are intensified, momentum, heat and vorticity balances examined, and energy integrals calculated. Statistical measures of significance and of uncertainty are computed where possible. Eddy energy is produced primarily by Reynolds stress work (barotropic instability) on the mean circulation shear in the recirculation and near-field region of the northern current system. Mean fluctuation correlation terms are presented in some regions at order 1 in the mean heat and vorticity balance and can be the leading ageostrophic effect in the mean momentum balance. The flow is non-quasigeostrophic in some parts of the intense boundary currents.

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A. R. Robinson
,
N. E. Huang
,
C. D. Leitao
, and
C. G. Parra

Abstract

Altimeter data obtained from GEOS-3 during the three year period 1975–78 for a region of the western North Atlantic which includes a portion of the Gulf Stream system and part of the open ocean area of the subtropical gyre are analyzed by a new technique which utilizes all the points along the satellite tracks. The physical phenomenon studied are the time-variable but almost geostrophic currents, or mesoscale eddies, so that geoid errors contaminate the scientific signal minimally and the dynamical interpretation is direct. Results presented include the spatial distribution of geostrophic eddy kinetic energy and examples of a synoptic map of the eddy field (April 1977) and of a time series at a point. These results are compared to and synthesized with a diverse and selected set of existing measurements and observations obtained in situ by a variety of instrumental techniques. The agreement is generally good, and the altimeter data analyzed provides new information on features in the map of mean eddy kinetic energy. The implications are that satellite altimetry will serve as a powerful quantitative tool in eddy current research and that even presently archived data contains further useful scientific information.

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