Geostrophic Eddies Research Articles

Geostrophic response near the coast

Geostrophic response of a two-layer fluid near a straight coast is investigated for a successive disturbance by the use of the inviscid, reduced gravity model. Poincare waves, coastal motion (which is trapped by the coast) and a geostrophic eddy are created. The energy of these motions is obtained. The manner in which the ocean responds is found to depend considerably on the way the disturbance is applied.

A Study of the Variability of Ocean Currents in the Northwestern Atlantic Using Satellite Altimetry

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.

Zonal Wavenumber Characteristics of Northern Hemisphere Transient Eddies

Abstract Eleven winters and twelve summers of twice-daily National Meteorological Center (NMC) analyses of geopotential height and temperature at five levels for the extratropical Northern Hemisphere are partitioned into long-wave fields with zonal wavenumbers m ≤ 5 and short-wave fields with zonal wavenumbers m ≥ 6. The data are also bandpass-filtered to investigate fluctuations of periods 2.5–10 days. From these fields, the standard deviation of 500 mb geopotential height and the poleward geostrophic transient eddy fluxes of sensible heat and zonal momentum are calculated and presented. The results are compared with theoretical and numerical modeling studies of nonlinear, baroclinic instability. Both the observations and modeling studies produce evidence that baroclinic eddies have a definite life cycle and that the strongest baroclinic eddies are of intermediate size (zonal wavenumbers 6–8). Some interesting differences between observations and modeling results also emerge.

A Note on Rotational and Divergent Eddy Fluxes

Abstract If the deviation of mean flow from mean temperature contours is small, it is shown that a part of the eddy heat flux can be separated out which circulates around eddy potential energy contours, and has a component up/down the mean temperature gradient if there is flow advection of eddy potential energy into/out of the region. If the mean flow is strong, this rotational flux is large and results in regions of up- and downgradient flux. It is a prominent feature of maps of geostrophic eddy fluxes in the ocean and atmosphere.

A Statistical Description of Temperature Finestructure in the Presence of Internal Waves

We have developed a statistical model describing a random field of internal waves passively oscillating a random, locally horizontally uniform temperature finestructure. Here we define finestructure to be non-internal wavelike temperature fluctuations of any vertical scale caused by phenomena such as horizontal intrusions or geostrophic eddies. The model allows one to examine the effects of finestructure upon all relevant measurable statistical quantities of the temperature field as a function of the vertical scale of the finestructure. Also, the effects of the time variation of the finestructure itself are considered. The model was fit to data obtained during the 3-week Mid-ocean Acoustic Transmission Experiment (MATE) in summer 1977 new Cobb seamount in the northeastern Pacific. Various spectra and coherences estimated from temperature time series and vertical profiles were used to make an assessment of the finestructure as well as establish the consistency of the model. Temperature variance measured at frequencies above the local Väisäiä frequency was used to estimate the magnitude of the high vertical wavenumber finestructure. This high wavenumber finestructure (0.05–1 m−1) with a (wavenumber)−−2.5 dependence can consistently explain all the observed variance in the measured high vertical wavenumber spectra. At lower wavenumbers (0.002–0.020 m−1) a (wavenumber)−2 dependence was observed, and the spectral level found to be approximately equally divided between finestructure and internal waves advecting a constant temperature gradient. The contribution of the finestructure effects to the internal wave frequency spectrum was found to be about a factor of 10 less than that of internal waves advecting a constant temperature gradient.

Direct Atmospheric Forcing of Geostrophic Eddies

Abstract To assess the role of direct stochastic wind forcing in generating oceanic geostrophic eddies we calculate analytically the response of a simple ocean model to a realistic model wind-stress spectrum and compare the results with observations. The model is a continuously stratified, β-plane ocean of infinite horizontal extent and constant depth. All transfer and dissipation processes are parameterized by a linear scale-independent friction law (Rayleigh damping). The model predictions that are least sensitive to this parameterization, the total eddy energy and the subsurface displacement, are in good agreement with observations in mid-ocean regions far removed from strong currents. Properties that depend crucially on the parameterization of nonlinearities and topographic effects are not well reproduced. Observed coherences and seasonal modulations provide direct evidence of wind forcing at high frequencies where motions have little energy. Direct evidence at the more energetic low frequencies will ...

On the Parameterization of Geostrophic Eddies in the Ocean

An attempt is made to incorporate into a two-layer, zonally averaged, channel ocean model the important transfers achieved by a geostrophic eddy field, using gross parameterizations rather than resolving individual eddy events. It is shown that a representation of the eddy field as an explicit diffuser of potential vorticity can give a reasonable description of the interaction between the eddies and mean flow, provided care is taken to satisfy the attendant constraints that the zonally invariant channel geometry imposes on the eddy fields.

The kinetic energy spectrum vis-à-vis a statistical model for geopotential

The relationship between a statistical model for geopotential height and the geostrophic eddy kinetic energy spectrum is elaborated. Ensemble spatial variability of the isobaric field is represented with a continuous linear stochastic model in the natural coordinates of a rotating sphere. The corresponding bi-frequency spectral density for the height field is derived, and a linear filter technique is applied to obtain a description of its implications for the eddy kinetic energy spectrum. The direct correspondence between spatially autoregressive models and bi-frequency spectra, and the companion correspondence between these autoregressive models and lag-correlation functions are discussed with regard to their combined potential for elucidating properties of the frequency (wave number) power spectrum. The development produces an analytic tool for further investigation of the dependence of geostrophic kinetic energy on latitudinal and longitudinal wave numbers. It is argued that the somewhat simplistic nature of the approach is compensated by the sensitivity of the representations of distinct frequency components and the parsimony of statistical analyses based on the result. Possible implications for the retention of forecast-relevant energy distributions in objective analyses are considered. DOI: 10.1111/j.2153-3490.1981.tb01767.x

On the Generation of Geostrophic Eddies by Surface Buoyancy Flux Anomalies

Abstract Stochastic fluctuations in the buoyancy flux at the air-sea interface create density anomalies in the oceanic surface layer, which drive quasi-geostrophic motions in the ocean interior. The efficiency of this forcing mechanism is evaluated by comparison with wind-stress forcing. Stochastic buoyancy forcing is found to be always negligible in the wavenumber-frequency range of oceanic geostrophic eddies. The effect of mass exchange anomalies at the surface is also found to be negligible. The conclusions seem applicable to time scales up to centuries.

On the internal wave variability during the internal wave experiment (IWEX)

The relation between internal wave variability and larger and smaller scales of motion is investigated, using the IWEX data set. To investigate the role of internal waves in the vertical diffusion of large scale momentum, the time variability of the vertical flux of horizontal internal wave momentum (estimated from temperature and current data) is compared to that of the mean vertical shear. It is found that internal waves cannot cause a vertical viscosity as large as proposed by Müller (1976), but that the data are too noisy to detect a possible wave‐induced viscosity in absolute value of the order of 10−2 m2 s−1 or less. Similarities in the time behavior of the total internal wave energy and that of the square mean vertical shear suggest that some kind of dynamical coupling exists between internal waves and larger scale flows. There is some evidence that the level of temperature finestructure activity also varies in a related way. An analysis of CTD station data taken during Mode demonstrates the mappability of the finestructure activity, and again suggests a relation with the geostrophic eddy flow.

Determination of Eddy Fluxes of Heat and Eddy Temperature Variances Using Weakly Nonlinear Theory

Drazin's (1972) weakly nonlinear theory of the self interaction of a single, slightly unstable, normal mode in a viscous, baroclinic fluid with a continuous density distribution is used to determine eddy fluxes of heat and eddy temperature variances as a function of rotation and internal thermal gradients. The calculations are applied to annulus experiments in that portion of the regular wave regime in which the observed flow is dominated by a single mode. V. Barcilon's (1964) linear theory for a viscous fluid is used for the purpose of mode selection at each point in dimensionless-parameter space. The geostrophic eddy heat flux and temperature variance are then determined from Drazin's lowest order nonlinear theory (which is a direct extension of Barcilon's theory). It is found that the eddy beat flux and temperature variance depend upon internal thermal gradients at marginal stability and upon the distance of the point in dimensionless-parameter space from marginal stability for each mode. This result is different from those obtained from linear theory (e.g., by Green, 1970; Saltzman and Vernekar, 1971; Stone, 1972) but agrees in essence with that obtained by Hart (1974) from a nonlinear analysis of a two-layer model. Numerical calculations from the theoretically derived formulas show that the eddy beat flux and temperature variance corresponding to each mode increase with increasing rotation rate when the imposed temperature contrast is held constant, but that there is an abrupt drop in the magnitude of both quantities when the wavenumber changes to the next higher integral value. Experimental evidence obtained by Pfeffer et al. (1978) verifies that real fluids behave in this way. Another feature observed in laboratory experiments and predicted by the theory is that at fixed rotation rate (or Taylor number) the eddy heat flux and the eddy temperature variance may be either larger or smaller at larger values of the imposed temperature contrast, depending on the location of the experiments in dimensionless-parameter space. If we think of seasons being brought about by changes in imposed temperature contrast at constant rotation rate, this result implies that only in certain regions of dimensionless-parameter space is winter (defined as the season with the highest imposed temperature contrast) the season with the largest eddy heat flux or eddy temperature variance.

Das Spektrum ozeanischer Turbulenz für den Bereich von 40 bis 1000 km

Networks of closely spaced hydrographic stations are used to calculate the structure function for the depths of the 20° isotherm over distances between 40 and 1400 kilometers. In the central North Pacific Ocean the 20° isotherm is situated near the center of the thermocline and these spectra have a typical peak near 200 kilometers, which is related to large geostrophic eddies of corresponding dimensions. The remainder of each spectrum increases monotonically in the range 400 to 1400 kilometers. There are appreciable differences between the spectra obtained on different cruises. In contrast to the spectra of the thermocline, the spectra of the mixed-layer depth often remain flat between 400 and 1400 kilometers, indicating that all energy is contained in fluctuations of small scale. These spectra show the inherent irregularities in the thermal structure of the ocean and are valuable for assessing the accuracy of predictions of thermal structure. The computed spectra cover the gap between ordinary turbulence and the large-scale ocean circulation, and show that turbulent energy is continuously distributed over the entire range from 50 meters to 1400 kilometers.