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N. P. Fofonoff

Abstract

Because of the strong variation with temperature of the thermal expansion coefficient of seawater, both horizontal and vertical mixing that perturb the gradients produce changes of volume, usually a decrease, that shift mass relative to the earth's gravitational field resulting in significant changes of gravitational potential energy (GPE). For sufficiently large temperature gradients, these changes may serve as energy sources to supplement the tidal, internal wave, and other processes that mix the ocean waters and limit the gradient magnitudes. The hypothesis examined here is that for overall stable ocean stratifications of temperature and salinity, the GPE conversions must be positive with respect to local perturbations by diffusion, mixing, or other disturbances. Thus, for the long-term steady state of the oceans to exist, the GPE structure must be stable, including the nonlinear effects.

A dynamical description of the conversion process for GPE changes to kinetic energy for mixing has not yet been developed. The evidence for the significance of the process is based on observational data. Examples are given for several oceans to show the limiting effects of the nonlinear Equation of State properties on the main thermoclines.

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N. A. Bray
and
N. P. Fofonoff

Abstract

Available, potential energy (APE) is defined as the difference between total potential plus internal energy of a fluid in a gravity field and a corresponding reference field in which the fluid is redistributed (leveled) adiabatically to have constant stably-stratified densities along geopotential surfaces. Potential energy changes result from local shifts of fluid mass relative to geopotential surfaces that are accompanied by local changes of enthalpy and internal energy and global shifts of mass (because volumes of fluid elements are not conserved) that do not change enthalpy or internal energy. The potential energy changes are examined separately by computing available gravitational potential energy (GPE) per unit mass and total GPE (TGPE) per unit area.

A technique for estimating GPF, in the ocean is developed by introducing a reference density field (or an equivalent specific volume anomaly field) that is a function of pressure only and is connected to the observed field by adiabatic vertical displacements. The full empirical equation of state for seawater is used in the computational algorithm. The accuracy of the estimate is limited by the data and sampling and not by the algorithm itself, which can be made as precise as desired.

The reference density field defined locally for an ocean region allows redefinition of dynamic height ΔD (potential energy per unit mass) relative to the reference field. TGPE per unit area becomes simply the horizontal average of dynamic height integrated over depth in the region considered. The reference density surfaces provide a precise approximation to material surfaces for tracing conservative variables such as salinity and potential temperature and for estimating vortex stretching between surfaces.

The procedure is applied to the MODE density data collected in 1973. For each group of stations within five 2-week time windows (designated Groups A–E) the estimated GPE is compared with the net APE based on the Boussinesq approximation and to the low-frequency kinetic energy measured from moored buoys. Changes of potential energy of the reference field from one time window to the next are large compared with the GPE within each window, indicating the presence of scales larger than the station grid.

An analysis of errors has been made to show the sensitivity of the estimates to data accuracy and sampling frequency.

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N. P. Fofonoff
and
R. M. Hendry

Abstract

A cooperative moored array experiment to measure currents and temperatures in the vicinity of the Southeast Newfoundland Ridge was carried out over a 14-manth period starting September 1979 and ending December 1980. Measurements were obtained from an incoherent exploratory army of nine moorings instrumented at 500, 800, 1500 and 4000 m levels. The objectives were to explore the amplitude, spatial variability and time scales of the eddy field in the decay region of the Gulf Stream. An additional experiment and mooring to examine the influx of Norwegian Sea Overflow Water into the western North Atlantic Basin was imbedded in the array. Supplementary CTD and XBT profiles and a single current profile were taken during deployment and recovery cruises to aid in interpretation of the moored measurements and identification of water masses.

A description of the array experiment and preliminary results are reported. Mean velocities were weak compared with rms eddy velocities and were not determined with statistical significance at many locations. Maximum daily-averaged speeds ranged from 0.3 to 0.6 m s−1 at 500 m and 0.09 to 0.3 m s−1 at 4000 m. Eddy kinetic energies at 500 m ranged from 0.034 J kg−1 near the Ridge to 0.005 J kg−1 in the southeastern corner of the array. Deep energies were reduced by a factor of 4 to 12 with the 4000 m level showing bottom intensification at most locations. Topographic effects were noted in the orientation of eddy velocities and reduction of low-frequency components near the Ridge. The eddy field showed no clear-cut evidence of wavelike propagation or advection in streamfunction plots. An increase in time scales is evident north and east of the Ridge towards the Newfoundland Basin and the Mid-Atlantic Ridge, where most of the eddy kinetic energy at 500 m is at time scales of 150–200 days compared with 100 days southwest of the Ridge and about 50–60 days in the central POLYMODE array at 55°W.

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N. P. Fofonoff
and
M. M. Hall

Abstract

Mass, momentum and kinetic-energy fluxes in the Gulf Stream have been estimated from hydrographic data taken by Fuglister in the Gulf Stream ’60 project; the data cover the Stream as it flows eastward, from south of Georges Bank to the Grand Banks. The results are compared to a two-layer, constant potential-vorticity inertial-jet model and reasonable agreement is found. Error estimates based on the model and the data indicate errors of up to about 30% for mass and momentum and 50% for kinetic energy fluxes. All three fluxes exhibit considerable downstream divergence; the dynamical implications of these divergences for the region are assessed, and importance of nonlinear effects in the Stream is discussed. It is suggested that there may be a significant conversion of kinetic to potential energy and that this mechanism ought not be excluded a prior by examining primarily linear models of the Stream.

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