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Carl Wunsch

Abstract

The time- and space-scale descriptive power of two-dimensional Fourier analysis is exploited to reanalyze the behavior of midlatitude variability as seen in altimetric data. These data are used to construct a purely empirical and analytical frequency–zonal wavenumber spectrum of ocean variability for periods between about 20 days and 15 yr and on spatial scales of about 200–10 000 km. The spectrum is dominated by motions along a “nondispersive” line, which is a robust feature of the data but for whose prominence a complete theoretical explanation is not available. The estimated spectrum also contains significant energy at all frequencies and wavenumbers in this range, including eastward-propagating motions, which are likely some combination of nonlinear spectral cascades, wave propagation, and wind-forced motions. The spectrum can be used to calculate statistical expectations of spatial average sea level and transport variations. However, because the statistics of trend determination in quantities such as sea level and volume transports depend directly upon the spectral limit of the frequency approaching zero, the appropriate significance calculations remain beyond reach, because low-frequency variability is indistinguishable from trends already present in the data.

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Carl Wunsch

Abstract

Small surface displacements appearing in tide gauge and altimetric records are used to detect hydrostatic baroclinic modes in the ocean. Those deflections are a small fraction of the interior isopycnal vertical displacements and are dependent directly upon the in situ stratification. Conversion of surface height to interior amplitudes and energies encounters significant spatial and seasonal shifts that need to be accounted for in quantitative use. This technical article analyzes the global-scale spatial variations in the relationship between surface deflections and interior motions. Similar considerations make it possible to use altimetric data to estimate the deep interior temperature variability as a function of position, calculations having a strong influence on abyssal trend determination in the presence of eddies.

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Carl Wunsch

Abstract

Atmospheric meridional heat transport is inferred as a residual from the Earth Radiation Budget Experiment (ERBE) data and in situ oceanic estimates. Reversing the conventional approach of computing the ocean as an atmospheric model residual is done to permit calculation of a preliminary uncertainty estimate for the atmospheric flux. The structure of the ERBE errors is itself an important uncertainty. Total energy transport is almost indistinguishable from a hemispherically antisymmetric analytic function, despite the great asymmetry of the oceanic heat fluxes. ERBE data appear sufficiently noisy so that a considerable range of atmospheric transports remains possible: the maximum atmospheric value lies between 3 and 5 PW in the Northern Hemisphere, at one standard deviation, although the values are sensitive to the noise assumptions made here. The Northern Hemisphere ocean and atmosphere carry comparable poleward heat fluxes to about 28°N where the oceanic flux drops rapidly, but does not actually vanish until the oceanic surface area goes to zero. Within the estimated error bars, there is a remarkable antisymmetry about the equator of the combined ocean and atmospheric transports, despite the marked oceanic transport asymmetry.

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Carl Wunsch

Abstract

Determining flow fields and mixing rates from chemical tracer distributions is a challenging and important oceanographic problem. Thus the conclusion, that solutions obtained for underdetermined systems were “devoid of physical content”, drawn by Fiadeiro and Veronis after attempting to “invert” a simple tracer distribution in a known advective-diffusive field, is particularly disturbing. The problem they formulated is reexamined here. The procedures used differ from theirs in making use of the full machinery of inverse methods; even in the grossly underdetermined case, it is possible to (i) obtain useful information about the underlying flow field, (ii) to deduce the structure of the parts that are not determinable, (iii) to find formal error bars arising both from data noise and from indeterminate components, (iv) to make use of a priori statistical information and a posteriori tests, and, in general, (v) to extract much useful information about the field.

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Carl Wunsch

Abstract

To produce an interpretation of the surface kinetic energy as measured by altimeters, a survey is made of the vertical structure of kinetic energy profiles in a large number of globally distributed long current meter records. Although the data are geographically confined primarily to a latitude band in the North Pacific, to the North Atlantic, and to a few moorings in the South Atlantic, the results show, generally speaking, that most regions are dominated by the barotropic and first baroclinic modes. Because of the near-surface intensification of baroclinic modes altimeters primarily reflect the first baroclinic mode, and thus the motion of the main thermocline. There is good quantitative agreement, with a few exceptions, with estimates of the surface kinetic energy obtained from the TOPEX/POSEIDON altimeter and from vertical extrapolations to the surface of the mooring profiles. These results are consistent with previous suggestions that barotropic models have little skill in depicting variability as seen in the altimeter data. An EOF analysis is shown to produce fictitious mode coupling unless the dynamical modes have very different energy levels.

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Carl Wunsch

Abstract

Difficulties remain with theoretical explanations of the apparent reduced zonal sea surface temperature gradient in the tropical Pacific of the Pliocene. One favored hypothesis is that it was a “permanent El Niño” state, with the warm phase of ENSO remaining fixed over millions of years. Here, an alternative is suggested—that there was a “perpetually running ENSO” with a shorter return time than is observed today, and that the apparently reduced zonal gradient is an alias–rectification of a high-frequency signal governed by the growth patterns of the foraminifera used to provide proxy temperatures. The hypothesis is probably testable in the modern ocean with comparatively modest measurements of foraminifera behavior in time.

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Carl Wunsch

Abstract

An infinitely deep stratified ocean on a equatorial beta plane is forced with a periodic wind system. The resulting linearized motion is shown to result in a deep cellular flow structure in rough agreement with recent observations. Because of the infinite depth, the vertical structure is dependent only on the horizontal structure and frequency of the wind-forced layer. The motion is a mechanism for carrying momentum downward from the surface. A western boundary is easily accommodated.

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Carl Wunsch

Abstract

The modal decomposition, proportional amplitudes, and lateral spatial scale of baroclinic motions in the North Atlantic Ocean are described as determined from more than 20 years of moored instrument data. The subtropical and subpolar gyres emerge naturally as regions of distinct energy levels, in the ratio of potential to kinetic energy, and in horizontal wavelength. Data in other ocean basins prove too sparse to use.

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Carl Wunsch

Abstract

Internal wave records from a variety of deep water locations in the North Atlantic have been reduced by common analysis methods in a search for systematic deviations from a universal spectral model. Such inhomogeneities are probably necessary conditions for sources and sinks of the motion. For a number of reasons, only records at 2000 m and below were used. Real variations in energy level of up to an order of magnitude were found, but the only clear inhomogeneities are associated with bottom topography, especially in those records obtained near Muir seamount. The effects, if any, on the internal wave field by the large velocity and shear of the Gulf Stream region are weak. Apparent topographic sources are inconspicuous at very short distances, suggesting a very rapid, nonlinear recovery of the spectrum to an equilibrium form.

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Carl Wunsch

Abstract

A new estimate is made using altimeter data of the rate at which the wind works on the oceanic general circulation. The value of about 1 TW is lower than previously estimated and is dominated by the work done by the mean zonal wind in the Southern Ocean. The meridional component of the mean wind contributes primarily in the eastern upwelling regions of the ocean. Fluctuating component contributions are small. A comparison with the results of a numerical model produces both the same total work as well as the same general geographical patterns but with detailed differences. Both observations and model show that the subtropical gyres are regions where the atmosphere is braking the ocean circulation. The input of wind energy is shown to be qualitatively consistent with estimates of the rates of decay of barotropic and baroclinic mesoscale variability. If most of the energy input into the Southern Ocean is dissipated there, this region could be a dominant factor in mixing the global ocean.

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