Carl Wunsch

Abyssal recipes II: energetics of tidal and wind mixing

Without deep mixing, the ocean would turn, within a few thousand years, into a stagnant pool of cold salty water with equilibrium maintained locally by near-surface mixing and with very weak convectively driven surface-intensified circulation. (This result follows from Sandstrom’s theorem for a fluid heated and cooled at the surface.) In this context we revisit the 1966 “Abyssal Recipes”, which called for a diapycnal diffusivity of 10-4m2/s (1 cgs) to maintain the abyssal stratification against global upwelling associated with 25 Sverdrups of deep water formation. Subsequent microstructure measurements gave a pelagic diffusivity (away from topography) of 10-5 m2/s — a low value confirmed by dye release experiments. A new solution (without restriction to constant coefficients) leads to approximately the same values of global upwelling and diffusivity, but we reinterpret the computed diffusivity as a surrogate for a small number of concentrated sources of buoyancy flux (regions of intense mixing) from which the water masses (but not the turbulence) are exported into the ocean interior. Using the Levitus climatology we find that 2.1 TW (terawatts) are required to maintain the global abyssal density distribution against 30 Sverdrups of deep water formation. The winds and tides are the only possible source of mechanical energy to drive the interior mixing. Tidal dissipation is known from astronomy to equal 3.7 TW (2.50±0.05 TW from M2 alone), but nearly all of this has traditionally been allocated to dissipation in the turbulent bottom boundary layers of marginal seas. However, two recent TOPEX/POSEIDON altimetric estimates combined with dynamical models suggest that 0.6–0.9 TW may be available for abyssal mixing. A recent estimate of wind-driving suggests 1 TW of additional mixing power. All values are very uncertain. A surprising conclusion is that the equator-to-pole heat flux of 2000 TW associated with the meridional overturning circulation would not exist without the comparatively minute mechanical mixing sources. Coupled with the findings that mixing occurs at a few dominant sites, there is a host of questions concerning the maintenance of the present climate state, but also that of paleoclimates and their relation to detailed continental configurations, the history of the Earth–Moon system, and a possible great sensitivity to details of the wind system.

Improved estimates of global ocean circulation, heat transport and mixing from hydrographic data.

Through its ability to transport large amounts of heat, fresh water and nutrients, the ocean is an essential regulator of climate. The pathways and mechanisms of this transport and its stability are critical issues in understanding the present state of climate and the possibilities of future changes. Recently, global high-quality hydrographic data have been gathered in the World Ocean Circulation Experiment (WOCE), to obtain an accurate picture of the present circulation. Here we combine the new data from high-resolution trans-oceanic sections and current meters with climatological wind fields, biogeochemical balances and improved a priori error estimates in an inverse model, to improve estimates of the global circulation and heat fluxes. Our solution resolves globally vertical mixing across surfaces of equal density, with coefficients in the range (3–12) × 10-4 m 2 s-1. Net deep-water production rates amount to (15 ± 12) × 106 m3 s -1 in the North Atlantic Ocean and (21 ± 6) × 10 6 m3 s-1 in the Southern Ocean. Our estimates provide a new reference state for future climate studies with rigorous estimates of the uncertainties.

Ocean acoustic tomography: a scheme for large scale monitoring

Abstract We consider the problem of monitoring ocean basins for mesoscale fluctuations, using acoustic inverse techniques. The procedure, which has much in common with conventional seismology, consists of measuring perturbations in travel time between acoustic sources and receivers. Because the number of pieces of information is the product of the number of sources, receivers, and resolvable multipath arrivals, the economics of the system is enhanced over usual spot measurements. The temporal resolution required to distinguish multipath arrivals is estimated at 50 ms; the precision required to measure mesosclae perturbations is estimated at 25 ms. The required resolution and precision can be achieved by existing low-frequency (100- to 200-Hz) broadband (> 20-Hz) sources, but we are ultimately limited to 1000-km ranges by the variable ocean finestructure and associated micropaths. There appear to be no practical range limits imposed by micropaths if such broadband sources could be centered at 30 Hz. Given the travel time measurements and their noise estimates, we show how actually to invert the system for the interior changes in sound speed and, by inference, for density. The method is analogous to the medical procedure called tomography (from the Greek ‘slice’). Measures of the spatial resolution and of formal error bars are obtained. We conclude that such a system is achievable now and has potential for development in a number of directions.

Ocean Acoustic Tomography

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The Interpretation of Short Climate Records, with Comments on the North Atlantic and Southern Oscillations

Abstract This pedagogical note reminds the reader that the interpretation of climate records is dependent upon understanding the behavior of stochastic processes. In particular, before concluding that one is seeing evidence for trends, shifts in the mean, or changes in oscillation periods, one must rule out the purely random fluctuations expected from stationary time series. The example of the North Atlantic oscillation (NAO) is mainly used here: the spectral density is nearly white (frequency power law ≈ s-0.2) with slight broadband features near 8 and 2.5 yr. By generating synthetic but stationary time series, one can see exhibited many of the features sometimes exciting attention as being of causal climate significance. Such a display does not disprove the hypothesis of climate change, but it provides a simple null hypothesis for what is seen. In addition, it is shown that the linear predictive skill for the NAO index must be very slight (less than 3% of the variance). A brief comparison with the South...

Two transatlantic sections: meridional circulation and heat flux in the subtropical North Atlantic Ocean

Abstract Transatlantic hydrographic sections were obtained in mid-1981 along latitudes 24.5° and 36.25°N. The tracks nearly duplicated sections made 23 years earlier as part of the International Geophysical Year. A total of 215 stations were occupied: data from a conductivity-temperature-depth (CTD) probe and water samples for analyses of oxygen, nutrient, and other tracer concentrations were collected from the ocean surface to near bottom. The 1981 sections are described and displayed, and the circulation is compared to that of the earlier survey. Large-scale meridional velocity and basin-integrated transport are compared in the 1981 and IGY sections, using a hierarchy of geostrophic models. In the simplest model, a reference level is based on the gross thermohaline flow and consideration of water mass characteristics. The only transport constraint is that the geostrophic plus Ekman flows sum to zero. Subsequent models impose mass conservation in a set of layers and then conservation of potential vorticity in a single layer. Distributions of salinity and potential vorticity on density surfaces are examined in order to identify layers in which an advective balance of tracer distribution is plausible and where gradients are strong enough to be of practical use. It was found that the 1981 and IGY sections have similar features in their large-scale velocity fields and similar zonally averaged meridional transport. In each case, net northward transports of approximately 17 Sv of surface and intermediate water (above σ 2 = 36.82) were balanced by equal southward flow in the deep water. A significant shift toward greater depth occurred in the depth distribution of the deep southward flow in the 1981 sections. The wind-driven subtropical gyre is superimposed on this thermohaline overturning, and transport calculations in the gyre interior show the Sverdrup relation to be of questionable direct applicability. Ocean heat transport in 1981 was found to be about 1.2 × 10 15 W across 24°N and 0.8 × 10 15 W across 36°N. These values are indistinguishable from those obtained from the IGY data and from computations of air-sea heat exchange. The steadiness of the heat transport is attributed to the invariance of the zonally averaged meridional circulation.

Large-Scale Ocean Heat and Freshwater Transports during the World Ocean Circulation Experiment

Hydrographic sections obtained during the World Ocean Circulation Experiment are combined using a geostrophic inverse model to estimate the global-scale horizontal transports and transport divergences of heat and freshwater with self-consistent error bars. The overall results are compared to bulk formula‐derived climatologies and estimates derived from atmospheric reanalyses. At 7.58N in the Atlantic, a previous estimate of the heat transport is modified. A recent atmospheric residual estimate from NCEP and the Earth Radiation Budget Experiment (ERBE) products is consistent with the present results for the heat budget, except at high northern latitudes where it falls outside error estimates. The freshwater transport divergence from hydrography is statistically significant only when integrated over very large areas and difficult to test—as extant climatological estimates differ substantially from each other. Hydrographic estimates can be improved through repeated observations to reduce the temporal aliasing, and by combining more detailed regional estimates using more data types. To permit a formal comparison and assimilation in ocean general circulation models, atmospheric estimates urgently require convincing error estimates for both heat and freshwater transports.

Atmospheric loading and the oceanic “inverted barometer” effect

The response of the ocean to fluctuating atmospheric pressure loads is reviewed in theory and in observations. Major theoretical issues lie primarily with oceanic boundary reflectivity and with rates of dissipation, generally. Analytical solutions for a stratified ocean show that a static (“inverted barometer”) response is anticipated at all frequencies and wavenumbers not coincident with certain dispersion curves. Nonstatic behavior of two types is predicted: zero motion of the sea surface and a resonant response. Two types of resonance emerge. The first type corresponds to barotropic modes which are long gravity waves or Rossby waves at high and low frequencies, respectively. The second type excites internal modes, either gravity waves or Rossby waves depending on frequency, but modified from a conventional resonant response by the immediate juxtaposition in frequency/wavenumber space of the rigid-lid modes. The extent to which these actual resonances are generated is obscure owing to the same uncertainties about oceanic dissipation and scattering which affect all other forced oceanic motions, especially including the tides and other barotropic motions. Zero sea surface motion is predicted at frequencies and wavenumbers corresponding to “rigid-lid” modes. Observations support the wide applicability of the static response except in the tropics and in the western boundary current extension regions; there, the signal-to-noise ratios may be inadequate. The only other known clear nonstatic response occurs near 5 days in the Pacific and South Atlantic Oceans, indicative of a low-Q resonance in the former area and a forced nonresonant response in the latter, but there are remaining problems with these interpretations.

The work done by the wind on the oceanic general circulation

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.

Accuracy assessment of recent ocean tide models

Over 20 global ocean tide models have been developed since 1994, primarily as a consequence of analysis of the precise altimetric measurements from TOPEX/POSEIDON and as a result of parallel developments in numerical tidal modeling and data assimilation. This paper provides an accuracy assessment of 10 such tide models and discusses their benefits in many fields including geodesy, oceanography, and geophysics. A variety of tests indicate that all these tide models agree within 2-3 cm in the deep ocean, and they represent a significant improvement over the classical Schwiderski 1980 model by approximately 5 cm rms. As a result, two tide models were selected for the reprocessing of TOPEX/POSEIDON Geophysical Data Records in late 1995. Current ocean tide models allow an improved observation of deep ocean surface dynamic topography using satellite altimetry. Other significant contributions include theft applications in an improved orbit computation for TOPEX/POSEIDON and other geodetic satellites, to yield accurate predictions of Earth rotation excitations and improved estimates of ocean loading corrections for geodetic observatories, and to allow better separation of astronomical tides from phenomena with meteorological and geophysical origins. The largest differences between these tide models occur in shallow waters, indicating that the current models are still problematic in these areas. Future improvement of global tide models is anticipated with additional high-quality altimeter data and with advances in numerical techniques to assimilate data into high-resolution hydrodynamic models.