The influence of geothermal sources on deep ocean temperature, salinity, and flow fields

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution May 1988This thesis is a study of the effect of geothermal sources on the deep circulation, temperature and salinity fields. In Chapter 1 background material is given on the strength and distribution of geothermal heating. In Chapter 2 evidence for the influence of a hydrothermal system in the rift valley of the Mid-Atlantic Ridge on nearby property fields and a model of the flow around such a heat source are presented, with an analysis of a larger-scale effect. Results of an analytical model for a heat source on a β-plane in Chapter 3 show how the response far from the source can have a structure different from the forcing because of its dependence on two parameters: a Peclet number (the ratio of horizontal advection and vertical diffusion), and a Froude-number-like parameter (the ratio of long wave phase speed to background flow speed) which control the relative amount of damping and advection of different vertical scales. The solutions emphasize the different behavior of a dynamical field like temperature compared to tracers introduced at the source. These ideas are useful for interpreting more complicated solutions from a numerical model presented in the final chapter.This study was supported by the National Science Foundation grants OCE8515642 to T. Joyce and OCE82-13967 to B. Warren, and by the WHOI-MIT Joint Program Ocean Ventures Fund

Topography, jets, and eddy mixing in the Southern Ocean

The relation between topography, and the 3D structure of patchy eddy-induced mixing in the Southern Ocean is analyzed descriptively by applying diagnostic methods to output from the Southern Ocean State Estimate. A localized cumulative probability density function method is developed to verify the use of Nakamura\u27s (2001) mixing efficiency in the ocean. Both methods reveal enhanced eddy mixing at mid-depths. The spatial pattern of the mid-depth enhancement of eddy mixing is primarily linked to the merging of multiple jets embedded in the Antarctic Circumpolar Current over topography. We suggest that enhanced eddy mixing over topography is due to locally enhanced baroclinicity and instability within the wake itself, partly due to the convergence of eddies in merging branches of the ACC. Interaction of barotropic and baroclinic eddies may be important to setting the strength and vertical structure of eddy mixing associated with topography

Deep Circulation in the Eastern South Pacific Ocean

We use an inverse method to compute the deep circulation in the eastern South Pacific Ocean, on isopycnal layers within the depth range of 1,000–3,000 m. The method gives probability density functions of the isopycnal and diapycnal velocities and diffusivities on a grid with a realistic eastern boundary, based on tracer fields. An oxygen consumption rate, assumed spatially constant, controls the solution effectiveness (minimum value of the model cost function) and the magnitude of the diffusivities. The isopycnal circulation is, by contrast, less dependent on the consumption rate. Direct deep float displacement data is used as a check on our solutions rather than a constraint. The model circulation agrees broadly with this independent data. Where the computed circulation does not agree, we argue that it is mainly because of different sampling, model resolution and model assumptions. The isopycnal circulation below 1,000 dbar is dominated by zonal flows and topographically controlled structures. A westward helium plume at 15°S seen on δ3He maps is reproduced by the model but is less intense than expected by the β-plume theory. The model did not find correlations between the diapycnal forcing and the known volcanically active part of the EPR, suggesting that the net large-scale effect of heating along the ridge on the circulation is small over the basin as a whole. Eastern boundary flow along the coast of South America appears as a main exit route for deep water, but the flow shows broader scales than suggested in previous studies, possibly due to the resolution of the model. The deep outflow of the southeastern Pacific is associated in the model with isopycnal PV stretching, rather than a Stommel-Arons (diapycnal) balance

Multiple Zonal Jets in a Differentially Heated Rotating Annulus

A laboratory experiment of multiple baroclinic zonal jets is described, thought to be dynamically similar to flow observed in the Antarctic Circumpolar Current. Differential heating sets the overall temperature difference and drives unstable baroclinic flow, but the circulation is free to determine its own structure and local stratification; experiments were run to a stationary state and extend the dynamical regime of previous experiments. Atopographic analog to the planetary β effect is imposed by the gradient of fluid depth with radius supplied by a sloping bottom and a parabolic free surface. New regimes of a low thermal Rossby number (RoT ~ 10-3) and high Taylor number (Ta ~ 1011) are explored such that the deformation radius Lp is much smaller than the annulus gap width L and similar to the Rhines length. Multiple jets emerge in rough proportion to the smallness of the Rhines scale, relatively insensitive to the Taylor number; a regime diagram taking the β effect into account better reflects the emergence of the jets. Eddy momentum fluxes are consistent with an active role in maintaining the jets, and jet development appears to follow the Vallis and Maltrud phenomenology of anisotropic wave-turbulence interaction on a ß plane. Intermittency and episodes of coherent meridional jet migration occur, especially during spinup

Deep circulation in the Lau Basin and Havre Trough of the western South Pacific Ocean from floats and hydrography

A system of meridional ridges in the western South Pacific Ocean frame the Lau Basin and Havre Trough, and form a barrier to direct communication between the far western South Pacific basins and the interior South Pacific Ocean. The eastern side of this system comprises the Tonga and Kermadec Ridges, the location of the main deep western boundary current entering the Pacific Ocean. Observations from floats released in the Lau Basin as part of the RIDGE2000 program suggested the presence of a western boundary current along the Lau Ridge exiting into the North Fiji Basin. Those observations, together with Argo sub-surface float data and repeat hydrographic sections, confirm and expand the boundary current observations along the Lau Ridge throughout the Lau Basin and into the Havre Trough, along the Colville Ridge. The observations also reveal two previously unrecognized westward flowing jets bisecting the Lau Basin and Havre Trough. Using an extension to the classic Stommel-Arons abyssal circulation model, the predicted strength and location of these boundary currents and their bifurcation is compared with the float observations. The model provides a simplified view of the dynamics controlling the boundary current structure in the deep basins. A comparison of transport within the western boundary current derived from float data, hydrographic sections, and the idealized analytical model indicates that roughly 4 Sv (below 1,000 db) is transported northward through the Lau Basin, exiting into the North Fiji Basin

The influence of geothermal sources on deep ocean temperature, salinity, and flow fields

Thesis (Ph. D.)--Joint Program in Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 1988.Includes bibliographical references (p. 142-146).This thesis is a study of the effect of geothermal sources on the deep circulation, temperature and salinity fields. In Chapter 1 background material is given on the strength and distribution of geothermal heating. In Chapter 2 evidence for the influence of a hydrothermal system in the rift valley of the Mid-Atlantic Ridge on nearby property fields and a model of the flow around such a heat source are presented, with an analysis of a larger-scale effect. Results of an analytical model for a heat source on a #-plane in Chapter 3 show how the response far from the source can have a structure different from the forcing because of its dependence on two parameters: a Peclet number (the ratio of horizontal advection and vertical diffusion), and a Froude-number-like parameter (the ratio of long wave phase speed to background flow speed) which control the relative amount of damping and advection of different vertical scales. The solutions emphasize the different behavior of a dynamical field like temperature compared to tracers introduced at the source. These ideas are useful for interpreting more complicated solutions from a numerical model presented in the final chapter.by Kevin G. Speer.Ph.D

Property distributions and circulation in the Angola Basin

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1986.Microfiche copy available in Archives and ScienceVita.Bibliography: leaves 87-93.by Kevin G. Speer.M.S

Conversion among North Atlantic surface water types

Climatological data sets are used to estimate the amount of water changing temperature and salinity at the sea-surface owing to air-sea exchanges. Considering this conversion as a function of sea-surface temperature and salinity leads to the definition of a conversion vector, which represents mass sources and sinks for each water type. The vector representation shows graphically the way the shape of the temperature-salinity relation is driven at the surface by the air-sea heat and fresh water fluxes