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Anders Stigebrandt and Jan Aure

Abstract

The rate of work against the buoyancy forces due to vertical mixing (W) has been determined from repeated measurements of vertical density profiles in a large number of fjordic sill basins (basins dammed by sills). It is found that there is a weak “background” rate of work W 0, probably driven by the local wind. Superposed upon this is work driven by the tide. Thus W = W 0 + RfE, where E is the mean energy flux from the surface tide to turbulence in the sill basin and Rf is an efficiency factor. We distinguish between “wave basins” and “jet basins.” In the former category progressive internal tides are generated in the mouths, while in the latter there are tidal jets at the mouths. For wave basins, about 5.6% of the energy flux E from the surface tide is used for work against the buoyancy forces in the basin water (i.e., Rf ≈ 0.056). The corresponding figure for jet basins appears to be less than 1%.

We have also studied the dependence of the vertical diffusivity κ upon the vertical stratification N. For well-behaved vertical distributions of N, it is found that κ ∼ N −1.5. A formula for κ, which appears to be applicable to many wave sill basins in fjords, is derived. From this, κ may be predicted if the vertical stratification NN(z), the characteristics of the topography and the sea level statistics are known.

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Signild Nerheim and Anders Stigebrandt

Abstract

Data from a moored buoy in the Baltic proper have been analyzed to study the ageostrophic wind-driven (Ekman) transport accounting for buoyancy fluxes in a stratified ocean. A model considering different dynamical regimes governed by wind stress τ, buoyancy fluxes, stratification, and rotation is used to determine the thickness of the mixed layer. For shallow layers in the regime of positive buoyancy fluxes (35% of the time) a transport of about 0.77τ/ρf directed about 30° to the right of the wind is observed. This is far from 1.0τ/ρf and 90°, given by the classical Ekman solution for homogeneous water. The result can be understood qualitatively as caused by drag between the well-mixed surface layer and the underlying layer if the time scale of decay is about 2 h−1. For negative buoyancy fluxes through the sea surface (53% of the time) the mean observed transport was 1.69τ/ρf directed about 60° to the right of the wind. Finite-depth equations for homogeneous water cannot explain this result. No simple explanation of this observational result is offered, but it should be connected to the simultaneously occurring thermohaline convection, which efficiently transmits momentum vertically to the whole mixed layer. The computed mean energy transfer to the wind current is about 12 mW m−2.

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Anders Stigebrandt and Jarle Molvaer

Abstract

A large number of historical vertical salinity profiles from the deep Holandsfjord are analyzed to investigate properties of the brackish surface layer. It is found that the mean salinity and mean thickness of the brackish layer in the fjord are almost horizontally homogeneous. By statistical regression it is shown that the freshwater content in the brackish layer, measured as the freshwater height H 1f varies with the freshwater supplyQ f as H 1fQ 2/3 f This result is predicted by a simple two-layer model, with a thin active brackish surface layer upon a thick passive layer of seawater, subject to baroclinic hydraulic control in a contraction at the fjord mouth. The observed freshwater height, however, is ∼50% greater than predicted. It is suggested that this, at least partly, is an effect of vertical stratification in the lower layer of seawater, focusing the estuarine compensation current toward the pycnocline.

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Lars Arneborg, Carol Janzen, Bengt Liljebladh, Tom P. Rippeth, John H. Simpson, and Anders Stigebrandt

Abstract

Two microstructure profilers, two ships, and four moorings with acoustic Doppler current profilers and conductivity–temperature loggers were used in an intensive effort to map the spatial and temporal variations of vertical mixing in the stagnant deep basin of Gullmar Fjord, Sweden. During three days in the beginning of August 2001 a continuous time series of turbulent kinetic energy dissipation profiles was obtained with one microstructure profiler at a fixed position near the deepest part of the fjord. During the same period the other microstructure profiler was used to obtain six sections of dissipation through the length of the basin. Two moorings were deployed in the fjord basin for one month from the end of July to the end of August. The mapping of dissipation rates reveals that the dissipation in the deep basin is confined to areas just inside the sill. More than 77% of the dissipation in the fjord basin happens above the sloping bottoms closest to the sill.

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Susan E. Wijffels, Raymond W. Schmitt, Harry L. Bryden, and Anders Stigebrandt

Abstract

The global distribution of freshwater transport in the ocean is presented, based on an integration point at Bering Strait, which connects the Pacific and Atlantic oceans via the Artic Ocean. Through Bering Strait, 0.8 × 106 m3 s−1 of relatively fresh, 32.5 psu, water flows from the Pacific into the Arctic Ocean. Baumgrtner and Reichel's tabulation of the act gain of freshwater by the ocean in 5&deg latitude intervals is then integrated from the reference location at Bering Strait to yield the meridional freshwater transport in each ocean. Freshwater transport in the Pacific is directed northward at nearly all latitudes. In the Atlantic, the freshwater transport is directed southward at all latitudes, with a small southward freshwater transport out of the Atlantic across 35°S. Salt transport, which must be considered jointly with the freshwater transport, is northward throughout the Pacific and southward throughout the Atlantic (in the same direction as the freshwater flux) and is equal to the salt transport through the Bering Strait. The circulation around Australasia associated with the poorly known Pacific-Indian throughflow modifies the above scenario only in the South Pacific and Indian oceans. A moderate choice for the throughflow indicates that it dominates the absolute meridional fluxes of freshwater and salt in these oceans. The global freshwater scheme presented here differs markedly from earlier interpretations and suggests the need for a careful assessment of the treatment of ocean freshwater and salt transports in inverse, numerical, and climate models.

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