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Richard Parsmar
and
Anders Stigebrandt

Abstract

Baroclinic wave drag, due to internal wave generation at steep topography, is shown to be a mechanism that effectively subdues barotropic seiches in fjords. A two-layer model for a fjord with a sill at the mouth is applied to the Gullmar Fjord, Sweden. The damping of the fundamental seiche mode observed from sea level records is well predicted by the model. This includes the observed seasonal variation in damping due to the corresponding variation in vertical stratification. It is shown that ordinary bottom friction should contribute less than 1% to the damping in this fjord.

Simultaneous current records from different depths, obtained on the slope of the sill in the fjord, are analyzed. Spectra of all records show a significant energy peak at the seiche frequency. The vertical variation of the phase of the current at this frequency shows that the motion is essentially baroclinic.

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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 1f Q 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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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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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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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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