Anders Stigebrandt

Transport of Freshwater by the Oceans

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 Reichels tabulation of the act gain of freshwater by the ocean in 5° 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 sal...

A Model for the Exchange of Water and Salt Between the Baltic and the Skagerrak

Abstract A model for the exchange of salt and water between the Baltic and the Sea (the Skagerrak) is presented. Because of strong inter-basin interactions in the Baltic entrance area, the model must include the Kattegat and the Belt Sea. These are modeled by horizontally homogeneous two-layer sub-models. The most prominent dynamical properties of the sub-models are wind-driven entrainment flows and rotational-baroclinic, hydraulic controls. The model is driven by a meteorologically forced barotropic transport Qh [calculated from the freshwater supply to the Baltic (Qf) and the sea level fluctuations in the Kattegat], and turbulent entrainment flows coupled to the wind speed W and, in the Belt Sea, also to the barotropic transport. The most important bathymetric features of the basins are included. The model equations are integrated numerically for a test period of 1½ years. The stratification in the Kattegat, as well as in the Belt Sea, is quite well predicted. It is found that approximately one-half of ...

A Model for the Vertial Circulation of the Baltic Deep Water

Abstract The time-dependent vertical circulation of the Baltic Proper is modeled using a horizontally integrated model of high vertical resolution. A seasonal pycnocline model computes the properties of the mixed layer. Below this an advection-vertical diffusion model computes the evolution of the salinity and temperature fields. A simple model for an entraining dense bottom current—which carries the intruding seawater and drives the vertical advection in the basin—is developed and used. In the derivation of entrainment velocity we it is shown that E(=we/u, where u is the speed of the bottom current) may be expressed in the well-known empirical constants m0 and Cd. The hypsographic features of the Baltic are accounted for in the model. The model is forced using realistic meteorological and hydrological time series. The inflow of dense seawater to the Baltic, with large fluctuations in flow rate and salinity, is realistically described and constitutes the upstream boundary condition for the bottom current....

Vertical Diffusion Driven by Internal Waves in a Sill Fjord

Abstract A new mechanism, the breaking of internal waves, is proposed to explain vertical mixing within the lower layers of sill fjords. The generation of the waves at the sill of a fjord is modelled assuming constant depth except at the narrow sill whose height is the thickness of the lower layer. A barotropic tide oscillating across the still creates internal waves which propagate both seaward and landward from the still. These waves break against the bottom, creating boundary turbulence which mixes water of different density in the lower layer. This is demonstrated experimentally. The mixture flows away from the boundaries into the interior of the fjord, causing an effective vertical mixing. The energy input into the internal waves and the damping of barotropic seiches are computed using linear theory. Possible instability, except at the bottom, is discounted by considering representative Richardson and Froude numbers. The theory is then qualitatively applied to the Oslofjord with particular attention ...

Vertical mixing in basin waters of fjords

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 W0, probably driven by the local wind. Superposed upon this is work driven by the tide. Thus W = W0 + 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 ...

The North Pacific: A Global-Scale Estuary

Abstract The atmospheric net flow of water from the Atlantic to the Pacific Ocean is supposed to maintain the salinity difference between the two oceans. Assuming the existence of a subsurface level of no horizontal pressure gradient in the ocean, the mean sea level in the northern Pacific must be higher than in the Arctic Ocean. This mean sea level difference is supposed to drive the observed mean flow through the Bering Strait. The estimated flow of freshwater through the Bering Strait is approximately equal to the estimated atmospheric net flow of water from the North Atlantic to the North Pacific. This justifies the formulation of a simple estuary model for the North Pacific in which the “brackish” water exits through the Bering Strait. The salinity difference between the two oceans is shown to be controlled by the topography of the Bering Strait. The estuary model gives residence times for water in the upper layer (∼1000 m thick) of approximately 1000 years and in the lower layer of approximately 400...

Regulating the local environmental impact of intensive marine fish farming I. The concept of the MOM system (Modelling-Ongrowing fish farms-Monitoring)

Abstract The paper describes the concept of a management system called MOM (Modelling-Ongrowing fish farms-Monitoring) which may be used to adjust the local environmental impact of marine fish farms to the holding capacity of the sites. The concept is based on integrating the elements of environmental impact assessment, monitoring of impact and environmental quality standards (EQS) into one system. The amount of monitoring is dependent on the level of the environmental impact. Two terms are introduced: (1) the degree of exploitation, which is an expression of how much the site is being utilised, and (2) the level of monitoring, which determines the amount of monitoring depending on the environmental impact. For Norwegian conditions, a monitoring programme, including EQS, has been developed concerning the impact on the sediment under fish farms. It consists of three types of investigations of increasing elaboration and accuracy. A model, which simulates the environmental impact on a site given information about the farms size and production and the hydrodynamic conditions and topography of the site, has been developed but not yet tested. The model and the monitoring programme with EQS are only briefly described, but will be published later. The MOM system should help to maintain satisfactory environmental conditions in and around fish farms and may be a valuable tool in site selection and coastal zone management.

Response of the Baltic Sea to climate change—theory and observations

The dynamics controlling the response of the Baltic Sea to changed atmospheric and hydrologic forcing are reviewed and demonstrated using simple models. The response time for salt is 30 times longer than for heat in the Baltic Sea. In the course of a year, the Baltic Sea renews most of its heat but only about 3% of its salt. On the seasonal scale, surface temperature and icecoverage are controlled by the atmospheric conditions over the Baltic Sea as demonstrated by e.g. the strong inter-annual variations in winter temperature and ice-coverage due to variations in dominating wind directions causing alternating mild and cold winters. The response of surface temperature and ice-coverage in the Baltic Sea to modest climate change may therefore be predicted using existing statistics. Due to the long response time in combination with complicated dynamics, the response of the salinity of the Baltic Sea cannot be predicted using existing statistics but has to be computed from mechanistic models. Salinity changes primarily through changes in the two major forcing factors: the supply of freshwater and the low-frequency sea level fluctuations in the Kattegat. The sensitivity of Baltic Sea salinity to changed freshwater supply is investigated using a simple mechanistic steady-state model that includes baroclinic geostrophic outflow from the Kattegat, the major dynamical factor controlling the freshwater content in the Kattegat and thereby the salinity of water flowing into the Baltic Sea. The computed sensitivity of Baltic Sea surface salinity to changes of freshwater supply is similar to earlier published estimates from timedependent dynamical models with higher resolution. According to the model, the Baltic Sea would become fresh at a mean freshwater supply of about 60000 m 3 s � 1 , i.e. a 300% increase of the contemporary supply. If the freshwater supply in the different basins increased in proportion to the present-day supply, the Bothnian Bay would become fresh already at a freshwater supply of about 37000 m 3 s � 1 and the Bothnian Sea at a supply of about 45000 m 3 s � 1 . The assumption of baroclinic geostrophic outflow from the Kattegat, crucial for the salinity response of the Baltic Sea to changed freshwater supply, is

A Model for the Thickness and Salinity of the Upper Layer in the Arctic Ocean and the Relationship between the Ice Thickness and Some External Parameters

Abstract This paper presents a dynamical model for the salinity and thickness of the upper layer in the Arctic. The parameters are the river runoff to the Arctic, the buoyancy supply through the Bering Strait, the export of ice from the Arctic and a parameter characterizing the vertical mixing. An ice model is formulated, having the following two important properties: 1) the horizontal surface area of the exported ice is essentially determined by external parameters (the wind field over the Arctic); and 2) there is a relationship between the ice thickness and the fraction of open water in the Arctic. The model for the upper layer and the ice model are used together with a heat budget for the Arctic, also including the effect of different albedo for ice and open water. A relationship between the freshwater supply and the ice thickness is derived. Also investigated are the effects on the ice thickness of a changed export of ice area and changed properties of the flow through the Bering Strait. It is found t...

Some aspects of tidal interaction with fjord constrictions

Tidal interaction with fjord constrictions is discussed. Two typical cases are thoroughly analysed. The first is the so called tidal choking problem where the effect of vertical stratification is insignificant. In the second case the influence of vertical stratification is crucial and the forcing barotropic tide generates internal waves at the constriction (a sill in this case). Some theoretical gaps in the field of tidal interaction with fjord constrictions are filled. First a theory for tidal choking in a fjord with freshwater runoff is presented. A Table is given which can be used for a quick determination of tidal choking effects in almost any fjord. Second a simple theory for internal wave generation by tides in a linearly stratified fjord is developed. The predictions are compared with measurements from the Herdlafjord (from Fjeldstad) with encouraging results.