TY - BOOK
T1 - Rotational Baroclinic Adjustment
T2 - Surface Fronts in the Kattegat
AU - Holtegård Nielsen, Steen Morten
PY - 1998
Y1 - 1998
N2 - In stratified waters like those around Denmark there is a close
correlation between the biology of the water masses and their
structure and currents; this is known as dynamic
biologicaloceanography. The currents are particularly strong near
the fronts, which can be seen in several places throughout the
area. When the rotation of the earth is considered, these frontal
flows can account for many hitherto unsolved features in the
Kattegat, a sea within the Danish inland waters. Important aspects
of the dynamics of rotating stratified systems are reviewed. These
include the time scale, which equals the reciprocal of the
socalled Coriolis parameter, and the length scale, which is known
as the Rossby radius. Also, because of their limited width
currents influenced by rotation are quite persistent. The flow
which results from the introduction of a surface level
discontinuity across a wide channel is discussed; this situation
resembles the conditions in the Kattegat where the transition
towards the Skagerrak consists of a front---the Kattegat/Skagerrak
front. The remarkable difference between changing the upstream and
the downstream boundary conditions in a rotating system is
discussed.A three-dimensional hydrodynamic model, MIKE 3, is used
to show that the features outlined above also apply to highly
perturbed systems like the Kattegat/Skagerrak front. Based on the
model results, a general flow pattern for the Kattegat is
discussed. This picture accounts for several dynamical features of
the area which have been unexplained so far. These include that
outflowing water from the Great Belt can often be found along the
east coast of Jutland, and that a comparatively large part of the
flow of brackish water through the Kattegat takes place in the
narrow channel west of the island Læsø.In connection with the
results of the numerical model a mechanism for the generation of
along-frontal instabilities and eddies is suggested. Also, the
effect of an irregular bathymetry is studied.Together with
observations of wind and water levels some of the oceanographical
observations from the old lightvessels are used to study the
baroclinic flows of the Kattegat. The observations support the
suggested picture of the general circulation. Also, they indicate
how weakly outflowing water masses from the Baltic are governed by
this circulation. In case of strong outflow, the buoyancy flux
from the brackish water overrules this circulation to produce
coastal currents flowing cyclonically through the Kattegat.Off the
headland Skagen, the lightvessel observations together with
earlier studies suggest that strong wind-driven currents are
responsible for the location of the Kattegat/Skagerrak frontin
this area.Observations from the interior of the Kattegat show that
strong baroclinic currents can be found also away from the major
frontal areas. This is also the impression when studying the shown
satellite image. Mechanisms for generation of these fronts are
discussed.Finally, MIKE 3 is used to study a conceptual model of
the Kattegat. This model does not include mixing and friction.
Because of the limited extent of the model compared to the actual
size of the Kattegat, baroclinic instabilities to some extent
conceal the general flow patterns. In spite of that, the
implications of the Kattegat/Skagerrak front for the dynamics of
the area are shown. In connection with this, some results of the
DYNOCS project are briefly discussed.It is expected that the
results of this work can contribute significantly to a progress of
the understanding of the dynamic biological oceanography of the
Kattegat. Furthermore, thestratification together with the
horizontal extent of many other parts of the Danish inland waters
implies that the dynamics of these should also be discussed in
terms of rotational effects.
AB - In stratified waters like those around Denmark there is a close
correlation between the biology of the water masses and their
structure and currents; this is known as dynamic
biologicaloceanography. The currents are particularly strong near
the fronts, which can be seen in several places throughout the
area. When the rotation of the earth is considered, these frontal
flows can account for many hitherto unsolved features in the
Kattegat, a sea within the Danish inland waters. Important aspects
of the dynamics of rotating stratified systems are reviewed. These
include the time scale, which equals the reciprocal of the
socalled Coriolis parameter, and the length scale, which is known
as the Rossby radius. Also, because of their limited width
currents influenced by rotation are quite persistent. The flow
which results from the introduction of a surface level
discontinuity across a wide channel is discussed; this situation
resembles the conditions in the Kattegat where the transition
towards the Skagerrak consists of a front---the Kattegat/Skagerrak
front. The remarkable difference between changing the upstream and
the downstream boundary conditions in a rotating system is
discussed.A three-dimensional hydrodynamic model, MIKE 3, is used
to show that the features outlined above also apply to highly
perturbed systems like the Kattegat/Skagerrak front. Based on the
model results, a general flow pattern for the Kattegat is
discussed. This picture accounts for several dynamical features of
the area which have been unexplained so far. These include that
outflowing water from the Great Belt can often be found along the
east coast of Jutland, and that a comparatively large part of the
flow of brackish water through the Kattegat takes place in the
narrow channel west of the island Læsø.In connection with the
results of the numerical model a mechanism for the generation of
along-frontal instabilities and eddies is suggested. Also, the
effect of an irregular bathymetry is studied.Together with
observations of wind and water levels some of the oceanographical
observations from the old lightvessels are used to study the
baroclinic flows of the Kattegat. The observations support the
suggested picture of the general circulation. Also, they indicate
how weakly outflowing water masses from the Baltic are governed by
this circulation. In case of strong outflow, the buoyancy flux
from the brackish water overrules this circulation to produce
coastal currents flowing cyclonically through the Kattegat.Off the
headland Skagen, the lightvessel observations together with
earlier studies suggest that strong wind-driven currents are
responsible for the location of the Kattegat/Skagerrak frontin
this area.Observations from the interior of the Kattegat show that
strong baroclinic currents can be found also away from the major
frontal areas. This is also the impression when studying the shown
satellite image. Mechanisms for generation of these fronts are
discussed.Finally, MIKE 3 is used to study a conceptual model of
the Kattegat. This model does not include mixing and friction.
Because of the limited extent of the model compared to the actual
size of the Kattegat, baroclinic instabilities to some extent
conceal the general flow patterns. In spite of that, the
implications of the Kattegat/Skagerrak front for the dynamics of
the area are shown. In connection with this, some results of the
DYNOCS project are briefly discussed.It is expected that the
results of this work can contribute significantly to a progress of
the understanding of the dynamic biological oceanography of the
Kattegat. Furthermore, thestratification together with the
horizontal extent of many other parts of the Danish inland waters
implies that the dynamics of these should also be discussed in
terms of rotational effects.
M3 - Book
BT - Rotational Baroclinic Adjustment
PB - Department of Hydrodynamics and Water Resources, Technical
University of Denmark
CY - Lyngby
ER -