Hydrographic investigation of the Northern Kattegat front

Pergamon

ContinentalShelfResearch, Vol. 17, No. 5, pp. 533-554, 1997 Copyright~) 1997 Elsevier ScienceLtd Printed in Great Britain. All rights reserved PII: S0278-4343(96)00044-1 0278-4343/97 $17.00+ 0.00

Hydrographic investigation of the Northern Kattegat front FLEMMING JAKOBSEN (Received 7 April 1995; in revised form 4 July 1996; accepted 20 August 1996) Abstract The Northern Kattegat front separates Kattegat surface water or around 26 PSU and Skagerrak water of around 34 PSU. The front plays an important role in the water exchange betweea Kattegat and Skagerrak, and consequently it is also important for the exchange of dissolw~,d and suspended matter such as nutrients, organic material and Gelbstoff. In order to investi~;ate the dynamics of the Northern Kattegat front two surveys were performed, one in the period 2-12 March, 1992 and in one in the period 21 September-2 October, 1992. Vertical profiles of temperature, salinity and current were measured along cross-sections. The data from the two surveys and the analyses of the data concerning the dynamics of the Northern Kattegat front are presented. First, a conceptual model of the current dynamics, based mainly on Stigebrandt, A. (1987) (Tellus, 39A, 170-177), is proposed, where the front is considered to be a 'rotational baroclinic control' guiding the current in the Kattegat. The model is evaluated on the basis of the two surveys. Secondly, an equation describing the outflow from the Kattegat to the Skagerrak is derived and evaluated. © 1997 Elsevier Science Ltd. All rights reserved

1. INTRODUCTION The water masses in the Belt Sea (including the Sound) and the Kattegat consist of low saline surface waters from the Baltic Proper, which flow northward in the Baltic Current, and high sa]line sub-surface water from the North Sea (Svansson, 1975, 1984; Stigebrandt, 1983; Anderson and Rydberg, 1993), see Fig. 1. The Baltic Current continues flowing northward from the Kattegat into the Skagerrak, where it forms a coastal current along the Swedish coast. A drastic decrease in width of the Baltic Current is observed in the Northern Kattegat and the separation of Kattegat surface water (-26 PSU) and Skagerrak water (-34 PSU) in this area is named the Northern Kattegat front. Along the western coast of Jutland flows a less saline, northerly current named the Jutland Coastal Current, see e.g. Richardson and Jacobsen (1990). The Baltic Current and the Jutland Coastal Current meet in the northern KattegatSkagerrak area. The less saline current (a mixture of the Baltic Current, the Jutland Coastal Current and local river inflow), which continues in a westward direction along the Norwegian coast, is, together with the barotropic cyclonic current in the Skagerrak, named the Norwegian Coastal Current (Svansson, 1975; Aure and S~etre, 1981; S~etre et al., 1988). These less saline currents together form a baroclinic cyclonic current in the Skagerrak, which is strongly influenced by wind-driven circulation (a southwesterly wind Danish Hydraulic Institute, Coastal & Environmental Division, Agern All6 5, DK-2970 H0rsholm, Denmark. 533

534

F. Jakobsen

|

,'NORWAY:

o °

GSteborg:

ll'rederlkshovn ~/Y~ Honsth¢

Les~

57*

5

:SWEDEN

KA TTE GA T

~ Anholt

Alborg Bctj

M ixtcte l -

grunden

:JUTLAND

N m

-55" ARKONA B A S I N

.qxx~x:\\\'sb~\\\] oo" Km

FILE:A:8-1995\8079-01.dwg\ced Fig. 1. A m a p of the observed area: the North Sea, the Kattegat, the Belt Sea (the Sound included) and the Baltic Proper, and the analysed sections in the literature are shown as cross sections 1--6.

The Northem Kattegat front

535

forces a cyclonic circulation) (Pingree et al., 1982; Rodhe, 1987; Fonselius, 1990; Poulsen, 1991). Rodhe (1992) used long-term measurements to investigate the surface salinity distributions in the Kattegat-Skagerrak area for both the winter (Nov.-Feb.) and the summer (May-Au~;.) periods. The distributions show similar features in the two periods, though the near-surface water has a lower salinity during the summer than during the winter. The Northern Kattegat front is found to be the most prominent and persistent front in the area. The averaged position is from Cape Skagen towards north-east. The findings by Rodhe (1992) support the fact that during the period from April to September/October the wind velocity is small in comparison to the yearly average (M¢ller and Hansen, 1994), while during the period from October/November to March the wind velocity is large compared to the yearly average, see also Stigebrandt (1984). During the fall and winter periods, when large inflows to the Baltic Proper occur (Stigebrandt, 1984; Matth/ius and Franck, 1992; Jakobsen, 1995), the Northern Kattegat front system may be destroyed and rebuilt several times, see iErtebjerg et al. (1991). A destructive; scenario often starts with strong westerly winds, which forces a high water level in the southern Kattegat and a low water level in the Arkona Basin. Thus, the water masses from the Belt Sea and the Kattegat are barotropically discharged into the Baltic Proper. The large production of turbulence from the wind and current almost homogenize the water masques in the southern Kattegat and the Belt Sea. Once the wind decreases, the outflow from the Baltic Proper starts. As the water masses no longer homogenize, the stratification and the Northern Kattegat front system is rebuilt. There is still much we do not know about the current in the Kattegat-Skagerrak area. This paper addresses some of the observed processes in the area, see also Jakobsen et al. (1994). Clearly, in-depth numerical modelling studies would also do much to extend the existing level of knowledge, but unfortunately no results are available yet. The paper is based on two surveys of the Kattegat-Skagerrak area, one for the period 2-12 March, 1992 and one for the period 21 September to the 2 October, 1992. The paper is outlined as follows. First, the theoretical background for a conceptual model of the large-scale baroclinic structure in Kattegat is described, and an outflow condition from the Kattegat to the Skagerrak is derived. Second, the dynamical features observed during the two surveys are presented and related to the conceptual model and the outflow condition. Thirdly, results presented and interpreted by Poulsen (1991) are discussed and re-interpreted on the basis of the present findings. 2. 2.1.

THEORETICAL BACKGROUND

The conceptual model

In Stigebrandt (1987) a conceptual model for the quasi-stationary current along the rim of the high density bottom layer/pool in the Arkona Basin is presented. Similar flow features are anticipated in the northern Kattegat, see Fig. 2. According to Stigebrandt (1987) the surface layer may be considered a pool of light water which is filled by the pulsating light water inflows from the Belt Sea, see also Dietrich (1951), Stigebrandt (1980, 198:3) and Jakobsen and Lintrup (1997). The light pool lacks a physical wall in the north and the leakage from the pool is controlled by the Northern Kattegat front, which acts as a 'rotational baroclinic control' guiding the current. This results in a concentration

536

F. Jakobsen 58.5"

~QtQ~ 40040Ot~O

58.0"

f/

20m

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,

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i.i..i..i:i..iI < •

.

10.5"

|

x

Y

T

i

11.0"

11.5'

r

X

12.0"

Fig. 2. Chart of the Kattegat and the Skagerrak, where an expected possible event of front, upwelling north of L~es0and current is shown. 1° latitude is approximately 111 km and 1° longitude approximately 60 km. of the outflow in the western part of the northern Kattegat. In open channel hydraulics the 'rotational baroclinic control' would be n a m e d 'the control section', which, for subcritical stationary currents, is known to be positioned downstream and to control the upstream flow conditions, see Chow (1986). The frontal system is m o v e d back and forth by the barotropic pulsating flow, but this should not alter the baroclinic dynamics. Pedersen and MOiler (1981) found the front to be primarily positioned at Frederikshavn and the m o v e m e n t of the front as a rule to be 40 km, while Romell and Stigebrandt (1985) found the front to be only occasionally north of Frederikshavn during an 18-month period. Situations without quasi-stationary conditions will occur in the Kattegat and infor-

The Northern Kattegat front

537

mation on this situation can be obtained from Jakobsen (1991), where a 'one-and-a-half layer' computer model of the lower layer of the Bornholm Basin is presented. The bottom layer is a pool of dense water, which is filled by the inflow from the Arkona Basin. The outflow from the eastern Bornholm Basin takes place through the Stolpe Channel into the Gotland Basin. Computer simulations show how the currents are determined under stationary conditions by a control section in the Stolpe Channel, which guides the current mainly through the northern half of the Bornholm Basin. This finding strongly supports the conceptual model outlined in Stigebrandt (1987). Transient currents were also investigated in the simulations, where it was found that the transient part of the current is not influenced by the downstream control before the control section is reached, but will instead be in local geostrophic balance and run through the southern half of the basin. It is therefore expected that during non-stationary conditions in the Kattegat, the flow will be concentrated in the eastern part, closer to the Swedish coast, and as the system develops into stationary conditions the flow will move from the eastern part to the western part.

2.2.

The outflow condition

It is possible to give a simple mathematical description of the current dynamics in the upper layer. We will consider the two-layer current system using a coordinate system with the y-axis pointing in the flow direction. Following the same path used when deriving the Margules equation (see e.g. Gill, 1982), but keeping the influence of the wind stress perpendicular to the front and assuming the shear stress at the interface to be small, the following equation can be derived: gI

_

me ha

g ' c]hl --[- g 2 --[-- -

f Ox

(1)

where g' (in s -2) is the reduced gravity acceleration, f ( ~ 1 . 2 . 1 0 -4 s -1) is the Coriolis parameter, h (m) is layer depth (indices 1 and 2 are for upper and lower layers, respectively), V (m s -1) is velocity in the y-axis and me (m 2 s -a) is the Ekman discharge parallel to the front. It is often reasonable to assume that the shear stress is small at the interface (Pedersen, 1986), because the strong stratification at the interface decreases the length scale', of the turbulence, i.e. the eddy viscosity decreases. The total Ekman discharge then takes place in the upper layer. The first term to the right of the equals sign is the baroclinic geostrophic velocity. If the term for the wind is omitted then this equation is called the ldargules equation. The Ekman discharge is related to the observed wind as follows:

me

-- 1 T x --

f Po

~C10 PA W l 0 W ~ 0

Po

(2)

where P0 (kg m -3) is the reference density, P.4 (--1/800 P0) is air density, r x (N m -z) is the wind shear stress component parallel to the x-axis, W10 (m s -1) is the wind speed, W~0 (m s -1) is wind component parallel to the x-axis, and C10 (-(0.75 + 0.067Wao). 10-3 (Garratt, 1!977)) is the surface drag coefficient. The discharge (Q) can be determined by integrating equation (1) across the cross section of the current. When integrating, the first term to the right of the equals sign is

538

F. Jakobsen

approximated by setting the sea surface elevation to zero, and the second and third terms are approximated by estimating the cross section area. The discharge then becomes: Q

_ g' ,t,2

mei] H1L I 2 nc HI

t.2 ' + { V

- ~ \t~lS - t t l E )

(3)

where Ha -- ½(has+ haE)and hxs (m) and hie (m) are the depths of the upper layer at the start and at the end of the integration, respectively, and L (m) is the width of the integration. In the following the outflow discharge from the Kattegat is determined by considering the coastal current along the Swedish coast in Skagerrak using a coordinate system with the x-axis towards E and the y-axis towards N, see e.g. Fig. 2. Several assumptions are required which we will investigate carefully later on the basis of two surveys. In equation (3) the upper layer depth is zero at the integration starting point and is assumed to be equal to the upper layer depth in the Kattegat (hK)at the integration end point. The width of the coastal current is assumed approximately equal to the baroclinic Rossby Radius in the Kattegat. The discharge of the coastal current is hereby determined to be:

OK=~fg'h2+ ( K V2 +2me]hKl'lhK 2 ]g'h£f

(4)

If equation (4) can be shown to be valid it will be quite useful in connection with the treatment of long-term measurements and/or simulation of longer periods. A cross-frontal discharge probably takes place due to an eddy formation on the front. This discharge is not quantified, as it is considered to be of only minor importance to the outflow from the Kattegat, even if it is believed to be of importance for the biology in the northern Kattegat. 3.

EQUIPMENT AND METHOD

During the two surveys, the vertical temperature and conductivity profiles were measured by a Niel Brown, Mark 3. The salinity was calculated from the conductivity and the temperature. Vertical current profiles were measured by a ship-mounted 600 kHz ADCP from RDI. The ADCP transducers were placed 4 m below sea surface, the sampling interval was set at 120 s and the bin length was set at 2 m. To a great extent the data presentation in this paper follows the same hydrographical cross-sections as those presented in earlier investigations by e.g. Svansson (1984), Rodhe (1987) and Andersson and Rydberg (1993), thus making it possible to compare the surveys, see Fig. 3. The first survey took place in the period 2-12 March, 1992. The survey involved 202 stations distributed over 18 cross-sections. The second survey took place from 21 September until 2 October, 1992 and it contained 236 stations on 25 cross-sections. The quality of the ADCP measurements during this survey was not high, probably due to a growth on the transducers. 4.

RESULTS

4.1. The conceptualmodel The conditions in the Kattegat during the second survey were heavily affected by the outflow from the Baltic Proper, see Figs 4 and 5. In the northem Kattegat/southeastern

539

The Northern Kattegat front

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Cross-sections used at presentation of data. Cross-section A continues further south than shown in the figure.

540

F. Jakobsen

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r.. -.... -.... • /

25

22,6

22.6

57.0* 10.0"

10.5 °

11.0"

11.5"

Fig. 4. Salinity distribution (PSU) in the northern Kattegat and the southeastern Skagerrak in 5 m depth during the first week of survey 2 (21-25 September, 1992). The framed salinities, from the end of the week, are excluded in the analysis.

12.0"

541

The Northern Kattegat front

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Fig. 5.

26

"

---I 10,5"

11.0 °

11.5 °

Salinity distribution (PSU) in the northern Kattegat and the southeastern Skagerrak in 5 m depth during the second week of survey 2 (28 September to 2 October, 1992).

12.0 °

542

F. Jakobsen 56.6 o

57.0 o

57.4 o

57.8 o

58.2 o •- -

O . O m

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--~

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--120.0

56.6 o

57.0 o

57.4 o

57.8 o

58.2 o

-110.0

I -120.0

Fig. 6. Salinity distribution (PSU) on cross-section A shown on Fig. 3 from a position SE of Anholt on 21 September, 1992 at 12:49 h and north to the Skagerrak on 22 September, 1992 at 05:47 h.

Skagerrak, a significant front area between the Kattegat and the Skagerrak waters is present. The front is observed in an area from Cape Skagen NE along the Swedish coast, and the appearance is in good agreement with expectations, see Rodhe (1992). In the Skagerrak, eddies (waves) are observed on the front with a longitudinal range of 10-30 km. There is little change from the first week to the second week, so conditions can be assumed to be quasi-stationary. In the Kattegat a significant interface is seen as a strong halocline at approximately 20 m, see Fig. 6. In the Skagerrak the interface rises to the surface of the sea over a distance of 55 km. Close to the front a local maximum in salinity is observed, which is due to the eddy formation on the front. The discharge in the front area is estimated using equation (3) at 1 0 7 . 1 0 3 m 3 s - t , and is therefore smaller than the inflow Table 1.

Crosssection

A C

Summary of data interpreted on the basis o f survey measurements for two cross-sections during the second survey and the discharges calculated using equation (3)

Date

his (m)

hie (m)

L (km)

21-22 Sep 22 Sep

0 0

20 20

55 cos 45 ° 12

AS V2 (PSU) (m s -1)

7 7

-0.02 0.00

melH 1 ( m s -x)

Q (103 m 3 s -I)

0.06 0.10

107 102

The winds are taken at the location from hind-cast simulations with H I R L A M obtained from DMI. The outflow through the Great Belt, Little Belt and the Sound to the southern Kattegat from 18-30 September, 1992 is estimated at 127 - 103 m 3 s - i , see e.g. Jakobsen (1995).

543

The Northern Kattegat front 10.1 ° O.Om

10.4 °

10.7 °

11.0 °

..

11.3 ° O.Om

t

-10.0

-10.0

-20.0

-20.0

-30.0

-30.0

-40.0

-40.0

-50.0

-50.0

-60.0

-60.0

-70.0

-70.0

-80.0

-80.0

-90.0

-90.0

-100.0

-100.0 ~

I~

-110.0

-

-120.0

-

-110.0 -120.0

10.1 °

10.4 °

10.7 °

11.0 °

11.3 °

Fig. 7. Salinity distribution (PSU) on cross-section C shown on Fig, 3 from the Swedish coast in the Skagerrak on 22 September, 1992 at 08:05 h and westward to the central Skagerrak on 22 September, 1992 at 16:05 h.

of 127 • 103 m 3 S - 1 to the southern Kattegat, see Table 1. In the stationary case the discharge in the front area should be bigger than the inflow to the southern Kattegat due to upwards entrainment, while during the non-stationary case the discharge might be bigger or smaller than the inflow due to reservoir (storage) effects in the Kattegat. The high discharge in the front zone shows that the outflow from the Kattegat follows the front from Cape Skag;en towards the northeast and therefore the current in the Kattegat south of Cape Skagen must flow towards Cape Skagen, so that it is in the western part. In the Skagerrak the Baltic Current is observed at the Swedish coast, see Fig. 7. It is possible to identify two water masses in the coastal current; the water mass closest to the coast is the outflow from the Kattegat and the other water mass is assumed to be Skagerrak water mixed with eddy-shedded water from the outflow. The discharge in the coastal current is estimated using equation (3) at 102 • 103 m 3 s -1 (smaller than at the front). A significant outflow of 24 PSU water both east and west of L~es0 is observed, see Fig. 8. An area north of L~es¢ is affected by upwelling (note the 23 PSU curve). The measured velocity distribution shows a wedge-shaped northbound current of 0.5 m s -1 in the 23 PSU water. Evidence of a strong current in the western Kattegat (west and just east of L~es¢) is also found by Svansson (1984) and Anderson and Rydberg (1993) as expected, as the current flows towards Cape Skagen. The increase in inflow to the southern Kattegat should, according to our conceptual model, cause a secondary front which, in its initial phase, should go l~hrough the eastern part of the Kattegat. This secondary front is observed in the measurements, see Figs 4 and 5. During the first week a secondary front developed NE of

544

F. Jakobsen 10.5 °

11.0 °

0.0m •

\x

~

/ j

:1

:::(

' a~ ; x "

-10.0

--..

-20.0

11.5 °

i " "" ~ '

---05

-40.0

-10.0

-.,:..,,, ; ,',.,

-20.0

,. _Zf -

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0.0m

I

.

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.

~

_

-30.0

-40.0

J

-50.0

-50.0

-60.0

-60.0

-70.0

-70.0

I

-80.o 10.5 °

11.0 °

'f}~ ['.":

I -80.0

11.5 °

Fig. 8. Salinity distribution (PSU) on cross-section B shown on Fig. 3 from the Danish coast on 24 September, 1992 at 01:40 h and to the Swedish coast on 24 September, 1992 at 09:02 h. A current of 0.5 m S-1 was m e a s u r e d east of L~es~, which is indicated on the figure.

L~es¢ (see the 23 PSU curve), and an upwelling north of L~es¢ was present, although this was not observed to the same extent during the second week of the survey. Several identified features found during the first survey therefore support the conceptual model. The outflow from the Great Belt and the Sound during the first week of the first survey takes place along the Swedish coast and it can be identified far north in the Kattegat in two parallel layers, see Fig. 9. In this non-stationary building-up situation the water discharge from the Great Belt through Laes¢ Channel (west of L~es¢) is, as expected, limited. During the second week of the survey the Kattegat surface water of 26-28 PSU was forced through the Lapse Channel into the area north of L~es¢, and a more characteristic front structure was developed, beginning close to Frederikshavn travelling NE, see Fig. 10. Cross-section B was measured three times during the survey, see Fig. 11. On 4 March the outflow from the Baltic was seen as a thin layer in the eastern part of the cross-section. The outflowing water mass from the Great Belt and the Sound was separated. The relatively small amount of Great Belt water compared to Sound water was apparently due to a time delay. On 6 March the area of outflowing water had increased compared with previous observations. Only an insignificant outflow from Laes¢ Channel could be identified in the western part of the cross-section. Using equation (3) the discharge in the coastal current can be calculated at approximately 90 • 103 m 3 s -1, see Table 2. The discharge was increased in the cross-section, and was therefore almost equal to the inflow of 88 • 103 m 3 s -1 to the southern Kattegat on 6 March. On 12 March the cross-section of the outflowing water had increased further, confirming that the discharge is stored in the Kattegat. This may be due to the time it takes for the water to flow from the Great Belt, which then forces the

The Northern Kattegat front

545

58.5"

f/ 58.0"

•

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•

I---

32

I •

e

0 0

20m

33 &4 2~4

57.5"

".." " •

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~t2.8

"'.' "*".'**'"*.'*"**';/ .!~;I / 24 57.0o L-10.0"

•

. 10.5"

11.0"

. 11.5"

Fig. 9 Salinity distribution (PSU) in the northern Kattegat and southeastern Skagerrak at 5 m depth during the first week of survey 1 (2-6 March, 1992). The framed salinities, from the beginning of the week, are excluded in the analysis.

12.0"

546

F. Jakobsen

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26

28.6 27.8 *

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iiiiiiiiiiiiiii!iiiiiiiiiiiii !l 80

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279

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z:0

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[.-i.i...i-.i..i-./ 57.0" 10.0" Fig. 10.

10.5"

11.0"

11.5"

Salinity distribution (PSU) in the northern Kattegat and southeastern Skagerrak at 5 m depth during the second week of survey 1 (8-12 March, 1992).

12.0"

The Northern Kattegat front

547

Table 2. Summary of data interpreted on basis of survey measurements for two cross-sections during the first survey and the discharges calculated using equation (3)

Crosssection B C

his

hlE

L

Date

(m)

(m)

(km)

6 March 9 March

0 0

20 18

25 12

AS

V2

(PSU) (m s-a) 8 7

-0.05 0.10

me/H1

Q

(ms -1) (103m3 s-1) 0.0 0.0

90 84

The winds are taken at the location from hind-cast simulationswith HIRLAM obtained from DMI. The ontflow through the Great Belt, Little Belt and the Sound to the southern Kattegat from 5 March, 1992 to 11 March 1992 is estimated at 88 - 1 0 3 m 3 s -1, see e.g. Jakobsen (1995).

Kattegat surface water northwards t h r o u g h / ~ l b o r g Bay west of La~sO. F r o m observing cross-section B it is clear that the discharge during the building-up period moves from the eastern to the western side, as predicted by the conceptual model. The Baltic Current was observed on 9 March along the Swedish coast in the Skagerrak, see Fig. 12. Using equation (3) the discharge in the coastal current is estimated at approximately 84 • 103 m 3 s -1. Therefore it is still smaller than the outflow to the southern Kattegat. 4.2.

The our[tow condition

The conditions during the second survey can be assumed to be quasi-stationary and give us a good basis for evaluating the assumptions used when deriving the outflow condition. In the Kattegat a significant interface is seen at 20 m, see e.g. Fig. 6. The baroclinic Rossby radius in the Kattegat is about 10 kin. The Baltic Current observed at the Swedish coast in Skagerrak has a m a x i m u m depth of 20 m and a width of 12 km, see Fig. 7. This shows that during the second survey it is reasonable to assume that the m a x i m u m depth of the coastal current is equal to the u p p e r layer depth in Kattegat and that the width of the coastal current is equal to the baroclinic Rossby radius. E v e n though the conditions were very non-stationary during the first survey (the front actually btdlt-up during the survey), it is also possible in this case to give an approximate evaluation of the assumptions. On 6 March the depth of the u p p e r layer in the Kattegat was 0-20 m and the Rossby radius was 0-9 km, see Fig. 11. On 12 March the u p p e r layer m e a n depth was 17.5 m (0-22 m) and the Rossby radius was 0-10 km. On 9 March the Baltic Current in the Skagerrak had a m a x i m u m depth of 18 m and a width of 12 kin, see Fig. 12. It seems that even in this non-stationary case the assumptions are useful, but information taken near the Swedish coast in the Kattegat should be used. Considering a typical m e a n situation the lower layer mean velocity (V2) is 0.12 m s -1 (Rodhe, 1987) and the m e a n E k m a n discharge (me) is estimated at - 0 . 1 9 m 2 s -1 on the basis of wind data collected from the Sound during the period 5 Nov., 1993 to 5 Nov., 1995. Southwesterly winds dominate the K a t t e g a t - S k a g e r r a k area. The layer depth (hK) is usually 14 m and the salinity increase is typically (33-26) 7 PSU, see e.g. light-vessel L~es¢ Nord/Trindel in Sparre (1984). The outflow equation (4) then shows an outflow of 49.103 m 3 s -1 with a lower layer velocity and an E k m a n velocity contribution of 10%. In Anderson and Rydberg (1993) the outflow close to L~es¢ was observed to be 51 • 103 m 3 s -1. It should be noted that on shorter time scales (days to weeks) the contribution from the lower layer current and the wind can be much greater.

548

F. Jakobsen 0.5 ° O.Om


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Fig. 11. Salinity distribution (PSU) on cross-section B shown on Fig. 3 from close to the Swedish coast north of L~es~close to the Danish coast on: (a) 4 March, 1992; (b) 6 March 1992; and (c) 12 March, 1992. The measured currents are shown and also indicated on the salinity measurements. (Continued opposite.)

This type of outflow equation was used by Stigebrandt (1983). However, the effect of the lower layer was assumed to be small and direct wind effects on the frontal system were not considered [equation (4) was applied with the second term on the fight equal to zero]. In contrast, a constant densimetric Froudes number condition for the outflow is used in Jacobsen and Hansen (1985), which led to the conclusion that the essential geostrophic discharge was overestimated with a constant factor of 16/9. The two different approaches lead to almost the same dependency on the parameters involved, except for a constant of 16/9 (the constant depends on the choice of the constant Froudes' number, as well as an

549

The Northern Kattegat front 10.5 °

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Fig. 12. Salinity distribution (PSU) on cross-section C shown on Fig. 3 from the Swedish coast on 9 March, 1992 at 08:45 h westward out into the central Skagerrak on 9 March, 1992 at 12:47 h. The measured currents are shown and also indicated on the salinity measurements.

550

F. Jakobsen 57.0 °

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Salinity distribution (PSU) on cross-section A shown on Fig. 3 from a position SE of

Anholt on 2 March, 1992 at 15:04 h and northwards to the Skagerrak on 3 March, 1992 at l 1:14 h.

estimate of the connection between the depth of the layer in the Kattegat and the depth of the layer in the coastal current along the Swedish coast).

4.3.

Other observed features

During the first survey the salinity front was observed in the Kattegat while it was developing rapidly towards the north, see Fig. 13. Near Middelgrunden SE of Anholt in the Kattegat, where the depth is only 20 m, a local maximum of 26 PSU in the upper salinity layer was seen. This water mass was aged Kattegat surface water, having been exposed to wind mixing and it was surrounded by water from the Great Belt and the Sound, see also Pedersen (1993). Near the intrusion front a 'head' in the low saline outflowing water was observed. This is especially evident when seen from the 33 PSU curve, which dives approximately 10 m at the front, see e.g. McClimans (1988) for a general discussion of estuarine fronts. This may be a sign that the current is super-critical (the densimetric Froude number is greater than 1). The phenomena were also observed on the cross-sections across the front that followed. During the second survey on cross-section D a three-layer structure was observed, where the third layer was cold, deep-lying water in the Skagerrak, see Fig. 14. The temperature was 15°C in the upper and middle layers, while it was only 8°C in the third layer. In the third layer there was a significant dome structure in the temperature distribution, probably due to a strong cyclonic current along the front (Danielssen et al.,

The Northern Kattegat front 10.4 ° O.Om (

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Fig. 14. (a) Salinity (PSU) and (b) temperature distribution(°C) on cross-section D shown on Fig. 3 from Cape Skagen on 25 September, 1992 at 04:15 h and to the Swedish coast on 25 September, 1992 at 10:22 h. 1991). This relatively small dome can, to some extent, be observed along the front. The importance of the dome is not known at present. 5.

DISCUSSION

The observations presented in Fig. 15, redrawn after Poulsen (1991), are described below. The density front is located near station 111, where the depth is more than 80 m. The lower layer current (V2) in the front area is 0.3-0.4 m s -1 towards the west. Poulsen (1991) analysed the observations and showed that the first and the second terms on the right of equation (1) balance each other (the other terms were omitted in advance). On the basis of this Poulsen (1991) concluded that the lower layer current in the Skagerrak determines the position of the front. In this paper the barotropic current field in the Skagerrak is not believed to determine the position of the front as outlined by Poulsen (1991). The current field in the Skagerrak on monthly and yearly time-scales shows a cyclonic circulation, while on daily time-scales it shows both barotropic cyclonic and anticyclonic circulation, see e.g. Poulsen (1991). The measured lower layer current is considered :simply to be the barotropic current in the Skagerrak, while the vanishing upper layer current close to the front is considered to be a sum of the barotropic current and an oppositely-directed baroclinic current of nearly equal speed. Thus, if the lower layer velocity ceases, a balance in equation (1) is obtained by the term on the left (the baroclinic current) and the first term on the right (the baroclinic pressure force).

552

F. Jakobsen

SKAGERRAK STATION 111

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Fig. 15. (a) The salinity distribution (PSU) and (b,c) the velocity distribution (cm s- l ) on cross-section 6 shown on Fig. I through the northern Kattegat front in the northern Kattegat on 22 October, 1990. Redrawn after Poulsen (1991).

The Northern Kattegat front

6.

553

CONCLUSION

A conceptual model for the current in the Kattegat-Skagerrak area based on Stigebrandt (1987) and Jakobsen (1991) is presented. The surface layer is considered to be a pool of ligh~t water which is filled by the outflows from the Baltic Proper. The light pool lacks a physical wall in the north and is instead controlled by the northern Kattegat front, which acts as a 'rotational baroclinic control' guiding the current through the western part of the northern Kattegat. In the non-stationary case the transient part of the current will not be influenced by downstream control before the control section is reached, but instead it will be in local geostrophic balance and will run through the eastern part of the Kattegat. Several features supporting the conceptual model were found when investigating the measurements from the two surveys. An outflow condition was proposed based on Margules' equation extended by the wind contribution and some assumptions, i.e. the maximum depth of the coastal current in the Skagerrak was set as being equal to the upper layer depth iin the Kattegat and the width of the coastal current was set as being equal to the baroclinic Rossby Radius in the Kattegat. The assumptions were evaluated on the basis of the two surveys and found to be fulfilled. Acknowledgements

The National Agency of Environmental Protection (Denmark) under the Marine Research Programme 1990 and the EC under the MAST II project DYNOCS (Contract No. MAS2-CT94-0088) is acknowledged for financing this project. Special thanks go to Dr A. Stigebrandt, University of Gothenburg for his much appreciated comments and advice regarding a draft of this manuscript. Thanks also go to the anonymous reviewers for helpful suggestions.

REFERENCES /Ertebjerg G., P. Sandbeck, S. Lund0er, F. Jakobsen, B. LCkkegaard, J. N. Jensen, O. L. Jensen and P. B. Christensen (1991) Marine Omrhder--Fjorde, kyster og ~bent hav: VandmiljCplanens overv~gningsprogram 1990. Faglig Rapport Fra Danmarks MiljCunderscgelser, 40, 132 pp. Andersson L. and L. Rydberg (1993) Exchange of water and nutrients between the Skagerrak and the Kattegat. Estuarine, Coastal and Shelf Science, 36, 159-181. Aure J. and R. S~etre (1981) Wind effects on the Skagerrak outflow. In: The Norwegian Coastal Current, R. S~etre and M. Mork, editors, pp. 263-294. Reklanlstrykk A. S., Bergen. Chow V. T. (1,)86) Open-channel hydraulics, 22nd printing, McGraw-Hill. Danielssen D. S., L. Davidsson, L. Edler, E. Fogelqvist, S. Fonselius, L. FOyn, L. Hernroth, B. H~kansson, I. Olsson and E. Svendsen (1991) SKAGEX: some preliminary results. Council Meeting, 1991/C:2, 33 pp. Dietrich G. (1951) Oberfl/ichenstrfmungen im Kattegat, im Sound und in der Beltsee. Deutsche Hydrographische Zeitschrift, Band 4, Heft 4/5/6, pp. 129-150. Fonselius S. H. (1990) S K A G E R R A K - - t h e gateway to the North Sea. SMHI Oceanography, 38, 29 pp. Garratt J. R. (1977) Review of drag coefficients over oceans and continents. Monthly Weather Review, 105,915929. Gill A. E. (1982) Atmosphere--ocean dynamics. International Geophysics Series, Vol. 30, Academic Press, 662 PP. Jacobsen T. S. and N.-E. O. Hansen (1985) Oxygen Depletion in the Kattegat. Nordic Hydrology, 16,237-256. Jakobsen F. (1991) The Bornholm basin---estuarine dynamics. Technical University of Denmark, Institute of Hydrodynamics and Hydraulic Engineering, Series Paper, 52, 199 pp. Jakobsen F. (1995) The major inflow to the Baltic Sea during January 1993. JournalofMarine Systems, 6(3), 227240. Jakobsen F. and M. J. Lintrup (1997) The exchange of water and salt over the Drogden Sill in Oresund during the period September 1993 to November 1994. Nordic Hydrology, in press. Jakobsen F., G. /Ertebjerg, C. T. Agger, N. K. H0jerslev, N. Holt, J. Heilmann and K. Richardson (1994) Hydrografisk og biologisk beskrivelse af Skagerrak-fronten. Havforskning fra MiljCstyrelsen, 49, 106 pp.

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F. Jakobsen

Matth/ius W. and H. Franck (1992) Characteristics of major Baltic inflows--a statistical analysis. Continental Shelf Research, 12(12), 1375-1400. McClimans T. A. (1988) Estuarine fronts and river plumes. In: Physical Processes in Estuaries, J. Dronkers and W. van Leussen, editors, pp. 55-69. Springer, Berlin. M¢ller J. S. and I. S. Hansen (1994) Hydrographic processes and changes in the Baltic Sea. Dana, 10, 87-104. Pedersen F. B. (1986) Environmental hydraulics, stratified flows. Lecture Notes on Coastal and Estuarine Studies, Springer-Verlag, Vol. 18,278 pp. Pedersen F. B. (1993) Fronts in Kattegat The hydrodynamic key factor for the biology. Estuaries Special Issue, 16, 104-112. Pedersen F. B. and J. S. M¢ller (1981) Diversion of the river Neva--How will it influence the Baltic Sea, the Belts and Kattegat. Nordic Hydrology, 12, 1-20. Pingree R. D., P. M. Holligan, G. T. Mardell and R. P. Harris (1982) Vertical distribution of plankton in the Skagerrak in relation to doming of the seasonal thermocline. Continental Shelf Research, 1(2), 209-219. Poulsen O. (1991) The Hydrography of Skagerrak and Kattegat--The Dynamics of the Skagerrak Front. Technical University of Denmark, Institute of Hydrodynamics and Hydraulic Engineering, Series Paper, 54, 164 pp. Richardson K. and T. S. Jacobsen (1990) JyllandsstrCmmen. NPo-forskning Fra MiljCstyrelsen, C6, 68 pp. Rodhe J. (1987) The large-scale circulation in the Skagerrak; interpretation of some observations. Tellus, 39A, 245-253. Rodhe J. (1992) Studies of currents and mixing in the Skagerrak; Paper V; The water masses of the Skagerrak, dynamics and large-scale mixing. Ph.D. Thesis, University of Gothenburg, Sweden. Romell L. and A. Stigebrandt (1985) Determination of the absolute sea level difference between Varberg and Frederikshavn. Geophysica, 21(1), 65-72. S~etre R., J. Aure and R. LjCen (1988) Wind effects on the lateral extension of the Norwegian Coastal Water. Continental Shelf Research, 8(3), 239-253. Sparre A. (1984) The climate of Denmark. Climatological papers No. 11 by the Danish Meteorological Institute. Stigebrandt A. (1980) Barotropic and baroclinic response of a semi-enclosed basin to barotropic forcing from the sea. In: Fjord Oceanography, H. S. Freeland, D. M. Farmer and C. D. Levings, editors, Plenum, pp. 151164. Stigebrandt A. (1983) A model for the exchange of water and salt between the Baltic and the Skagerrak. Journal of Physical Oceanography, 13,411-427. Stigebrandt A. (1984) Analysis of an 89-year-long sea level record from the Kattegat with special reference to the barotropically driven water exchange between the Baltic and the sea. Tellus, 36A, 401-408. Stigebrandt A. (1987) Computations of the flow of dense water into the Baltic Sea from hydrographical measurements in the Arkona Basin. Tellus, 39A, 170-177. Svansson A. (1975) Physical and chemical oceanography of the Skagerrak and the Kattegat. Fishery Board of Sweden, Institute of Marine Research, Report No. 1, 88 pp. Svansson A. (1984) Hydrographic features of the Kattegat. Rapp. P.-v. Rdun. Cons. int. Explor. Met, 185, 78-90.