Simulation Experiments of the Evolution of Mesoscale Circulation Features in the Norwegian Coastal Current

303

SIMULATION EXPERIMENTS OF THE EVOLUTION OF MESOSCALE CIRCULATION FEATURES IN THE NORWEGIAN COASTAL CURRENT P.M. HAUGAN, J.A. JOHANNESSEN. K. LYGRE, S. SANDVEN & O.M.JOHANNESSEN Nansen Remote Sensing Center, Edvard Griegsvei 3A, N-5037 Solheimsvik, Bergen (Norway)

ABSTRACT

Synoptic observationsof the mesoscale circulation in the Norwegian Coastal Current off the west coast of Norway are used to test and improve model forecasts. Temporal and spatial mappings of the current (Acoustic Doppler Current Profiler) during a field investigation period in February - March 1986are obtained using objective analysis techniques. An exceptional sequence of cloud-free NOAA satellite IR images is shown to be correlated with the in situ data set, and used for initializationof a two-layer quasi-geostrophicmodel. Sensitivity studies are performed to quantify important physical mechanisms, and identify which types of data are most important for precise model forecasts. Baroclinic and barotropic instabilities and interactions with some major topograhic features are shown to be capable of producing meanders and detached eddies like those observed in the in situ and remote sensing data set. Varying initial conditionsin the southern part of the model domain have little influence on the evolution in the northern part over a two week period. However, the baroclinic structure of perturbations is important for the evolution locally. Recirculation of Atlantic water along the western slope of the Norwegian Trench and topographic steering of Atlantic and coastal water contributesto the mesoscale flow pattern. INTRODUCTION It is well established from NOAA AVHRR satellite images that mesoscale (30-100 km) eddies are dominant features in the Norwegian Coastal Current, hereafter referred to as the NCC (Johannessen and Mork, 1979, Audunson et al., 1981, Johannessen et al., 1983). In order to quantify this variability, and assess the importance of various effects which have been proposed to be responsible for eddy generation, propagation and decay, such images were recently used in near real time to locate eddy features for detailed three-dimensional in situ investigation (Johannessen et al., 1988). The unique high quality data set obtained is suitable for relating surface IR informationto subsurface structure, and allows combination with and testing of eddyresolving numerical models. A relatively simple two-layer quasi-geostrophic (QG) model, expressing conservation of potential vorticity in an inviscid fluid overlying gentle bathymetry on an f-plane, is used. In the absence of strong local winds, this model has been shown to capture the essential dynamics of mesoscale circulation in the NCC (Ikeda et al., 1988). The same QG model was used to predict the drift and eddy induced spreading of toxic algae (Johannessen et al., 1988b) in late May 1988, giving input to daily algae forecasts issued to the fish farmers and general public, Again IR images were used for initialization of the model, and ADCP data from R/V Hkon Mosby provided in situ current information used directly in the forecasts and for veritication of the model. In section 2 we review the major results from Johannessen et al. (1988a). In section 3 we present the simulation experiments initiated with different interpretations from IR images, and discuss the main results from the various case studies. Finally section 4 contains the conclusions.

304

Fig. 1. Overview of the experiment area: Ship transects 1-26 perfonned from 24 February to 5 March 1986, bottom topography (meters) and model domain (section 3).The in situ data was obtained from the R/V HAkon Mosby, supplemented by three current meter moorings and seven Argos drifters with drogues. With an average ship speed of 7 knots, the 3-D temperature, salinity and velocity structure was simultaneously mapped with a towed, undulating CTD (SeaSoar) and a ship mounted 150 kHz Acoustic Doppler Current Profiler (ADCP).

305

2 OBSERVATIONS The Coastal Eddy Tracking Experiment performed in February-March 1986 was the first dedicated mesoscale circulation experiment in the Norwegian Coastal Current off the west coast of Norway. Clear weather allowed the collection of an exceptional sequence of cloud-free NOAA AVHRR images in the NCC area The images were processed in near real time, and used to direct the ship into interesting areas for eddy mapping (Fig. 1 and 2).

b

a 6:

3

6

60

50

50

.

4

10 km:1 m/3:-

Fig. 2. a) OA velocity maps from ADCP data (ship tracks shown) at 25m depth obtained from 26 February to 1 March. 5 km grid spacing. Correlation functions from Freeland and Could (1976): longitudinal velocity correlation f(r) = (1 + br)exp(-br), where b is chosen as 0.032, i.e. length scale is 50 km. Signal to noise ratio is 5. Original data are subsampled to 6-7 km minimum distance to avoid problems with correlated noise (Clancy, 1983). b) Contoured sea surface temperatures (Contour interval 0.5 O C) from IR images on 25 & 27 February. The area north of approximately 60 ON was cloudfree on 27th, and is combined with the image from the 25th south of 60 ON. Temperature gradient across the frontal zone is 0.2 "C/km. Significant features are labelled: corresponds to A 1 , I to C1,Ato C2,Xto A2, and+to C3.

The ADCP data collected during the experiment provides a dense sampling of reliable instantaneous velocities along the ship tracks from about 10m below the surface to 10-20m above the bottom. In order to obtain an interpolated velocity field suitable for temporal and spatial comparison to IR images and QG models, objective analysis (OA) techniques (Brethertonet d., 1976. McWilliams et al., 1986. Carter and Robinson, 1987), have been applied. Figure 2a shows in the northern part, a well mapped asymmetric cyclonic eddy C1 on the offshore side of the main jet and weaker indications of an anticycloneA1 further offshore. (The star-shaped ship tracks were primarily designed to map selected features. The resulting data coverage gives increased uncertainty near the boundaries of the domain.) In the southern part, the current jet is meandering with an anticyclone A2 towards the shore and two cyclones C2 and C3 offshore. The cyclones have a significant bmtropic flow component of 0.2- 0.3 mls, with the ratio of baroclinic to barompic flow ranging from 0.4 to 1.3. However, the anticyclone A2 has a much weaker barompic circulation. Observations in general characterizecyclonic eddies as barompic and anticycloniceddies as baroclinic (Johannessen et al., 1988, McClmans, personal communication). Comparison of the OA current fields and the IR data (Fig. 2a-b) shows that in the southern part of the investigation area, the thermal surface front is located somewhat offshore of the main current jet at 25m depth, while they coincide more closely, both lying quite near the coast in the northern part. Offshore extension of the surface front in the southern part has previously been associated with northerly wind conditions (Saetre et al., 1988), while Ikeda et al. (1988) suggested topographic steering due to the ridge south of 59 O N (Fig.1). Further north, onshore advection and cyclonic turning of Atlantic water (Johannessen et al., 1988, Fumes et al., 1986) may contribute to keeping the jet near the coast. 3 SIMULATION EXPERIMENTS The numerical model is the two-layer quasi-geostrophic model by Ikeda and Ape1 (1981). The model was applied to the NCC in Ikeda et al. (1988), where details about the model and results of many test cases can be found. Governing equations are given in Ikeda and Lygre (1989, this volume). The model geometry, shown in Fig. 3, is basically a straight channel with simplified representation of the main topographic features: The ridge at 58.5 O - 59 N, the cross-channel slope deepening eastward in the region of (58.5 - 59.5 O N,3 - 4 O E), and the gentle northward deepening of the Trench north of 60 O N. The boundary conditions are given with fixed streamfunction on the (closed) side boundaries, and on the southern, upstream boundary which has fixed inflow. At the northern, downstream boundary, the streamfunction and potential vorticity are assumed to propagate out of the domain at the phase velocity of the fastest growing wave predicted by linear stability theory. We discuss 5 cases here (Table l), primarily designed for testing the sensitivity on interpretation of the IR images used for initializing the model. The first case is similar to the simulation case in Ikeda et al. (1988) with initial condition based on the IR image of 13 February, cases 2-4 are variations of this, and case 5 is initialized on 16 February based on the IR image at that date.

307 X

-10

J.

Fig. 3. Model geometry. Distances are in multiples of Rossby deformation radius, L = 5.7 km. Grid resolution is 1/2 L.The domain covers 114 x 430 km (40 x 150 grid) geographically located as shown in Fig. 1. H I = 50m, H2=250m, Hr = 60m. The ridge has an e-folding distance of 2L in the x-direction and 2.24 L in the y-direction. Bottom slopes are tan(&) = 0.39 m/km, tan(0y) = 3.9 mkm. TABLE 1 Summary of cases. Length scale L is 5.7 km. Velocity scale U is 0.3 m/s. Variations from the basic case 1 are underlined. Case

1

Anomaly #

X 111

Location

rLi

5.0

1 2 3 4 5 6

53.5 46.0 38.0 32.0 26.0 15.0

2

It o 4

as case I

3

1 to6

as case1

4

1 2 3 4 to 6

5

1 2 3 4

Y

I,

It I,

,,

53.5

38.0

r

Inflow/ outflow

4.0 I,

11

Yes

I1 I, I,

Q!!!

5.0

1.5

au

1.5

ILz_5

11

1.5

4.0

4.0

I' 1,

6aau

u

1.0

1.5 -1.5 1.5 -1.5 1.0 1.0

E-folding scale ri 1

Yes

as c a w 1

5Q-Q

1.5 -1.5 1.5 -1.5

1.0

I,

as case 1

Streamfunction anomaly upper lower RJLl ruu

49

n

2 7 . 5 u

1.5 -1.5 1.0 1.0

-1.5

1.0 1.0

11

u 2 4.0

Yes

Yes

308

In case 1, the base case, the flow field on 13 February (Fig. 4a) is interpreted as a straight upper layer jet with two cyclonic and four anticyclonic, barotropic eddies superimposed, and a barotropic inflow/outflow at the northern boundary diminishing southwards, representing the retroflection of Atlantic water (Fig. 4d,e). The straight jet is located east of the surface thermal front in the southern part of the domain in qualitative agreement with the analysis in the previous section. The main features of the development (Fig. 5a-c and Fig. 6b) reveal a growth of the anticyclonic meanders ml and m2, a strenghtening of the cyclone C1 which entrains ml, and finally (on 27th) a strong vortex pair A1 - C1 is created. On 27th three cyclones C1, C2,C3 at the offshore side of the jet are observed, in qualitative agreement with the combination of the IR images on 25th and 27th, Fig. 6a. Interaction with the seamount is suggested to be responsible for the generation of the cyclone C3 (Ikeda et al., 1988). The anticyclones A1 and A2 are less pronounced in the IR data. An anticyclone assumed to be A2 is also observed in the OA current fields (Fig. 2a). On the other hand, the current fields reveal minor evidence of the anticyclone A]. Case 2 is based on case 1, but with m3 and mq removed. As seen in Fig 6c, this removal has practically no effect on the development of the vortex pair A1 - C1. Thus, the evolution in the downstream region is found to be very weakly dependent on initial conditions upstream. The cyclone C3 observed on 27th in the base case is not seen in this case. This cyclone was generated in the trail of m q (A4), which has now been removed. Hence we conclude that the initialization of the model with the correct number of meanders, is important for a complete description of the flow field. Case 3 is again based on the basic case 1, but the barotropic (Atlantic water) inflow/outflow in the northern part of the domain has now been removed. As seen from the figures, this hardly influences the southern region. In contrast, the development in the northern region differs from the base case in the following ways: Firstly, the strong anticyclonic eddy A1 has now reached the artificial offshore boundary on the 27th, and secondly, it has not rotated so much around its companion cyclone C1 as in the base case; the cyclone is weaker and located more offshore. This is interpreted as that the onshore advection caused by the topographic steering of Atlantic water in the downstream part was responsible for pushing the vortex pair towards the coast, showing the importance of Atlantic inflow/outflow for obtaining a realistic flow pattern in the northern part of the domain. Also the stronger cyclone C1 in the base case may be explained by the contribution of cyclonic vorticity from the recirculation of Atlantic water. Case 4 is also based on case 1, but ml and 11-12 are now made baroclinic by halving the intensity of the vortex signal in the lower layer. The anticyclone A1 previously found on the 27th is now totally absent. This implies that the model is very sensitive to the vertical structure of the initial flow field. Note that the decreased lower layer circulation in ml and m 2 will reduce topographic effects, so that in this case a developing anticyclone A1 is not kept in place. As is also commented by Ikeda et al. (1988), A1 is not so obvious in the IR data, or in the ADCP data, Fig. 2a. Hence this case study suggests that the initial meander should have some baroclinicity. Johannessen et al. (1988a) found that the ratio of baroclinic to barotropic speed at the core of the coastal current in the vicinity of the cyclonic eddy C1 was 0.86.

309

b

a

C

3

d

e

4

f

--ml

-cl -- rn2

0

-m3

Fig. 4.a,b,c) Contoured IR images on 13, 15 and 16 February, d) Initial streamfunction (13th) i n the upper layer for the basic case 1, e) Initial streamfunction (13th) in the lower layer for case 1, f) Initial streamfunction (16th) in the upper layer for case 5 .

310

a

C

b

ml

c1 A2

c2

E2 A3

A4

Fig.5. Evolution of upper layer streamfunction in the basic case 1. a) 15, b) 19, c) 23 February.

Case 5 is initialized from the situation as seen from the IR image on the 16th (Fig.4c and 0. There is an anticyclonic meander ml in the downstream region and a cyclonic meander c l upstream, somewhat displaced towards the shore. A small anticyclonic meander m2, and a larger anticyclonic meander m3 displaced offshore, are located further upstream. On the 27th this case also shows the developed vortex pair A1 - C1 (Fig. 60, although not as strong as in the base case. The upstream part hardly shows any similarities in comparison to the base case. Interactions with the seamount upstream are lost in this realitively short integration period as there is initially only an upper layer jet in this region.

311

3

C

b

a 4

-A1

-c1

3

4

d

e

f

!-A1

-c1

Fig. 6. a) Same as Fig. 2b., b) -0Upper layer streamfunction on 27th for the various cases: b) Base case 1, c) Case 2, d) Case 3, e) Case 4, 0 Case 5 .

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4 CONCLUSIONS

A relatively simple numerical model has shown some skill in reproducing mesoscale circulation patterns in the NCC. There is an apparent decoupling between the evolution of the flow in the upstream and downstream parts of the domain over a two week period. The rapid growth and almost steady location of a cyclonic eddy over the northward deepening of the Norwegian Trench north of 60 N is suggested to be associated with topographic steering of both Atlantic and coastal water. The model is very sensitive to the vertical structure, indicating that initializationfrom IR images should be supplemented by in situ data. Coast-fjord exchange and local wind effects are not taken into account in these model simulation studies. For the time period considered, winds were generally low and not expected to influence the mesoscale circulation significantly. For more general situations, local wind should be included. Indirect wind effects occur through slackeningof west and southwesterlywinds in the Skagerrak, followed by outbreaks in the NCC. Time variable inflow boundary conditions both from the Skagmak and of Atlantic water should be further explored. In order to test and validate models of the mesoscale circulation in the NCC, more data sets from different episodes are needed. Repeated near synoptic 3D mapping of the kind obtainable from research vessels like HAkon Mosby, coupled with remote sensing observations, can give high quality data with the resolution and coverage required. With sampling strategiesdictated by objective analysisrequirements, and further developing OA to integrate the various data types, the improved data base obtained will allow the discrimination and quantification of various effects, required for model improvements. 5 ACKNOWJXDGEMENTS Thanks to Statoil, Norwegian Space Center/NTNF (Royal Norwegian Council for Scientific and Industrial Research) and University of Bergen. 6 REFERENCES Audunson, T., Dalen, V., Krogstad, H., Lie, H.N. and Steinbakke, O., 1981. Some observations of ocean fronts, waves and currents in the surface along the Norwegian coast from satellite images and drifting buoys. In: Saetre, R. and Mork, M. (Editors), The Norwegian Coastal Current, University of Bergen, pp. 20-56. Aure, J. and Saetre, R., 1981. Wind effects on the Skagerrak outflow. In: Saetre, R. and Mork, M. (Editors), The Norwegian Coastal Current, University of Bergen, pp. 263-293. Bretherton, F.P., Davis, R.E and Fandry, C.B.. 1976. A technique for objective analysis and design of oceanographicexperiments applied to MODE-73. Deep-sea Res. 23,559-582. Carter, E.F. and Robinson, A.R., 1987. Analysis models for the estimation of oceanic fields. J. Atm. Oceanic Techn. 4,49-74. Clancy, R.M., 1983. The effect of observational error correlations on objective analysis of ocean thermal structure. Deep Sea Res. 30,9A, 985-1002. Freeland. H.J. and Gould, W.J., 1976. Objective analysis of meso-scale ocean circulation features. Deep Sea Res. 23,915-923. Fumes, G.K., Hackett, B. and Sztre, R., 1986. Retroflection of Atlantic Water in the Norwegian Trench. Deep Sea Res., 33, 2, pp. 247-265. Ikeda, M. and Apel, J.R., 1981. Mesoscale eddies detached from spatially growing meanders in an eastward-flowingoceanic jet using a two-layer quasi-geostrophicmodel. J. Phys. Oceanogr., 11, pp. 1638-1661. Ikeda, M. and Lygre, K., 1989. Eddy-current interactions using a two-layer quasi-geostrophic model. In: Nihoul, J.C.J. and B.M. Jamart (Editors), Mesoscale/Synoptic Coherent

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Structuresin GeophysicalTurbulence, Elsevier Oceanography Series, Elsevier, Amsterdam (this volume). Ikeda, M., Johannessen, J.A., Lygre, K. and Sandven, S., 1988. A process study of mesoscale meanders and eddies in the Norwegian Coastal Current. J. Phys. Oceanogr., 18, No. 12. Johannessen, O.M. and Mork, M., 1979. Remote sensing experiments in the Norwegian coastal waters. Report 3/9. Geophysical Institute, University of Bergen, Norway. Johannessen, O.M., Johannessen, J.A. and Farrelly, B., 1983. Application of remote sensing for studies, mapping and forecasting of eddies on the Norwegian Continental Shelf. In Proceedings from EARSEWESA Symposium on Remote Sensing for Environmental Studies, Brussel, Belgium, 26-29 April, 1983. Johannesssen, J.A., Svendsen, E., Sandven, S., Johannessen, O.M. and Lygre, K., 1988a. Three dimensional structure of mesoscale eddies in the Norwegian Coastal Current J. Phys. Oceanogr., 18, No. 12. Johannessen, J.A., Johannessen. O.M. and Haugan, P.M., 1988b. Remote sensing and model simulation studies of the Norwegian Coastal Current during the algal bloom in May 1988. Submitted to Int. J. Rem. Sens. McWilliams, J.C., Owens, W.B.and Hua, B.L. 1986. An objective analysis of the POLYMODE Local Dynamics Experiment Part I General formalism and statistical model selection. J. Phys. Oceanogr. 16, No. 3, 483-504.