North sea Model Advection Dispersion Study

Objective (1), the Realistic Test Case
This focusses on the period November 1988 to October 1989 and spatially on the southern North Sea. The models are taken as they are presently used, i.e. varying from one another in the detail of included physics (2D/3D, barotropic/baroclinic, dispersion formulation), numerical solution techniques, grid sizes, calibration and underlying hydrodynamics. Thus the one constraint is that the models must all have the common spatial coverage of the North Sea basin between 51deg N and 55deg 40’N. To enable model simulations to be carried out a comprehensive [data set] has been constructed which includes bathymetry, tides, time- varying meteorology, boundary forcing and river discharge. Within the intercomparison there are four 2D models and nine 3D models, some of these are tidally-resolving (mesoscale) (9) and some are not (macroscale) (3).

Four experiments were defined:
– Experiment 1 (for 2D models only): continuous release of conservative and non-conservative tracers from the six locations (see figure) for 180 days starting 1 March 1989. Parameters examined include the age and concentration of the tracer.
– Experiment 2: single particle release from each location at four intervals in the year (releases at surface, mid-depth and bottom for 3D models). Parameters examined include distance travelled and relative tracer position.
– Experiment 3: instantaneous release of a 1kg tracer from each location at 2 times, 1 March and 1 August 1989. Parameters examined include trajectory of the centre of mass, total mass and patch shape and area.
– Experiment 4: Salinity and accumulated volume flux across 2 east-west sections at 52.5deg N and 54.5deg N (see figure). Parameters examined include time slice values and spatial variability.
The 2D models were run for the full 12 month period, the 3D models were run for either a) a six-month period, March-September 1989, or b) two 1-month periods, March and August 1989, depending on partners computational ability. Grid sizes of the models ranged from 2.4 km up to 20 km.

Objective (2), the Idealised Test Case
The experiment is the development of a fresh water eddy. A cylinder of fresher water is placed in the surface layer of an ambient fluid and allowed to mix. As the eddy forms under the influence of rotation, instabilities (secondary eddies) develop, the order of which depends on the friction (numerical and interfacial) in the system. The experiment is based on laboratory measurements (Griffiths & Linden, 1981, Journal of Fluid Mechanics, 105, 283- 316) and the numerical design of James (1996, Journal of Marine Systems, in press).
Five partners have participated in this experiment, producing 2 distinct sets of instability related to their numerical schemes.
Together, both experiments have generated more than 900 datasets for intercomparison. The intercomparison is presently in the analysis phase.
NOMADS will produce four Technical Reports:
• TR-1 Project rationale and experimental setup
• TR-2 Overview of the models to be used
• TR-3 Partner interpretations of the simulations
• TR-4 Intercomparison and conclusions.

The NOMADS dataset

The dataset has been constructed to enable partners to run the NOMADS simulations for the period 1 November 1998 to 31 October 1989. It contains the following data:

1) Covering the European Continental Shelf
Gridded bathymetry, 1/12deg latitude by 1/8deg longitude

Ten tidal constants around the shelf break

Gridded meteorological data at 3-hourly intervals (approx. 75km resolution) –

  • Surface winds
  • Atmospheric pressure
  • Relative humidity
  • Air temperature
  • Daily estimates of cloud cover derived from AVHRR imagery

Annual mean discharge of 43 rivers.

2) Covering the Common Area (Southern North Sea, 51deg N to 55deg 40’N)
Thirty tidal constants across north and south boundaries

Residual elevation and current at hourly intervals across the boundaries

Monthly discharges from 16 rivers

Initial conditions for Temperature and Salinity for March and August 1989.

The data is available via anonymous ftp, subject to owner conditions. Please contact the NOMADS co-ordinator for details (

Description of the STOWASUS-2100 project


The overall objective of STOWASUS-2100 is to study severe storms, surges and waves in the present climate and in a scenario with increased CO2-concentration. More specifically the project is a joint atmospheric/oceanographic numerical modelling effort aiming at constructing and analysing storm, wave and surge climatologies for the North Atlantic/European region in a climate forced by increasing amounts of greenhouse gases and to compare with present day conditions. It is investigated whether any systematic anomalies regarding frequency, intensity or area of occurrence are found for these extreme events. Also physical mechanisms responsible for possible scenario anomalies are investigated.


Off-shore industries, fisheries, shipping companies, and the insurance business are highly sensitive to extra-tropical strong wind events and the associated ocean waves and surges. It is likely that impacts of possible future changes in the occurrence of extreme type events like these and others will be more severe than modulations of the long term mean climate. This is the rationale behind the STOWASUS-2100 project which aims at setting up climate change scenarios for storms, waves and surges on a variety of spatial scales. On the larger scales, studies on storminess in the North Atlantic region will be performed, while detailed studies on storminess, surges and waves will be carried out in the Adriatic, The North Sea and the Norwegian Sea. On the local scales, storms and surges will be studied in estuaries, low lying coastal areas along the North Mediterranean and North-western European coasts.

The project builds to considerable degree on the results obtained in another project called WASA, which has been described by “The WASA Group” (1998). In WASA it was found that the storm and wave climate has roughened in recent decades, but that the present intensity of the storm and wave climate seems comparable with that at the beginning of the 20th century. The WASA project furthermore analysed and used the output from a high-resolution (T106 spectral truncation) climate change scenario experiment, mimicking global warming due to increase greenhouse gas concentrations. It was found that storm and extreme wave activity was slightly increased in the Bay of Biscay and in the North Sea in a warmer climate, while this activity was slightly weakened at several other places. The experimental set-up of the climate model simulations on which these results were based has been described by Beersma et al. (1997) who pointed out that the projected anthropogenic changes in storm activity fall well within the limits of variability observed in the past considering the length of the (control and scenario) simulations which was only 5-years.

Recently, two so called time slice simulations with the ECHAM4 model also at T106 horizontal resolution have been performed at the Danish Meteorological Institute (DMI) in a collaboration between the Max Planck Institute for Meteorology in Hamburg and DMI. These simulations each covered a period of 30 years, i.e. 6 times longer than the simulations used in WASA. Thus they should be much more suited for studies of storminess and associated impacts since the sampling problem is considerably reduced. The STOWASUS-project therefore uses these new simulations as a backbone to drive very high resolution regional atmospheric climate models and wave and surge models of different resolution. It is these secondary simulations which will be used to set up climate change scenarios of storms, waves and surges along European shelf and in European Estuaries and to compare these with present day conditions. The project is logically divided into 12 working tasks some of which will be described briefly in the following three sections together with preliminary results – to the extend they are available at this early state of the project.


As mentioned above the backbone in STOWASUS-2100 consists of two 30 year time slice simulations with the ECHAM4 atmospheric climate model at T106 horizontal resolution. The experimental design of these simulations which are not part of the project is described in May and Roeckner (1998). The project includes an analysis of the storm and extreme wind climate in these simulations. Fig. 1 (left column) shows the long term mean sea level pressure (MSLP) in winter (DJF) as obtained from the European Re-Analysis data (ERA), from the control simulation and from the scenario simulation. It is seen, that the ECHAM4 model simulates the atmospheric mass field well except for a too high pressure over and immediately to the west of the Iberian peninsula which leads to a too zonal flow over NE Atlantic region. The figure also shows the difference in the MSLP between the scenario and control simulations in the bottom panel, and it is seen that there is a significant increase in the zonality over the northern part of the area associated with a decrease in the high latitude MSLP in the scenario. The right column in Fig. 1 similarly shows the standard deviation of the band pass filtered (2.5-6 day filter) 500 hPa winter height fields which is commonly used as an estimate of storm activity. Also here the model behaves well and a significant increase in storm activity is seen over Northern Europe in the scenario simulation relative to the control simulation together with a corresponding decrease in storm activity around the east coast of US. Fig.2 shows the 1% percentile wind speed for the winters (DJF). Comparing the ERA data and the control run shows that the ECHAM4 model has more severe storms along the south and east coast of Greenland than the ERA data. The difference between the scenario run compared to the control run (fig. 2d) shows more severe storms in the Atlantic north of 60N, and less severe storms south of this latitude. These changes are in accordance with the changes in the 500 hPa variability (fig. 1d). The changes in near surface wind (Fig. 2) are important to the wave and surge simulations (see below) as one can expect that the enhanced wind speeds will also lead to more severe wave and surge activity. This has, however, not yet been shown in the project.

The atmospheric investigations will also cover atmospheric modelling with very high resolution regional climate models (HIRHAM and BOLAM) to perform of intensive systems that are not well resolved at T106 resolution: intensive extra-tropical baroclinic developments, polar lows and highly convective systems (with some apparent similarities to polar lows) in the Mediterranean. Fig. 3 shows the orography in the T106 model and in the BOLAM model over the Mediterranean region and it is seen that one may expect much larger impact from orographic effects at very high resolution than at T106. All the simulations with HIRHAM and BOLAM will take boundary conditions from the T106 time slice simulations and will to considerable degree focus on analysing and understanding the processes associated with possible changes in scenario cases relative to control cases.