MARE - Proposals

MARE Proposal

Programme title
Programme outline
Composition of the research group
Detailed description of the programme

Rationale
Main goals
Methods and time table
Key papers

Innovative character
Clivar(net) specifics
Relation to other international research programmes.
International cooperation
Economic and societal relevance
Feasibility

1. Programme title:

Mixing of Agulhas Rings Experiment (MARE)

2. Programme outline (outline scientific objectives):

Climate variability at interannual, decadal to millennial time scales is coupled to variations in the oceans thermohaline circulation (THC) (1,2). Interocean exchange of water around South Africa is thought to be a key link in the maintenance of the THC (3,4). As a result, variability in interocean exchange induces variability in the global THC and can affect the production of North Atlantic Deep Water (NADW) (5) and associated climate variability over Europe. Interocean exchange, or Agulhas leakage, occurs on an intermittent basis. It is determined largely by the shedding of Agulhas Rings which is extremely variable on interannual time scale (6,7). As a result Agulhas Rings seem to be the most likely source of South Atlantic circulation anomalies that influence NADW production. However at present it is unclear what proportion of Agulhas Ring Water is transferred by mixing to the THC. The main objective of the MARE programme is therefore to determine the proportion of Agulhas leakage that contributes to the northward branch of the THC, estimate its variability at interannual to millennial time scales and determine the impact of anomalous Agulhas leakage on the strength of the Atlantic overturning circulation and associated (actuo- and paleo-) climate fluctuations over the northeast Atlantic sector.

3. Composition of the research group

Names and titles	Specializations	Affiliation	Fraction of working time spent on the programme
Prof.dr.W.P.M. de Ruijter	phys. oceanography	Utrecht Univ.	25%
Dr. P.J. Van Leeuwen	phys. oceanography	Utrecht Univ.	25%
Dr. S.S. Drijfhout	oceanogr./clim. dyn.	KNMI	25%
Dr. J.H.F Jansen	paleoceanography	NIOZ	25%
Dr. C. Veth	phys. oceanography	NIOZ	25%
Dr. G.-J.A. Brummer	paleoceanography	NIOZ	20%
Dr. G.J.M. Versteegh	organic chemistry	NIOZ	10%
Dr. H.M. van Aken	phys. oceanography	NIOZ	10%
Dr. G.M. Ganssen	paleoceanography	Free. Univ. Amsterdam	10%
Prof.dr.J.R.E. Lutjeharms	phys. oceanography	Univ. Cape Town	5%
Dr. M. Rouault	air-sea interaction	Univ. Cape Town	5%
Dr. H.A. Dijkstra	phys. oceanography	Utrecht Univ.	5%
OIO (NWO)	phys. oceanography	Utrecht Univ.	100%
Postdoc (NWO)	phys. oceanography	NIOZ	100%
OIO (NWO)	phys. oceanography	KNMI	100%
Postdoc (NWO)	paleoceanography	NIOZ	100%
Postdoc (NOP II)	phys. oceanography	Utrecht Univ.	100%
Dr. A. Kattenberg	climate dynamics	KNMI	50%

4. Detailed description of the programme:

4.a. Rationale

Climate and its variability over Western Europe depends crucially on the warm north-eastward branch of the North Atlantic Current. Variations in the strength and position of that branch are directly related to variations of the global overturning, so-called thermohaline, circulation (THC) of the ocean.

In the upper branch of the THC a critical link is formed by the heat and salt exchanges between the Indian and Atlantic oceans around South Africa (3, 4). Varying fluxes at thermocline and intermediate water levels ('Agulhas leakage') at the Agulhas Retroflection are related to a weakening or strengthening of the Atlantic overturning circulation (5) and associated heat transport variations towards the North-East Atlantic sector. As a consequence, the South Atlantic plays a unique role in the present day climate in that it transports heat towards and across the equator. Proxy data suggest that during glacial periods the Atlantic heat transport was reversed and poleward (e.g. 9, 10). It is unknown to what degree this was related to a reduction or shut off of the heat input by Agulhas leakage. Recent observations have shown that Agulhas leakage varies strongly on seasonal and interannual time scales. Its variability is directly linked to the intermittent shedding of large Agulhas Rings at the Agulhas retroflection (6, 11) that carry their anomalous Indian Ocean properties into the South Atlantic. A part of this leakage merges into the northward branch of the Atlantic THC. The remainder recirculates in the wind-driven 'super'-gyre that connects the subtropical gyres of the South-Indian and South-Atlantic Oceans (4, 12). As a consequence, variability of heat and salt transport by Agulhas leakage may lead to climate variations at regional scale (Africa), at basin scale (across the South Atlantic) and at global scale via the THC.
The division between water that recirculates and the part that feeds into the THC takes place mainly in the Benguela area, in the Southeast Atlantic. It is largely unclear what processes determine this subdivision and its variability and what proportion of Agulhas leakage eventually contributes to the THC.

Detailed descriptions of Agulhas rings have been and are still being made (e.g. 13) as a result of shipborne observations. However, each individual ring has different properties on being shed and has gone through a different history of transformation and mixing when investigated. Such one time hydrographic surveys provide very useful snapshots of individual rings. But, to quantify the partitioning of their heat and mass transfer between the subtropical gyre system and the THC one has to re-examine the same ring and its surroundings several times. The latter is due to the strongly non linear decay of the rings during the first year of their existence, when they travel on an irregular path through a Ring corridor (6) towards the Walvis Ridge. This is probably due to a shift of the dominant mixing and transformation processes from initial vigorous air-sea interaction and convection (14) to double diffusive interleaving after the rings have left the shedding area (e.g. 11).

A reliable budget estimate of the total input of Agulhas Ring leakage into the THC and its variability can only be established if the operation and effect are known of the dominant mixing processes that determine the decay of the Rings. This has to form the basis of a correct representation or parameterization of these processes in a detailed regional model in which the repeated in situ observations as well as satellite altimeter data are assimilated. Paleoceanographic data of previous Agulhas Ring systems under largely different climatic conditions may provide an additional data source to support the validation of the ring system. These proxy data may also answer the question as to whether abrupt global climate fluctuations that appeared in the past were associated with a sudden shut-off or reduction of Agulhas Rings and leakage.

The degree, to which the interbasin exchange is determined by or determines the Atlantic overturning circulation is still an unresolved problem. This can only be answered by coupled ocean-atmosphere modelling studies that involve realistic feedbacks between varying interbasin exchanges and changing global scale buoyancy fluxes across the air sea interface. More locally a related question is whether Agulhas leakage is related to intrinsic modes of ocean-only variability or whether there are coupled modes of varying interbasin exchange and associated atmospheric pressure field responses.

4b. Main goals

In view of the above the central objectives proposed for this programme are:
(I) to estimate the proportion of Agulhas (Ring) leakage that contributes to the northward branch of the global THC and identify the dominant mixing processes that determine that proportion;
(ii) to estimate its variability at interannual and decadal time scales in the present climate system;
(iii) to reconstruct variability of Agulhas Ring shedding and leakage in the past from paleo (proxy)- records;
(iv) to determine the impact of varying Indian-Atlantic interocean exchanges on variability on regional scale as well as on the strength of the Atlantic overturning circulation and associated climate fluctuations over the North Atlantic sector.

These objectives will be addressed in the following programme components:

Component A: Observations of Agulhas Ring mixing
Component B: Modelling and data assimilation of Agulhas Ring mixing
Component C: Paleoceanographic observations of the Agulhas Ring Corridor
Component D: Impact of Agulhas leakage on South Atlantic anomalies
Component E: Impact of South Atlantic anomalies on European climate variability

The first objective, (i), involves the programme components A and B. A consists of a sea-going programme in which the same carefully selected Agulhas Ring and its surroundings will be examined three times at different stages of its nonlinear decay. These in situ data, together with satellite altimeter observations, will be used in component B to determine optimal parameter settings in a high resolution model of the southeast Atlantic. This data assimilation system will then be applied for a budget study to estimate the total time varying input of the Agulhas Rings and associated leakage into the THC, and to determine its sensitivity to changing environmental conditions (objective ii). Objective (iii) is addressed in programme component C which aims at paleo- reconstructions of variability of Agulhas leakage and its relation to climate variability in the past. The data from components A and C will serve as input for programme components D and E, in which objective (iv) will be addressed. Component D aims at answering the question how anomalous Indian- to Atlantic Ocean fluxes affect the THC in the South Atlantic and what the relation is between local forcing and basin scale circulation anomalies.
Finally, in component E we will study the impact of basinscale circulation and watermass anomalies in the South Atlantic, such as determined from programme components A, B and C, on the strength of the Atlantic overturning circulation and associated climate variability over the European sector (objective iv). The initial phase of this component is presently being carried out as part of the so-called NOP-II programme, a Dutch National programme on Global Change. It will be extended in this proposed programme with studies using an Ocean GCM and coupled General Circulation Models (ECBILT and ECHAM) jointly by the staff of KNMI and IMAU (So, no additional support is requested for this component).

4c. Methods and time table

Components A and C consist of sea-going observations and subsequent analysis. Components B, D and E consist of numerical modelling and assimilation of the collected data and satellite altimeter data. After the data from the first cruise have been assimilated in the regional high-resolution model we try to predict the path and decay of the selected ring with the partly validated model. As explained above, at least three cruises are necessary to examine the same Agulhas Ring in three different stages of decay. Also, the paleoclimatological aspects will be studied from data and samples collected at all 3 cruises. The three cruises are motivated further by the fact that we need to measure and model:
1) the role of air/sea interaction in mixing and modifying Ring Water
2) the role of interaction with bottom topography and "ridge-induced" mixing
3) the role of small-scale turbulent mixing, double diffusion and interleaving.
4) the fate and robustness of proxy data in Agulhas Rings while they pass through the eastern South Atlantic (a calibration and reconstruction objective).

The timing of the cruises is guided by the wish to optimally measure the relevant mixing. Rings will be tracked by satellite altimetry in cooperation with DEOS (this is on-going as part of the NOP-II programme), to determine the course of the cruises and to monitor the Ring and its decay in between the cruises.

The concurrent modelling components consist of a data-assimilation system involving a high-resolution eddy- resolving isopycnic ocean model of the south-east Atlantic (component B), and a lower resolution eddy-permitting version of the same model for the whole South Atlantic (component D). The assimilation system is (among others) meant to be used to optimize the model parameters. In comp. D the model will be coupled to a particle tracking model to provide for a Lagrangian description of the spreading of the relevant water masses. Component E will use of global coupled ocean-sea ice-atmosphere models of intermediate complexity (ECBILT) and state of the art (ECHAM) as well as a global ocean general circulation model.

4d. Key papers

1) Sutton, R.T., and M.R. Allen, Nature, 388, 563-567, 1997.
2) Broecker, W.S.: Science, 278, 1582-1588,1997.
3) Schmitz, W.J.:, Rev. Geophys., 33, 151-173, 1995.
4) Gordon, A.L., R.F. Weiss, W.M. Smethie Jr. and M.J. Warner: J. Geophys. Res., 97, 7223-7240, 1992.
5) Thompson, S.R., D.P Stevens, and K. Doos:, J. Geophys. Res., 102, 3303-3315, 1997.
6) Goni, G.J., et al, J. Mar. Res., 55, 861-883, 1997.
7) Garzoli, S.L., et al, J. Mar. Res., 54, 1039-1071, 1996.
8) Weijer, W., W.P.M. de Ruijter, H.A. Dijkstra and P.J. van Leeuwen, J. Geophys. Res., Submitted.
9) Miller, J.R. and G.L. Russell: Paleoceanography, 4, 141-155, 1989.
10) Wefer, G. L. Labeyrie, M. Paterne, S. Hovine, T. Fichefet, J. Duprat and M. Labracherie: in The South Atlantic; present and past circulations, eds G. Wefer, W.H. Berger, G. Siedler and D.J. Webb, Springer, 1996.
11) Van Ballegooyen, R.C., M.L. Grundlingh and J.R.E. Lutjeharms: J. Geophys. Res., 99, 14053-14070, 1994.
12) De Ruijter, W.P.M. de Ruijter, A. Biastoch, S.S. Drijfhout, J.R.E. Lutjeharms, R.P. Matano, T. Pichevin, P.J. van Leeuwen and W. Weijer: J. Geophys. Res., Submitted.
13) Boebel, O., C. Duncombe Rae, S. Garzoli, J.R.E. Lutjeharms, P. Richardson, T. Rossby, C. Schmid and W. Zenk: EOS, Trans. Am. Geophys. Un., 79, 1, 1998.
14) Olsen, D.B., R.A. Fine and A.L. Gordon: Deep-Sea Res., 39S, 163-181, 1992.

5. Innovative character

The main new aspect of this proposed programme is that we will use both actuo-and paleo-oceanographic data, obtained from the observational components (A and C) of this programme, to develop and validate a detailed diagnostic and predictive model/data assimilation system of the Southeast Atlantic (component B) and that it will be applied to make budget estimates of the time varying input of Agulhas leakage into the Atlantic overturning circulation. The resulting estimates will then serve as input for a series of basin scale and global scale simulations on the impact of varying interbasin fluxes on climate variations over the Atlantic (comp. D, E). In the observational programme three cruises are planned to measure the same Agulhas Ring in different stages of decay and to estimate the relevant mixing and exchange processes.

Occasionally, a few eddies in the North Atlantic have been revisited before by research vessels to compare the hydrograpy at different times. We propose the timing of our cruises such that an optimal sampling can be made of the effect of the most relevant mixing processes and to estimate the water exchange with the surroundings. Agulhas Rings have been revisited before by coincidence at a WOCE-section. In that case, both the resolution and timing were insufficient to address the questions of this programme.

A related innovative aspect is the scientific organisation and cohesion of the programme. Detailed process- oriented measurements are embedded in a series of numerical impact studies that consider increasingly larger scales. By doing so, we address the global climate impact without neglecting the scales and processes that contain the relevant physics. Measurements of features on a 1-10 km scale (comp. A) will be interpreted and generalized in a numerical model that resolves the 10-100 km scale (comp. C). These results will be used within an impact study on the regional scale (1000 km, Comp. D), and the global scale (Comp. E). If not novel, this is one of the first programmes in which such a scale overarching effort is pursued.

A fourth innovative aspect is the cooperation between paleo- and actuo-climate research in one fieldwork programme. Although such a cooperation is both self-evident and desirable, it is certainly not common practice. This programme contributes to the integration of the description of paleodata and their interpretation within physically based numerical models.

A final innovative aspect is related to the integration and combination of a series of state-of-the-art methods into the present programme:
a) The development of a new state-of-the-art data-assimilation system for the South-east Atlantic.
b) Interpretation and evaluation of the larger scale ocean model (Comp. D) (among others) with the use of Lagrangian techniques. The use of particle tracking techniques in eddy-resolving ocean models is rather new, and both methods for calculating trajectories and seeding techniques to ensure statistical robustness of the results are currently being developed (EC-project TRACMASS, starting in 1998).
c) The fate and robustness of the proxies during settling and burial will be studied with time series from sediment traps moored below the travelling Agulhas Ring's thermocline, in combination with paleodata from multi cores taken from the bottom at the same locations. It is the first time that such an approach will be followed.
d) Impact studies will be performed with ECHAM and the coupled model ECBILT, which is one of the first models of intermediate complexity. It is one to two orders of magnitude faster than state-of-the-art coupled models and still shows realistic mid-latitude atmospheric variability. This model is ideally suited for impact studies as proposed here. Moreover, we will couple this atmospheric model to a higher resolution ocean model of the Atlantic. Such a coupled model is completely new.

6. Clivar(net) specifics

This programme carries out the Clivar main objective (Clivarnet p.3): "to describe and understand physical processes responsible for climate variability and predictability on interannual, decadal and centennial time scales through the collection and analysis of observations and application of models of the coupled climate system", for the following reasons:
We try to establish the relation between variability in Agulhas leakage, variability in the thermohaline circulation and variability over the North Atlantic sector. We use both observations and models for process studies and a coupled climate model.

Two of the five principal research areas considered by Clivar -Dec-Cen are covered in the present programme: The North Atlantic Oscillation and the global thermohaline circulation. In particular we focus on the role of interocean exchange in the South Atlantic on the global THC (Clivar Implementation Plan, chapter D3) which is addressed in section 1.5 "link to other oceans". The main objective of the present programme coincides with one of the Clivar - Dec-Cen objectives for the thermohaline circulation: "to investigate the sensitivity to changes in interocean exchange" (sec. 1.6). The observational component of our programme meets one of the "observing needs" (sec. 3.5): "Inventories of hydrographic and tracer properties to assess volumetric changes and spreading rates of Antarctic Intermediate Water and Subtropical Indian Mode Water in the South Atlantic will be necessary".

Moreover, the present programme addresses the Clivar objective (Clivarnet p.3): "to extend the record of climate variability over time scales of interest through the assembly of quality-controlled paleoclimatic and instrumental data sets" through its observational components that consists of both paleodata and ocean observations. We will use the paleodata to validate the simulated natural variability in our coupled model (Clivar -ACC objective; Clivarnet p.4) with emphasis on the NAO and the link between variations in interocean exchange and THC shut down events. The paleodata will produce boundary conditions for the coupled general circulation models (comp. E) with Agulhas leakage and a THC that is greatly different from the actual system.

This programme contributes to a national coherent programme as it is relevant for Clivar (see arguments above), it builds on expertise in the Netherlands and South Africa (it extends ongoing activities at IMAU, NIOZ and VUA like the NOP/NWO financed programs "Interbasin exchange", "Mode water variability", "Eddy-induced heat KNMI, transport" and the VVA program "Images"). It consists of a collaboration of several universities (UU, VUA, UCT) and institutes (IMAU, KNMI and NIOZ) and three disciplines (meteorology, oceanography, paleoceanography). Moreover, it brings together the specific expertise of these groups (NIOZ, VUA, UCT: observations, IMAU: theory and satellite observations, KNMI: modelling) into one programme. Also, it focuses on a region (Agulhas) that is only slightly covered abroad and it supplements recent programmes in that region (KAPEX and BEST) by focusing on Rings and mixing processes and using this data for impact studies in larger-scale numerical models. It provides for a balance between modelling and observations. In particular, we focus on understanding of Atlantic /European climate variability and we use paleo-data for its study. We provide for a link between interocean exchange, THC variability and the climate of Europe.

In short, the present programme builds a bridge between modelling and observations (both seagoing and remote sensing), it provides for a link between different disciplines (oceanography, meteorology, paleoclimatology) and it creates a research platform in which all national institutes that perform physical oceanography cooperate in a seagoing programme that combines their specific expertise.

7. Relation to other international research programmes.

MARE is embedded within Clivar . It builds on the results of the World Ocean Circulation Experiment (WOCE, in particular sections A10, A11, A12 and A13; 1992-1995), the Agulhas Retroflection Cruise (ARC, 1983), the Subtropical Convergence and Agulhas Retroflection Cruise (SCARC, 1987) and various cruises from South African scientists. These cruises have provided for a hydrographic mapping of the region and of various Agulhas Rings, but could not address the evolution of mixing and watermass transformation within those Rings. The contribution from Agulhas leakage to the South Atlantic was estimated from the South Atlantic Ventilation Experiment (SAVE, 1989-1990) and the Benguela Sources and Transport (BEST; 1992-1993) programme. The main objective of BEST was to measure the transport of the Benguela Current, monitor its variability and determine its sources. It was found that the current consists of a more or less steady part that derives water from both the Indian and South Atlantic Oceans and a transient part associated with the passage of Rings. As the Agulhas Ring corridor crosses the pathway of the steady component of the Benguela Current it is not clear to which extent mixing of Agulhas Rings contributes to the more steady part. Also variability on interannual and longer time scales is not well described yet.v There are two seagoing physical oceanography programmes being carried out in the South Atlantic which are relevant to the present programme. The South Atlantic Climate Change (SACC) programme focuses on the western part of the basin. Contacts have been established by the IMAU, our contact person is Dr. Matano (OSU), one of the PI's of SACC. It is the intention as part of SACC to study the interaction of Agulhas rings with the boundary currents of the Western Atlantic. This would complement MARE. The second programme is KAPEX (Cape of Good Hope Experiment). Its objective is to trace the flow of intermediate water around southern Africa with RAFOS floats. KAPEX will give a larger scale picture of the spreading of water masses in the eastern South Atlantic than the seagoing component of the present programme. MARE will investigate in detail by what processes, and how much, water is transferred from a Ring to the THC. MARE and KAPEX supplement each other. Our contact person in KAPEX is Prof. Lutjeharms who will participate also in MARE. MARE offers in addition to KAPEX a modelling component to interpret/generalize the measurements and to calculate their climate impact.

A related seagoing paleoceanographic programme is NeBrOc, an initiative of NIOZ and the University of Bremen. The South Atlantic paleoceanography is a cornerstone of this programme, that makes use of the cores collected during recent expeditions as (IGBP/IMAGES, 1996) and the Ocean Drilling Program (1997). The paleoceanographic component of MARE extends some of the results of this programme.

Related modelling studies are:
1) TRACMASS; an EC-funded programme which will start in 1998. It uses a method that employs Lagrangian trajectories to investigate the North Atlantic and Mediterranean water mass circulation as they result from numerical simulations of the global ocean. Coordinator of this programme is Dr. Doos (MISU). KNMI participates in both MARE and TRACMASS. MARE supplements the subproject "Origin and formation of North Atlantic Deep Water" in TRACMASS by studying the spreading of Subtropical Indian Ocean Water and Antarctic Intermediate Water in the South Atlantic.
2) SINTEX is a programme on Interannual and decadal climate variability: Scale interaction experiments. KNMI participates in this programme with the ECBILT model. Our contact person at KNMI is Dr. Opsteegh. The experiments with ECBILT in component E are relevant for SINTEX. Also Comp. E will use results of the EC-project MILENNIA which recently has been finished.
3) DIADEM is a EU-funded programme, in which a series of sophisticated data-assimilation schemes is used with the same eddy permitting numerical model to describe and predict the ocean circulation in the North Atlantic. Leading data-assimilation experts from all over Europe are involved. Coordinator of the programme is Dr. Evensen. IMAU participates by studying the performance of ensemble smoothers, which will be used in component B.

In short, MARE has established several working contacts with other ongoing and related international programmes and supplements these. It is, however, an independent programme. To continue and extend international cooperation the execution of the programme is essential.

8. International cooperation

International cooperation exists with researchers participating in SACC, KAPEX, SINTEX and TRACMASS. In addition to these, working contacts with Dr. Evensen (Bergen, data assimilation), Dr. Chassignet (Miami, isopycnic model), Dr. Dewar (Tallahasee, theoretical models of Rings), and Dr. Von Storch (GKSS, coupled models) should be mentioned. National cooperation exists between the parties directly involved. Relevant for this programme is the cooperation with the Atmospheric predictability section on KNMI (dr. Opsteegh), the section Satellite altimetry and applied physical oceanography of Delft University (DEOS), and the isotope laboratory of the geomarine center of the Free University of Amsterdam (Dr. Ganssen)

9. Economic and societal relevance

MARE is embedded in Clivar which addresses future climate change. As such it has a tremendous economic and societal relevance. Additional economic and societal relevance compared to other Clivar projects results from its impact in contributing to the understanding of African climate variability and to the capacity building in South Africa.

10. Feasibility

MARE is feasible for several reasons. It builds on the experience of various experts who have proven their capability in similar research projects in the past. The numerical models and methods required are all either developed or being developed at the moment. In all cases these are expected to be available in the course of 1998, that is long before the start of the observational part of the programme. Dr. Veth (cruise leader) and Dr. Van Aken (assistant cruise leader), both from NIOZ, have plenty of experience with seagoing research (European Polarstern-; JGOFS-Southern Ocean-, Nansen-, WOCE Hydrographic-, and Triple B-project). Also the assistance of prof. Lutjeharms is appreciated in this respect. At NIOZ, a great experience has been built up (dr. Brummer) in the employment of moored sediment traps and the exploitation of the data, with successful participation in a number of legs of the Netherlands Indian Ocean Program (NIOP) and the APNAP and JGOFS programmes in the North Atlantic. NIOZ has also proven its ability to take cores.

To reduce the risks of the seagoing components we assign postdocs to these components. If the amount of collected data would be disappointing, the objectives of these components could be adjusted more easily without the constraint of producing a thesis. In that case, these components would rely more on historical data, available in the WOCE special analysis centre in Hamburg. This would make these components less exciting, but still valuable for the scientific community. To a lesser extent, this also holds for comp. B, D and E. These modelling studies could be executed separately from the seagoing components, but without these, they would only become more qualitative and less specific.

Sometimes Agulhas Rings are adsorbed, stay for a long time in their parent region, or follow a very anomalous track. With the interpretation of satellite altimetry an Agulhas Ring corridor has been identified that depicts the envelope of pathways that the bulk of Agulhas Rings follow. At present, the course and propagation velocity of Agulhas Rings seems rather predictable, as are the characteristic features of Rings that behave, or will behave extraordinary. To reduce the risk of starting to map a Ring which will behave in a peculiar, unrepresentative way, it is envisaged to examine two rings in detail during the first cruise. In the following cruises we can than select the most promising ring of the two.

The modelling component B will be performed on the CRAY of SARA Amsterdam. The modelling components D and E will be performed on the Fujitsu from the ECMWF in Reading, the multiprocessor Power Challenge of KNMI and also on the CRAY.