Written by the Netherlands Clivar Committee (Dr. B. van Geel, Dr. G.J. Komen [chairman], Prof. J.W. de Leeuw and Prof. C. Schuurmans) with input from: Dr. H. van Aken, Dr. G. Burgers, Dr. H. Dijkstra, Dr. S. Drijfhout, Prof. J. van Hinte, Prof. H. Hooghiemstra, Dr. P.J. van Leeuwen, Dr. H. Lindeboom, Prof. J. Meulenkamp, Prof. H. Oerlemans, Dr. Th. Opsteegh, Dr. H. Ridderinkhof Prof. W. de Ruijter, Dr. U Sass-Klaassen, Prof. J.S. Sinninghe Damsté, Dr. R. Tol, Dr. S. Troelstra, Prof. J. Vandenberghe, Prof. H. Visscher, Dr. Tj. van Weering, Prof. ir. K.F. Wakker, Dr. N. Weber

Recent progress in climate research has confirmed the need for a multidisciplinary approach in which theorists, modellers, observationalists, meteorologists, oceanographers and palaeoclimatologists closely collaborate. This recently led to the formulation of Clivar, a new international programme for the study of climate variability and predictability. Clivar has an envisaged duration of 15 years. The present document is a proposal for Clivarnet, an initial, five-year, Dutch contribution to Clivar. The idea is to build a strong Dutch Clivar programme out of existing elements. To this end we identify Clivar-related research in the Netherlands and make suggestions for collaboration and reinforcement. This will increase national coherence further.

Understanding climate is a major scientific challenge. Society has a strong interest because natural climate variations have large impacts and because (quote from IPCC) "the balance of evidence suggests a discernible human influence on global climate".

Climate research has been stimulated in many countries. In Europe, extra funding came from the Fourth Framework Programme, through the Environment and Climate Programme, and - to a lesser extent - through the Marine Science and Technology programme. In the Netherlands stimulation came from programmes such as VVA and NOP-1 and -2. This world-wide stimulation has resulted in spectacular progress. However, this very progress also revealed the enormous complexity of the problem. It is clear now that solution of the climate problem requires a more multidisciplinary collaboration than has been realised so far.

Climate and climate change are global phenomena. Therefore, international coordination of climate research is essential. This has been realised at an early stage by organisations such as WMO, ICSU, UNEP and IOC, who started a World Climate Research Programme with major elements such as TOGA and WOCE. Although these programmes were quite successful by themselves, a certain amount of integration was lacking. For example, there was hardly any collaboration between the oceanographic community and the meteorological community. Also use of palaeo-information for model validation was limited. Therefore, when WCRP started to define a follow-up for WOCE and TOGA, it aimed at a broad programme, with input from theorists, modellers, observationalists, meteorologists, oceanographers and palaeoclimatologists. The result is Clivar. Central to Clivar is the problem of climate variability and predictability. The design of Clivar has taken many years, but now (1997) it has matured, and the programme is in the process of world-wide implementation. An outline of Clivar will be given in section 3.

In 1996, Euroclivar was established to help implement Clivar in Europe. Technically, Euroclivar is a concerted action under the Environment and Climate Programme. Its core objective is to increase efficiency and coherence of climate research in Europe. Similar efforts are being made at national levels. This also needs to be done in the Netherlands, and this is the time to do it.

In the Netherlands important steps towards increased coherence were made when in 1995 three institutes (IMAU, KNMI and RIVM) decided to establish a joint Netherlands Centre for Climate Research (CKO), and when the Research School Sedimentary Geology selected Palaeoclimate Research as one of its major research activities. However, to fully implement Clivar in the Netherlands, these initiatives need to be extended, for example, 1. to link modelling within CKO with oceanographic research elsewhere; 2. to bridge the gap between meteorologists and oceanographers on the one side and the palaeoclimatic research community on the other side; and 3. to better involve the community which processes earth observation data taken by satellites. This is made the more urgent by the fortunate fact that the Netherlands is in a unique position among the smaller countries in Europe, in having strong research groups in palaeoclimatology along with a high expertise in meteorology, oceanography and earth observation data processing and analysis.

The present document, written on the initiative of a group of researchers, aims at increasing national coherence further, by identifying Clivar-related research in the Netherlands and by making suggestions for collaboration and reinforcement. The philosophy is not so much to propose a completely new type of research, but rather to build a strong Dutch Clivar programme out of existing elements. Specific purpose of this document is to present the contours of a Dutch Clivar-oriented research programme (Clivarnet).

The "added value" of Clivarnet is manifold:

We will first give brief descriptions of Clivar and existing Clivar-related research in the Netherlands. This will be followed by our recommendations. Details on ongoing activities are given in Annex A.

• To describe and understand physical processes responsible for climate variability and predictability on seasonal, interannual, decadal and centennial time scales through the collection and analysis of observations and the development and application of models of the coupled climate system,
• To extend the record of climate variability over time scales of interest through the assembly of high-quality palaeoclimatic and instrumental datasets,
• To extend the range and accuracy of seasonal to interannual climate prediction through the development of global, coupled predictive models.
• To understand and predict the response of the climate system to the growth of the radiatively active gases and aerosols and to compare these predictions to the observed climate record in order to detect the anthropogenic modification of the natural climate signal.

The CLIVAR programme is initially organised in three component programmes:

• CLIVAR-GOALS: a study of natural seasonal-to-interannual global climate variability and predictability,
• CLIVAR-DecCen: a study of natural decadal-to-centennial global climate variability and predictability,
• CLIVAR-ACC: a study of the response of the climate system to the addition of radiatively active gases and aerosols to the atmosphere.

However, it is intended that these component programmes merge when CLIVAR evolves, with projects that cut through time scales and programme components. Indeed, many tools and techniques are common to the different programme components. These are
1. observing systems (both in situ and remote)
2. numerical modelling and prediction
3. analytical and diagnostic studies
4. dataset development (including historic and palaeo-data).

CLIVAR is administered by an International Project Office, which is located in Hamburg. The Initial Implementation Plan for CLIVAR will be finalised in the beginning of 1998. It will serve as a background document for the first CLIVAR Conference, which is scheduled for December 1998. A summary document (CLIVAR, 1997) is available from the International CLIVAR Project Office. This Implementation Plan identifies a number of Principal Research Areas. CLIVAR-GOALS consists of four areas. The first is "ENSO: Extending and Improving the Predictions". Two other areas are concerned with the interannual variability of the Monsoon Systems (Asian/Australian and American). The fourth area focuses on the "Variability of the African Climate System". CLIVAR-DecCen considers five Principal Research Areas. Four are related to the dominant modes of decadal variability: the Pacific-Indian mode, the Tropical Atlantic Variability, the Southern Ocean Climate Variability and the North Atlantic Oscillation. A fifth focus is on the Variability of the Atlantic Thermohaline Circulation. CLIVAR-ACC consists of two Principal Research Areas, namely Climate Change Prediction and Climate Change Detection and Attribution.

Common elements of all of these areas are:

• the identification of patterns and mechanisms of variability,
• extension of the data record of past climate data to study natural climate variability and to validate the models,
• the implementation of observing, modelling, computing and data collection systems to understand and predict climate variability and climate change,
• focused process studies both in the atmosphere (clouds, radiative feedbacks, etc) and in the ocean (watermass transformation, boundary currents, interbasin exchange, mixing, the role of eddies, deep convection, the role of sea ice, etc).

With respect to the palaeo-data there is a strong focus on high resolution (annual) data from the late holocene, and on high resolution data during extreme or abrupt events

We refer to the Initial CLIVAR Implementation Plan (CLIVAR, 1998) for more details.

In this section we will give an overview of ongoing Clivar related research in the Netherlands. This information might have been organised in several ways, for example, by institute or by Clivar Principal Research Area. We have chosen for an initial summary presentation by principal investigator (PI) followed by a more global overview by institute. We start with the PI's because they are the principal actors, who will have to detail and supervise activities. So, it is important that they are identified right at the beginning.

Table 1 gives the overview by PI. The first and second column of this table give names and affiliations. Column three gives a short description of the focus of each activity and columns four and five indicate the Clivar area for which this activity is relevant. Obviously, a summary table of this type can not be exhaustive. Nevertheless, we believe that it gives a good indication of what is going on. Details at the PI level are given in Annex A. Here we will continue with short descriptions of the relevant institutional activities.

In 1995 the Koninklijk Nederlands Meteorologisch Instituut (KNMI), the Institute for Marine and Atmospheric Research (IMAU) and the Rijksinstituut voor Volksgezondheid en Milieuhygiëne (RIVM) started to collaborate in climate research in the Netherlands Centre for Climate Research (CKO). CKO is active in all areas of Clivar, as follows:

Two groups within CKO are working on ENSO. Both try to understand the basic dynamical mechanisms. The group of Dijkstra studies Nonlinear Dynamics of the Coupled Equatorial Ocean-Atmosphere System The group of Burgers at KNMI has developed a simple ENSO forecasting model. In addition, work is going on concerning the initialisation of ocean models for ENSO forecasting. A simple method has been developed jointly with IMAU (van Leeuwen) and DEOS (Vossepoel). A more advanced model using the adjoint of HOPE has been developed at KNMI in collaboration with MPIM in Hamburg.

The emphasis within CKO is on numerical studies. Modelling activities centre around three themes:

The group of Oerlemans is studying the response of glaciers and ice caps to changing climatological circumstances. Models, field experiments and remote sensing are essential. These studies are highly relevant both for Clivar-DecCen and for Clivar-ACC. In the framework of the ESF project EISMINT projections of glacier behaviour are being made by running numerical models of real glaciers for a set of prescribed climate change scenarios.

Modelling studies directly aiming at the prediction of climate change due to human activities do not exist in The Netherlands. Quite a lot of research however, is related to such studies, either by improving our knowledge of the anthropogenic forcing mechanisms, or by analysing output of predictions, produced elsewhere. Also studies on model validation are made, mainly in relation to the analysis of predictions. Research on detection and attribution is limited to the statistical analyses of long series of local climate and global mean temperature, except for studies of glaciers and sea level rise, being of a global nature.

Present CKO activities related to Clivar-ACC are: 1. RIVM (F. de Leeuw) and KNMI (Opsteegh) are working on the improvement of the IMAGE model, which is used as a tool for the study of feedback mechanisms and interrelations in the climate system; 2. Most of the work of the group of Können (KNMI) on data analysis; 3. Part of the model validation work in other KNMI groups; 4. the EU-funded WASA project in which the effect of CO₂ doubling on the wave and storm surge climatology has been studied; 5. a recent study by van Ulden and van Dorland who have corrected estimates of anthropogenic global warming for effects related to variations in solar activity; 6. part of the work of the group of Oerlemans (IMAU) on glacier fluctuations and sea level rise; 7; a project on natural variability and change of storm tracks (Schuurmans, IMAU).

KNMI contributes to the Voluntary Observing Ship programme of the World Weather Watch. Under this programme, operated by the WMO, over 7000 ships make meteorological and oceanographic observations, some with the help of XBT's. Data are disseminated in real-time and archived in global collecting centres. Dutch ships contribute some 90000 observations annually. Recently KNMI has developed a software package (TURBO1) to facilitate compilation, encoding and transmission. This package will be introduced for all seafaring nations of WMO. KNMI also contributes to the European Group of Ocean Stations (EGOS). As part of this programme 18 drifting buoys are deployed in data sparse areas in the North Atlantic Ocean. These buoys measure sea level pressure and sea surface temperature. Some measure winds and currents as well.

ECMWF has performed a fifteen-year reanalysis. Komen is involved in an assessment of the quality of global air/sea fluxes resulting form this project, by studying the response of both an ocean wave prediction model and an OGCM. Knowledge of these fluxes is vital for Clivar. KNMI is also active in data archeology (Können, Komen) and in the analysis of a global multi-proxy palaeodataset (Weber).

The Delft Institute for Earth-Oriented Space Research (DEOS) of Delft University of Technology (DUT) covers a wide field of research in space geodesy, geophysics and physical oceanography. Topics included are Earth rotation, crustal dynamics, tectonics, gravity field, sea surface topography, medium-scale and large-scale ocean currents, and sea level rise. All investigations make explicit use of observations gathered by satellites and the research requires the precise computation of the orbits of the satellites involved. One of the major research themes that is of interest to the Clivar programme is satellite altimetry. This theme includes the acquisition, preprocessing and correction of the satellite altimeter measurements, the processing of the measurements in order to obtain information about the Earth's gravity field, sea surface topography, sea level rise, ocean currents, and ocean energy transportation. Both global and regional studies are performed; the regional studies focus on the North Atlantic, on the region of the Agulhas Current and on the seas around Indonesia. Apart from the actual data processing and the modelling of the observed phenomena, the physical interpretation of the results is performed in cooperation with IMAU, KNMI, NOAA and some foreign university teams.

The following activities of NIOZ are relevant for Clivar, in particular for Clivar DecCen.

The physical oceanography department at NIOZ, Texel, is involved in observational programmes, mainly in the North Atlantic Ocean. Since 1990 these sea-going programmes are performed within the framework of the WOCE hydrography programme. Research is focused on the thermohaline transport and exchange of water between the northeastern North Atlantic and the Norwegian Sea. At present attention is also directed to ocean margin bound processes in the eastern North Atlantic. From 1992 to 1998 research cruises are carried out in the Bay of Biscay. Research items here are the hydrography of the eastern boundary current, kinematics and dynamics of the slope current, internal tides, tide topography interaction, and boundary mixing. From 1997 to 1999 mixing due to internal waves near the margin of the Faroer-Shetland Channel will be studied extensively.

Ongoing climate related research in the N. Atlantic Ocean is directed towards establishment of high resolution late glacial and holocene variability and oscillation of the thermohaline circulation and watermass exchange between the Atlantic Ocean and the Norwegian-Greenland seas, through analysis of high resolution (decadal to centennial) core data. Attention is paid to the causes and occurrence of abrupt climate changes (Heinrich events/Dansgaard Oescher cycles), their relationship to variability of deep circulation, the (palaeo)oceanographic consequences and the land-ocean linkage. In 1987 and from 1993-1997 research cruises were carried out related to the above in combination with a study of the palaeo-oceanographic effects of development of boundary currents following changes in thermohaline circulation. Phytoplankton biomarker studies are incorporated to study the relationship with variability of the carbon cycle following short duration events.

Long-term, on-going research within the departments of Biological Oceanography and Marine Ecology has resulted in biological data series of the North Sea and the Waddensea covering a period of several decades. Frequency analyses on these data as well as on growth ring data of shells indicates incidental and periodical climate changes. The continuation of this type of research is of utmost importance to confirm statistically and otherwise the phenomena observed so far and to evaluate the usefulness of palaeo-data as climate proxies. Several investigations performed within the departments of Marine Chemistry and Geology and Marine Biogeochemistry and Toxicology concentrate on high-resolution analyses of sediment cores applying palaeontology, isotopes and inorganic and organic biomarkers.

The research programme of the Netherlands Research School of Sedimentary Geology (NSG) emphasises two interrelated research themes: (1) Sedimentary Basins and the underlying Lithosphere, and (2) Palaeo-environments and Earth System History. Since the significance of the short-term environmental changes that concern us today can only be understood in the context of longer timescales, notably the latter theme includes research topics that are relevant to the Clivar programme (DecCen/ACC). To place constraints on models of climate change, and to test the degree to which they are capable of simulating reality, high-resolution records of past environmental parameters are obtained from the integrated analysis of physical, chemical and biological characteristics of marine and terrestrial sediments (Van Hinte/Troelstra; Meulenkamp; Visscher). Especially the successful development of work on the interface of earth science and biology (e.g., recognition of astronomic and sub-astronomic cyclicity; correlation of marine and continental records; recognition of atmospheric CO₂ fluctuations; Global Emiliania Modelling Initiative) provides a promising starting point for Dutch Clivar-oriented research.

NSG operates in close collaborative contact with various universities and research institutions all over the world. Because of complementary expertise, there are numerous links between NSG and NIOZ research programmes (e.g., common responsibility in NWO-subsidised ODP projects; integration of biogeochemical research in NSG activities). Largely related to Clivar targets, together with NIOZ, NSG has recently started an NWO-sponsored collaboration with the University of Bremen.

The RING Foundation/Dutch Centre for Dendrochronology (established in 1991 at the State Service of Archaeological Research (ROB) in the Netherlands), is mainly involved in the research of subfossil wood from the Netherlands. Research topics include the dating of wood (Jansma et al, 1996), the development of ultra-long tree-ring chronologies of oak on an annual scale (Jansma 1995a; Jansma 1995b) and the assessment of the climate signal in these chronologies (Jansma, Maessen 1995). Funding is received from the archaeological community as well as from national and international organisations (e.g., the Netherlands Organisation for Scientific Research (SHW/NWO) and the EU). At present, the data set of absolutely dated tree-ring series from the Netherlands is comprised of approximately 2000 measurement series from subfossil logs (oak trunks that have been preserved in marine and fluvial deposits and in (former) bogs) and oak timbers from archaeological and historical contexts. This data set covers the interval 3000 BC to present (Jansma 1995a; RING unpublished data). Moreover, RING participates in the PEP II working group of IGBP-PAGES with research on tropical tree species (teak, dipterocarpaceae) from South-East Asia (Eckstein et al, 1995; Sass et al, 1995).

The following activities are relevant for Clivar, in particular for Clivar-DecCen:

The research group Palynology and Palaeo/Actuo-ecology (PPA) of the Netherlands Centre for Geo-ecological Research (ICG, UvA) is active in 14C-wiggle-match dating. This dating strategy is used as a tool for high resolution dating of peat deposits (B. van Geel, M. R. Kilian, NWO-funded; cooperation with J. van der Plicht, RuG). In first instance the focus was on the period of the sharp rise of atmospheric 14C between 850 and 760 cal BC (which is around 2650 BP on the 14C time scale). Based on palaeo-ecological, archaeological and geological evidence, the conclusion could be made that this period represents an extreme and abrupt climate change. In European raised bog deposits, the changing spectrum of peat forming mosses and a sharp decline in decomposition of the peat indicate a sudden change from relatively dry and warm to cool, moist climatic conditions. The rise of the ground-water table in low-lying areas in The Netherlands resulted in abandonment of settlement sites. There is strong evidence for synchronous climate change in Europe and on other continents (climatic teleconnections on both hemispheres ) around 2650 BP. Reduced solar activity and related increase of cosmic rays as a cause for the observed climatological phenomena and the contemporaneous rise in the 14C-content of the atmosphere are considered. Cosmic rays may have been a factor in the formation of clouds and precipitation, and in that way changes in solar wind were amplified and the effects induced abrupt climate change.

Several key questions in palaeoclimatology concern the tropics. International projects, such as PEP-1 (PAGES), BIOME, COHMAP, but also in the model-model comparisons of PMIP, a significant shortage of data on climate variability in the tropics, especially at low elevation, is noticed. Therefore PPA/ICG focused research objectives during the last 4 years on high-resolution pollen records of climatic variability from the lowlands (0-1000 m) and lower montane belt (1000-2300 m) of Colombia (post-doc H. Behling, oio M. Wille; GOA-funded). Based on the pollen analysis of newly collected cores (> 20 new sites) there is growing evidence, based on high-resolution pollen analysis, that Holocene climatic conditions were not as stable as previously thought. New coring equipment has also contributed to better quality sediment cores. The high quality and relevance in palaeoclimate studies of tropical pollen records is, e.g., illustrated by the high-resolution record of H. Mommersteeg (VVA-funded; thesis defence scheduled January 1998) where a detailed correlation of the pollen record with oxygen isotope and ice-core records, and astronomical forcing is shown in great detail over the last 150 kyr. AMS wiggle match dating is scheduled for a Colombia record in order to study, a.o., the 2650 BP climate change and to evaluate teleconnections between hemispheres and different climate belts.

The section Quaternary Geology (VU) is specialised in the reconstruction of palaeoclimates using proxy-data of different nature (so-called multi-proxy-approach). The main proxies are specific periglacial features, ecological data, palaeohydrological characteristics and sedimentological properties. A multi-proxy database (MPDB) is constructed for the last interglacial-glacial cycle (up to the Younger Dryas) for northwestern Europe. Palaeoreconstructions are compared with model simulations. A particular topic is the climatic reconstruction by means of the grain size of wind blown material carried out on the Chinese Loess Plateau. The different projects involve several PhD studies and postdoc projects financed by the Vrije Universiteit, GOA, NOP, the EU and the Dutch and Chinese Academies.

The Institute for Environmental Studies, Vrije Universiteit Amsterdam has expertise in statistical time series analysis of climate data. Regression and cointegration techniques have been applied to global and hemispheric records of temperature and atmospheric composition, while Bayesian techniques have been used to include the influence of sulphate aerosols and long-term natural variability. Most attention has been paid to distinguishing between, and attributing degrees of belief to the various hypotheses explaining the observed global warming.

In this section we will describe existing and potential cross relation between the activities summarised in section 4. We also will indicate international contacts.

IMAU (PI: Dijkstra) and KNMI (PI: Burgers) closely collaborate in their studies of the variability and predictability of the equatorial ocean-atmosphere system. their objective is to understand the mechanisms that generate ENSO's and to help develop numerical tools for their prediction. CKO will not make its own ENSO prediction system, but it does collaborate with groups that intend to make such predictions. The deliverables from the Dutch research activity will be a better understanding of ENSO dynamics, to be published in the literature, but also improved methods for ocean data initialisation, which are crucial for forecasting. Input from DEOS (Wakker, satellite data) and Van Leeuwen (data assimilation techniques) will be essential. When the decadal variability of ENSO is going to be investigated there will be an interesting link with the work of Opsteegh on the ECBILT model. There is international collaboration with groups in the USA (J.D. Neelin, UCLA; F.F. Jin, Univ. Hawaii at Manoa) and in Europe (Latif, MPIM; Anderson, ECMWF). KNMI will provide results directly to ECMWF, who are in the process of setting up an operational forecasting system based on a full coupled O/AGCM. In turn, ECWMF will make trial forecasts available to KNMI, where the quality will be assessed.

Most activity is in this area. This is not surprising because decadal variability is strong in the extratropics, and also because our ability to detect anthropogenic change depends to a large extend on our understanding of natural decadal variability. There are many interrelations. These have been indicated in figure 1.

Central to our understanding of decadal variability are simulations with coupled atmosphere/ocean/sea-ice models. In the Netherlands such simulations are made with ECBILT (Opsteegh). ECBILT is a model of intermediate complexity. Studies with ECBILT are complimentary to those with full state-of-the-art OGCM's. The advantage of ECBILT is that many long runs can be made to investigate sensitivities and feedback mechanisms. Simulations with global models are also complimentary to the more regional studies, involving the variability of glaciers, ocean waves and tree rings based reconstructions of temperature.

There is a wealth of such data available from tree rings, shell rings, biological time series, sediment studies and fossil records. Climatic reconstructions, based on these proxy data, are potentially very useful. They can be applied for the determination of the magnitude, spatial patterns and timescales of climatic variability. This is essential for the validation of climate models and for the detection of climatic change. Some of the data required for climate reconstructions will be obtained through international collaborations (IGBP/PAGES and bilateral collaboration with the Bremen institute (NEBROC)).

It is generally believed that ocean variability plays an important role. Therefore, several groups are studying ocean variability. DEOS concentrates on the detection of seasonal and long-period variations of the mean sea level of the North Atlantic, the location of the Gulf Stream and the trajectories of large eddies. The work by Dijkstra focuses on a reconstruction of the bifurcation structure of the ocean circulation. An important global scale process in the ocean is the overturning thermohaline circulation (thc). Variability of this circulation can contribute significantly to variability in the North Atlantic Ocean. To understand this variability one must understand the different aspects of the thermohaline circulation. An important aspect is the formation of bottom water. This is studied by several groups in Europe (for example, by the Institut für Meereskunde, Kiel, within a long-term programme of the Sonderforschungsbereich). In the Netherlands, studies concerning the thc have focused on the problem of interbasin exchange in the South Atlantic (de Ruijter) and through-flow in the North Atlantic (van Aken). The exchange of water between the Indian Ocean and the South Atlantic Ocean is of a fascinating geophysical complexity, and is believed to affect variability of the surface temperature in the North Atlantic. Sea-going research has focussed on (variability in) the hydrography of the eastern Atlantic Basin with special emphasis on the role of the eastern boundary current. The data obtained form part of the internationally accessible dataset of the WOCE Hydrographic Programme. This global dataset will be used for the operation of OGCM's.

Existing ocean models are reasonably correct in an average sense but they are far from realistic in their description of important details. Therefore, several groups are trying to improve our understanding of the ocean and our ability to model ocean processes (mixing, air/sea interaction, the role of eddies). Also here observations both present (monitoring, satellites, ships) and past (historic data and palaeo-records) are being used. Data assimilation methods are being developed in order to make optimum use of observations and to prepare for true ocean forecasting.

For the climate research community in the Netherlands the Clivar ACC priorities 2 (Model validation) and 3 (Detection of climate change) are both of direct relevance. As a common factor in these priority areas we have to consider the magnitude and structure of natural climate variability. Better knowledge of this, particularly at 10 - 100 year time scales is needed. Clearly, the results of the Clivar-DecCen programme will be of vital importance in improving this situation. Therefore, Clivar-ACC will have to be conducted in close collaboration with Clivar-DecCen, also in the Netherlands. This means that in improving the reliability of a climate model (model validation) by using available palaeoclimatic reconstructions, direct links should be established between palaeo-research groups and climate modelling groups, exactly as foreseen in figure 1 for Clivar-DecCen. For the interpretation of numerical climate predictions and the detection of the anthropogenic signal (Interpretation and detection) Clivar-ACC is a direct user of the knowledge of natural variability acquired in Clivar-DecCen. As an example we may mention the present situation in regard to changes in storm tracks and storm intensity as a result of the increasing greenhouse effect. IPCC (1995) clearly states that as yet no definite conclusions may be drawn, due to contradictory results of models, but also as a result of incomplete insight in the natural variability of this phenomenon. Up till now, validation and detection research in the Netherlands did not show much coherence. By involving palaeoclimatologists and focussing upon aspects of climate, being of particular relevance to our country (storms, hydrological parameters, sea level) this could be improved drastically.

An overview of available and planned models is given in table 2a for Goals and in table 2b for DecCen. Full state of the art coupled models should not be implemented, both because they are extremely expensive to run and because there is no purpose in duplicating the work in this field going on elsewhere in Europe. Results from state-of-the-art models are normally available on the basis of collaborations with institutes outside the Netherlands.

HOPE, LSG, MOM, MICOM are public domain OGCM's that have been developed elsewhere and have proven their capabilities. These models are generally well described. HOPE and MOM are similar in physics (primitive equations). LSG is developed for its fast performance. MICOM is an isopycnic model.

Van Leeuwen has developed the present Agulhas model. Drijfhout developed ISOP. ECBILT is a coupled model, with an atmospheric component based on the quasi geostrophic potential vorticity equation and a MOM type PE ocean component (Haarsma). ECBILT-1 was developed by the group of Opsteegh. In ECBILT-2 the ocean component will be replaced by the LSG model.

Through bilateral contacts CKO has access to the large state-of-the-art models of MPIM/DKRZ and the Hadley Centre. DKRZ has provided computing facilities for model development and for simulation with General Circulation Models. The European Centre for Medium-range Weather Forecasting (ECMWF) should also be mentioned. The relation with this centre is very useful. It is special, because ECMWF is co-funded by KNMI jointly with other national European Met Services.

This work is all done in international collaboration. The VOS programme in which KNMI participates is coordinated by WMO. KNMI also participates in the reconstruction of global air/sea flux fields through the so-called re-analysis projects, carried out at ECMWF. Satellites are always multinational undertakings. The glacier data are made available to international data centres, as are the palaeo proxy data. Some samples will become available through the existing ODP collaboration. Data resulting from the NIOZ sea-going programmes are made available to the relevant data centres (WHP-DAC, ICES, WODC).

Clivar is a component of the World Climate Research Programme (WCRP). Therefore, Clivarnet will be a Dutch contribution to WCRP. But it will also contribute to the International Geosphere Biosphere Programme, in particular to the IGBP core project PAGES, because internationally Clivar and PAGES have established the so called "Clivar/PAGES" intersection". Clivar also contributes to IMAGES.

In the previous sections we have given a short overview of on-going Clivar related activities in the Netherlands, their interrelations, and their international embedding. The challenge now is to build a strong Dutch Clivar programme, which is able to address the Clivar objectives in a focused way, building on existing work and expertise. To this end we will first propose a number of criteria that can be used to define an extension of Clivar-related research in the Netherlands. On the basis of these criteria we have selected a number of specific scientific objectives. These will be followed by an outline of how these objectives can be achieved.

The following criteria have been used

This leads us to formulate the following specific research objectives for a Dutch Clivar programme: