An EU MAST project

CLIVAMP is a research program whose objective is to investigate the long term variations of the thermohaline circulation of the Mediterranean Sea for time scales ranging from interannual up to changes during the ice ages. CLIVAMP will thus study the different states of the Mediterranean Sea during the past, its paleo-circulation and the processes affecting the ventilation and the overturning of the water masses.

The CLIVAMP strategy is based on an integrated approach involving theoretical studies, numerical modelling and interpretation of transient tracer and paleo-oceanographic observations.

The CLIVAMP proposal is divided into four major tasks:

1) Evidence and interpretation of past circulation and forcing;
2) Modelling of externally forced circulation variations;
3) Internal variations in thermohaline circulation and stability;
4) Towards an assessment of paleo-oceanographic circulation scenarios.


CLIVAMP will contribute to understanding the mechanisms of the Mediterranean thermohaline circulation and consequently those of the global conveyor belt. CLIVAMP will assess the possibility of detecting climatic signature in the Mediterranean Sea.

CLIVAMP will benefit and enhance the observations taken during other EC supported programmes such as PALEOFLUX, EUROMAGE and MTP II .

The global oceans plays a crucial role in controlling climatic variations on time scales of months to centuries. The role is due to its enormous heat capacity and its ability to ensure meridional heat transports via the global conveyor belt as first described by Gordon (1986). We still know very little about the natural variability of the ocean and how it is controlled by the external forcing and internal dynamics. It is unclear how representative the present state of the ocean is compared with past states and how the ocean might evolve in the future. Consequently, it is important to infer how the ocean has varied at long time scale from the paleo-oceanographic records and to use modelling experiments to identify which aspects of the circulation are robust or sensitive to different climatic conditions. This will provide a context for understanding the present ocean and a reference point for speculating on how the ocean might subsequently evolve through natural variability or anthropogenic-induced change.


Due to its relatively small size and its well defined boundaries the Mediterranean provides a natural laboratory for studying the different ocean thermohaline circulation regimes which may be possible under given surface climatic conditions:


- i) It contains analogues of the major deep and intermediate water formation processes which occur in the global oceans.


- ii)Its thermohaline circulation is similar to a reduced scale `conveyor belt' with a recirculation time of around 100 years, making it accessible to modern modelling techniques.


- iii) Its small volume as a `marginal basin' means it can both amplify the climatic signal imposed at the surface (Kennett, 1985), and record evidence for changes in circulation on very short time-scales in its basin wide sediments.


- iv) It is semi-enclosed allowing closed budget studies of E-P and heat through the surface and through various straits.


- v) Observational evidence (direct and indirect) for the Mediterranean climate and circulation is available from historical and fossil records, which go back over thousands of years.




Over the past decade considerable advances have been made in observing, modelling and understanding the circulation of the Mediterranean Sea under present day climatic conditions (POEM, PRIMO, MAST). The thermohaline overturning is key to understanding the circulation.

At present, a net surface increase of salt (and density) due to evaporation leads to a surface inflow of fresher, lighter waters, to both eastern and western basins (at Sicily and Gibraltar) with deeper outflow of dense, salty waters referred to as an anti-estuarine or lagoonal circulation. However, the formation sites and properties of these dense water outflows are strongly controlled by the regional circulation patterns and seasonal surface fluxes, and are thus likely to be very sensitive to climatic variations.


A case in point is the recent observation showing a fundamental change in the deep waters of the Eastern Mediterranean with the most dense water switching from being formed in the Adriatic to the Aegean (Rohling et al., 1995). It is unclear whether this change is induced through variations in the external forcing or through internal variability. If the present generation of circulation models are to be useful in any predictive sense, e.g. for understanding anthropogenic effects or the consequences of global warming scenarios, the models must be tested against such events as seen in the present observations and inferred from past records.


Paleo-oceanographic records also suggest that there have been fundamental changes in the Mediterranean circulation. Deep sea cores obtained from several oceanographic cruises, such as DSDp and ODP international projects, reveal that the hydrological characteristics of the Mediterranean have continuously changed during the past million years in response to changes in global climate associated with variability in global ice volume. Paleoclimatologists have now accumulated enough data to propose contrasting hypothesis explaining past circulation of the Mediterranean Sea. This data set constitutes an exciting opportunity to test our physical insight and numerical models by performing experimental studies under a range of climatic conditions. The paleo-oceanographic record provides a wide range of data, although it is sparse in space. Sea surface salinity changes through the past, which will partly drive the Mediterranean convective circulation, can be quantified by measuring the oxygen isotope composition of the carbonaceous planktonic foraminifera (Duplessy et al., 1991). Sea surface temperatures can be estimated using a similar technique (Hutson, 1979). Eastern Mediterranean sediment cores reveal several intervals enriched in organic carbon (sapropels), suggesting that the deep circulation in the eastern basin has been stagnant or unventilated according to biological production for periods of several thousands for years. Sapropels have now also been found in the western basin, although less well developed and/or preserved. There are two contrasting explanations for these events involving different thermohaline circulations:

- i) The `conveyor belt' switched sign to an estuarine circulation with net surface westward flow at Sicily and Gibraltar. This circulation would require the surface buoyancy forcing to water flux.


- ii) The `conveyor belt' became shallower, than in the present day anti-estuarine circulation with only intermediate waters being formed. This circulation change would lift the nutricline into the euphotic zone, which would fuel an increase in biological activity.




During MAST3, CLIVAMP proposes to evaluate the sensitivity of the Mediterranean circulation to various well-defined perturbations (e.g. sea level lowering, change in E-P), to investigate the likelihood of the two contrasting scenarios for sapropel formation. Attention will be focused on the last glacial cycles. To compare the model response with geological data, CLIVAMP also proposes to increase the spatial and geographic resolution of paleoceanographic proxies for hydrography and fresh-water fluxes especially in the western Mediterranean by analyzing deep sea cores extracted from recent oceanographic campaigns in the Mediterranean Sea (Etna, Dedale, Promethe, Valpamed, Paleoflux). A database will be formed by combining these additional measurements with the existing evidence, which will be used to validate our modelling experiments.


Given the range of possible paleo-circulations which could embody the above two scenarios, CLIVAMP first proposes to investigate the fundamental dynamical processes which would be involved in establishing and maintaining the gross aspects of these different paleo-circulations. Previous paleo-oceanographic modelling studies have tended to use coarse resolution ocean general circulation models (OGCM), however small scales are crucial for capturing water formation and dispersal events and for modelling flow through straits. While the climatic state may have varied in the past it is unlikely that the controlling processes will have altered their fundamental scales. Consequently, our modelling strategy aims to incorporate or parameterize these fine-scale processes within the paleo-oceanographic study. The modelling study will employ a hierarchy of approaches ranging from simple box models to high resolution OGCMs in order to identify which aspects of the ocean system within a paleo-oceanographic environment are robust or sensitive to changes in external forcing or internal dynamics.


In the second phase, CLIVAMP will use the OGCMs to numerically study climatic changes of the paleo-circulation and compare the integrations with the database formed by reconstructing the paleo sea surface forcing and salinity and temperature of the Mediterranean Sea.


The CLIVAMP proposal is divided into four major tasks:

1) Evidence and interpretation of past circulation and forcing;
2) Modelling of externally forced circulation variations;
3) Internal variations in thermohaline circulation and stability;
4) Towards an assessment of paleo-oceanographic circulation scenarios.


CLIVAMP will benefit and enhance the observations taken during other EC supported programmes such as PALEOFLUX, EUROMAGE and MTP II .

We propose to study the sensitivity of the Mediterranean circulation to climatic change by investigating the full range of conditions from the current situation back through the last glacial cycle, and the transition between those states. This range contains two extreme states of climate, the Last Glacial Maximum (LGM; 18 ky BP) and the Holocene Climatic Optimum (about 8-6 ky BP). The time in between, the deglaciation, was characterized by complex, but well documented reorganiazations of climatic zones on the Northern Hemisphere, which caused strong temperature changes (up to 6 degrees C for winter in some areas; cf. Bigg, 1994) and humidity increase then decrease in the Mediterranean. A humidity minimum was achieved over the (eastern) Mediterranean between 11 and 10 kyrs BP, during the younger Dryas cold period (Cheddadi et al., 1991). Globally, see level rose spasmodically by some 120 m in response to melt water additional to the world ocean, which also caused a drop in average world ocean salinity by 1 to 1.2 psu (Fairbanks, 1989; Blanchon and Shaw, 1995).


Task1 is devoted to the analyze of observations of past climatic conditions of the Mediterranean Sea. First CLIVAMP will concentrate on the more recent evolution, using (historical) measurements of water properties and tracer distributions. Then, we will direct our interest to longer-term changes, focusing on the last glacial cycle by analyzing sediment records. This time interval is chosen because it benefits from support of a well-defined time-stratigraphic framework based on numerous AMS 14C datings (e.g., Jorissen et al.,1993).

1-1 - Recent Evolution of the Mediterranean Sea

The current conditions of the Mediterranean Sea have been described extensively through programs such as POEM, PRIMO and MAST. The amassed observations clearly exhibit strong seasonal cycles and some interannual variability. The observed thermohaline circulation has widely been considered to be quasi-stationary, characterizing also the recent past. However, very recent observations portray a completely changed deep-water regime in the eastern Mediterranean (Roether et al., 1995), which challenges the latest generations of models.


We propose to investigate variability of the Mediterranean thermohaline circulation by analyzing transient tracer (CFCs, tritium) and 14C observations. Transient tracers provide detailed information on the structure and rate of interior circulation, on the ventilation and the overturning of the Mediterranean waters and will also contribute to the calibration of 3D models. Transient tracer data indicate that renewal times for the deep and bottom waters in the Eastern Mediterranean are approximately 100 years (Roether and Schlitzer, 1991). Are they of the same order in the western Mediterranean Sea? Suitable data for the Mediterranean are available (Broecker and Gerard, 1969; Roether et al., 1992; 1994; SEMAPHORE 2 cruise, September 1994; METEOR cruise M31/1, Jan. 1995; and unpublished data). A systematic analysis of them will be made.


A further item is the role that salt releases from deep sea sediments could have played during past climate-induced transients in reducing the strength of deep water renewal by creating a stable density gradient in the bottom and deep waters (Van Santoort et al., 1994). May this gradient be eroded by convective renewal from above? A useful tracer in this context is terrigenic helium (Roether et al., 1994) because in the Mediterranean salt and helium released from the sediments are strongly linked. Subsurface pathways of both properties should therefore be the same. The known distribution of terrigenic helium will be used to study the current recirculation of the released salt through the deep waters, in order to help understand its potential role in making the deep waters stagnant under past climatic conditions.


An assessment of stable isotope (deuterium, O-18) data for the Mediterranean will also be made, in the context of both local water-cycle variations in time and for obtaining oxygen isotope-based paleo-information. Stable isotopes provide information on the water balance through special isotopic signatures of precipitation and run-off, as well as isotopic separation during evaporation of water from the sea (Anati and Gat, 1989). The former carry stable-isotope signatures that depend on the water cycle of the greater Mediterranean region. CLIVAMP proposes to analyze recent data for the Mediterranean and the surrounding region (Gat et al., 1994), (METEOR M31/1, Jan-Feb. 1995) to clarify to which extend the Mediterranean can serve as an integrator for climate effects on the water cycle in the region.


These analysis (Bremen team, W. Roether; LODYC-Bethoux) will provide essential information on the ventilation of the Mediterranean waters and will help to calibrate numerical models.




1-2 - Paleoceanographic Data Base of the Mediterranean Sea

A paleoceanographic database for the whole Mediterranean will be gathered and developed. It will consist of proxy-data for climatic and hydrographic changes taken from a wide range of disciplines including marine micropalaeontology, palynology, paleobotany, limnology, organic and inorganic geochemistry. The data will be used to portray changes in the Mediterranean region from the last glacial maximum to today, with particular emphasis on the climatic extremes of the last glacial maximum and early Holocene climatic optimum. The database is intended to include relevant data from outside the Mediterranean, such as the drainage histories of major rivers that discharge into the basin, propagating variability from their catchment areas. At the present time, this database may be started immediately for AMS 14C dated fossil evidence over the eastern Mediterranean and the Alboran Sea. We will perform analyses to bring paleo-oceanographic/- climatological evidence to similar spatial and temporal resolutions as that from the eastern Mediterranean and Alboran Sea.


In addition, Mediterranean-wide maps of (quasi-)conservative properties such as stable oxygen isotope composition of foraminiferal carbonate will be produced for selected time-slices. First the isotopic maps will be used per se to characterize changes in the processes of advection and mixing of water masses of various sorts (e.g. surface inflow through the straits, river discharge), in combination with changes in isotopic composition of the these source areas (Van Os and Rohling, 1993; Bigg, in press). Thus, it will be possible to describe regional variability as well as changes in the overall E-P balance (c.f., for the Red Sea, Rohling, 1994b). The necessary core-material for the proposed new analyses is available from mainly European core-repositories (e.g., Cardiff, Southampton, Utrecht, Paris, Bordeaux, Barcelona). The production temperature and salinity maps obtained from the conversion of the isotopic measurements will be made in cooperation of the studies of section 1-3.


The database will be compiled with addition of the proposed new analyses, by the Southampton based team (Rohling and Bryden) and LODYC (Behtoux, Vergnaud-Grazzini).




1-3 - Paleo-Hydrology of the Mediterranean Sea

Sea surface salinity changes through the past, that partly characterize the Mediterranean thermohaline circulation, may be quantified using micropaleontological and oxygen isotope analyses following the method of Duplessy et al. (1991) for the North Atlantic Ocean. This offers relevant quantitative information for theoretical and numerical studies of natural climatic variations.


The oxygen isotope composition of the planktonic foraminifera depends on the oxygen isotope composition in the ambient sea water and the temperature-related isotope fractionation between carbonate and water (Shackelton, 1974). Furthermore, the oxygen isotope composition of sea water is often perceived to be linearly linked to salinity in response to evaporation and precipitation (e.g. Thunell and Williams, 1989), although non-linearities may develop in marginal basins due to advection processes and some influences on isotopic fractionation at the sea surface (Van Os and Rohling, 1993; Rohling, 1994b; Bigg, in press). Nevertheless, assuming an idealised linear relation, past salinity changes can be estimated using the oxygen isotope composition of planktonic foraminifera, and correcting for the influence of sea surface temperature changes.


Sea surface temperatures are determined with the modern analogue technique (Hutson, 1979) which compares multivariate fossil planktonic foraminiferal samples to a modern calibration data base and identifies the best modern analogues for each fossil association (Prell, 1985). The modern analogues are globally related to prevailing sea surface temperatures using the Levitus atlas. Thus, relating Mediterranean fossil assemblages to modern assemblages allows for rather accurate calculation of sea surface paleo-temperatures (Kallel et al., in prep.).


To quantify sea surface temperatures (SST) and salinity (SSS) changes in the Mediterranean Sea since the last glacial maximum, CLIVAMP (via the C.F.R. group) plans:

i) to perform micropaleontological studies on some 20 deep sea sediment cores collected in the different Mediterranean basins. The C.F.R. group benefits for a large Mediterranean-wide collection of marine cores from several deep-sea coring campaigns (Etna, Dedale, Promethe, Valpamed). In addition, analyses will be included from new marine cores collected during the January/February 1995 PALEOFLUX cruise of M/V Marion Dufresne.


ii) to extend the modern analogue database by studying modern foraminiferal assemblages in additional core tops, to improve paleo-temperature estimates.


iii) to combine the previous information with isotopic measurements, to derive the oxygen isotope composition of the sea water, which may then be translated into salinity using an idealized linear relation between salinity and isotopic composition. Thus, a record will be generated of quantitative estimates of SST and SSS variations during the last glacial cycle.




The SST-SSS reconstructions will be performed by the team based in Paris (LODYC - CFR group).




1-4 - Inferred Variability

The combined database of existing and newly added evidence will be used to characterize the changes in hydrography and climatic forcing of the Mediterranean over time. The fossil evidence will be used to develop concepts of possible circulation-patters, which can then be employed to validate the results of the models proposed in this document. A special aspect is in the influence of sea level change. The proposed model validation will be made with a view towards distinguishing between changes in forcing that were relevant during the past or for future predictions of anthropogenic-induced changes in the Mediterranean.


We are particularly interested in the mode of circulation during the formation of organic-rich sediments (sapropels) deposited in the eastern Mediterranean, and also found less prominently in the western basin. It is unclear whether these layers correspond to periods of reverse conveyor belt circulation. The fossil evidence appears to be more supportive of an anti-estuarine circulation, but one which is weakened (compared with the present day) in response to reduced E-P (see review in Rohling, 1994). Accordingly, a further question is; what did the nutrient budget of the Mediterranean look like during the deposition of those sediments. An answer might be relevant for assessments of a potential anthropogenic-induced eutrophication. Also, recent suggestions (Bigg, 1994) of unexpected origins of deep water in the glacial eastern Mediterranean (deep water from the western basin flowing into the eastern basin at depth through the Strait of Sicily) need to be investigated with models of the type and resolution proposed here. Fossil evidence, especially maps of (quasi-) conservative properties at depth for discrete time slices, will help to evaluate the likelihood of past changes in the origin of deep waters.


Although fossil evidence allows one to develop scenarios of past circulations, robust answers will need an evaluation by modelling including the physics of the system. The modelling studies proposed in tasks 2 and 3 are designed to assess how robust the present day thermohaline circulation is to changes in the climatic conditions.


This compilation and interpretation will be performed by the teams based in Southampton (Rohling & Bryden), Paris (Crepon, Vergnaud-Grazzini - CFR group), Edinburgh (Haines and Williams).

The state of the ocean is strongly influenced by the external forcing from the atmosphere and the exchange of heat, moisture, salt and momentum through the lateral and bottom boundaries. The response of the oceans is sensitive to both the spatial and temporal variability of the forcing. It is important to understand how robust the Mediterranean circulation is to variability in the external forcing given the concern over the impact of possible greenhouse warming. For example, it is uncertain whether warming would reduce or enhance deep water formation in the Mediterranean, as both heat and fresh water fluxes are crucial. While a reduction in dense water formation might be expected from warming alone, this might be overcompensated by the extra evaporation from a warmer ocean.


The major themes of the externally-forced studies proposed here are as follows:

- The mean thermohaline conveyor belt: its sign and vertical structure under different surface forcing scenarios from the present day.


- Transient thermohaline responses: timescales of changes to surface boundary conditions will be investigated to determine the `buffering' and `memory' potential embodied in the water properties and circulation of the Mediterranean.


- The role of short extreme weather events (e.g. anomalous winters) in modifying the water properties and thermohaline circulation.


- Inferred implications of the modelled circulations on the nutrient profiles will be assessed to determine the likelihood of forming organic rich sediments under different circulation scenarios. This will allow the evidence for past circulation regimes to be assessed both for physical realism and consistency with available data.






2-1 - Circulation and Ventilation Changes Induced by Surface Forcing

The principle external changes known to have occurred over the last 18,000 years are in: (i) mean sea level, (ii) sea surface salinity, (iii) surface air humidity, and (iv) winter-time surface air temperatures (see task 1 for further discussion).


These changes may all have altered the circulation and ventilation of the Mediterranean. Some of these changes might be correlated e.g. changes in sea level were accompanied by salinity changes world wide; changes in humidity around the Mediterranean basin would also affect surface salinity within the basin through E-P changes. With a modelling approach we are able to separate the possible competing effects of these changes and assess the contribution of each individually.




2.1.1) Paleo-Modelling Experiments

The present thermohaline circulation in the Mediterranean has been modelled using MOM OGCM within the MTP Mermaids II project and the LODYC OGCM within the MTP EUROMODEL2. The dispersal of water masses, the connection between intermediate and deep water formation, and the exchange of water through straits have been studied using isopycnal diagnostic techniques using the MOM by Haines and Wu (1994) and Wu and Haines (1995). Our modelling experience makes us ideally placed to study how robust or sensitive the thermohaline circulation is to altered climatic conditions, such as those experienced over the last 18,000 years.


We propose to use an OGCM at 1/4 by 1/4 degree resolution, so allowing multiple model runs. The OCGM will be integrated with the modified topography and coastlines consistent with the Holocene Optimum period (8-6 ky), with a sea level about 50 m lower than today, and at last glacial maximum conditions (120 m lower sea level). We will use a variety of surface forcing parameters obtained either from the present day situation or from past climate simulations done by the British Met Office and LMD (Laboratoire de Meteorologie Dynamique). `Perpetual' year boundary forcing will be used to capture seasonal water formation processes which drive the thermohaline circulation. Some OGCM integrations will last for several 100 years, so as to assess the response of the thermohaline circulation. Such long runs are now feasible on recent computers as the CRAY98. In order to improve the efficiency of the coarse grid model and to simulate deep water formation which is an essential ingredient of the thermohaline circulation some studies will use a finer grid mesh in the deep water formation zones. Such model embedding techniques have already been developed by EUROMODEL with the LODYC OGCM.


OGCM experiments will be conducted with the following changes in the surface forcing or boundary conditions:




i) Changes in mean sea level are easiest to assess as they will not have strong regional variations. Sea level is though to have risen by about 120 m over the past 18,000 years. This could influence Mediterranean thermohaline circulation flows in several ways. The changes in depth of straits could influence the required flow speeds in the straits necessary to maintain heat and salt budgets, Bethoux (1984), Bryden and Kinder (1992), Garrett (1994). This could influence both Gibraltar and Sicily exchanges and also thereby affect the depth of the surface-intermediate and intermediate-deep water interfaces. The reduced depth in sub-basins such as the north Adriatic (which becomes land) and the Aegean could strongly influence the water properties formed nearby. The volume of water formed each year may also be altered (Bethoux, 1993).




ii) Past salinities of the Mediterranean waters are thought to have been higher during the glacial maximum and lower during the Holocene Optimum. The intervening salinity reduction might have led to the deep waters in the eastern Mediterranean becoming unventilated for long periods (Bethoux, 1993, Rohling, 1994). This hypothesis may be examined by increasing the density difference between the deep and intermediate waters to reduce further formation of deep water. It is unclear how the circulation might respond to the changes in deep water formation; for example, if dense water formation ceased in the Adriatic, then the intermediate waters may not be drawn into that part of the basin. This change in the ventilation may alter the interfaces between the surface, intermediate and deep waters (which might alter the sapropel formation, as argued by Rohling, 1994).




iii) Temperature effects on circulation are harder to assess using OGCMs because of the sensitivity of the thermohaline circulation to regional variations particularly in the winter season. The regions of deep water formation in the present Mediterranean are determined by extreme winter cooling periods, e.g. associated with Mistral and Bora winds, combined with subsurface salinity maxima due to advection of intermediate waters, Roether and Schlitzer (1991). We propose a series of sensitivity experiments to assess the memory of the Mediterranean circulation under present day conditions, to extremes of winter cooling. Cooling variations in both deep and intermediate water formation regions in the east and western basins will be studied. These same spectra of extreme winter events will also be examined with the modified hydrography used for the stagnant deep water experiment discussed above to assess the likely persistence of a stagnant deep pool. Such experiments are very relevant to the Younger Dryas period when the gap between deep and intermediate water densities may have been reduced.




These studies will be conducted by Haines and Williams (G.B.), Crepon-Bethoux and Martel (Paris).




2.1.2) Tracer and nutrient experiments

In each of these OGCM investigations, idealized tracer experiments will be conducted to reveal the circulation pathways for different water masses; these tracer experiments are conducted in an off-line manner (Williams et al, 1995; Stratford and Williams, 1995). The tracer experiments may be used to quantify the ventilation rate and flushing timescale for each of the water masses. The tracers may be preferentially seeded at the upper or lower boundaries or certain geographical regions. The tracer model is then used to advect and mix the tracer distributions using the off-line OGCM fields. The resulting tracer distributions may be used to highlight the pathways for different water masses over the basin.


As the sapropels provide crucial paleoceanographic evidence of the biological activity, we wish to examine how the various climate scenarios alter the supply of nutrients to the euphotic zone. This investigation will be achieved by integrating a nutrient model using a similar approach to the above tracer model and in the carbon study by Follows et al., (1995). The model will be initialized with a nutrient distribution taken from the present day. The nutrient fields should subsequently evolve through the physical control by the circulation and mixing, and the biological control by consumption and fallout. The biological activity will be simply assumed to be limited by light and the availability of new nutrients. When there is biological activity in the euphotic zone, nutrients will be consumed at a specified rate, fall out as detritus and be re-mineralized within the underlying water column. Thus, the take-up of new nutrients may be converted into fluxes of organic carbon by assuming that the present Redfield ratio still holds. While there is much uncertainty in the present biological models used in OGCMs, the resulting diagnostics of the biological activity should hopefully identify which circulation regime might lead to an environment favourable to the formation of sapropels.


These studies will be conducted by Haines and Williams (G.B.)




2-2 - The Role of a Strait in the Response of a Semi-Enclosed Basin to Surface Forcing

In a classical paper Phillips (1966) showed that in a semi-enclosed basin separated from the ocean by a sill, and under steady-state conditions, the circulation type of the basin (lagoonal or estuarine) is determined by the mean buoyancy budget of the basin. Under such conditions, a two-layer flow system develops at the strait which exactly compensates for the net buoyancy gain or loss to the atmosphere. For the present Mediterranean, there is an annual gain of buoyancy from the atmosphere (with annual heat and water losses of 7 W/m2 and of 0.65-0.95 m/yr; Harzallah et al., 1993; Bethoux and Gentili, 1994; Garrett, 1994), which indeed suggests the observed lagoonal circulation should occur. However, it is unclear whether this circulation should persist given either the seasonal variation in the surface heat fluxes of +- 200 W/m2 or the decadal variation of +/- 25 W/m2 (Garrett et al., 1993). The sapropel layers in the sediments of the Mediterranean have been interpretated by some authors as suggesting a switch from lagoonal to estuarine circulation in past geological times (Stanley et al., 1975, Thunell and Williams, 1989; Sarmiento et al., 1988) while others considered climatic forcing as a strong constraint against.




2.2.1) Control of the frequency of forcing

If there has been a reversal of the overturning circulation in the past geological record, it raises the issue of whether the timescale of the forcing influences the exchange of fluid between neighbouring basins. Lascaratos and Garrett (1995) investigate this issue using an idealised two dimensional model of a vertical section of a basin with a strait. In the limit of "high frequency" forcing, the strait responds to the mean forcing, while in the "low frequency" limit the strait responds to the instanteous forcing. The controlling factor appears to be related to the ratio of the time scale of the forcing versus the advective time scale of the basin.


It is unclear how important these ideas are to the thermohaline circulation in the Mediterranean or its separate basins both now and in the past. This will be addressed using a hierarchy of numerical experiments ranging from simple rectangular "one" and "two" sub-basin idealizations to a real representation of the Mediterranean. Initially the experiments will be driven only by thermal forcing, which will be followed by incorporating both thermal and haline forcing. In this context, we will also study the case of competing buoyancy forcing, which is relevant for example to the Adriatic basin with a buoyancy loss from heat fluxes, but a gain from water fluxes.




2.2.2) Signal at straits of the thermohaline circulation within a basin

The buoyancy flux through the strait and its variability does not necessary reflect the full variability of the surface buoyancy fluxes. This is clearly the case for the Mediterranean at seasonal time scales. The buoyancy flux through Gibraltar seems to reflect only the annual mean and not the seasonal cycle of the surface forcing. On the other hand, the seasonal cycle is responsible for deep water production. There is therefore a connection between the flow system at the strait and the recirculation of the deep waters produced within the basin. Assuming that the strait flow responds to only very low frequency forcing, it is clear that any deep waters produced within the basin have to be completely recirculated and mixed in the interior of the basin. In the limit of flow at the strait having a very strong seasonal cycle, all the dense water formed within the basin may pass through the strait and there is no interior recirculation. These opposing limits may be controlled by the relative size of the basin with the latter case being most relevant for a very small basin.


We will examine this aspect of the thermohaline circulation by conducting a series of experiments with basins of different dimensions. In each experiment, the same amount of deep waters will be produced in winter and we will examine the thermohaline circulation within the basin, as well as monitoring the volume of fluid passing through the strait to estimate what percentage of these waters leave the basin at different times of the year.


These studies will be conducted by Lascaratos (Athens) and Crepon (LODYC).




3-2 - Recent Radical Changes to Mediterranean Waters and Sedimentary Salt Input

An analysis of very recent changes to the Eastern Mediterranean deep waters (Roether et al., 1995) finds bottom waters originating from a new source (the Aegean instead of the Adriatic), with much faster renewal rates than had been found previously (Roether and Schlitzer, 1991). Moreover the changes imply net downward transfers of heat and salt, and an upward displacement of the previous deep waters. We aim to find out to what extend the cause could be an internal instability of the system (e.g. by salt advection, with a possible role of stochastic forcing, such as by the severe winter of 1991-92; Sur et al., 1992), a change in the water budget (e.g. diversion of the Nile), or a time-integrated effect of a climatic trend. A further item is the presumed transitory nature of the change. The assessment will primary use conceptual circulation models and property balances, with an option for using an OGCM.


A related subject, to be studied by a similar approach, is potential initiation, or enhancement, of deep-water stagnation by salt released from the sediments. The present role of this extra salt, studied under section 1.1 of this proposal, will be translated to scenarios of past climates, and in particular, transients of climate, as these may have a significant influence.


These studies will be conducted by W. Roether (Bremen).


3-3 - Long Term Variability of the Mediterranean Sea Climate

By using a similar OGCM at 1/4 x 1/4 degree resolution as in task 2 we will investigate the effect of the internal dynamics of the Mediterranean sea on its long term variability:


i) We will assess the impact of stochastic forcing and the importance of extreme events versus climatological forcing in controlling the circulation, ventilation and basin exchange in the Mediterranean for the 8-6 ky Holocene Optimum period.


ii) We will investigate the interannual variability of the Mediterranean modal waters. Do they form every year? What controls the variability of their formation? Is is the variability of the atmospheric fluxes alone or does the internal dynamics of the Mediterranean sea play a role? Is the memory of past events important? At what time scale? What is the role of the wind stress?




These studies and those of task 2 will lead to an understanding of the ability of the Mediterranean Sea to detect climatic changes. They will be conducted by M. Crepon-F. Martel-J.P. Bethoux team.

While the state of the ocean clearly depends on external forcing, this state is by no means uniquely defined. Multiple equilibria have been shown to be possible for a given forcing. There are also transients, which rely on internal dynamics, mean responses to stochastic forcing. Changes in response to forcing may be subject to thresholds, beyond which a qualitative change of the ocean will be induced. It is by no means clear that current numerical models can predict such features realistically. Large deep-water transients have undoubtedly occurred in the course of the last glacial cycle. Transients will be assymetric because under conditions of raised surface density, renewal will be accelerated, and vice versa. Forcing stochastics may play a role in the proposed intermittent renewal of mid-depth waters during deep water stagnation periods, but vertical mixing (in view of the limited depth of the Mediterranean) will also limit stagnation. A number of approaches will be used to study the interplay of all these processes.




3-1 - Theoretical Studies of Internal Variability

We will study the possibility that the Mediterranean Sea circulation has multiple equilibrium states, as recently emphasized by several authors for the global ocean circulation (Rooth, 1982, Broecker et al., 1985, Bryan, 1986, Marotzke et al., 1988, Marotzke and Willebrand, 1991). A potential candidate for inducing multi-equilibrium states is fluid motion at the Strait of Gibraltar which can condition the circulation, as noted by Bryden and Stommel (1984), through hydraulic control. The salt budget may also crucially condition multiple equilibria in OGCMs. A salinity- relaxation (Haney, 1971) boundary condition overconstrains the range of surface changes possible; a recent account was given by Huang (1993). Salt may also govern, through advection, the role played by different deep water formation sites in the past. Salt addition from the sediments (see above, Task1) may also be important. The impact of these and other critical influences on the Mediterranean conveyor belt will be investigated here.


First the thermohaline circulation of the Mediterranean will be studied as a dynamical system in terms of dynamical system theory (Lorenz, 1963). Topics will be exponential divergence from initial states, and predictability of long-term variability in a statistical sense, under a variety of external conditions. Can we identify pseudo-cycles; what are the attractors of the Mediterranean system? (In these studies it is supposed that the Mediterranean Sea does not react onto the Atmosphere).


In a complementary approach various scenarios will be implemented in an OGCM, simplified with respect to forcing and basin geometry, to study;

i) effects of different boundary conditions at the Straits (Gibraltar and Sicily)


ii) salt fluxes at the upper boundary


iii) strong wind forcing events.




These investigations will be conducted by V. Artale, Rome - M. Crepon, F. Martel, Paris.


Competition between different intermediate and deep water formation areas and their interplay under different climatic conditions will be studied. Use will be made of the simple by instructive box modelling technique. This approach has already been successfully applied to the Atlantic (Stommel, 1961; Weaver et al., 1991). More specifically we will

i) investigate the possibility of existence of several distinct equilibria and/or self-sustained thermohaline oscillations under the same boundary conditions;


ii) we will assess the effect of the competing roles of the thermal and haline surface surfacing in driving the thermohaline circulation. (High evaporation rates in the east together with higher temperatures);


iii) we will study the possibilities for synergistic or competing roles of the individual thermohaline cells.




These studies will be conducted by A. Lascaratos, Athens.


To study thermohaline circulation in an OGCM with the possibility of radical changes, flux boundary conditions are needed. The MOM model will be modified to run with those T and E-P fluxes consistent with recent thermohaline circulation conditions, i.e. those derived from the Haney relaxation run. The full year to year or longer time-scale variations in circulation will be noted. When the model reaches statistically steady conditions, similar to present day, the surface boundary conditions can then be changed by introducing a flux anomaly. The magnitude of E-P fluxes will be halved to study the linear or non-linear response of the circulation corresponding to the possible conditions during the Holocene Optimum (Rohling, 1994). Will both deep and intermediate circulations still occur with reduced amplitude or can the feedback through intermediate waters switch off the deep water formation? These integrations may be continued for periods of up to several centuries to detect the establishment of any new thermohaline regimes. The timescale for this to occur will be studied along with sensitivity to flux variations in individual winters. This will therefore address the issue of stability of the current thermohaline circulation as well as the possibility of alternative circulation regimes more akin to paleoclimatic conditions.


These studies will be conducted by K. Haines, Edinburgh.




3-2 - Recent Radical Changes to Mediterranean Waters and Sedimentary Salt Input

An analysis of the very recent changes to Eastern Mediterranean deep waters (Roether et al., 1995) finds bottom waters originating from a new source (the Aegean instead of the Adriatic), with much faster renewal rates than had been found previously (Roether and Schlitzer, 1991). Moreover the changes imply net downward transfers of heat and salt, and an upward displacement of the previous deep waters. We aim to find out to what extent the cause could be an internal instability of the system (e. g. by salt advection, with a possible role of stochastic forcing, such as by the severe winter of 1991-92; Sur et al, 1992), a change in the water budget (e. g. diversion of the Nile), or a time-integrated effect of a climatic trend. A further item is the presumed transitory nature of the change. The assessment will primarily use conceptual circulation models and property balances, with an option for using a GCM.


A related subject, to be studied by a similar approach, is potential initiation, or enhancement, of deep-water stagnation by salt released from the sediments. The present role of this extra salt, studied under section 1.1 of this proposal, will be translated to scenarios of past climates, and in particular, transients of climate, as these may have a significant influence.


These studies will conducted by W. Roether ( Bremen).




3-3 - Long Term Variability of the Mediterranean Sea Climate

By using a similar OGCM at 1/4 x 1/4 degree resolution as in task 2 we will investigate the effect of the internal dynamics of the Mediterranean sea on its long term variability :

i) We will assess the impact of stochastic forcing and the importance of extreme events versus climatological forcing in controlling the circulation, ventilation and basin exchange in the Mediterranean for the 8-6ky Holocene Optimum period.


ii) We will investigate the interannual variability of the Mediterranean modal waters. Do they form every year? What controls the variability of their formation? Is it the variability of atmospheric fluxes alone or does the internal dynamics of the Mediterranean sea play a role? Is the memory of past events important ? At what time scale?




These studies and those of task 2 will lead to an understanding of the ability of the Mediterranean Sea to detect climatic changes. They will be conducted by M. Crepon-F. Martel-J.P Bethoux team

Over the last few decades, it has become evident that the most valuable contribution of oceanographers to paleoceanography is to describe the gross features of basin scale oceanic state. Paleoceanographic data can stimulate hypothetical descriptions of likely hydrographic configurations and circulations in the past that seem to fit the data, the central features of which can then be more rigorously tested. At the present stage it is still premature to hope that paleoceanographic data may be directly assimilated into numerical models to produce a realistic representation of regional- scale phenomena in the past (e.g., intermediate/deep water formation). Fossil data simply are not available with the required spatial resolution, and even historical measurements over the last century are generally too sparse.


These `past circulation scenarios' may be investigated with the sophisticated modelling tools used for studying present circulations, especially in small areas like the Mediterranean. Present day circulations, which the models have been tuned to reproduce well, can be perturbed in appropriate ways (as discussed in tasks 2 and 3) to determine the likely response to known climatic conditions in the past. These responses can then be compared with the hypothetical `past circulation scenarios' from which much of fundamental importance can be learnt concerning how robust or sensitive the circulation is. Results will be relevant to climatic changes on any time-scales, such as those recorded in historic measurements (since about A.D. 1900), those related to glacial cycles, and those which may result from future climate change. This approach is highly innovative, and is likely to identify which hypotheses are robust and dynamically plausible.




4-1 - Concept Development

In the framework of this research, a real attempt will be made to validate or refute some of the hypothetical models offered for the paleo-circulation of the Mediterranean and to develop new conceptual models, where necessary, based both on gathering a unified data base of relevant paleo-data, and the recreation of some elements of past climate with models. When there are more than one possible explanation for the assembled data, for any discrete time-slice, the multiple interpretations will each be worked out in detail, including listings of the correspondences and discrepancies between concept and data. For example, in identifying the relevance of the different scenarios for sapropel formation involving either a reversal of the present circulation or a weaker and shallower present circulation. Where possible there will be an effort to present results in as quantitative as a manner possible including likely error margins, based on both data and modelling results.


This research will be conducted by Bremen, Edinburgh Southampton teams.




4-2 - Evaluation, Robustness - `Climate' Index

The modelling approaches outlined in tasks 2 and 3 will provide a clear understanding of the sensitivity of the thermohaline circulation to modified forcing or basin conditions which may have occurred in the past. A sound physical understanding of the controlling processes will also be provided both from the simplified box model studies and the idealised GCM studies which will be valuable in further interpretation of historical and present day data.


Our modelling results will be used to try and validate the various proposed circulation scenarios used to explain events in the paleo-oceanographic record. We will investigate the interaction of the various external forcings and internal dynamics when they are all applied together (they are separately described in tasks 2 and 3), which would have been the case in past eras; e.g. the reduced sea level occurring with modified surface flux conditions.


Tasks will involve :

1) Assess the physical consistency of past Mediterranean circulation scenarios.


2) Assess the stability of these conditions, under the natural range of interannual forcing conditions.


3) Assess the likely secondary effects of past circulations on nutrient cycling on a basin-wide and regional basis to understand possible sediment formation conditions.


4) Assess the accessibility of past circulation conditions from the present circulation state, i.e. the transient response to changes in present climatic forcings.


5) Determine the most robust indication of changing circulation conditions, i.e. a possible `climate index'.




This research will be conducted by all teams.

TASK 1

Section 1.1

1.1.1 Investigate the variability of the thermohaline circulation of the Mediterranean Sea by analyzing transient tracer observations (e.g., helium, CFM) [Bremen] .0 - 24 months.

1.1.2 Assess the water balance of the Mediterranean through research on stable isotopes in water [Bremen] 12 - 36 months.

Section 1.2

1.2.1 Compile a comprehensive database of proxy-data on climatic and hydrographic changes in the Mediterranean area/basin through geologic time [Southampton, LODYC-CFR] 0-24 months

1.2.2 Add new micropaleontological and isotopic analyses to the database of new paleoceanographic/-climatological evidence, to bring studies from the western Mediterranean basin and the Atlantic Ocean just outside the Strait of Gibraltar to similar spatial and temporal resolutions as that from the eastern Mediterranean and the Alboran Sea [Southampton, LODYC-CFR]. 0-24 months.

1.2.3 Produce Mediterranean-wide maps of (semi-)conservative properties - such as the stable oxygen isotope composition of foraminiferal carbonate - for selected time-slices, to characterise changes in the processes of advection and mixing of water masses from various sources (e.g. surface inflow though the straits, river discharge) [Southampton, LODYC-CFR]. 12 - 36 months

Section 1.3

1.3.1 Derive original sea surface temperature estimates from a new Mediterranean planktonic foraminiferal transfer function based of basin wide Mediterranean sediment cores for the last glacial cycle [LODYC-CFR]. 12 - 36 months

1.3.2 Derive original oxygen isotope composition from sediment cores and convert into estimates of original water column salinity [LODYC-CFR]. 0 - 36 months

Section 1.4

1.4.1 Use the combined database of existing and newly added evidence to characterise changes in hydrography of the Mediterranean through time and the related climatic forcing. The fossil evidence will be used to develop concepts of possible circulation-patterns, which can then be employed to validate the results of the models proposed in this document [Southampton, LODYC-CFR, Edinburgh, Liverpool]. 18-36 months

TASK 2

Section 2.1 Circulation and Ventilation changes induced by surface forcing

2.1.1 Paleo-modelling experiments



2.1.1.1 Assess impact of sea level change alone on the circulation, ventilation and basin exchange in the Mediterranean for the 8-6ky Holocene Optimum period and last glacial maximum, 18ky BP. [Edinburgh, Liverpool, Southampton ] 0-18 months



2.1.1.2 Assess impact of salinity changes alone between the higher values in the glacial maximum and lower values in the Holocene Optimum on the circulation, ventilation and basin exchange in the Mediterranean. [Edinburgh, Liverpool, Southampton] 0-18 months



2.1.1.3 Assess impact of temperature changes alone on the circulation, ventilation and basin exchange in the Mediterranean for the 8-6ky Holocene Optimum period. [Edinburgh, Liverpool, Southampton] 12-24 months



2.1.2 Tracer and nutrient experiments



2.1.2.1 Perform off-line tracer experiments with GCM experiments in 2.1.1 to reveal how the different climatic conditions alter the circulation pathways, the ventilation rate and flushing timescale for Mode waters. [Liverpool, Southampton, Edinburgh] 0-18 months



2.1.2.2 Assess the supply of nutrients to the euphotic zone with GCM experiments in 2.1.1 to reveal favourable environments for sapropel formation under different climatic conditions. [Liverpool, Edinburgh, Southampton] 18-36 months



Section 2.2 Response of a strait to the surface forcing

2.2.1 Assess how the frequency of surface forcing influences the exchange of fluid between basins via a narrow strait using idealised box models and GCMs. Examine the competing roles of surface heat and water fluxes, and how they alter the response of a strait. [Athens] 0-18 months

2.2.2 Assess how the exchange through a strait influences the intermediate and deep water formation within a basin. Diagnose what proportion of dense water annually formed remains within the basin and examine whether this is controlled by the relative size of the basin. [Athens, LODYC] 18-36 months.

TASK 3

Section 3.1 Idealized and OGCM Model Studies of Internal Variability

3.1.1 Assess internal variations in an idealized GCM induced by different boundary conditions at the straits, by the surface salinity boundary condition and by high-wind events [Roma, 0 - 24 months]

3.1.2 Assess the behaviour of the thermohaline circulation in terms of dynamical systems theory. [Roma, 0 - 36 months]

3.1.3 Assess competition between different water mass formation areas and different climatic conditions by means of conceptual models. [Athens, 12- 36 months]

3.1.4 Assess the internal causes of the recently observed change in the deep waters of the Eastern Mediterranean. [Bremen, 0 - 36 months]

Section 3.2 Recent radical changes to Mediterranean Waters and Sedimentary Salt Input

3.2.1 Assess the role of the sedimentary salt source to induce stagnation under current and past climates. [Bremen, 0 - 18 months]

3.2.2 Determine the stability and tendency to drift, of the present thermohaline circulation, under flux-corrected boundary conditions. [Edinburgh, 12-24 months]

3.2.3 Determine response to reduced salt fluxes with present day distributions, particularly influence on deep water formation. [Edinburgh, 18-36 months]

Section 3.3 Long term variability of the Mediterranean Sea Climate

3.3.1 Assess impact of stochastic forcing and the importance of extreme events versus climatological forcing in controlling the circulation, ventilation and basin exchange in the Mediterranean for the 8-6ky Holocene Optimum period. [Edinburgh, Liverpool, LODYC] 18-36 months

3.3.2 Investigate the interannual variability of the Mediterranean modal waters. Do they formed every year? What controls the variability of their formation? Is it the variability of atmospheric fluxes alone or does the internal dynamics of the Mediterranean sea play a role? Is the memory of past events important? At what time scale? [Edinburgh, Liverpool, LODYC] 18-36 months

TASK 4

Section 4.1 Concept development

4.1.1 Provide assessment of physical consistency of integrated Mediterranean circulation scenarios at


(i) 18ky-10ky (deglaciation)


(ii) 8-6ky Holocene Optimum. [LODYC, Southampton, Edinburgh, Liverpool, Athens] 18-36 months



4.1.2 Assess stability of these conditions (in 4.1) under range of seasonal forcing extremes. [Athens, Edinburgh, Roma] 18-36 months

4.1.3 Assess past circulation influences on nutrient cycling. [Southampton, Liverpool, Edinburgh ] 24-36 months

4.1.4 Accessibility of past circulation states from present conditions. [Athens, Roma, Edinburgh] 12-36 months

Section 4.2 Evaluation, Robustness - 'Climate' Index

4.2.1 Determination of robust indications of changing circulations which might appear today/ climate index. [Bremen, Edinburgh, LODYC] 18-36 months

CLIVAMP will benefit and enhance the observations taken during other EC supported programmes as PALEOFLUX, EUROMARGE and MTP II


CLIVAMP research will contribute to improved knowledge of the functioning of the Mediterranean sea thermohaline circulation and its long term variability by investigating the major mechanisms with theoretical and numerical models and appropriate observations.


Owing to similarities in the processes driving the thermohaline circulation in the Mediterranean sea and in the global ocean CLIVAMP research will contribute to a better knowledge of the different processes driving the global ocean conveyor belt which drives the oceans meridional heat transfer and controls the climate of the Planet.


Thanks to the CLIVAMP studies, the understanding of the scenarios of the Paleocirculation of the Mediterranean Sea will progress. CLIVAMP studies will give information on the ability of the Mediterranean sea to detect climatic changes and to be considered as a test basin for climatic signatures.

CLIVAMP is a fundamental research programme. First the CLIVAMP results will be discussed with the scientists directly or indirectly involved in CLIVAMP such as PALEOFLUX, EUROMARGE, MTP II. The CLIVAMP results will then be disseminated in high standard scientific revues and presented in scientific meetings.


The database information described information described in Task 1-2 will benefit the scientific paleo-oceanographic and oceanographic community; they will be disseminated through a CD-ROM which will be available on request (at a mail charge) to members of non profit organizations and for scientific use only.


Owing to the quality and the pertinence of the results we will obtain, we envisage to publish an Atlas both for the database and the results of the numerical modelling scenarios.