Western Mediterranean Deep Water Research Articles

The Western Mediterranean Deep Water: A proxy for climate change

Reconstructions of Mediterranean ocean temperature fields back to 1950 show a proxy relationship between heat content changes in the North Atlantic and the Western Mediterranean Deep Water (WMDW) formed in the Gulf of Lions in winter, because of consistent air‐sea heat fluxes over these areas, strongly correlated to the North Atlantic Oscillation (NAO).

Impacts of the Nile River damming on the thermohaline circulation and water mass characteristics of the Mediterranean Sea

The long-term changes in the thermohaline circulation and water mass characteristics of the Mediterranean Sea caused by the damming of the Nile in 1964 are investigated using a 3-D primitive equation model (Princeton Ocean Model, POM). The model is first integrated for 65 years under climatological forcing and without taking into account the Nile runoff to obtain an initial steady state. Then the model is integrated for 75 years with the Nile outflow included until it reaches a steady state representative of that existing in the Mediterranean prior to the damming. Finally, the model is integrated for another 75 years eliminating the Nile runoff to reach a new steady state. The model results show that the absence of the Nile freshwater input induces a saltier surface layer in the vicinity of the Rhodes Gyre thus favoring the preconditioning for the formation of the Levantine Intermediate Water (LIW). This results in about 30% increase of the LIW formation rate. Intermediate waters become saltier, and as they are transported westward they reduce the stability of the water column in the deep-water formation sites, namely the South Adriatic and the Gulf of Lions. Thus saltier and larger amounts of dense waters are formed filling the deep parts of the Mediterranean Sea. According to the model, the Nile damming explains about 45% of the observed salinity increasing trend occurring over the last 40 years in the Western Mediterranean Deep Water (WMDW). Furthermore, model results demonstrate that the Nile damming played a crucial role in the long-term salt preconditioning of the surface/intermediate layers of the Cretan Sea, thus contributing in triggering the Eastern Mediterranean climatic transient in the late 1980s.

Acoustic thermometry of the western Mediterranean basin

Ocean acoustic tomography is used to obtain heat-content estimates for the western Mediterranean basin. Travel-time data from 13 tomography sections of the Thetis-2 experiment (January–October 1994) are analyzed with a matched-peak inversion approach. The underlying analysis involves the use of peak arrivals and nonlinear model relations between travel-time and sound-speed variations. Slice inversion results are combined with temperature covariance functions for the western Mediterranean to obtain heat-content estimates for the basin. These estimates compare favorably with ECMWF data over the nine-month period of the Thetis-2 experiment. Furthermore, estimates for the basin-average temperature of the western Mediterranean deep water are obtained.

Large-Scale Spreading of Deep Waters in the Western Mediterranean Sea by Submesoscale Coherent Eddies

Abstract Two large-scale free-drifting isobaric-floats experiments, “SOFARGOS”/Marine Science and Technology Programme, phase 2 (MAST2) and Mass Transfer and Ecosystem Response (MATER)/MAST3, undertaken in 1994–95 in the northwestern Mediterranean Sea and in 1997–98 in the Algerian Basin, respectively, have revealed for the first time that Western Mediterranean Deep Water, newly formed by deep convection in the Gulf of Lion (the so-called Medoc site), can be advected several hundreds of kilometers away from the formation area by anticyclonic submesoscale coherent vortices (SCVs). This behavior implies that SCVs participate actively in the large-scale thermohaline circulation and deep ventilation of the western Mediterranean Sea. These SCVs are characterized by small radius (∼5 km), very low potential vorticity, high aspect ratio (∼0.1), and extended lifetime (>0.5 yr).

Deep water in the western Mediterranean: peculiar 1999 and 2000 characteristics, shelf formation hypothesis, variability since 1970 and geochemical inferences

Time series of hydrological data acquired in the Ligurian Sea (western Mediterranean) over the 1993–2000 period allowed us to complement the monitoring of the multi-annual increases in temperature and salinity (3.4×10 −3 °C and 1.05×10 −3 per year, mean values at depth greater than 2000 m since 1959). They also revealed peculiarities of dense water characteristics. Following a 7-year period (1993–1998) with potential temperature and salinity continuously decreasing between 1000-m depth and the bottom (∼2700 m), the 1999 and 2000 data evidenced a positive anomaly, i.e., increases of potential temperature and salinity in the last hundred meters above the bottom. Records of currents and temperature on the continental slope of the Gulf of Lions, particularly in the Lacaze-Duthiers canyon, as well as wind and heat fluxes data indicated that cascading of dense water occurred in February 1999. The transport of turbid shelf water and re-suspended sediment might explain the near-bottom turbidity anomaly found concomitantly with the hydrological anomaly. Besides, hydrological data collected during summer 1999 in the Algero-Provençal basin showed a spreading of the anomalous bottom water, radiating from the Gulf of Lions at a mean speed of 3.5 cm s −1 over the whole basin. The analysis of historical data suggested that over the 1971–2000 period, four events of intense cascading of dense water from the Gulf of Lions' shelf (in 1971, 1980, 1988 and 1999) affected the hydrology of the basin and contributed to the formation of western Mediterranean deep water. Though these cascading events contribute little to the overall dense water formation, which mostly occurs off-shelf (MEDOC area), they probably have a rather important role in the transfer of matter.

Hydrographic characteristics and interannual variability of water masses in the central Mediterranean: a sensitivity test for long-term changes in the Mediterranean Sea

Hydrographic observations repeated from 1993 to 1999 in the central Mediterranean Sea, between the Sicily Strait and the Sardinia Channel, allowed us to define the water masses exchanged between the Eastern and the Western Mediterranean and the long-term variability of their properties. Besides the well known Modified Atlantic Water (MAW) and the Levantine Intermediate Water (LIW), other water masses are involved in this exchange: the waters of the upper deep layer in the Ionian Sea (transitional Eastern Mediterranean Deep Water (tEMDW)), which form a density current flowing at the bottom, and, from the Western Mediterranean, the Western Mediterranean Deep Water and a stream of LIW that has re-circulated in that basin (old LIW). The sub-surface water masses flow into the Tyrrhenian Sea, an intermediate basin, where they are subject to intense mixing, which modifies them or even makes them disappear. Accordingly, the outflow is formed by LIW and by the modified Tyrrhenian Deep Water (mTDW), both of them contributing to the exchange with a stream directed to the western Mediterranean. An interesting aspect indicated by the hydrographic time series is that, although the temperature and salinity of LIW in the Sicily Strait showed a prevailing trend towards lower values consistent with the changes produced by the recent climatic transient in the Eastern Mediterranean, the temperature and salinity of mTDW increased progressively throughout the whole period. This apparent anomaly was related to the behaviour of tEMDW in the Tyrrhenian Sea. While sinking at the Tyrrhenian entry, tEMDW represents a source of heat and salt for the colder and less saline resident waters at depth, thus progressively raising their temperature and salt content. The small, long-term tendency of this was intensified by the arrival of the new waters produced by the climatic transient. In our opinion, the effects of this process may add a component to the well known positive trend affecting the temperature and salinity of deep waters of the western Mediterranean.

Simulation of water mass formation processes in the Mediterranean Sea: Influence of the time frequency of the atmospheric forcing

Numerical experiments are conducted to investigate the interannual variability of water mass formation processes in the Mediterranean Sea. The experiments are conducted with a general circulation model which includes a set of calibrated air‐sea heat flux bulk formulae to account for the correct surface heat budget of the Mediterranean Basin. An analysis of the model response to different atmospheric forcing frequencies (monthly versus 12‐hour) for the period 1980–1988 is conducted in the areas of different water mass formation processes: the Western Mediterranean Deep Water (WMDW), the Eastern Mediterranean Deep Water (EMDW) (only the Adriatic component), the Levantine Intermediate Water (LIW), and the Levantine Deep Water (LDW). All the experiments are able to simulate well the LIW and LDW; however, the 12‐hour forced experiments (D experiments) tend to produce more LDW than LIW compared to the monthly forced experiments (M experiments). On the other hand, only the D experiments, with a salinity enhancement, are able in some years to model the WMDW deep convection. The WMDW and EMDW are formed approximately every 3 years depending on the model parameterizations; however, LIW is formed each year but with large‐amplitude fluctuations in the volume of water formed. LDW has a formation rate similar to that of deep waters.

XBT monitoring of a meridian section across the western Mediterranean Sea

During the Thetis-2/MAST-2 tomography experiment, T7-XBT calibrated (accuracy ∼0.05°C) probes were launched ∼28 km apart between France and Algeria, twice a month from Feb. to Sep. 1994. Combined with infrared images, altimetric data and ship drifts, they provide definite information on the structure, drift and role of the eddy-like mesoscale phenomena generated by the Algerian Current instability. When embedded in this alongslope current, these phenomena generally propagate downstream at a few km/day and are markedly asymmetrical. Because of the topography in the eastern part of the Algerian Basin, they separate from the current, become more symmetrical and follow an anticlockwise circuit in the open basin. These phenomena are deeper than ∼750 m and entrain seaward pieces of the Levantine Intermediate Water (LIW) vein flowing along the Sardinian slope, thus being responsible of the large spatial and temporal variability of the LIW distribution in the open basin. The non-existence of a LIW vein flowing westward across the Algerian Basin is definitely demonstrated. In the Gulf of Lions, new insights are provided into the formation and spreading of the Winter Intermediate Water (WIW), which is the Western Mediterranean counterpart of LIW. Considering the large amount of WIW formed during this mild winter, it is clear that this water has not received enough attention yet, and is certainly a major component of the Mediterranean outflow at Gibraltar. Finally, the XBT data account for the eastward flow of the Western Mediterranean Deep Water (WMDW) off Algeria.

Interbasin deep water exchange in the western Mediterranean

Owing to its nearly enclosed nature, the Tyrrhenian Sea at first sight is expected to have a small impact on the distribution and characteristics of water masses in the other basins of the western Mediterranean. The first evidence that the Tyrrhenian Sea might, in fact, play an important role in the deep and intermediate water circulation of the entire western Mediterranean was put forward by Hopkins [1988]. There, an outflow of water from the Tyrrhenian Sea into the Algero Provençal Basin was postulated in the depth range 700–1000 m, to compensate for an observed inflow of deeper water into the Tyrrhenian Sea. However, this outflow, the Tyrrhenian Deep Water (TDW), was undetectable since it would have hydrographic characteristics that could also be produced within the Algero‐Provençal Basin. A new data set of hydrographic, tracer, lowered Acoustic Doppler Current Profiler (LADCP), and deep float observations presented here allows us now to identify and track the TDW in the Algero‐Provençal Basin and to demonstrate the presence and huge extent of this water mass throughout the western Mediterranean. It extends from 600 m to 1600–1900 m depth and thus occupies much of the deep water regime. The outflow from the Tyrrhenian is estimated to be of the order of 0.4 Sv (Sv = 106 m3 s−1), based on the tracer balances. This transport has the same order of magnitude as the deep water formation rate in the Gulf of Lions. The Tyrrhenian Sea effectively removes convectively generated deep water (Western Mediterranean Deep Water (WMDW)) from the Algero‐Provençal Basin, mixes it with Levantine Intermediate water (LIW) above, and reinjects the product into the Algero‐Provençal Basin at a level between the WMDW and LIW, thus smoothing the temperature and salinity gradients between these water masses. The tracer characteristics of the TDW and the lowered ADCP and deep float observations document the expected but weak cyclonic circulation and larger flows in a vigorous eddy regime in the basin interior.

Hydrology and currents observed in the channel of Sardinia during the PRIMO-1 experiment from November 1993 to October 1994

The channel of Sardinia is one of the less-known areas of the Western Mediterranean sea. The paths of the Levantine Intermediate Water (LIW), the Tyrrhenian Dense Water (TDW) and the Western Mediterranean Deep Water (WMDW) mainly, in this wide passage, are still debated. Two major aims of the PRIMO-1 (Programme de Recherche International en Mediterranée Occidentale) experiment were to specify the mean circulation paths of these intermediate and deep waters there and to verify hypotheses about the migration of mesoscale eddies from Algeria to Sardinia and their effect on the circulation paths of all the water masses. Despite an important loss of moored instruments, current time series have been collected from November 1993 to October 1994 at four different locations and two CTD cross-sections have been performed. Current meters on the Sardinian slope indicate, during 1 year, an almost permanent alongslope flow toward the northwest in the intermediate and deep layers. Currents recorded on the African slope are always directed alongslope eastward down to 2500 m. Coherently, in the mid-western part of the channel, the bottom flow is northward along the isobaths, while the 200-m flow is southward due to the influence of anticyclonic eddies. At the bottom in the central part of the channel, the current is low and changing: episodes of northeast current along the orientation of the deepest channel axis alternate with westward flows. No seasonal variations have been clearly detected, but the mesoscale variability is large and has a deep signature in the western part of the channel.

On the upper layer circulation in the Alboran Sea

We present new experimental results and assemble them with previous results in order to develop an improved picture of the upper layer circulation in the Alboran Sea. It is suggested that the key idea to understanding this upper layer circulation is the tendency of the Atlantic jet (AJ) to have negative curvature. Local interactions with the western Alboran gyre (WAG) or the African coast can, however, counterbalance this tendency and modify the anticyclonic path of the AJ. It is also proposed that the density gradients in the WAG can be maintained in part by means of an intermittent surface cross‐gyre current which results in an input, mixing, and renewal of Atlantic water. The static stability at the bottom of the gyre increases because of the mixing of Atlantic water with Western Mediterranean Deep Water which is uplifted close to the African coast. This mixing process thereby acts as a local source of potential vorticity. We also report the existence in the Alboran Sea of subsurface anticyclonic eddies (located between 100 and 400 m) of relatively cold water that appear to be detached from the Iberian shelf. Regarding the large‐scale variability of the AJ‐WAG system, we present evidence of an eastward migration of the WAG and the subsequent formation of a new anticyclonic gyre in the western Alboran basin on a timescale of 1 month. This eastward gyre migration process temporarily allows the simultaneous presence of three anticyclonic gyres in the Alboran Sea.

On the importance of the choice of wind stress forcing to the modeling of the Mediterranean Sea circulation

A 1/4° degree ocean general circulation model is used to examine the role that four different wind stress climatologies play on the circulation of the Mediterranean. The wind stress climatologies examined are those derived from numerical weather prediction models (National Meteorological Center (NMC) and European Centre for Medium‐Range Weather Forecasts (ECMWF)) and one based on observations (Southampton Oceanography Centre (SOC)). Significant differences exist between the wind climatologies over the Mediterranean and in the response of an ocean general circulation model forced by the different climatologies. Excessive coastal upwelling/downwelling is found to be associated with the extreme zonal nature of one of the climatologies. Surface circulation differences include the position and penetration of the Mid‐Mediterranean Jet into the Levantine, the Ionian, and the Tyrrhenian circulations. Significant differences exist in the pathways for dispersal of Levantine Intermediate Water. Under the SOC forcing, there is a reduction in Eastern Mediterranean Deep Water formation in the Southern Adriatic, compensated by the production of intermediate or deep water in the Aegean. The ECMWF climatology is found to be associated with much more cyclonic doming in the Gulf of Lions, leading to better formation of Western Mediterranean Deep Water.

Deep water formation in the western Mediterranean

Wintertime deep convection was studied in the Gulf of Lions (northwestern Mediterranean) using the chlorofiuoromethane (CFM) distributions (components F11 and F12), from December 1991 and February–March 1992. Convection occurred on small spatial and temporal scales and it was not complete: only convection to 1900 m depth was observed. The vertical mixing of the water column above the convection depth turned out to be of major influence regarding the characteristic of newly formed deep water and regarding the boundary conditions for CFMs in modeling deep water formation. Air‐sea gas exchange and entrainment were found to be negligible. These findings are important in estimating deep water formation rates for all water masses renewed by convection (e.g., Greenland Sea Deep Water, Labrador Sea Water) and in evaluating the spreading of deep water masses along the western boundary current. A box model is presented which uses CFM and tritium distributions to estimate formation of Western Mediterranean Deep Water below 1000 m. The model starts with zero tracer concentrations in 1945, the integration step is 1 year, and the deep water formation rate is assumed to be constant from 1945 to 1992. The best fit between CFM model concentrations and measurements were obtained at a mean deep water renewal time of 5–7 years, equivalent to a conversion of 0.46 Sv Modified Atlantic Water (0‐ to 150‐m depth) and 0.76 Sv Levantine Intermediate Water (150‐ to 400‐m depth) into deep water. However, the tritium measurements are only partly matched by the model, by assuming that convection stopped in the years 1964–1968.

Man‐induced salinity and temperature increases in western Mediterranean deep water

The historical data base is used to study property changes in both the Western Mediterranean Deep Water (WMDW) and the Levantine Intermediate Water (LIW). Changes in WMDW properties during the past century have been described previously, although on a more limited data base. We are not aware of any previous study of changes in LIW properties. In the extensive data base we used, increases appear in both WMDW temperature and salinity, from 1909 to the present, which substantiate previously reported observations. In addition, we find that the density of WMDW seems to have increased as well, which disagrees with previous suggestions that it has remained constant. We observe that the WMDW temperature increase displays a distinct acceleration starting about 1955 and that a similar, although less conspicuous, acceleration occurs in the WMDW salinity increase. From our study of historical data on LIW properties, the LIW salinity also appears to have increased since 1909. We argue that the warming trend in WMDW may well be a response to the salinity increase, which seems to be imported from the eastern Mediterranean by LIW, and as such our observations endorse a recently published hypothesis. The increase in LIW salinity, in turn, is attributed to changes in the eastern Mediterranean freshwater budget, resulting from damming of major rivers that drain either directly or indirectly into the eastern Mediterranean. Finally, we demonstrate that the basin has not yet reached a new steady state after this freshwater disturbance and that the response time of the system seems to be of the order of 100 years.

The occurrence and significance of Pleistocene and Upper Pliocene sapropels in the Tyrrhenian Sea

Numerous sapropels and sapropelic strata from Upper Pliocene and Pleistocene hemipelagic sediments of the Tyrrhenian Sea show that intermittent anoxia, possibly related to strongly increased biological productivity, was not restricted to the eastern Mediterranean basins and may be a basin-wide result of Late Pliocene-Pleistocene climatic variability. Even though the sapropel assemblage of the Tyrrhenian Sea clearly originates from multiple processes such as deposition under anoxic conditions or during spikes in surface water productivity and lateral transport of organic-rich suspensates, many “pelagic sapropels” have been recognized. Stratigraphic ages calculated for the organic-rich strata recovered during ODP Leg 107 indicate that the frequency of sapropel formation increased from the lowermost Pleistocene to the base of the Jaramillo magnetic event, coinciding with a period when stable isotope records of planktonic foraminifera indicate the onset of climatic cooling in the Mediterranean. A second, very pronounced peak in sapropel formation occurred in the Middle to Late Pleistocene (0.73-0.26 Ma). Formainifers studied in three high-resolution sample sets suggest that changes in surface-water temperature may have been responsible for establishing anoxic conditions, while salinity differences were not noted in the faunal assemblage. However, comparison of sapropel occurrence at Site 653 with the oxygen isotopic record of planktonic foraminifers established by Thunell et al. (Proc. ODP, Sci. Results 107, 1990) indicates that sapropel occurrences coincide with negative δ 18O excursions in planktonic foraminifers in thirteen of eighteen sapropels recognized in Hole 653A. A variant of the meltwater hypothesis accepted for sapropel formation in the Late Pleistocene eastern Mediterranean may thus be the cause of several “anoxic events” in the Tyrrhenian as well. Model calculations indicate that the amount of oxygen advection from Western Mediterranean Deep Water exerts the dominant control on the oxygen content in deep water of the Tyrrhenian Sea. Inhibition of deep-water formation in the northern Adriatic and the Balearic Basin by increased meltwater discharge and changing storm patterns during climatic amelioration may thus be responsible for sapropel formation in the Tyrrhenian Sea.

Biological and geochemical signatures associated with the water circulation through the Strait of Gibraltar and in the western Alboran Sea

Previous hydrological investigations of the Gibraltar Strait have left some questions unanswered as regards the composition of the Mediterranean outflow into the Atlantic Ocean. In the present study, nutrient, oxygen, and chlorophyll distributions are used to characterize the main water masses flowing through the Strait of Gibraltar and the Alboran Sea, with particular reference to Levantine Intermediate Water (LIW) and Western Mediterranean Deep Water (WMDW). High chlorophyll a concentrations, reaching 3 μg L−1 at the subsurface, are observed in the northern part of the anticyclonic Alboran Sea gyre. The salinity subsurface minimum (≃36), located temporarily in the strait, corresponds to high nutrient concentrations (7–8 μg at. L−1). Diagram analysis shows that in the upwelling area off the Spanish coast, nutrients are largely transported into the photic zone, and that photosynthetic nitrate consumption actually takes place. Consequently, the formation of the Alboran Sea O2 extraminimum can be related to the enhanced new production. Most of the lowest O2 values (<4.00 mL L−1) also correspond to a slight nutrient maximum (PO4 and NO3). Comparative diagram analysis of biologically affected parameters (oxygen and nutrients) versus salinity as a conservative property confirms and expands results from the classical θ‐S analysis. Oxygen and silicate exhibit the best tracer qualities for the circulation pattern, demonstrating in particular that during the period of observations, water rich in LIW feeds the Mediterranean outflow.

Warming trend in the western Mediterranean deep water

THE western Mediterranean Sea comprises three water masses: a surface layer (from 0 to ∼150 m depth), an intermediate layer (∼150–400 m) issuing from the eastern basin, and a deep water mass at depths below 400 m. The deep water is homogeneous and has maintained a more or less constant temperature and salinity from the start of the century until recently1. Here we report measurements from the Medatlante cruises of December 1988 and August 1989, which show the deep layer to be 0.12 °C warmer and ∼0.03 p.s.u. more saline than in 1959. Taking these data together with those from earlier cruises, we find a trend of continuously increasing temperatures over the past three decades. These deep-water records reflect the averaged evolution of climate conditions at the surface during the winter, when the deep water is formed. Consideration of the heat budget and water flux in the Mediterranean2,3 leads to the possibility that the deep-water temperature trend may be the result of greenhouse-gas-induced local warming.

Direct measurements of the flow of western Mediterranean deep water over the Gibraltar sill

The Gibraltar Pilot Experiment was carried out in June 1984 in order to obtain preliminary moored current measurements in the vicinity of the sill in the Strait of Gibraltar. Bottom‐mounted Doppler current measurements and moored temperature measurements from this experiment show directly the westward transport of Western Mediterranean Deep Water (WMDW) over the sill. The net westward transport of WMDW over the sill is roughly estimated to be 0.2×106 m3 s−1.

Aluminium as a tracer of the deep outflow from the Mediterranean

Dissolved aluminium has been determined in sea water samples from the southwestern Alboran Sea and the Gulf of Cadiz. Partial dissolution of eolian dust is responsible for the elevated levels of dissolved Al that were found in both the Western Mediterranean Deep Water (WMDW) (111.5 nM) and Levantine Intermediate Water (LIW) (86.2 nM) masses in the Alboran Sea. The Al concentration in the outflowing plume of Mediterranean water sampled in the Gulf of Cadiz is also elevated (72 nM). These enhanced signals and the differences between them are used in a three water mass system to calculate a maximum 10% contribution of Western Mediterranean Deep Water to the outflow plume, considerably less than has been previously supposed. While the synoptic nature of this study and the possibility of involvement of other water masses in the outflow plume leave scope to increase the participation of WMDW in the outflow plume, these measurements indicate the potential of a nontransient tracer in elucidating the component parts of complex flow systems.

Yes, some of the Mediterranean outflow does come from great depth

In 1973 Stommel et al. predicted that the Western Mediterranean Deep Water (WMDW) contributes directly to the Mediterranean outflow through the Strait of Gibraltar (Strommel et al., 1973). Hydrographic data obtained in November 1985 showed unambiguous evidence of WMDW west of the major Gibraltar sill, thus confirming the prediction.