Winter Intermediate Water Research Articles

Assessing atmospheric and oceanic teleconnections between the eastern and western Mediterranean over the past 8000 years

Holocene climate records from the Mediterranean are marked by pervasive millennial to centennial-scale climate variability. Here, we investigate East-West Mediterranean atmospheric and oceanic teleconnections by computing phase-relationships between oxygen isotope (δ18O) records generated on Soreq (East) and Chorchia (West) spelaeothems, as well as between δ18O and carbon isotope (δ13C) records from planktonic and benthic foraminifera from core PS009PC (East, Levantin Basin), ODP Site 963D (Central, Sicily Strait), and core KESC9-14 (West, Ligurian Basin). These marine sites are all located at intermediate water depths (560–460 m depth). Hence, the benthic foraminiferal δ18O records reflect mainly the intermediate ocean temperature/δ18O of the water mass, and the benthic δ13C is a proxy for the intensity of water flowing at the studied depth called Levantine Intermediate Water (LIW). For both western and eastern cores, the planktonic stable isotopic records reflect the climate-induced activity of the nearby river system. We find broadly in-phase relationships between the spelaeothem δ18O records and between the planktonic δ18O and δ13C records at most multi-centennial and millennial periodicities. This is indicative of closely linked (hydro-) climatic conditions in Southern Europe, the Levant, and North Africa over the last 8000 years. Conversely, at intermediate water depths, we find a distinct out-of-phase relationship between the East/Central and West Mediterranean benthic δ18O and δ13C records at 1000–2000 years periodicities. We interpret this see-saw pattern as indicative of a persistent regional influence of LIW on oceanographic conditions in the intermediate depths of the eastern basin. Conversely, we suggest a strong influence of the modified Atlantic Ocean inflow (MAW) in the intermediate water formation in the Western Mediterranean (‘Winter Intermediate Water’; WIW). This WIW overprints, at least partially, the LIW signal that reaches the western Mediterranean causing the out-of-phase relationship between the east and the west oceanographic signals at intermediate depths.

Hydrography and circulation west of Sardinia in June 2014

Abstract. In the frame of the REP14-MED sea trial in June 2014, the hydrography and circulation west of Sardinia, observed by means of gliders, shipborne CTD (conductivity, temperature, depth) instruments, towed devices, and vessel-mounted ADCPs (acoustic doppler current profilers), are presented and compared with previous knowledge. So far, the circulation is not well-known in this area, and the hydrography is subject to long-term changes. Potential temperature, salinity, and potential density ranges as well as core values of the observed water masses were determined. Modified Atlantic Water (MAW), with potential density anomalies below 28.72 kg m−3, showed a salinity minimum of 37.93 at 50 dbar. Levantine Intermediate Water (LIW), with a salinity maximum of about 38.70 at 400 dbar, was observed within a range of 28.72<σΘ/(kg m−3) < 29.10. MAW and LIW showed slightly higher salinities than previous investigations. During the trial, LIW covered the whole area from the Sardinian shelf to 7°15′ E. Only north of 40° N was it tied to the continental slope. Within the MAW, a cold and saline anticyclonic eddy was observed in the southern trial area. The strongest variability in temperature and salinity appeared around this eddy, and in the southwestern part of the domain, where unusually low saline surface water entered the area towards the end of the experiment. An anticyclonic eddy of Winter Intermediate Water was recorded moving northward at 0.014 m s−1. Geostrophic currents and water mass transports calculated across zonal and meridional transects showed a good agreement with vessel-mounted ADCP measurements. Within the MAW, northward currents were observed over the shelf and offshore, while a southward transport of about 1.5 Sv occurred over the slope. A net northward transport of 0.38 Sv across the southern transect decreased to zero in the north. Within the LIW, northward transports of 0.6 Sv across the southern transects were mainly observed offshore, and decreased to 0.3 Sv in the north where they were primarily located over the slope. This presentation of the REP14-MED observations helps to further understand the long-term evolution of hydrography and circulation in the Western Mediterranean, where considerable changes occurred after the Eastern Mediterranean Transient and the Western Mediterranean Transition.

Seasonal variations in the vertical structure of pelagian water stratum in the Southern Baikal

For the first time, T,S-analysis was used to determine the specifics of seasonal variations in the vertical structure of Lake Baikal active layer. In the under-ice period, the active layer includes the under-ice, top winter, and upper intermediate water masses. The under-ice water mass, unlike other masses, shows an increase in mineralization to 100.74 mg/kg, which corresponds to a release of 71.1 g salt under 1 m2 of water surface in a layer 0–40 m in the process of salt freezing out during ice cover formation and accretion. In the phases of mixing (homothermy), the water masses of the active layer transform into a surface homogeneous mass. In summer and autumn, surface and upper intermediate water masses, separated by a water mass of summer thermocline can be identified. A specific feature of the summer thermocline water mass is the increased sum of ions because of an increase in HCO 3 - concentration at the decay of organic matter accumulating in the bottom part of the thermocline. The existence of the under-ice water mass and the water mass of summer thermocline was established in Lake Baikal for the first time. In the deep-water zone (>250 m), except for the bottom parts, the lower water masses (the lower intermediate and the deep) are permanent, their characteristics remaining stable during the year. The changes in the bottom water mass are due to the character of the processes of bottom water renewal.

Fingerprinting Northeast Atlantic water masses using neodymium isotopes

Dissolved neodymium (Nd) isotopic composition (expressed as εNd) has been analysed for 82 seawater samples collected from 13 stations stretching from the Alboran Sea to the Iceland Basin. The distribution of the εNd values of water masses was thus investigated for the first time along the western European margin in order to explore whether the water masses flowing in the eastern subpolar and subtropical Atlantic reveal distinct isotopic patterns. The Modified Atlantic Water (MAW) in the Alboran Sea displays εNd values (between −9.2±0.2 and −8.9±0.2) that are significantly more radiogenic than those reported in previous studies (−10.8±0.2 to −9.7±0.2), suggesting temporal variations in the Nd isotopic composition of the water that enters the Mediterranean Sea from the Strait of Gibraltar. The εNd value of the underlying modified Winter Intermediate Water (WIW) has been established for the first time (−9.8±0.3) and is compatible with a Nd signature acquired from the sinking of MAW in the northwestern Mediterranean Sea. Within the Gulf of Cádiz, southern Mediterranean Sea Water (MSW) (−10.6±0.2) differs slightly from the northern MSW (−9.9±0.4) owing to a significant contribution of modified East Antarctic Intermediate Water (EAAIW) (−10.9±0.2). In the northeast Atlantic, the North Atlantic Current surface water located in the inter-gyre region (north of 46°N) displays εNd values of between −14.0±0.3 and −15.1±0.3, reflecting the subpolar gyre signature. Along the western European margin, εNd values of surface water decrease toward the north (from −10.4±1.6 to −13.7±1.0) in agreement with the gradual mixing between subtropical and subpolar water. At intermediate depth, εNd values decrease from −9.9±0.4 within the Gulf of Cádiz to −12.1±0.2 within the Porcupine Seabight, indicating a strong dilution of the MSW with subpolar water. Within the Rockall Trough and the Iceland Basin, the more negative εNd values at mid-depth (<−13.5±0.3) indicate that the MSW has no influence, even during periods of low NAO index. Water masses deeper than 1200m in the northeast Atlantic are clearly influenced by the less radiogenic Labrador Sea Water (LSW) (εNd between −13.4±0.3 and −14.0±0.3) that mixes locally in the Iceland basin with the Iceland-Scotland Overflow Water (ISOW) (between −10.3±0.2 and −11.3±0.3).

Mediterranean waters along and across the Strait of Gibraltar, characterization and zonal modification

Hydrological data collected in the Strait of Gibraltar have been used to examine the distribution and spatial–temporal evolution of the water masses in the area. The spatial variability has been addressed by means of a clustering method that determines the affinity of a collection of temperature–salinity samples to one of the water masses involved in the exchange. The method, which has been applied to a nearly-synoptic data set, highlights the clear evolution of the Mediterranean Waters as they flow westward through the Strait. While up to four different Mediterranean Waters are spatially distinguishable east of the main sill of Camarinal in the Strait, most of their differentiating characteristics are eroded after flowing over this restrictive topography due to mixing. West of the sill, therefore, speaking of a unique Mediterranean Water seems more appropriate. The same applies to the North Atlantic Central Water flowing in the opposite direction, which is noticeably modified along its path to the Mediterranean Sea, most of its transformation taking place in the Camarinal sill surroundings. A series of repeated transects carried out in the eastern and western sides of the Strait, provided a temporal analysis of the water masses evolution: the temporal variability manifests seasonality in the surface waters, while interannual signal is mainly detected in the deeper water masses. It is worth remarking the statistically significant positive trend of Western Mediterranean Deep Water (0.009°C/year) and Winter Intermediate Water (0.03°C/year), with the latter showing also intermittent occurrence in the Strait.

Spatio-temporal distribution patterns of Mediterranean cold-water corals (Lophelia pertusa and Madrepora oculata) during the past 14,000 years

This study presents newly obtained coral ages of the cold-water corals Lophelia pertusa and Madrepora oculata collected in the Alboran Sea and the Strait of Sicily (Urania Bank). These data were combined with all available Mediterranean Lophelia and Madrepora ages compiled from literature to conduct a basin-wide assessment of the spatial and temporal occurrence of these prominent framework-forming scleractinian species in the Mediterranean realm and to unravel the palaeo-environmental conditions that controlled their proliferation or decline. For the first time special focus was placed on a closer examination of potential differences occurring between the eastern and western Mediterranean sub-basins. Our results clearly demonstrate that cold-water corals occurred sparsely in the entire Mediterranean during the last glacial before becoming abundant during the Bølling-Allerød warm interval, pointing to a basin-wide, almost concurrent onset in (re-)colonisation after ~13.5ka. This time coincides with a peak in meltwater discharge originating from the northern Mediterranean borderlands which caused a major reorganisation of the Mediterranean thermohaline circulation. During the Younger Dryas and Holocene, some striking differences in coral proliferation were identified between the sub-basins such as periods of highly prolific coral growth in the eastern Mediterranean Sea during the Younger Dryas and in the western basin during the Early Holocene, whereas a temporary pronounced coral decline during the Younger Dryas was exclusively affecting coral sites in the Alboran Sea. Comparison with environmental and oceanographic data revealed that the proliferation of the Mediterranean corals is linked with enhanced productivity conditions. Moreover, corals thrived in intermediate depths and showed a close relationship with intermediate water mass circulation in the Mediterranean sub-basins. For instance, reduced Levantine Intermediate Water formation hampered coral growth in the eastern Mediterranean Sea during sapropel S1 event as reduced Winter Intermediate Water formation did in the westernmost part of the Mediterranean (Alboran Sea) during the Mid-Holocene. Overall, this study clearly demonstrates the importance to consider region-specific environmental changes as well as species-specific environmental preferences in interpreting coral chronologies. Moreover, it highlights that the occurrence or decline of cold-water corals is not controlled by one key parameter but rather by a complex interplay of various environmental variables.

Origin and pathways of Winter Intermediate Water in the Northwestern Mediterranean Sea using observations and numerical simulation

The study of water masses worldwide (their formation, spreading, mixing, and impact on general circulation) is essential for a better understanding of the ocean circulation and variability. In this paper, the formation and main pathways of Winter Intermediate Water (WIW) in the Northwestern Mediterranean Sea (NWMED) are investigated during the winter‐spring 2011 using observations and numerical simulation. The main results show that the WIW, formed along the continental shelves of the Gulf of Lion and Balearic Sea, circulates southward following five preferential pathways depending on the WIW formation site location and the oceanic conditions. WIW joins the northeastern part of the Balearic Sea, or flows along the continental shelves until joining the Balearic Current (maximum of 0.33 Sv in early‐April) or further south until the Ibiza Channel entrance. Two additional trajectories, contributing to water mass exchanges with the southern part of the Western Mediterranean Sea, bring the WIW through the Ibiza and Mallorca Channels (maxima of 0.26 Sv in late‐March and 0.1 Sv in early‐April, respectively). The circulation of WIW over the NWMED at 50–200 m depth, its mixing and spreading over the Western Mediterranean Sea (reaching the south of the Balearic Islands, the Algero‐Provencal basin, the Ligurian and the Alboran Seas) suggest that the WIW may have an impact on the ocean circulation by eddy blocking effect, exchange of water masses between north and south subbasins of Western Mediterranean Sea through the Ibiza Channel or modification of the ocean stratification.

Changes in planktic and benthic foraminifer assemblages in the Gulf of Lions, off south France: Response to climate and sea level change from MIS 6 to MIS 11

AbstractA multidisciplinary study involving micropaleontological and geochemical tools was carried out in borehole PRGL1 (Promess 1), with the aim of reconstructing the impact of climate change and sea level variation between 133 ka and 406 ka in the upper slope of the Gulf of Lions. We used factor analysis to obtain three main benthic assemblages related to eutrophic, mesotrophic, and oxygenated environments; planktic foraminifers were grouped as warm‐water and cold‐turbulent species. These results were compared with records of CaCO3 and major and trace elements (Al, Ca, K, Sr) as well as the C/N ratio of organic matter. Power and cross‐spectral analysis showed a straightforward relationship between precession minima and thermal stratification of the water column as well as the occurrence of eutrophic bottom conditions during lowstand periods and mesotrophic environments at times of highstand. These eutrophic‐mesotrophic oscillations, usually driven by global eustatic change, also involved regional variations in CaCO3 source to this environment. During periods of precession maxima, enhancement of northwesterly winds increased primary productivity by mixing, enhancing the percentage of cold‐turbulent species in the water column and the proportion of oxygenated benthic species on the bottom. During interglacial stages, these events were recorded by lower biogenic carbonate at the expense of higher silicate‐related components most likely due to a higher supply from Pyrenees rivers. The record of oxygenated benthic species can be a good proxy to monitor past changes in Winter Intermediate Water dynamics driven by northwesterly winds.

Another description of the Mediterranean Sea outflow

Papers about the outflow in the Strait of Gibraltar assume that (i) it is composed of only two Mediterranean Waters (MWs), the Levantine Intermediate Water (LIW) and the Western Mediterranean Deep Water (WMDW) from the eastern and western basins, respectively, (ii) both MWs are mixed near 6°W, hence producing a homogeneous outflow that is then split into veins, due to its cascading along different paths and to different mixing conditions with the Atlantic Water (AW). A re-analysis of 1985–1986 CTD profiles (Gibraltar Experiment) indicates two other MWs, the Winter Intermediate Water (WIW) from the western basin and the Tyrrhenian Dense Water (TDW) basically originated from the eastern basin. In the central Alboran subbasin, these four MWs are clearly differentiated, roughly lying one above the other in proportions varying from north to south. Proportions also vary with time, so that the outflow can be mostly of either eastern or western origin. While progressing westward, the MWs can still be differentiated and associated isopycnals tilt up southward as much as being, in the sill surroundings, roughly parallel to the Moroccan continental slope where the densest MWs are. The MWs at the sill are thus juxtaposed and they all mix with AW, leading to an outflow that is horizontally heterogeneous just after the sill (5°45′W) before progressively becoming vertically heterogeneous as soon as 6°15′W. There can be little LIW and/or no WMDW outflowing for a while. An analysis of new 2003–2008 time series from two CTDs moored (CIESM Hydro-Changes Programme) at the sill (270 m) and on the Moroccan shelf (80 m) confirms the juxtaposition of the MWs, their individual and generally intense mixing with AW, as well as the large temporal variability of the outflow composition. Only LIW and TDW were indicated at the sill while, on the shelf, only LIW, TDW sometimes denser there than ∼200 m below, and WMDW were indicated; but none of the MWs has been permanently outflowing at one or the other place. The available data can be analyzed coherently. Intermediate and deep MWs are formed in both basins in amounts that, although variable from year to year, allow their tracing up to the strait. Four major MWs circulate alongslope counterclockwise as density currents and as long as they are not trapped within a basin, which is necessarily the case for the deep MWs. In the Alboran, the intermediate MWs (WIW, LIW and upper-TDW) circulate in the north while the deep MWs (lower-TDW and WMDW) are uplifted, hence relatively motionless and mainly pushed away in the south. Since both the intermediate and deep MWs outflow at the sill, they are considered as light and dense MWs, the light–dense MWs interface possibly intersecting the AW–MWs interface in the sill surroundings. Considering an outflow east of the sill composed of only two (light–dense) homogeneous layers gives significant results. Across the whole strait, the outflow has spatial and temporal variabilities much larger than previously assumed. The MWs are superposed in the sea and lead at the sill to juxtaposed and vertically stratified suboutflows that will cascade independently before forming superposed veins in the ocean. These veins can have similar densities and hydrographic characteristics even if associated with different MWs, which accounts for the features permanency assumed up to now. The outflow structure downstream of the sill depends on its composition upstream and, more importantly, on that of AW in the sill surroundings where fortnightly and seasonal signals are imposed on the whole outflow.

Changes in the diet and feeding of the hake Merluccius merluccius at the shelf-break of the Balearic Islands: Influence of the mesopelagic-boundary community

Short spatio–temporal variations in the feeding intensity and the diet of the European hake, Merluccius merluccius, together with the abundance of their potential prey were studied between August 2003 and June 2004 at two locations, northwest (Sóller) and south (Cabrera), off the island of Mallorca (Balearic Islands, Western Mediterranean) at depths between 150 and 750 m. The two areas present different oceanographic conditions. Hake was mainly distributed along the shelf-slope break and the upper slope (between 166 and 350 m) where recruits (TL<18 cm) were dominant. The hake's diet varied as a function of size. Recruits fed mainly on micronektonic prey, and the diet was influenced primarily by seasonality, with two dietary patterns (identified by MDS analyses) corresponding to August–September 2003 (summer) and to November 2003/February–April 2004 (autumn–winter). The summer pattern was consistent with a thermally stratified water column, while November and April were consistent with homogenized temperature and salinity throughout all the water column. The main prey of recruits were the euphausiid Meganyctiphanes norvegica and the midwater fish Maurolicus muelleri in autumn–winter and myctophids (mainly Ceratoscopelus maderensis) in summer. In contrast to recruits, the geographic factor (NW vs. S) was the main factor influencing the diets of post-recruits (TL between 18 and 21.9 cm) and adults (TL⩾22 cm). Hake recruits (and to a lesser extent post-recruits) and their preferred prey occupied different depth ranges during daylight periods. Meganyctiphanes norvegica and Ceratoscopelus maderensis were, for instance, distributed as much as 500 m deeper than hake that had eaten them. All these trends were especially obvious at NW, an area with a more abrupt slope and with a greater influence by northern winter intermediate water (WIW) inflow in early spring than the S area. These factors probably enhanced micronekton aggregation in April, when feeding intensity (stomach fullness) increased among recruits and post-recruits only at NW. All these factors may have a crucial role in the diet, distribution and probably recruitment success of small hake. Biological factors were also important in trophic shifts in the diet and feeding of hake. Multi-linear regression models pointed to a trend of higher fullness with higher hepato-somatic index (HSI). Therefore greater food consumption by hake may enhance its metabolic condition. Within the framework of shelf-break and slope ecology, we show how the ‘boundary’ mesopelagic community inhabiting the middle slope sustains the trophic requirements of hake, a species distributed at shallower depths along the shelf-slope break. Mesopelagic euphausiids and myctophids are often found in the diets of shelf-break fish. Because the boundary mesopelagic community is distributed worldwide, the high levels of fish biomass often found at shelf-slope breaks could be sustained trophically by deeper, offshore mesopelagic communities, an inverse energy transfer from deep to shallow-water marine ecosystems.

Two-way nested model of mesoscale circulation features in the Ligurian Sea

A coarse resolution primitive equation model of 1/4° resolution is implemented covering the whole Mediterranean Sea. Within this grid a 1/20° resolution model of the Liguro-Provençal basin and the northern part of the Tyrrhenian Sea is embedded. A third fine resolution model of 1/60° is nested in the latter one and simulates the dynamics of the Ligurian Sea. Comparisons between one-way and two-way nesting in simulating the Northern Current (NC) are made. The properties of the Eastern and Western Corsican Current and the Northern Current are investigated with this nesting system. Special attention is given to the variability of the NC. Meanders and interactions with Winter Intermediate Water lenses are shown. Topographic features also lead to a highly variable NC.

On the presence of a cadmium-rich subsurface water mass in the western Mediterranean and its influence on the distribution of cadmium in the surface waters

Vertical distribution of cadmium (27–133 pM) at 13 stations in the Alboran Sea (western Mediterranean) showed profiles that were different from those typically known from oceanic waters. A subsurface cadmium-rich layer, related to the presence of Winter Intermediate Water (WIW), was a prominent feature. The WIW is formed in the northwestern coastal sector of the western Mediterranean Sea, and the high cadmium concentrations in this water mass are therefore likely to be of coastal origin. The surface concentrations of cadmium at all stations in the Alboran Sea were higher and increased, but not progressively, from west to east, with a substantially higher concentrations around the Almeria–Oran front than on either side. As shown by the cadmium–salinity diagram and the rates of diffusive transfer of cadmium from the subsurface into the surface waters, flux from WIW is an important source of cadmium for surface waters in the Alboran Sea, with a marked increase in the rates near the front.

Simulation and analysis of the mesoscale circulation in the northwestern Mediterranean Sea

Abstract. The large-scale and mesoscale circulation of the northwestern Mediterranean Sea are simulated with an eddy-resolving primitive-equation regional model (RM) of 1/16° resolution embedded in a general circulation model (GM) of the Mediterranean Sea of 1/8° resolution. The RM is forced by a monthly climatology of heat fluxes, precipitation and wind stress. The GM, which uses the same atmospheric forcing, provides initial and boundary conditions for the RM. Analysis of the RM results shows that several realistic features of the large-scale and mesoscale circulation are evident in this region. The mean cyclonic circulation is in good agreement with observations. Mesoscale variability is intense along the coasts of Sardinia and Corsica, in the Gulf of Lions and in the Catalan Sea. The length scales of the Northern Current meanders along the Provence coast and in the Gulf of Lions’ shelf are in good agreement with observations. Winter Intermediate Water is formed along most of the north-coast shelves, between the Gulf of Genoa and Cape Creus. Advection of this water by the mean cyclonic circulation generates a complex eddy field in the Catalan Sea. Intense anticyclonic eddies are generated northeast of the Balearic Islands. These results are in good agreement with mesoscale activity inferred from satellite altimetric data. This work demonstrates the feasibility of a down-scaling system composed of a general-circulation, a regional and a coastal model, which is one of the goals of the Mediterranean Forecasting System Pilot Project. Key words. Oceanography: physical (currents; eddies and mesoscale processes; general circulation)

Water masses and circulation between the eastern Algerian Basin and the Strait of Sicily in October 1996

The investigation is based on data collected between the eastern Algerian Basin and the Strait of Sicily and in the southern Tyrrhenian Sea. The major pathways of water masses are identified by the core method and geostrophic currents are derived from the objectively analysed density field. Between the Sardinia Channel and the Strait of Sicily, the large-scale circulation of Modified Atlantic Water and Winter Intermediate Water is found to be cyclonic. Inflow into the gyre occurs via the Sardinia Channel by means of a boundary current attached to the Algerian coast, and from the northern Tyrrhenian. The outflow is accomplished via the Strait of Sicily and to the Tyrrhenian. The Levantine intermediate water flow resembles that of Modified Atlantic Water/Winter Intermediate Water in the southern Tyrrhenian, but it is opposed in the Strait of Sicily and off Tunisia. Outflow to the Algerian Basin occurs south of Sardinia. In the eastern Algerian Basin the flow direction of all water masses is eastward close to the Algerian shelf. Farther offshore, Modified Atlantic Water flows mainly southwest whereas the Levantine Intermediate Water is opposed to that supporting northward transport along the Sardinian shelf. The large-scale flow of all water masses is perturbed by mesoscale eddies. The impact of topographic obstacles is investigated.

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.

Winter intermediate water lenses in the Ligurian Sea

Hydrographic measurements carried out in the Ligurian Sea in early spring 1991 revealed the existence of mesoscale lenses at intermediate depths that were remarkably different from the environmental waters. The lens temperature was up to 0.7°C colder, salinity 0.2 lower and the oxygen content 0.8 ml/l higher than the waters outside. The most significant of these lenses was found along the Ligurian coast between the surface layer of Modified Atlantic Water (MAW) and the Levantine Intermediate Water (LIW) below it. This lens was clearly the result of the sinking of water of the central region of the basin along the isopycnal slope associated with the large scale cyclonic circulation. The precise identification of its characteristics allowed us to obtain some basic information on the processes responsible for lens formation and evolution. This can bring new elements for the study of dense water formation and spreading in the Northwestern Mediterranean.

A Sensitivity Study of the General Circulation of the Western Mediterranean Sea. Part II: The Response to Atmospheric Forcing

Abstract This paper investigates the influence of sea surface thermohaline fluxes and wind stress on the circulation of the Western Mediterranean Sea using a high-resolution 3D primitive equation model. An 18-year experiment was forced with the daily output of a fine grid mesh numerical weather prediction model. The major characteristics of the circulation are well reproduced. The basin surface circulation is cyclonic over all of the basin. The two anticyclonic Alboran gyres are present. The instabilities of the Algerian Current generate large anticyclonic eddies that invade the whole Algerian Basin. The Liguro–Provencal–Catalan Current is well marked. Deep water convection down to the bottom only occurs during the first 3 years, then winter intermediate water is produced. The north–south gradient of the atmospheric thermohaline fluxes induces a northward surface transport of water from the Algerian Basin into the Liguro–Provencal Basin. This pattern can be associated with the Balearic front. Sensitivity ...

Climatological Coastal Jet Collision, Intermediate Water Formation, and the General Circulation of the Red Sea*

The authors present climatologies of a numerical model of the Red Sea, focusing on the dynamics of winter intermediate water formation. Northward flowing boundary currents are identified as the major dynamical elements. At the northern boundary, the eastern current follows the geometry, eventually turning back to the south. At ;268N and the western wall the two boundary currents collide. At the collision site, the denser eastern current subducts under the western boundary current. The subduction forces the western boundary current eastward into the interior. Convection communicates the surface fluxes to the downwelled plume and intermediate water forms. The estimated rate, 0.11 Sv (Sv [ 106 m3 s21), agrees with previous estimates. The authors identify basin-scale sea-surface tilt to the north due to variable thermohaline forcings as the key dynamical variable. The resultant geostrophic eastward cross-channel flow interacts with the boundaries and creates upwelling and surface topography spatial patterns that drive the coastal jets. Upwelling-induced vortex stretching dominates the vorticity balance and governs the separation of the western boundary current from the western wall. The process ceases in the summer.

Characteristics and circulation of the surface and intermediate water masses off Algeria

During the Médiprod-5 experiment, conducted off Algeria between ∼ 0 and 5°E in 1986–1987, ∼100 CTD casts were made in May–June 1986 down to ∼ 800 decibars. Their analysis is supported by results from current meters set in place for ∼ 9 months in the same area. The flow of Modified Atlantic Water (MAW) off Algeria, namely the Algerian Current, was initially (near 0–2°E) structured as a narrow (∼ 30 km) and deep (∼ 400 m) coastal vein (transport ∼ 1.7 Sv). It became wider and thinner to the east, with its core slightly detached from the coast. This current was first expected to generate coastal anticyclonic eddies and second to interact strongly with open sea anticyclonic eddies. Two such mesoscale eddies were sampled. Geostrophic calculations and drifting buoy trajectories clearly show that the whole Algerian Current was deflected seawards due to interaction with an open sea eddy. Spatial gradients of the temperature minimum of the Winter Intermediate Water (WIW), and of the temperature and salinity maxima of the Levantine Intermediate Water (LIW), are consistent with historical data. They indicate that both waters flow eastwards along the Algerian continental slope. These data sets allow completion of our circulation diagrams. In the eastern Alboran Sea, the old MAW, the WIW and the LIW, flowing from the north along the Spanish continental slope, more or less collide with the MAW that has recently entered through the Strait of Gibraltar. Therefore, all or a part of them is entrained off Algeria and, as a whole, they roughly flow cyclonically along the continental slope in most of the western Mediterranean Sea. The distribution of these water masses in the Algerian Basin is strongly dependent on the Algerian Current and on the various kinds of mesoscale anticyclonic eddies encountered there.

Hydrodynamics of an open sea Algerian eddy

During the Médiprod-5 cruise, conducted in May–June 1986 in the Algerian Basin, an anticyclonic open sea eddy, similar to those depicted previously with infrared images, was shown deflecting the Algerian Current seawards for months. CTD profiles collected ∼10 km apart down to ∼ 800 decibars throughout the eddy allow a close examination of the density, velocity, vorticity and energy fields. All the major water masses were recognized in the eddy with rather heterogeneous characteristics. Intrusions of surface water, directly issued from the Algerian Current in the form of streamers, were found in places across the eddy. A lens of Modified Atlantic Water, locally cooled during the preceding winter and trapped within the central region of the eddy, was isolated from outside and was in near solid-body rotation. Relatively unmixed Winter Intermediate Water and Levantine Intermediate Water also encountered in the eddy could be more or less exchanged with the surroundings. In a ∼ 50 m mixed layer, the water in the central region of the eddy was denser than in the outer region. As the reverse occurs below, this leads to relatively large velocities (up to 25–30 cm/s) at 50–100 m and ∼ 25 km from the eddy axis. Beneath, all the isopleths were depressed by 100–250 m. Although the eddy was mainly a cylinder of 120–140 km diameter and much more than ∼ 800 m deep, the distributions of the period of rotation and of the potential vorticity reveal an overall conical structure of the whole eddy. The inner and upper part (diameter of 80–100 km at the surface and depth of ∼300 m near the axis) were relatively isolated from, and rotated more rapidly than, the remainder of the eddy. Effective Rossby numbers for the eddy arc significantly <1, and the centrifugal force cannot be neglected. Strong interactions occurred between the eddy and the Algerian Current. The whole current first deviated from the coast seawards, but then streamers crossed the eddy and carried water back towards the coast. These interactions account for the markedly asymmetrical shape of the eddy. They might also account for its relatively long persistence, through transfers of energy from the Algerian Current itself.