Mid-Mediterranean Jet Research Articles

Twenty years of in-situ monitoring in the south-eastern Mediterranean Levantine basin: Basic elements of the thermohaline structure and of the mesoscale circulation during 1995-2015

The in-situ data gathered over two decades since mid-1990s in the South-Eastern Levantine Basin from 34 hydrographic campaigns and from 35 bi-weekly and monthly Voluntary Observing Ships (VOS) transects, after the pioneering POEM cruises, have provided insight on the dominant meso-scale coherent circulation features of this sub-basin. The most pronounced feature, the anticyclonic Cyprus Eddy, migrates over the broad region of the Eratosthenes seamount and exhibits significant seasonal and inter-annual spatial-temporal variability. Another prominent structure of the sub-basin is the anticyclonic Shikmona Eddy generated by instabilities of the strong northward flowing jet along the south-easternmost shelf and slope of the Levantine basin. Its evolution and co-existence with the Cyprus Eddy, cause the periodic re-establishment of the Shikmona Gyre, which was first observed during the POEM cruises in mid-1980s. The offshore, cross basin, eastward flowing Mid-Mediterranean Jet (MMJ) defines the northern and southern flanks of these sub-basin scale anticyclonic eddies and transports the lower salinity Modified Atlantic Water (MAW) in the subsurface layer, through the warmest and most saline waters of the Mediterranean. Periodically, the MMJ bifurcates and/or is diverted northward, along the western coast of Cyprus due to westward and northward spatial fluctuations of the Cyprus Eddy’s western boundaries. The current long term in-situ monitoring of the sub-basin provides an overview of the extent of the main water masses and characterizes their variability throughout the period considered. The analysis of the data indicates an increase of the temperature and salinity of the Levantine Surface Water (LSW) and of the subsurface MAW. The Eastern Mediterranean Transient Water (EMTW) is shown to occupy the deep cavities in the vicinity of the Eratosthenes seamount while its upper boundary was found to be lifted to shallower depths over a period of two decades.

Surface circulation properties in the eastern Mediterranean emphasized using machine learning methods

Abstract. The eastern Mediterranean surface circulation is highly energetic and composed of structures interacting stochastically. However, some main features are still debated, and the behavior of some fine-scale dynamics and their role in shaping the general circulation is yet unknown. In the following paper, we use an unsupervised neural network clustering method to analyze the long-term variability of the different mesoscale structures. We decompose 26 years of altimetric data into clusters reflecting different circulation patterns of weak and strong flows with either strain or vortex-dominated velocities. The vortex-dominated cluster is more persistent in the western part of the basin, which is more active than the eastern part due to the strong flow along the coast, interacting with the extended bathymetry and engendering continuous instabilities. The cluster that reflects a weak flow dominated the middle of the basin, including the Mid-Mediterranean Jet (MMJ) pathway. However, the temporal analysis shows a frequent and intermittent occurrence of a strong flow in the middle of the basin, which could explain the previous contradictory assessment of MMJ existence using in-situ observations. Moreover, we prove that the Levantine Sea is becoming more and more energetic as the activity of the main mesoscale features is showing a positive trend.

A study on causes and recurrence of the Mid-Mediterranean Jet from 2003 to 2015 using satellite thermal and altimetry data and CTD casts

ABSTRACT The existence of the Mid-Mediterranean Jet (MMJ) as part of the pathway of the Atlantic Water (AW) in Eastern Mediterranean is still debated and the reasons for its formation are unknown. The MMJ was investigated over the 2000–2015 summers using thermal and altimetry satellite data and ‘in situ’ casts. In the Levantine basin, the AW flowing along the Libyan-Egyptian coast splits into an along-shore and an offshore branch, the latter following the northern border of the warm anti-cyclones located near the African coast (Libyan-Egyptian Eddies, LEEs). The MMJ is triggered when a surface cold water mass formed south-east of Crete, in part outflowing the Aegean Sea and in part generated within the Rhodes Gyre, moves southward and intrudes the middle Levantine basin. The surface cold water overlays the AW flowing along the northern periphery of LEEs that sinks dozens of metres below increasing its budget of anti-cyclonic vorticity and causing the formation of the jet. Satellite data show that the MMJ formed every summer from 2003 to 2015, sometimes for limited periods of time and at different latitudes, with the exception of summers 2005–2006, probably due to the weakness of the African anti-cyclones that in those years did not advect AW offshore.

The controversial path of Atlantic Water in the Eastern Mediterranean

The Atlantic Water (AW) path suggested by two decades of thermal satellite data observations in the Eastern Mediterranean is similar to the path followed by the AW in the Western Mediterranean but is in contrast with the scheme proposed after the Physical Oceanography of the Eastern Mediterranean (POEM) experiment during the 1990s. Evidence of the AW path along the Libyo-Egyptian coast (Libyo-Egyptian Current; LEC) and around anti-cyclonic eddies of variable size and position (Libyo-Egyptian Eddies; LEEs) is reported in XBT transects, high resolution models and drifter trajectories. In the POEM scheme, the AW was carried eastwards by the Mid-Mediterranean Jet (MMJ) flowing in the middle of the Levantine basin. The MMJ is still noted in recent drifter trajectory analyses, and seasonal surveys and glider missions report that the AW is transferred towards the Levantine basin by the MMJ in subsurface layers.The issue is investigated in this study by considering whether the determination of the AW path and the resolution of the surface circulation are, unlike in the Western Mediterranean, two distinct problems. Historical hydrographic datasets and numerical experiments illustrated in this study demonstrate that satellite observations in the Eastern Mediterranean reveal the path of the surface water of Atlantic origin, which is modified by heating and evaporation into the saltier and warmer Levantine Surface Water (LSW), but do not reveal the path of the submerged AW that maintains the distinctive character of fresh AW in the Levantine basin. Differences between the surface and the submerged circulation in the Mersa-Matruh area explain the reason for the discrepancy.In summer the AW departs from the Sicily Channel at the sea surface and crosses the Ionian Sea in a surface layer 30m deep. At the entrance to the Levantine basin, part of the AW re-circulates westwards in two separate branches, one directed to the Southern Ionian and the other towards the Central Ionian, causing meanders in the incoming AW. In the Levantine basin, before the Cretan Passage, the AW is close to the Libyan coast with surface layers already modified into LSW and a subsurface fresh AW core centered at 50m depth. Surface and subsurface flows diverge within the Cretan Passage as they approach the Mersa-Matruh area. According to satellite observations, the LSW flows along the Libyo-Egyptian coast (LEC) and around anti-cyclonic LEEs of variable size and position. In the subsurface layers, the blend and fragmented anti-cyclonic circulation found at the surface is augmented by the AW itself, which becomes saltier and negatively buoyant, and veers offshore joining the MMJ. In summer, the jet assumes the aspect of a submerged injection of fresh AW into the Levantine basin below a depth of 60–70m. From the Ionian eastwards, the depth of the fresh core of AW sinks by tens of meters from the beginning to the end of summer. In the Sicily Channel, the AW in winter is saltier than in other periods of the year due to evaporation and mixing occurring between the fresh AW and the saltier Ionian Surface Water intruding into the southern side of the Channel in summer.

On the surface circulation of the Levantine sub-basin derived from Lagrangian drifters and satellite altimetry data

The surface currents of the Levantine sub-basin (Mediterranean Sea) are described using 18 years (1992–2010) of drifter data and satellite-derived sea level anomalies. The combination of drifter and satellite data allowed to estimate maps of surface geostrophic circulation and to obtain more accurate pseudo-Eulerian velocity statistics for different time periods. Seasonal and interannual variability of surface currents are investigated with particular focus on the main sub-basin eddies of the eastern Levantine. The mean velocity field depicts the typical patterns of the along-slope and offshore currents and outlines the sub-regions where eddies are generated recurrently (west Egyptian coast, Ierapetra, Mersa-Matruh, south-west of Cyprus, Israel–Lebanon coast, Latakia) or persist steadily (Rhodes Gyre). Highly variable and energetic currents are observed between the Ierapetra and Mersa-Matruh regions, as the result of the interaction of the Mid-Mediterranean Jet meandering in between, and interacting with, the eddies generated by the instability of the coastal current. Seasonal pseudo-Eulerian maps show the current field stronger in summer and weaker in winter, mainly in the western Levantine and in the Cyprus–Syria Passage. The Shikmona Eddy displays a periodic nature with higher intensities during the cold months and an enhanced activity in the period 1998–2005. The Cyprus Eddy has a less periodic nature, characterised by events of high activity and periods in which it dominates as a single enlarged eddy in the southeast Levantine, eventually including the Shikmona Eddy. The Latakia Eddy is mainly cyclonic with higher intensities in summer and fall; occasional weekly or monthly inversions of circulation from cyclonic to anticyclonic are triggered by the interaction between the MMJ and the northward coastal meandering current.

Surface circulation in the Eastern Mediterranean using drifters (2005–2007)

Abstract. Within the framework of the EGITTO/EGYPT programs, the spatial structure and the temporal variability of the surface circulation in the eastern basin of the Mediterranean Sea was studied with satellite-tracked drifters. A total of 97 drifters drogued to 15-m nominal depth were released between September 2005 and March 2007, regularly along ship-of-opportunity routes in the Sicily Channel and within specific structures during dedicated campaigns in the Levantine sub-basin. This study complements previous ones in the former and in the Ionian, but it is the first one in the latter. After editing and low-pass filtering, the drifter trajectories were used to estimate pseudo-Eulerian statistics: mean current, variance ellipses, mean and eddy kinetic energies. A statistical analysis was also performed dividing the dataset in two extended seasons (winter and summer). A branching behaviour of the surface water after passing through the Sicily Channel is evidenced, together with a seasonal variability inducing a reversal of the circulation in the southern part of the Ionian. In the Levantine, the surface circulation describes an eastward flow along the Libyan and Egyptian slopes (the Libyo-Egyptian Current: LEC) that continues in a cyclonic circuit along the Middle East and Turkish slopes. This general alongslope circuit can be perturbed locally and temporally by the numerous anticyclonic eddies that co-exist in the Levantine, mainly created by the instability of the LEC in the south (Libyo-Egyptian Eddies: LEEs), but also by the wind (Ierapetra and Pelops), and by the topography (over the Eratosthenes Seamount and off Latakia). The most frequent perturbation is the entrainment of part of the flow seaward: the LEEs close to the slope can interact with the LEC, which then spreads more or less around them, so that a series of contiguous LEEs (paddle-wheel effect) can possibly result in an eastward offshore transport (the so-called Mid Mediterranean Jet). Additionally, when LEEs are close enough to the slope, most of the surface flow is spread seaward. Along the slope the current is then induced by the LEEs southern side which results in a westward current. Locally and temporally the circulation along the slope can thus be reversed.

A Mean Dynamic Topography of the Mediterranean Sea computed from altimetric data, in-situ measurements and a general circulation model

In the Mediterranean Sea, where the mean circulation is largely unknown and characterized by smaller scales and less intensity than in the open ocean, the interpretation of altimetric Sea Level Anomalies (SLA) is rather difficult. In the context of operational systems such as MFS (Mediterranean Forecasting System) or MERCATOR, that assimilate the altimetric information, the estimation of a realistic Mean Dynamic Topography (MDT) consistent with altimetric SLA to be used to reconstruct absolute sea level is a crucial issue. A method is developed here to estimate the required MDT combining oceanic observations as altimetric and in-situ measurements and outputs from an ocean general circulation model (OGCM). In a first step, the average over the 1993–1999 period of dynamic topography outputs from MFS OGCM provides a first guess for the computation of the MDT. Then, in a second step, drifting buoy velocities and altimetric data are combined using a synthetic method to obtain local estimates of the mean geostrophic circulation which are then used to improve the first guess through an inverse technique and map the MDT field (hereafter the Synthetic Mean Dynamic Topography or SMDT) on a 1/8° resolution grid. Many interesting current patterns and cyclonic/anticyclonic structures are visible on the SMDT obtained. The main Mediterranean coastal currents are well marked (as the Algerian Current or the Liguro–Provenço–Catalan Current). East of the Sicily channel, the Atlantic Ionian Stream divides into several main branches crossing the Ionian Sea at various latitudes before joining at 19°E into a unique Mid-Mediterranean Jet. Also, strong signatures of the main Mediterranean eddies are obtained (as for instance the Alboran gyre, the Pelops, Ierapetra, Mersa-Matruh or Shikmona anticyclones and the Cretan, Rhodes or West Cyprius cyclones). Independent in-situ measurements from Sea Campaigns NORBAL in the North Balearic Sea and the North Tyrrhenian Sea and SYMPLEX in the Sicily channel are used to validate locally the SMDT: deduced absolute altimetric dynamic topography compares well with in-situ observations. Finally, the SMDT is used to compute absolute altimetric maps in the Alboran Sea and the Algerian Current. The use of absolute altimetric signal allows to accurately follow the formation and propagation of cyclonic and anticyclonic eddies in both areas.

Variability of the Cyprus warm core Eddy during the CYCLOPS project

The circulation of the southeastern Levantine Basin (Eastern Mediterranean) as determined from several oceanographic cruises carried out in April, May, and August 2001 and in March, May, and August 2002 was investigated within the framework of the CYCLOPS experiment. The May cruises were part of CYCLOPS while the others, which provide supporting data, were part of the ongoing CYBO project. The May 2002 data provided the location of the eddy and physical description of the circulation, which were crucial for the in situ phosphate/SF6 addition experiment of CYCLOPS. The major flow features observed in the region south of Cyprus displayed strong variability within the period of the observations. Throughout the entire study period, the warm core, anticyclonic eddy, known as the Cyprus eddy, was a dominant component of the circulation. During the 18 months covered by the cruises, the eddy drifted slowly to the southwest. At the end of the CYCLOPS cruises, the center of this persistent, recurrent eddy was found approximately 100–150 km west of its previously observed location since the 1980s. The mid-Mediterranean jet (MMJ), associated with the rim of this eddy, followed a complex path south of Cyprus depending upon the eddy’s shape and location. During most of the study period the MMJ surface currents near the northern periphery of the rim of the eddy were as high as 35–45 cm/s. In May 2002, during the in situ experiment, the near surface seasonal thermocline inhibited downward mixing of the added tracers while the strong horizontal current around the rim of the clearly defined eddy core limited the lateral spreading and dilution of the patch. Finally, the westward shift of the Cyprus eddy allowed space for the generation of a secondary anticyclonic eddy in the eastern part of the domain. The co-existence of the two anticyclonic eddies during 2002 along with the development of a third anticyclonic eddy closer to the coast of Egypt provides strong evidence for the re-establishment of the previously observed, multi-pole, sub-basin scale Shikmona gyre. r 2005 Elsevier Ltd. All rights reserved.

A new hypothesis about the surface circulation in the eastern basin of the mediterranean sea

In time, the circulation of the Atlantic Water (AW) in the eastern basin of the Mediterranean Sea has been described differently, according to two major representations. The historical one, which began with the scheme from Nielsen in 1912 and has been refined up to the 1980s, favours a counterclockwise circulation in the whole basin, with AW flowing in its southern part as a broad flow off Libya and Egypt (from the Ionian to the Levantine subbasins), then continuing along Middle-East and Turkey before flowing back westwards. The more recent one, issued in the 1990s, favours a clockwise circulation in the northern part of the Ionian continuing offshore across the basin from the Cretan to the central part of the Levantine as the so-called “Mid-Mediterranean Jet”. This jet is depicted then as splitting both clockwise in the southeastern part of the basin and counterclockwise off Turkey (where this representation agrees with the former). Because the recent representation cannot be considered as a refinement of the historical ones, we have been interested in understanding why a given data set available to everybody is interpreted in such different ways. In the Algerian subbasin, the combined use of satellite infrared images and a significant amount of in situ data sets (hydrology and both Eulerian and Lagrangian current measurements) allowed us to solve a similar controversy. Therefore, we examined the circulation features in the eastern basin, undertaking the detailed analysis of ∼1000 daily and weekly composite images spanning the period 1996–2000, and of monthly composite images available since 1985. Whenever in situ observations were available, we have confronted them with the satellite thermal signatures and have shown that both are consistent. This paper focuses on the overall (basin scale) results while the detailed ones are published in an other paper. The new scheme we propose is basically a refined version of the historical ones: the circulation of AW is counterclockwise in the whole eastern basin but it is more constrained alongslope than previously thought, and the broadening historically schematised appears to be due to intense mesoscale eddies mainly generated by the instability of this circulation.

Surface circulation of the Levantine Basin: Comparison of model results with observations

The eastern Mediterranean (Levantine Basin) hydrography and circulation are investigated by comparing the results of a high-resolution primitive equation model with observations. After a 10-year integration, the model is able to reproduce the major water masses and the circulation patterns of the eastern Mediterranean. Comparisons with the POEM hydrographical observations show good agreement. The vertical distribution of the water masses matches that of the observations quite well in terms of monthly mean. The model surface circulation is in agreement with circulation schemes derived from recent observations. Some well-known mesoscale features of the upper thermocline circulation are also realistically reproduced. In agreement with satellite observations, the model shows that high-energy mesoscale eddies dominate the upper thermocline circulation in the southern and the central parts of the Levantine Basin. Most of the Atlantic Water follows the north African coast and forms a strong coastal jet near the Libyan coast rather than forming the Mid-Mediterranean Jet described by several authors. The sub-basin circulation shows a strong seasonal signal. A strong and stable current flows along the isobaths in winter, becoming weaker and with more meanders in summer. The mesoscale eddies throughout the whole basin are more energetic in summer than in winter.

Simulation of the Mediterranean Sea circulation from 1979 to 1993: Part I. The interannual variability

The interannual variability of the Mediterranean (MED) circulation from 1979 to 1993 is studied with a 1° 8 × 1° 8 resolution OGCM. The surface forcing used is 6 hourly ECMWF (European Center for Medium Range Weather Forecast) reanalysis data. Two different periods in the surface forcing and model variability are identified during 1981–1993: the first, Period I (1981–1987) and the second, Period II (1988–1993). Changes in the model response between the two periods are driven by corresponding differences in the surface forcing, which presumably are a result of the decadal scale changes of the Northern Hemisphere (NH) atmospheric regimes, related to the intensification of North Atlantic Oscillation (NAO) at the end of the 1980s. During the second period (1988–1993), the Mediterranean circulation reveals an overall weakening of the kinetic energy in the Western Mediterranean (WMED) basin and significant changes in the structure of circulation in the Eastern basin. In the latter region, the anticyclonic activity increases in the southern Ionian and in the southern area of the mid-Mediterranean Jet. These anticyclonic eddies, present during different years of Period II with variable intensity, have an important impact on the transport of Modified Atlantic Waters (MAW) and Levantine Intermediate Waters (LIW) in the Eastern Mediterranean (EMED). This change of the circulation modified the salinity and the amount of Levantine Intermediate Waters transported towards the Aegean Sea and the Adriatic Sea, which are important factors for deep water formation processes there. These model results are in good agreement with available observational results. The deep water formation event observed in the Aegean Sea [Science 271 (1996) 333] is produced by the model, but at a shallower depth. We interpret this event as the result of circulation changes between Period I and Period II and anomalous surface atmospheric forcing over the Aegean Sea.

Major changes in Mediterranean Sea level variability from 7 years of TOPEX/Poseidon and ERS-1/2 data

Seven years of combined maps of TOPEX/Poseidon (T/P) and ERS-1/2 altimeter data are used to describe the surface circulation variability in the Mediterranean Sea. As described in the past, the Mediterranean Sea level variability is a complex combination of a wide range of spatial and temporal scales. However, this paper gives an improved estimation of sea level statistics (rms, annual, semiannual cycle). Moreover, the longer period (1993–1999) and the merging of T/P and ERS-1/2 altimeter data allows us to observe with a good accuracy the major changes that occurred in the Mediterranean Sea and, in particular, at basin and sub-basin scales. First, important interannual signals were found in the Ionian basin where the cyclonic circulation has clearly intensified since 1997. In the Levantine basin, although the Ierapetra eddy exhibits a clear seasonal cycle, it is not always present during our period of observation. These interannual changes seem to be correlated with the variability of the Mid-Mediterranean Jet, which produces strong meanders. A new view of the circulation in the south of the basin is indeed suggested by considering that the eddies observed in the Mersa-Matruh and Shikmona areas represent meanders of the Mid-Mediterranean Jet rather than permanent structures as commonly described. Secondly, the seasonal cycle of the Alboran gyres is confirmed. Moreover, these gyres and the Ierapetra eddy constitute the most intense signals of the Mediterranean Sea variability. Finally, this descriptive study illustrates the need to continue monitoring the surface circulation in order to better understand the dynamics of the Mediterranean Sea.

Hydrography and circulation south of Cyprus in late summer 1995 and in spring 1996

The hydrodynamical characteristics of the area south of Cyprus in the Levantine Sea were examined in late summer 1995 and in spring 1996. Analysis of the CTD data collected, provided a new dynamic picture of the Cyprus Basin and revealed the existence of the Cyprus Basin cyclonic eddy. The latter circulated to the south of Cyprus and exhibited marked seasonality. Production of Levantine Intermediate Water in the area of the latter eddy occurred. Occasionally, between the Cyprus Basin cyclonic eddy and the southern coast of Cyprus a warm current, the Cyprus coastal current appears to intrude from the east. In late summer 1995, the spatial fluctuations of the mid-Mediterranean jet, the main current of the Eastern Mediterranean Sea, caused the splitting of the Cyprus Basin cyclonic eddy, into two smaller cyclonic ones. During spring 1996, however, the Cyprus Basin cyclonic eddy appears as a single large vortex, occupying the greater part of the area south of Cyprus, and flanked to the south by the Cyprus anticyclonic eddy.

Comparaison des données altimétriques Topex-Poséidon en Méditerranée avec un modèle de circulation générale

Outputs of a primitive-equation numerical model of the Mediterranean forced by 6-h ECMWF winds are compared with Topex-Poseidon surface topography measurements in 1993. The comparisons show a fair qualitative agreement on the structure of the general surface circulation and on a number of large-scale features. The high-frequency wind forcing is thought to objectively contribute to the control of the model trajectory. There is good agreement on the variations of the surface transport of the Algerian Current and Mid-Mediterranean jet. As expected, the medium model resolution ( 1 0/4 ) does not allow for a satisfying representation of the smaller scales in the model. The limited resolution accounts for large errors on the response to wind forcing, on the spindown of seasonal gyres, and on the instabilities and meanders of coastal currents. Nevertheless, this study can serve as a reference for a future step involving higher resolution modelling and assimilation.

Dynamical studies of the Mersa Matruh Gyre: intense meander and ring formation events

Abstract A study of the dynamics of the Mersa Matruh Gyre and the Mid-Mediterranean Jet flow system in the southwestern Levantine basin is presented. Data-driven simulations in the Levantine basin, using an eddy-resolving quasigeostrophic model initialized with two quasi-synoptic hydrographic data sets, reveal intense mesoscale meander and ring formation events involving the Mid-Mediterranean Jet, the Mersa Matruh Gyre and the Rhodes Gyre. The dynamics of these events are quantified via local energy and vorticity budget analyses. The dominant processes are investigated and compared with previously studied events in the Gulf Stream Ring and Meander region.