Elsevier

Progress in Oceanography

Volume 197, September–October 2021, 102636
Progress in Oceanography

Characterization of fronts in the Western Mediterranean with a special focus on the North Balearic Front

https://doi.org/10.1016/j.pocean.2021.102636Get rights and content

Highlights

  • Thermal and haline fronts differ in the central part of the Algero-Provençal basin.

  • Thermal fronts form a seasonal frontal zone from the Pyrenees almost toward Corsica.

  • Salinity fronts form a quasi-permanent frontal zone from Mallorca to South Sardinia.

  • The main salinity frontal zone shows a marked inter-annual variability.

  • Deep Water Formation and Algerian Eddies spreading shift the haline front.

Abstract

We focus on the characterization of thermohaline fronts in the Western Mediterranean, with a particular focus on the North Balearic Front (NBF), separating the Atlantic Waters that spread into the Algerian Basin from the saltier and colder waters of the Liguro-Provençal area. We use a simple gradient-based method of front detection applied to a 20-year (from June 1993 to June 2013) reanalysis of the Mediterranean. Statistics of daily frontal indices are used to identify areas of recurrent fronts, i.e., frontal zones. Comparisons with data from glider transects and remotely sensed sea surface temperature and altimetry data have been used to validate our approach from daily to seasonal and interannual time scales. Our method yielded two co-existing frontal zones in the area of the NBF. One with an almost permanent haline surface frontal zone extending southeastward from the Balearic Islands to Sardinia, representing the northern limit of the fresher Atlantic Water that spreads via instabilities of the Algerian Current and associated Algerian Eddies. Between years, its position shifts by about 1° of latitude, possibly due to processes associated with both the deep water formation in the Provençal Basin and the spreading of southern Algerian Eddies. The second frontal zone is seasonal and thermally driven extending off the northeast of Menorca to the northwest of Corsica. The Pyrenees Front, a sharp thermal front off Cap de Creus which marks the boundary between the warm surface waters of the Balearic Sea and the cooler waters in the Gulf of Lion in late summer, appears to facilitate the formation of the aforementioned seasonal thermal front through the northeastward advection of its waters toward the West Corsican Current. The divergent eastward extensions of the two frontal zones, and their differences in nature, structure, and spatio-temporal variability, call for a revisit of the NBF appellation and a clarification of its dynamics.

Introduction

The Mediterranean Sea is a mid-latitude, semi-enclosed sea and a concentration basin where evaporation exceeds precipitation and run-offs due to a dry, windy, and relatively warm regional climate (Nof, 1979, Béthoux, 1980, Mariotti et al., 2002). The resulting salinity increases and water deficit are compensated by an anti-estuarine circulation through the Strait of Gibraltar that shows an inflow of Atlantic Water (AW) at the surface and an outflow of salty Mediterranean water at depth (Nof, 1979, Millot, 1987). The AW flows cyclonically around all Mediterranean sub-basins including the easternmost Levantine Basin and is continuously modified along its path by mixing with resident waters and marked air-sea exchanges. This leads to the notion of modified Atlantic Water (mAW) that will be used in this study to denote the older and saltier AW mainly present in the northern part of the Western Basin. In addition, wind forcing in autumn and winter causes intermediate to deep convection along nearly the entire northern Mediterranean coast, from the Gulf of Lion, over the Adriatic and Aegean Seas to the Levantine Basin, leading to the formation of several water masses and the establishment of a specific thermohaline circulation (Robinson and Golnaraghi, 1994, Bergamasco and Malanotte-Rizzoli, 2010, Waldman et al., 2018, Pinardi et al., 2019). This Mediterranean thermohaline circulation (MTHC) has a time scale of about 100 years, i.e., 10 times less than the THC in the global ocean, and was shown to have responded rapidly to climate variability during the last glacial period (Cacho et al., 2000). More recently, an abrupt shift in the intermediate part of the MTHC during the 1990s, the so-called Eastern Mediterranean Transient (EMT), has affected the water mass characteristics in both the eastern and western basins (Roether et al., 2007 for a thorough review of the EMT). All climate models, including IPCC-2007, predict an increase in the interannual rainfall variability in addition to strong warming and drying (Somot et al., 2008, de Sherbinin, 2014, Cramer et al., 2018) in the Mediterranean. Moreover, several recent studies have shown that the Mediterranean surface waters have been warming at ~0.04 °C.y−1, i.e., four times the global rate over the last four decades, and that this trend affects both the surface, intermediate, and deep waters throughout the Mediterranean (Béthoux et al., 1998, Vargas-Yáñez et al., 2008, Nykjaer, 2009, von Schuckmann et al., 2019, Pisano et al., 2020).

One of the main challenges to improve our current understanding of these dynamics in the Mediterranean is the wide range of scales of motion (from sub-meso to basin scale) that drive and regulate general circulation. The impact of sub-mesoscale (1–10 km) to mesoscale (10–100 km) variability on ocean circulation and associated water and heat fluxes is far from clear, neither at the global scale (Lévy, 2008, Griffies et al., 2014) nor at the basin scale for the Mediterranean (Millot, 1991, Robinson et al., 2001, Schroeder et al., 2011, Tintoré et al., 2019, Aguiar et al., 2020). In this (sub-) mesoscale range, oceanic fronts are ubiquitous features that separate water masses of different origins and, hence, different thermohaline characteristics (Fedorov, 1986). In the open ocean, they are often associated with regional geostrophic currents and are areas of increased vertical motions due to frontal or current instabilities that affect the productivity and fate of biomass (Pinot et al., 1995b, Lévy et al., 1998a, Lévy et al., 1998b, Zakardjian and Prieur, 1998, Stemmann et al., 2008, Olita et al., 2014, as examples for the Mediterranean). Here, we will focus on the characterization and variability of thermohaline fronts in the Western Mediterranean and, more specifically, on the North Balearic Front (NBF) that is thought to separate the Liguro-Provençal and Algerian sub-basins (Fig. 1). Both sub-basins are characterized by marked differences in hydrology and dynamics. The regional circulation of the Liguro-Provençal sub-basin is mainly governed by the Northern Current (NC), a major pathway of the mAW circulation in the northwestern Mediterranean (Millot, 1991), and sustained marked wind forcings (i.e., the Tramontane and Mistral winds) that generate intermittent but pronounced coastal upwellings and, above all, the formation of deep water during winter convection events (MEDOC GROUP, 1970, Schott et al., 1996, Millot, 1999, Smith et al., 2008, Estournel et al., 2016b, Herrmann et al., 2017; Somot et al., 2018; Waldman et al., 2018, Waldman et al., 2017a, Waldman et al., 2017b, Testor et al., 2018). The Algerian sub-basin is dominated by the AW inflow through the Strait of Gibraltar which becomes the Algerian Current (AC) between the eastern end of the Alboran Sea and the Strait of Sicily. AC instabilities generate large (~100 km) anticyclonic open sea eddies, called Algerian Eddies (AEs), that follow a cyclonic path and can cause some AW to accumulate in the southern sub-basin with residence times ranging from months to years (Millot et al., 1990, Millot, 1999, Puillat et al., 2002, Isern-Fontanet et al., 2003, Testor et al., 2005, Escudier et al., 2016a, Garreau et al., 2018, Pessini et al., 2018, Pessini et al., 2020). The boundary between these two dynamical systems is clearly visible on maps of averaged dynamic height (Rio et al., 2007) with associated geostrophic currents that suggest a near-closure of the mAW regional circulation between the Balearic Current (BC) toward the West Corsica Current (WCC). While numerical models generally reproduce this mAW cyclonic re-circulation over the northern sub-basin relatively well (Béranger et al., 2005, Béranger et al., 2010, Beuvier et al., 2010, Beuvier et al., 2012), the few regional studies focusing on the NBF have not been able to demonstrate any barrier effect or zonal training of mAWs along the NBF (Schroeder et al., 2008). Thus, both the closing of the circulation of the saltier mAW over the northwestern sub-basin along the NBF and the supply of the WCC remain largely unexplained issues.

This dynamic and hydrological frontier is also apparent in remotely sensed chlorophyll-a concentrations and associated proxies of primary production, which show marked gradients between the northern and southern sub-basins, i.e., within the NBF area (Bosc et al., 2004, D’Ortenzio and Ribera d’Alcalà, 2009, Uitz et al., 2012, Mayot et al., 2016, Salgado-Hernanz et al., 2019). The higher productivity of the north-western sub-basin is traditionally explained by a more efficient replenishment of the surface layers with nutrients during winter convection, especially when compared to the waters of the south-western sub-basin which contains more oligotrophic and highly stratified AW. Separating these two biological sub-systems, the NBF area has been characterized as an area of intermittent productivity in response to frontal instabilities (Uitz et al., 2012, Olita et al., 2014, Mayot et al., 2016), but with an important interannual component that has yet to be elucidated (Olita et al., 2011, Mayot et al., 2016). While increases in secondary production is routinely assessed in the Balearic, Ligurian, and Alboran fronts (Boucher et al., 1987, Thibault et al., 1994, Stemmann et al., 2002, Fernández de Puelles et al., 2003, Fernández de Puelles et al., 2014), current knowledge on the biomass distributions or zooplankton diversity in the offshore NBF remains much more elusive (F. Carlotti, pers. com.), where a significantly increased secondary production can be deduced from a higher abundance of Bluefin tuna and cetaceans (Gannier and Praca, 2007; Fromentin et al., 2014).

These uncertainties regarding the possible role of the NBF in the regional circulation and biological productivity in the western Mediterranean, are mostly due to the high variability in surface conditions in the area of the suspected location of the NBF. The thermal front often exhibits meandering and eddy shedding on remotely sensed SST (Deschamps et al., 1984) and chlorophyll-A images (Olita et al., 2014). Several studies have shown that deep and intermediate eddies could cross the entire basin, e.g., as sub-mesoscale eddies consisting of Western Mediterranean Deep Water (WMDW) formed after convection events and spreading southeastward (Testor and Gascard, 2003, Beuvier et al., 2012, Bosse et al., 2016) or of Levantine Intermediate Water (LIW) moving northwestward from Sardinia (Sardinian Eddy, SE; Testor et al., 2005, Bosse et al., 2015). In contrast, vortex structures overlaying or interacting with the NBF are much less documented (Fuda et al., 2000, Pessini et al., 2018, Garreau et al., 2018) as are their interactions with the deep and intermediate ones. In addition, it has been shown that the NBF may rapidly shift by tens of kilometers under prevailing cross-front wind forcing (Estrada et al., 1999) and this horizontal displacement may be even more pronounced with strong northerly winds during Deep Water Formation (DWF) events (Estournel et al., 2016a).

Moreover, some ambiguities exist regarding the definition of the NBF in the literature. Early definitions used remote sensing data on sea surface temperature (SST) to identify the NBF as a thermal front northeast of the Balearic archipelago extending toward the north of Corsica (Deschamps et al., 1984). For Millot, 1987, Millot, 1999, the NBF represented the northern edge of the AW reservoir in the Algerian Basin, while Lopez-Garcia et al., 1994, Salat, 1995 considered it as the eastward continuation of the Balearic Front (BF) toward Corsica, hence as the southern branch of the northwestern gyre of mAW re-circulation (see also Garreau et al., 2018, Seyfried et al., 2019). Estrada et al. (1999) defined the NBF as the northern edge of the Balearic Current, hence of the BF. Pascual et al. (2002) identified the BF (and consequently the NBF) based on AW-related salinity gradients, and distinguished it from the Pyrenees Front (PF) which marks the boundary between the warm surface waters of the Balearic Sea and the cooler waters in the Gulf of Lion in late summer. Most of these studies had a limited spatial coverage and focused mainly on the waters off the north and northeast of Menorca and the suggested eastward extension of the NBF toward 5°E only rests on the ordinary interpretation of the remotely sensed SST data.

The fuzzy definition of the NBF and the uncertainties regarding its possible role in the regional circulation of the western Mediterranean are finally due to the scarcity of dedicated observations in the NBF eddy-like complex area and the lack of objective criteria to define the front itself. The present study attempts to remedy this issue by providing a more precise characterization of the frontal interfaces associated with the NBF and their daily, seasonal, and pluri-annual variability. We used a simple but robust approach of normalized gradients to detect haline and thermal fronts in a high-resolution and long-term (20 y) reanalysis of the Mediterranean Sea (the MEDRYS reanalysis in Hamon et al., 2016, Beuvier et al., 2016). By using a reanalysis, i.e., a numerical model coupled with a multi-platform data assimilation system, allows to limit the potential bias and long-term tendencies often inherent in such hindcasts due to uncertainties in forcing and initial conditions and unresolved sub-grid processes (Balmaseda et al., 2015). This approach makes it possible to construct the missing climatological statistics associated with the occurrences of temperature and salinity fronts, to characterize their main modes of variability, and identify the links with the main regional circulation patterns. This document is organized as follows. The in-situ, remotely sensed, and reanalysis data sets as well as the normalized gradient method are described in Section 2. In Section 3, we apply our method to detect temperature and salinity fronts in both modelled and observed data and evaluate their variability from the daily to the pluri-annual time scales. In Section 4, we discuss our method and the results, specifically the nature and variability of the most recurrent fronts in the NBF area, and we present our overall conclusion in Section 5.

Section snippets

Modeling and observational datasets

The present study uses data from the MEDRYS1V2 reanalysis that covers the period from October 1992 to June 2013 (Hamon et al., 2016, Beuvier et al., 2016). The reanalysis is based on the NEMO-MED12 model (spatial resolution of 1/12° or ~7 km and 75 z levels with higher vertical resolution near the surface) and the SAM2 assimilation scheme, as described in Lellouche et al., (2013) using satellite SST, altimetry, and in situ profiles of temperature and salinity (TS). The model was forced with

Daily surface fronts

We first describe how the reanalysis simulates surface thermohaline fronts and assess the results by comparing them with remotely sensed SST fronts, ADT, and derived currents. As the assimilation improved as the reanalysis progressed in time (due to ARGO data becoming available after 2005), the particular examples in Fig. 3, Fig. 4 were chosen to cover contrasting seasonal situations and the entire 20-year reanalysis period. All these examples show that the reanalysis captured the patterns of

About the front detection method

In this study, we choose to use the Fedorov (1986) gradient-based method for two reasons. First, it is a low computing-costs method given the numerous (7304) daily fields to be analyzed. Secondly, scaling the front intensity with the climatological gradients between the AW and mAW seemed best suited to capture temperature and salinity fronts relevant to the NBF. Nevertheless, when used with the 1 km resolution remotely sensed SST data, the gradient method also detected many scattered and

Conclusions

This study shows that using a simple gradient-based method allowed the detection of spatially coherent and realistic temperature and salinity frontal structures in a reanalysis of the Western Mediterranean. The comparisons with glider data and satellite-derived SST showed that most of the detected fronts were similar in shape and scales than those detected in simulated fields. Furthermore, the long-term statistics of frontal occurrences exhibited the same regional and seasonal patterns for

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was part of the PhD thesis of Q.-B. Barral funded by the French Ministère de l'Enseignement Supérieur, de la Recherche et de l'Innovation (MESRI). It is a contribution to the MISTRALS (Mediterranean INtegrated STudies at Regional And Local Scales) program through the CLOSCHEMED (CLOsure SCHEme of the MEDiterranean gyre) project funded by the French CNRS/INSU program LEFE-GMMC. Glider data were collected in the framework of the MISTRALS/Hymex and MOOSE (Mediterranean Ocean Observing

References (153)

  • J.L. Fuda et al.

    XBT monitoring of a meridian section across the western Mediterranean Sea

    Deep-Sea Res. Pt. I

    (2000)
  • Z.Y. Hu et al.

    Study of a mesoscale anticyclonic eddy in the western part of the Gulf of Lion

    J. Marine Syst.

    (2011)
  • A.G. Kostianoy et al.

    Fronts in the Southern Indian Ocean as inferred from satellite sea surface temperature data

    J. Marine Syst.

    (2004)
  • M. Lévy et al.

    The onset of a bloom after deep winter convection in the northwestern Mediterranean sea: mesoscale process study with a primitive equation model

    J. Marine Syst.

    (1998)
  • P. Miller

    Composite front maps for improved visibility of dynamic sea-surface features on cloudy SeaWiFS and AVHRR data

    J. Marine Syst. Special Issue Observ. Stud. Oceanic Fronts

    (2009)
  • C. Millot

    The Gulf of Lions’ hydrodynamics

    Cont. Shelf Res.

    (1990)
  • C. Millot

    Mesoscale and seasonal variabilities of the circulation in the western Mediterranean

    Dyn. Atmos. Oceans

    (1991)
  • C. Millot

    Circulation in the Western Mediterranean Sea

    J. Marine Syst.

    (1999)
  • C. Millot et al.

    The Algerian eddies

    Earth Sci. Rev.

    (1990)
  • K. Nieto et al.

    Mesoscale frontal structures in the Canary Upwelling System: New front and filament detection algorithms applied to spatial and temporal patterns

    Remote Sens. Environ.

    (2012)
  • A. Olita et al.

    Surface circulation and upwelling in the Sardinia Sea: A numerical study

    Cont. Shelf Res.

    (2013)
  • Y. Ourmières et al.

    Assessment of a NEMO-based downscaling experiment for the North-Western Mediterranean region: Impacts on the Northern Current and comparison with ADCP data and altimetry products

    Ocean Model.

    (2011)
  • F. Pessini et al.

    Life history of an anticyclonic eddy in the Algerian basin from altimetry data, tracking algorithm and in situ observations

    J. Marine Syst.

    (2020)
  • A. Petrenko et al.

    Circulation in a stratified and wind-forced Gulf of Lions, NW Mediterranean Sea: in situ and modeling data

    Cont. Shelf Res.

    (2005)
  • J.M. Pinot et al.

    The CANALES experiment (1996–1998). Interannual, seasonal, and mesoscale variability of the circulation in the Balearic Channels

    Prog. Oceanogr.

    (2002)
  • E. Aguiar et al.

    Multi-platform model assessment in the Western Mediterranean Sea: impact of downscaling on the surface circulation and mesoscale activity

    Ocean Dynam.

    (2020)
  • M.A. Balmaseda et al.

    The ocean reanalyses intercomparison project (ORA-IP)

    J. Oper. Oceanogr.

    (2015)
  • N. Barrier et al.

    Strong intrusions of the Northern Mediterranean Current on the eastern Gulf of Lion: insights from in-situ observations and high resolution numerical modelling

    Ocean Dyn.

    (2016)
  • K. Béranger et al.

    Impact of the spatial distribution of the atmospheric forcing on water mass formation in the Mediterranean Sea

    J. Geophys. Res.

    (2010)
  • A. Bergamasco et al.

    The circulation of the Mediterranean Sea: a historical review of experimental investigations

    Adv. Oceanogr. Limnol.

    (2010)
  • J. Béthoux

    Mean water fluxes across sections in the mediterranean-sea, evaluated on the basis of water and salt budgets and of observed salinities

    Oceanolog. Acta

    (1980)
  • J.P. Béthoux et al.

    Warming and freshwater budget change in the Mediterranean since the 1940s, their possible relation to the greenhouse effect

    Geophys. Res. Lett.

    (1998)
  • J. Beuvier et al.

    Spreading of the Western Mediterranean Deep Water after winter 2005: time scales and deep cyclone transport

    J. Geophys. Res.

    (2012)
  • Beuvier, J., Hamon, M., Greiner, E., Drévillon, M., Lellouche, J.M., 2016. New version of MEDRYS, a Mediterranean Sea...
  • J. Beuvier et al.

    Modeling the Mediterranean Sea interannual variability during 1961–2000: focus on the Eastern Mediterranean Transient

    J. Geophys. Res.

    (2010)
  • E. Bosc et al.

    Seasonal and interannual variability in algal biomass and primary production in the Mediterranean Sea, as derived from 4 years of SeaWiFS observations

    Global Biogeochem. Cycles

    (2004)
  • A. Bosse et al.

    Scales and dynamics of Submesoscale Coherent Vortices formed by deep convection in the northwestern Mediterranean Sea

    J. Geophys. Res. Oceans

    (2016)
  • A. Bosse et al.

    Spreading of Levantine Intermediate Waters by submesoscale coherent vortices in the northwestern Mediterranean Sea as observed with gliders

    J. Geophys. Res. Oceans

    (2015)
  • J. Boucher et al.

    Daily and seasonal variations in the spatial distribution of zooplankton populations in relation to the physical structure in the Ligurian Sea Front

    J. Mar. Res.

    (1987)
  • J. Bouffard et al.

    Coastal and mesoscale dynamics characterization using altimetry and gliders: a case study in the Balearic Sea

    J. Geophys. Res.

    (2010)
  • C. Cabanes et al.

    The CORA dataset: validation and diagnostics of in-situ ocean temperature and salinity measurements

    Ocean Sci.

    (2013)
  • X. Capet et al.

    Mesoscale to submesoscale transition in the California current system. Part II: Frontal processes

    J. Phys. Oceanogr.

    (2008)
  • E. Capó et al.

    Energy conversion routes in the western Mediterranean Sea estimated from eddy-mean flow interactions

    J. Phys. Oceanogr.

    (2019)
  • J.F. Cayula et al.

    Edge detection algorithm for SST images

    J. Atmos. Oceanic Technol.

    (1992)
  • W. Cramer et al.

    Climate change and interconnected risks to sustainable development in the Mediterranean

    Nat. Clim. Change

    (2018)
  • A. de Sherbinin

    Climate change hotspots mapping: what have we learned?

    Clim. Change

    (2014)
  • D.P. Dee et al.

    The ERA-Interim reanalysis: configuration and performance of the data assimilation system

    Q. J. Roy. Meteor. Soc.

    (2011)
  • P.Y. Deschamps et al.

    Sea surface temperatures of the coastal zones of France observed by the HCMM satellite

    J. Geophys. Res.

    (1984)
  • F. D’Ortenzio et al.

    On the trophic regimes of the Mediterranean Sea: a satellite analysis

    Biogeosciences

    (2009)
  • S. Dong et al.

    Location of the Antarctic polar front from AMSR-E satellite sea surface temperature measurements

    J. Phys. Oceanogr.

    (2006)
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