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