On the following subject:
Characterisation of the North Balearic front: variability and role of water mass circulation in the western Mediterranean
under the direction of
Mr Bruno ZAKARDJIAN, Professor, University of Toulon (France), Thesis supervisor
Co-supervised by Mr Franck DUMAS, Research Officer, SHOM, Brest (France) and Mr Pierre GARREAU, Research Officer -HDR, LOPS, IFREMER, Brest (France)
before a jury made up of
Mrs Claude ESTOURNEL, Director of Research-CNRS, LEGOS-Toulouse Rapporteur
Ms Sabrina SPEICH, Professor, LMD-ENS-IPSL Paris Rapporteur
Ms Karina von SCHUCKMANN, Research Fellow, Mercator-Ocean, Toulouse Examiner
Mr Baptiste MOURRE, Research Fellow, SOCIB, Palma de Mallorca (Spain) Examiner
Samuel SOMOT, Research Fellow, CNRM, Météo-France Toulouse (France) Examiner
Mr Franck DUMAS, Research Officer, SHOM, Brest (France), Invited
Mr Pierre GARREAU, Research Fellow -HDR, LOPS, IFREMER, Brest (France), Invited
Mr Bruno ZAKARDJIAN, University Professor, University of Toulon (France), Thesis supervisor
Summary:
In the western Mediterranean, the Balearic front (BF) marks the confrontation between the less salty Atlantic waters (AWs) and the more salty Mediterranean waters (mAWs); the Liguro-Provençal front results from the transport of warm Tyrrhenian waters in the North Current; the Pyrenees front (PF) is the result of the north-south contrast in wind forcing around the Pyrenees. The North Balearic Front (NBF), located between the Balearic Islands, Corsica and Sardinia, appears to separate the Provençal and Algerian basins, and is less well defined. Some studies consider it to be a thermal front, others a salinity front, or associated with a possible closing circulation between Minorca and Corsica of the "Northern Gyre" (a cyclonic loop of currents), or even as a southern limit to winter convection. This thesis characterises this front. A detection method is applied to sea surface temperature (SST) data observed by satellite and those from a simulation (the MEDRYS1V2 reanalysis, 1992-2013), as well as to its simulated sea surface salinity (SSS) data. Two zones of recurring fronts appear: two frontal zones. The first links the Pyrenees and Corsica via thermal fronts in summer and autumn near the surface (0-50m). It is the eastern extension of the PF when its water masses are advected to the subsurface by the regional circulation. The second links Menorca and Sardinia by haline fronts with no seasonal variability. It characterises the eastern continuation of the BF and the confrontation between mAWs and AWs displaced by Algerian eddies (AEs). Consequently, these two distinct front zones, previously confused in the sense of the NBF, are here named the Pyrenees-Corsica and Balearic-Sardinia Frontal Zones. The Pyrenees-Corsica Thermal Front is not influenced by Deep Water Formation (DWF). On the other hand, the Balearic-Sardinian Haline Frontal Zone shows spatial variability on an interannual scale. It moves southwards by around a hundred kilometres in DWF years, and returns to its stable position to the north under the influence of the AEs. A diagnostic algorithm for detecting water masses in the western basin has been developed to quantify them in the reanalysis. It assesses their characteristics, cores, volumes and specific circulations. The first result is the seasonal latitudinal oscillation of the North Gyre closing current. Unrealistic episodes of deep water circulation and mixing (1998-1999, 2003 and 2010) were also detected. Nevertheless, the reanalysis is resilient to them, so that an interannual dynamic is observed over the deep waters: the DWF dislocates the deep and cyclonic Algerian Gyre. Differentiating between these two regimes (with or without DWF) shows that all water masses are affected by the destabilisation of the gyre. At the surface, the AWs are no longer subject to cyclonic entrainment and their zonal circulation migrates southwards, explaining the displacement of the Balearic-Sardinian frontal zone. Alternatively, the West Mediterranean Transition (WMT, due to the DWF of 2005) led to a realistic overflow of deep water into the Sardinia Channel in 2009. The algorithm shows that this deep west-east transit caused an east-west return to the surface. This event brought mAWs through the Sardinia Channel instead of the usual East-Corsica Channel. It brought some of the AWs towards the Provençal basin in 2009, moving the Balearic-Sardinian Frontal Zone as far as Corsica in an atypical but realistic way.