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3rd HyMeX workshop 1-4 June 2009 Heraklion (Gournes), Crete-Greece

Monitoring, detection and attribution of thermohaline stability and variability in the Western Mediterranean Sea

Katrin Schroeder (CNR-ISMAR); Gian Pietro Gasparini (CNR-ISMAR), Stefania Sparnocchia (CNR-ISMAR), Marine Herrmann (CNRM/Météo-France)

Since the '50 the heat and salt contents of the western Mediterranean Sea have been increasing almost constantly, with an acceleration in the second half of the '80. More recent observations have revealed further significant salinity and temperature increases, in the surface, intermediate and deep layers, in many western subbasins. The long-term monitoring of the hydrographic and circulation properties of water masses in the Sicily Channel and in other key positions of the western Mediterranean during the last 20 years permitted to follow the interannual variability of the east-west exchanges, considering the propagation of the Eastern Mediterranean Transient (EMT) signature toward the western basin. During recent years, those changes were able to play a key role in the western Mediterranean deep water production (in terms of properties and volume), which induced a large renewal in the deep layer of that basin, an event that is comparable to the EMT, both in terms of intensity and observed effects. For this reason a continuous monitoring of the Mediterranean circulation would be necessary, to permit the detection of possible changes when they happen, to understand their time scales and to provide elements of comparison and verification to the models. A strategy to achieve this objective is proposed, addressing advances and gaps in current knowledge on the western Mediterranean deep-sea system and proposing priorities for future research.

1. Documentation and understanding of the climatic oscillations in the western Mediterranean Sea, studying the physical tracers and the factors that modulate this variability.
- Internal variability: to set the background of the more recent changes, thus improving the interpretation and synthesis capability of the new available observations; to identify knowledge gaps and design of an observational system to allow critical considerations and to diagnose future changes.
- Modulation of the variability: to define the relationship between exchanges through the straits and internal variability of the western basin; to formulate and test hypotheses on the possible mechanisms and causes of the internal variability in the western basin; to assess the possible feedbacks on the Mediterranean dynamics on the global climatic system.

2. Reconstruction and monitoring of the transient event which is occurring in the western Mediterranean Sea.
- Evolution in time and space: to contribute to the knowledge of the evolution of this climatic event, reconstructing its spatial and temporal variability, the transit times and the mixing fraction of the involved water masses, both resident and newly formed.
- Causes and forcings: to identify processes at different time-scales and to test hypotheses on the mechanisms and causes of the abrupt alterations in the physical properties of deep water masses, by using on-purpose models and the time-series of observations.
- Possible impacts and repercussions: to identify and quantify the effects the anomaly observed in the western basin induces on adjacent areas (eastern Mediterranean and Atlantic Ocean).