MEDITERRANEAN BASIN AND MEDITERRANEAN SEA

Friday 31 July 2009 by rascol

Motivations

The climate of the Mediterranean basin is controlled by physical processes which have regional spatial scales due to: the complex orography that surrounds the basin, the land-sea contrast, complex coastal features, Mediterranean cyclones with sub-synoptic scales, regional winds channelled by relief (Mistral, Bora, Etésiens), and the presence of numerous islands. As for the climate, the study of the Mediterranean Sea is also a regional problem. Indeed, the water circulation is forced by the same coast line but also by shallow and narrow straits and numerous meso-scale processes (eddies, barocline instabilities, jets, open-sea deep convection, cascading, deep water formation on the continental shelf). Among these physical processes, the open-sea deep convection that generates the Mediterranean thermohaline circulation is important because it is a process that occurs in very few localised places and its occurence in the Mediterranean makes it rather less difficult to study than in the open ocean. The study of the formation and of the characteristics of the Mediterranean Waters that come out through the Gibraltar Strait and influence the Atlantic Ocean at mid-depth is also fundamental. The Mediterranean basin and the Mediterranean Sea moreover experience long-term trends observed for several decades (drying, warming, weakening of the cyclones, increase in temperature and in salinity of the deep layers of the Mediterranean). Among the motivations to study the climate of the Mediterranean basin, one can add high interannual natural variability and the presence of extreme events (hot days, drought, intense precipitation), all having a strong sociological impact. In addition, the Mediterranean basin constitutes one of the areas of the planet where one of the strongest model agreement concerning climate change at the 21st century is found. The various models used in the last report of the IPCC envisage an important drying and warming of the Mediterranean basin. Apart from the direct impact on people, this could have an important impact on the Mediterranean Sea, its thermohaline circulation and its biogeochemistry.

Methods: regional climate models and regional coupling

The physical constraints of the Mediterranean basin need the use of climate models and of oceanic models with high resolution. For the climate, one speaks about regional models. Currently, we use two types of regional models for the representation of the atmosphere and of the surface: the variable resolution version of ARPEGE-Climate which is a global model with a stretched grid to increase the resolution over the area of interest (50 km of resolution over the Mediterranean basin for example) and ALADIN-Climate which is a limited area model only covering the part of the sphere to be studied. ALADIN-Climate is used with a horizontal resolution of 50 and 25 km. The NEMO-med model of the Mediterranean Sea is also used. It has a resolution of approximately 10 km. A coupled regional model was developed from the ARPEGE-Climate model zoomed over the Mediterranean and from the regional oceanic model representing the Mediterranean sea. This model, called S.A.M.M. (Sea-Atmosphere Mediterranean Model) is used to study physical processes strongly influenced by air-sea interactions (the Mediterranean cyclones, water mass formation) and for the simulation of climate change over the basin. Another regional coupled model is used, with ALADIN-Climate and NEMOMED8.

Researchers dealing with this subject:

Samuel Somot, Florence Sevault, Michel Déqué, Marine Herrmann, Clotilde Dubois

Publications associated with this subject:

Sanchez-Gomez et al. 2009, Somot et al. 2008, Herrmann et al. 2008, Tsimplis et al. 2008, Somot et al. 2006, Somot 2005 (Thèse), Gibelin and Déqué 2003