Abstract. The mistral is a northerly low-level jet blowing through the Rhône valley in southern France and down to the Gulf of Lion. It is co-located with the cold sector of a low-level lee cyclone in the Gulf of Genoa, behind an upper-level trough north of the Alps. The mistral wind has long been associated with extreme weather events in the Mediterranean, and while extensive research focused on the lower-tropospheric mistral and lee cyclogenesis, the different upper-tropospheric large- and synoptic-scale settings involved in producing the mistral wind are not generally known. Here, the isentropic potential vorticity (PV) structures governing the occurrence of the mistral wind are classified using a self-organizing map (SOM) clustering algorithm. Based upon a 36-year (1981–2016) mistral database and daily ERA-Interim isentropic PV data, 16 distinct mistral-associated PV structures emerge. Each classified flow pattern corresponds to a different type or stage of the Rossby wave life cycle, from broad troughs to thin PV streamers to distinguished cutoffs. Each of these PV patterns exhibits a distinct surface impact in terms of the surface cyclone, surface turbulent heat fluxes, wind, temperature and precipitation. A clear seasonal separation between the clusters is evident, and transitions between the clusters correspond to different Rossby-wave-breaking processes. This analysis provides a new perspective on the variability of the mistral and of the Genoa lee cyclogenesis in general, linking the upper-level PV structures to their surface impact over Europe, the Mediterranean and north Africa.

Abstract. The mistral is a northerly low level jet blowing through the Rhône valley in southern France, and down to the Gulf of Lions. It is co-located with the cold sector of a low level lee-cyclone in the Gulf of Genoa, behind an upper level trough north of the Alps. The mistral wind has long been associated with extreme weather events in the Mediterranean, and while extensive research focused on the low-tropospheric mistral and lee-cyclogenesis, the different upper-tropospheric large- and synoptic-scale settings involved in producing the mistral wind are not generally known. Here, the isentropic potential vorticity (PV) structures governing the occurrence of the mistral wind are classified using a self-organizing map (SOM) clustering algorithm. Based upon a 36-year (1981–2016) mistral database and daily ERA-Interim isentropic PV data, 16 distinct mistral-associated PV structures emerge. Each classified flow pattern corresponds to a different type or stage of the Rossby wave life-cycle, from broad troughs, thin PV streamers, to distinguished cut-offs. Each of these PV patterns exhibit a distinct surface impact in terms of the surface cyclone, surface turbulent heat fluxes, wind, temperature and precipitation. A clear seasonal separation between the clusters is evident and transitions between the clusters correspond to different Rossby wave-breaking processes. This analysis provides a new perspective on the variability of the mistral, and of the Genoa lee-cyclogenesis in general, linking the upper-level PV structures to their surface impact over Europe, the Mediterranean and north Africa.

Abstract. Deep convection in the Gulf of Lion is believed to be primarily driven by the Mistral winds. However, our findings show that the seasonal atmospheric change provides roughly 2/3 of the buoyancy loss required for deep convection to occur, for the 2012 to 2013 year, with the Mistral supplying the final 1/3. Two NEMOMED12 ocean simulations of the Mediterranean Sea were run for the Aug. 1st, 2012 to July 31st, 2013 year, forced with two sets of atmospheric forcing data from a RegIPSL coupled run within the Med-CORDEX framework. One set of atmospheric forcing data was left unmodified, while the other was filtered to remove the signal of the Mistral. The Control simulation featured deep convection, while the Seasonal did not. A simple model was derived, relating the anomaly scale forcing (the difference between the Control and Seasonal runs) and the seasonal scale forcing to the ocean response through the Stratification Index. This simple model revealed that the Mistral's effect on buoyancy loss depends more on its strength rather than its frequency or duration. The simple model also revealed that the seasonal cycle of the Stratification Index is equal to the net surface heat flux over the course of the year, with the stratification maximum and minimum occurring roughly at the fall and spring equinoxes.

Deep convection in the Gulf of Lion is believed to be primarily driven by the mistral winds. However, our findings show that the seasonal atmospheric change provides roughly two-thirds of the buoyancy loss required for deep convection to occur for the year 2012 to 2013, with the mistral supplying the final third. Two NEMOMED12 ocean simulations of the Mediterranean Sea were run from 1 August 2012 to 31 July 2013, forced with two sets of atmospheric-forcing data from a RegIPSL coupled run within the Med-CORDEX framework. One set of atmospheric-forcing data was left unmodified, while the other was filtered to remove the signal of the mistral. The control simulation featured deep convection, while the seasonal simulation did not. A simple model was derived by relating the anomaly scale forcing (the difference between the control and seasonal runs) and the seasonal scale forcing to the ocean response through the stratification index. This simple model revealed that the mistral's effect on buoyancy loss depends more on its strength rather than its frequency or duration. The simple model also revealed that the seasonal cycle of the stratification index is equal to the net surface heat flux over the course of the year, with the stratification maximum and minimum occurring roughly at the fall and spring equinoxes.

Abstract. Deep convection in the Gulf of Lion is believed to be primarily driven by the mistral winds. However, our findings show that the seasonal atmospheric change provides roughly two-thirds of the buoyancy loss required for deep convection to occur for the year 2012 to 2013, with the mistral supplying the final third. Two NEMOMED12 ocean simulations of the Mediterranean Sea were run from 1 August 2012 to 31 July 2013, forced with two sets of atmospheric-forcing data from a RegIPSL coupled run within the Med-CORDEX framework. One set of atmospheric-forcing data was left unmodified, while the other was filtered to remove the signal of the mistral. The control simulation featured deep convection, while the seasonal simulation did not. A simple model was derived by relating the anomaly scale forcing (the difference between the control and seasonal runs) and the seasonal scale forcing to the ocean response through the stratification index. This simple model revealed that the mistral's effect on buoyancy loss depends more on its strength rather than its frequency or duration. The simple model also revealed that the seasonal cycle of the stratification index is equal to the net surface heat flux over the course of the year, with the stratification maximum and minimum occurring roughly at the fall and spring equinoxes.