Large-scale drivers of the mistral wind: link to Rossby wave life cycles and seasonal variability

Abstract

Abstract. The mistral is a northerly low-level jet blowing through the\nRhône valley in southern France and down to the Gulf of Lion. It is\nco-located with the cold sector of a low-level lee cyclone in the Gulf of\nGenoa, behind an upper-level trough north of the Alps. The mistral wind has\nlong been associated with extreme weather events in the Mediterranean, and\nwhile extensive research focused on the lower-tropospheric mistral and\nlee cyclogenesis, the different upper-tropospheric large- and synoptic-scale\nsettings involved in producing the mistral wind are not generally known.\nHere, the isentropic potential vorticity (PV) structures governing the\noccurrence of the mistral wind are classified using a self-organizing map\n(SOM) clustering algorithm. Based upon a 36-year (1981–2016) mistral\ndatabase and daily ERA-Interim isentropic PV data, 16 distinct\nmistral-associated PV structures emerge. Each classified flow pattern\ncorresponds to a different type or stage of the Rossby wave life cycle, from\nbroad troughs to thin PV streamers to distinguished cutoffs. Each of these\nPV patterns exhibits a distinct surface impact in terms of the surface\ncyclone, surface turbulent heat fluxes, wind, temperature and precipitation.\nA clear seasonal separation between the clusters is evident, and transitions\nbetween the clusters correspond to different Rossby-wave-breaking processes.\nThis analysis provides a new perspective on the variability of the mistral\nand of the Genoa lee cyclogenesis in general, linking the upper-level PV\nstructures to their surface impact over Europe, the Mediterranean and north\nAfrica.\n