How a warmer Mediterranean preconditions the upper-level environment for the development of Medicane Ianos

Abstract. Medicane Ianos in September 2020 was one of the strongest medicanes observed in the last 25 years. It was, like other medicanes, a very intense cyclone evolving from a baroclinic mid-latitude low into a tropical-like cyclone. The dynamical elements necessary to improve the predictability of Ianos are explored with the use of simulations with the Met Office Unified Model (MetUM) at 2.2 km grid spacing for five different initialisation times, from four to two days before Ianos's landfall. Simulations are also performed with the Sea Surface Temperature (SST) uniformly increased and decreased by 2 K from analysis to explore the impact of enhanced and reduced sea-surface surface fluxes on Ianos's evolution. All the simulations with +2 K SST are able to simulate medicane Ianos, albeit too intensely. The simulations with control SST initialised at the two earliest times fail to capture intense preceding precipitation events at the right locations, and the subsequent development of Ianos. Amongst the simulations with -2 K SST, only the one initialised at the latest time develops the medicane. 

Links between sea-surface fluxes and upper-level baroclinic processes are investigated. We find (i) a bubble of low-valued potential vorticity (PV) formed within a trough above where Ianos developed, diabatic processes associated with a preceding precipitation event triggered a balanced divergent flow in the upper-levels which contributed to the creation and maintenance this low-PV bubble, as shown by results from a semi-geostrophic inversion tool, (ii) Upper-level geostrophic vorticity advection associated with the low-PV bubble forced quasi-geostrophic ascent during Ianos's cyclogenesis, and (iii) diabatic processes dominated by deep convection formed a vertical PV tower in Ianos and continued to produce diabatically-induced divergent outflow aloft, thus sustaining Ianos's development. Simulations missing any of these three elements do not develop medicane Ianos.

Our results imply that  preceding convection was essential for the subsequent development of Ianos, highlighting the importance of the interactions between near-surface small-scale diabatic processes and the upper-level quasi-geostrophic flow. A warmer SST strengthens the processes and thus enables Ianos to be predicted in simulations initiated at the earlier times that failed to generate the medicane with control SSTs.