The upstream-downstream mechanism of North Atlantic and Mediterranean cyclones in semi-idealized simulations

Abstract

Abstract. Cyclogenesis in the Mediterranean typically follows an archetypal scenario with the intrusion of a potential vorticity (PV) streamer over the Mediterranean, which results from a preceding Rossby Wave breaking (RWB) upstream over the North Atlantic. The RWB is amplified by the presence of warm conveyor belts (WCBs) in at least one North Atlantic cyclone about 4 days prior to Mediterranean cyclogenesis. This scenario has been found in several case studies of intense Mediterranean cyclones with a variety of different North Atlantic cyclone tracks and intensities, and different locations of PV streamers intruding the Mediterranean. While four main events (namely North Atlantic cyclone, WCBs, RWB and the resulting PV streamers) are suggested to be necessary for Mediterranean cyclogenesis, they rarely occur in a spatially consistent, fully repetitive pattern. To more systematically study this link between the upstream North Atlantic cyclone dynamics and the Mediterranean cyclogenesis, we perform a set of semi-idealized simulations over the Euro-Atlantic domain. For these simulations, we prescribe a constant climatological atmospheric state in the initial and boundary conditions. To trigger the downstream Mediterranean cyclogenesis scenario, we perturb the climatological polar jet through the inversion of a positive upper-level PV anomaly. The amplitude of this perturbation determines the intensity of the triggered North Atlantic cyclone. This cyclone provokes RWB, the intrusion of a PV streamer over the Mediterranean, and the formation of a Mediterranean cyclone. Therefore, our results show a direct causality between the presence of a North Atlantic cyclone and the downstream intrusion of a PV streamer into the Mediterranean, which causes cyclogenesis about 4 days after perturbing the polar jet, which we refer to as the upstream-downstream mechanism of cyclogenesis. To investigate the sensitivity of this mechanism, we vary the position and amplitude of the upper-level PV anomaly. In all simulations, cyclogenesis occurs in the Mediterranean. Nevertheless, the evolution, track and intensity of the Mediterranean cyclones are sensitive to the dynamical structure and intensity of the intruding PV streamer, which itself is sensitive to the interaction of the upstream cyclone and the RWB. By applying different seasonal climatological atmospheric states as initial conditions we show that the seasonal cycle of Mediterranean cyclogenesis indeed depends on the large-scale atmospheric circulation. In particular, we show that the Mediterranean cyclones in these semi-idealized simulations show characteristics that agree with the observed climatology of Mediterranean cyclones in the respective season.

Comment on egusphere-2024-1054

“…This results in anticyclonic Rossby wave breaking downstream, the southern flank of which extends into a cyclone-inducing PV streamer over the Mediterranean. This process was demonstrated for a set of a hundred intense Mediterranean cyclones in reanalysis (Raveh-Rubin and Flaounas, 2017) as well as several semiidealized simulations (Scherrmann et al, 2023). Additional works further highlighted specific links within this mechanism:…”