Vortex vortex interaction between Hurricane Nadine (2012) and an Atlantic cut off dropping the predictability over the Mediterranean
Operational ensemble forecasts for the western Mediterranean exhibited high uncertainty while Hurricane Nadine was moving slowly over the eastern North Atlantic in September 2012. The forecasts showed a bifurcation in the track of Nadine a significant fraction predicting its landfall over the Iberian Peninsula and a high spread in the synoptic conditions downstream. The forecast uncertainty was a major issue for planning observations during the first special observation period (SOP1) of the Hydrological cycle in the Mediterranean eXperiment (HyMeX) Clustering the ensemble forecast of the European Centre for Medium Range Weather Forecasts reveals two scenarios for interaction between Nadine and an Atlantic cut off, which controls both the track of Nadine and the synoptic conditions downstream. The observed scenario of weak interaction is characterized by Nadine moving westward and the cut off moving eastward. The cut off then triggers precipitation over the Cevennes in southeastern France. The contrasting scenario of strong interaction is characterized by Nadine and the cut off rotating around each other over the Atlantic. Nadine then either merges with the cut off or makes landfall over the Iberian Peninsula. The interaction between Nadine and the cut off mimics the vortex vortex interaction previously observed between two tropical cyclones or two upper level vortices. It differs from the usual interaction between a tropical cyclone and a larger trough during extratropical transition. A critical distance of about 1000 km between Nadine and the cut off distinguishes between the cases of weak and strong interaction in the ensemble forecast. Shifting the initial position of Nadine in Meso NH numerical experiments confirms the critical distance and suggests a bifurcation point in the relative position of Nadine and the cut off. The high forecast sensitivity to the vortex vortex interaction resulted in the lowest predictability over the Mediterranean during the whole HyMeX SOP1.