The Impact of North Atlantic Hurricanes on Mediterranean Intense Rainfall during Fall 2012

Fall is the most favorable season for hurricanes to undergo Extratropical Transition (ET) over the North Atlantic, thanks to the northward extension of hurricane tracks and the southward migration of the baroclinic zone. ETs can result in the reintensification of hurricanes and the amplification of the extratropical flow, which has been suggested as a source of downstream wave breaking. Fall is also the season when the combination of warm sea and the breaking of an upper-level trough is the most likely and often triggers intense rainfall in the western Mediterranean. Such events were recently the focus of the first HyMeX (Hydrological cycle in Mediterranean eXperiment) special observation period in Fall 2012. The impact of ETs over the North Atlantic on the breaking of troughs in the western Mediterranean during Fall 2012 is investigated here in the framework of the DRIHM (Distributed Research Infrastructure for Hydro-Meteorology) EU project. Four hurricanes were followed by the trigger of intense rainfall in the western Mediterranean, a few days after and downstream of ET. Simulations of hurricanes Leslie, Nadine, Rafael and Sandy were performed with the Meso-NH model on a domain encompassing the North Atlantic and the Mediterranean. In order to assess the impact of each hurricane on the extratropical flow, a one-week control experiment was compared to a sensitivity experiment where the hurricane was filtered out. Leslie, Rafael and Sandy all reintensified over the western North Atlantic during the breaking of an upstream trough. Their outflow delayed the eastward propagation of the upstream trough and amplified a downstream ridge. This local modification quickly propagated downstream with the group velocity of a Rossby wave train and impacted a trough over the eastern North Atlantic. Downstream of Leslie and Rafael, the trough was shifted westward and its breaking was delayed. As a consequence, the location of intense rainfall in the Mediterranean was modified. The strong outflow of Sandy accelerated a jet-stream over the central North Atlantic. Though Sandy had the highest intensity among the four hurricanes, its impact on the Mediterranean was unclear, because of both its remote location and the complex downstream flow. In contrast, Nadine was the weakest hurricane but had the strongest impact on the Mediterranean, because of its proximity that was related to its unusual track over the eastern North Atlantic. The outflow of Nadine eroded an upstream trough and prevented it to move eastward and trigger intense rainfall in the Mediterranean. This study suggests that most hurricanes that reintensify over the North Atlantic and amplify the extratropical flow during ET are solely perturbations in the downstream wave breaking. They do not induce intense rainfall in the Mediterranean, though they modulate its location. Only in rare configurations, a hurricane can significantly modify the evolution of the downstream flow and impact the trigger of intense rainfall.