Remote impact of North Atlantic hurricanes on the Mediterranean during episodes of intense rainfall in autumn 2012

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1 QuarterlyJournalof theroyalmeteorologicalsociety Q. J. R. Meteorol. Soc. 141: , April 2015 A DOI: /qj.2419 Remote impact of North Atlantic hurricanes on the Mediterranean during episodes of intense rainfall in autumn 2012 Florian Pantillon, a,b * Jean-Pierre Chaboureau a and Evelyne Richard a a Laboratoire d Aérologie, Université de Toulouse and Centre National de la Recherche Scientifique, Toulouse, France b Institut für Meteorologie und Klimaforschung, Karlsruher Institut für Technologie, Karlsruhe, Germany *Correspondence to: F. Pantillon, IMK-TRO, KIT, Kaiserstrasse 12, Karlsruhe, Germany. florian.pantillon@kit.edu Autumn is the most favourable season for tropical cyclones to undergo extratropical transition and interact with the midlatitude flow over the North Atlantic. Autumn is also the season when intense rainfall over the Mediterranean is often triggered by Rossby wave breaking. The impact of tropical cyclones on downstream wave breaking is investigated here during three episodes of intense rainfall which were the target of HyMeX (Hydrological cycle in Mediterranean experiment) in autumn Five-day simulations of hurricanes Leslie, Rafael and Sandy were performed with the Meso-NH model in a domain encompassing the North Atlantic and the Mediterranean. Control simulations were compared to simulations in which the hurricanes were filtered out from the initial conditions. In each case, the hurricane locally impeded the forward progression of an upstream trough, then reintensified as an extratropical cyclone during the wrap-up of the trough. The local impact of Leslie and Rafael on the midlatitude flow quickly propagated downstream along a polar jet and amplified Rossby wave breaking but decreased the intensity of the forecast precipitation over the Mediterranean. The local impact of Sandy propagated downstream along a subtropical jet in addition to the polar jet and resulted in a weak impact of the forecast precipitation on the Mediterranean. This study suggests that the interaction of tropical cyclones with the midlatitude flow over the western North Atlantic may be considered a perturbation to, rather than a source of, downstream wave breaking. Key Words: tropical cyclone; Rossby wave breaking; HyMeX; downstream development; Meso-NH; filtering Received 26 June 2013; Revised 18 June 2014; Accepted 1 July 2014; Published online in Wiley Online Library 11 August Introduction A tropical cyclone is likely to interact with the extratropical flow when it moves out of the Tropics. Both the tropical cyclone and the extratropical flow can be strongly impacted by this interaction. On the one hand, the axisymmetric tropical cyclone acquires a frontal structure and loses its warm core. The transformation is referred to as extratropical transition (Jones et al., 2003, give a review). The ex-tropical cyclone may reintensify as a midlatitude cyclone if its cyclonic circulation phases with large-scale forcing for ascent in its new baroclinic environment, and if its outflow phases with the entrance of an upper-level jet (Klein et al., 2002; Ritchie and Elsberry, 2007). The reintensified cyclone may have a direct, dramatic impact, during landfall on the western side of the oceanic basin. Over the North Atlantic, Floyd (1999) triggered a significant amount of precipitation during extratropical transition, which produced catastrophic flooding over large regions of the US East Coast (Atallah and Bosart, 2003). More recently, the strong winds of Sandy (2012) resulted in a storm surge which flooded New Jersey and New York (Blake et al., 2013). On the other hand, the extratropical flow is modified by the transitioning tropical cyclone. The strong cyclonic circulation and the supply of tropical warm, moist air may enhance a pre-existing midlatitude baroclinic zone, where slantwise ascent adds to upright convection in the remnants of the tropical core. The diabatic heating that occurs in slantwise ascent and upright convection redistributes potential vorticity (PV) vertically to produce a negative PV anomaly at upper levels. The negative PV anomaly is then advected towards the upper-level jet by the divergent outflow from both slantwise ascent and upright convection. It accelerates the jet and contributes to the building of a ridge downstream (Agusti-Panareda et al., 2004; Torn, 2010; Gramset al., 2013b). The acceleration of the downstream jet again depends on the phasing between the cyclone and its baroclinic environment (Klein et al., 2002; Grams et al., 2013a). The local acceleration of the jet can in turn accelerate the jet farther downstream, via transport of eddy kinetic energy, and induce surface cyclogenesis a process that is referred to as downstream baroclinic development (Orlanski and Sheldon, 1995). In the context of extratropical transition, this process was first documented in the influence of Irene (1999) over the North Atlantic on the development of an extratropical cyclone over the Bay of Biscay (Agusti-Panareda et al., 2004). The amplification of the midlatitude flow during extratropical transition and the c 2014 Royal Meteorological Society

2 968 F. Pantillon et al. remote triggering of surface cyclogenesis were emphasized in idealised simulations (Riemer et al., 2008). In these simulations, downstream baroclinic development was alternatively described as the amplification and dispersion of a Rossby wave train. A systematic amplification of the extratropical flow was recently shown during the extratropical transition of tropical cyclones over the western North Pacific (Harr and Dea, 2009; Cordeira and Bosart, 2010; Archambault et al., 2013). On the eastern side of the North Atlantic basin, Rossby wave breaking is known as a precursor of extreme weather. Breaking occurs when a Rossby wave attains an irreversible amplitude (McIntyre and Palmer, 1983, give a discussion of Rossby wave breaking). It results in an elongated upper-level trough, described as a stratospheric PV streamer (Martius et al., 2006, give a climatology). Along the southern slope of the Alps, heavy precipitation is frequently triggered by an elongated upper-level trough that results from wave breaking over the eastern North Atlantic (Massacand et al., 1998; Martius et al., 2006). In autumn, heavy precipitation is also frequently triggered by an elongated upper-level trough over the Mediterranean, where the sea remains warm (Nuissier et al., 2011). The trough induces strong low-level flow which transports warm moist air from over the sea towards the surrounding topography. The local dynamics and the forecast of such events at short range were recently investigated during HyMeX (Hydrological cycle in Mediterranean experiment) first Special Observation Period (SOP1) in autumn 2012 (Drobinski et al., 2013; Ducrocq et al., 2013). Autumn is also the most favourable season for tropical cyclones over the North Atlantic to undergo extratropical transition (Hart and Evans, 2001). It has long been suggested (Pinto et al., 2001) that tropical cyclones may have a remote impact on Mediterranean intense rainfall. However, few studies have addressed this connection. Hurricane Hanna (2008) amplified the midlatitude flow over the North Atlantic, from which an upperlevel trough elongated into a PV streamer and induced surface cyclogenesis and intense precipitation over the Mediterranean (Grams et al., 2011). The process involved in this complex case differed from downstream baroclinic development, in that the trough which elongated and initiated the Mediterranean cyclogenesis was located upstream of Hanna. Hurricane Helene (2006) amplified a Rossby wave train over the North Atlantic, modifying wave breaking downstream (Pantillon et al., 2013a). The elongation of a trough resulting from the wave breaking was crucial for the explosive development of a mesoscale low into a Mediterranean cyclone with tropical characteristics (Chaboureau et al., 2012). Grams et al. (2011) and Pantillon et al. (2013a) assessed the impact of Hanna and Helene on the midlatitude flow by removing the tropical cyclones from the initial conditions of numerical model forecasts. The same approach is applied here to selected tropical cyclones of the 2012 season in order to investigate their link with the Mediterranean during HyMeX SOP1. This investigation of far upstream precursors of intense rainfall complements the local approach of HyMeX. The purpose of the study is to assess the local impact of tropical cyclones on the midlatitude flow and their remote impact on synoptic conditions during episodes of intense rainfall over the Mediterranean. The local impact is defined as the modification of the midlatitude flow directly by the tropical cyclone, while the remote impact is defined as the result of this modification on Rossby wave dispersion and breaking downstream. Previous studies concluded that the amplification of the midlatitude flow was more dependent on the phasing of the tropical cyclone with the flow, rather than on the intensity or size of the tropical cyclone during extratropical transition (Harr and Dea, 2009; Grams et al., 2013a; Archambault et al., 2013). These studies discussed the impact of tropical cyclones over the western North Pacific only. To the authors knowledge, the present study is the first one that investigates both local and remote impact of several tropical cyclones over the North Atlantic during the same season. The selection of tropical cyclones is explained along with an overview of autumn 2012 in section 2. The setting of simulations and the filtering out of tropical cyclones are described in section 3. Results are shown in section 4; the evolution of tropical cyclones in the simulations, their local impact on the extratropical flow as well as their remote impact on the Mediterranean region are presented. Summary and conclusions are finally given in section Selected tropical cyclones from autumn 2012 The 2012 North Atlantic hurricane season contained 19 named storms between May and October. The focus here is on those that interacted with the midlatitude flow in September and October and could impact the Mediterranean during HyMeX SOP1. Four tropical cyclones were selected for investigation: hurricanes Leslie and Nadinein September and hurricanes Rafael and Sandy in October. Although they entered the midlatitudes, the other two tropical cyclones of September hurricanes Kirk and Michael were not selected because they could not impact the Mediterranean. Kirk interacted with a Rossby wave train far north of the Mediterranean, while Michael dissipated upstream of Leslie. The other three tropical cyclones of October tropical storms Oscar, Patty and Tony were not selected because they did not leave the Tropics. No tropical cyclone occurred in November. Hurricane Leslie recurved eastward on 9 September over the western North Atlantic, as a Rossby wave train originating from the North Pacific was migrating across the North Atlantic (Figure 1). Leslie moved towards a southerly upper-level jet on 10 September, on the eastern side of an upstream trough within the Rossby wave train (U on Figure 2(a)). Leslie became an extratropical cyclone early on 11 September (Stewart, 2013) then reintensified during the cyclonic wrap-up of the upstream trough on 12 September (Figure 2(b)). Meanwhile, a downstream trough Figure 1. Time longitude (Hovmöller) plot of the 250 hpa meridional wind averaged between 40 Nand60 N from 1 September to 10 November 2012, from 6 h analyses of ECMWF. Solid curves show the longitude of hurricanes, from their time of recurvature, defined as the time when hurricanes start tracking eastward. The position of hurricanes is given by the minimum of MSLP in the ECMWF analysis. Dotted lines show the zonal group speed of Rossby waves trains which are discussed in the text. Relevant HyMeX Intense Observation Periods (IOPs) are also marked.

3 Remote Impact of North Atlantic Hurricanes on the Mediterranean (a) (d) (g) (b) (e) (h) 969 (c) (f) (i) Figure 2. Evolution of the upper-level flow in the ECMWF analysis during the interaction with hurricanes (a c) Leslie, (d f) Rafael and (g i) Sandy; potential vorticity (shading, 1 pvu = 10 6 K m2 kg 1 s 1 ) and wind (vectors over 30 m s 1 ) on the 330 K isentropic level, and MSLP (solid contours below 1000 hpa, every 10 hpa). The 330 K isentropic level was chosen to illustrate the evolution because it captures relevant features of the upper-level flow. When relevant, labels mark the position of the hurricane (H), and the upstream (U) and the downstream (D) troughs. (labelled D on Figure 2(a c)) elongated over central Europe. It eventually evolved into a cut-off low over Italy on 14 September (Figure 2(c)). The cut-off low induced cyclonic low-level flow over Italy, which transported moisture from over the sea towards the surrounding topography (not shown). Intense precipitation up to 300 mm (24 h) 1 was locally recorded on 14 September and produced floods on the eastern side of the Italian Abruzzo range (Ducrocq et al., 2013, electronic supplement). The fourth HyMeX Intense Observation Period (IOP4) was dedicated to this event (Figure 1). Hurricane Nadine took an unusual, long and complex track over the eastern North Atlantic (Figure 1). After recurving eastward on 14 September over the western North Atlantic, Nadine weakened to below hurricane strength on 17 September then moved more slowly over the eastern North Atlantic, where it lacked the required convective organization to be considered a tropical cyclone on 21 September. On 23 September, Nadine turned westward while redeveloping organized convection and regaining tropical cyclone status, then acquired hurricane intensity for a second time on 28 September. The storm recurved eastward again on 1 October before its deep convection dissipated due to cooler waters and stronger vertical wind shear. Nadine eventually merged with a cold front on 4 October and ended its historic 23 day lifespan (Brown, 2013). The relevant period for investigating the impact of Nadine on the Mediterranean is when the storm moved slowly over the eastern North Atlantic around 20 September. A Rossby wave train was propagating over the eastern North Atlantic from the North Pacific (Figure 1). However, forecasts of Nadine exhibited high uncertainty during this period. For instance, the operational forecast from the European Centre for Medium-range Weather Forecasts (ECMWF) initialized at 0000 UTC on 20 September erroneously predicted Nadine to reintensify to 970 hpa and pass c 2014 Royal Meteorological Society the Strait of Gibraltar (not shown). Ensemble forecasts also showed a bifurcation between eastward and westward tracks (Brown, 2013). The high forecast uncertainty then reached the northwestern Mediterranean and was an issue for the HyMeX field campaign during IOP6 (Figure 1). Obtaining reliable simulations of the evolution of Nadine and of the synoptic conditions in the Mediterranean was also an issue for this study. The case of Nadine is therefore not discussed further in this article. Hurricane Rafael recurved eastward on 16 October over the western North Atlantic in a high-amplitude flow pattern (Figure 2(d)). This flow pattern was associated with a Rossby wave train that propagated from eastern North America towards Europe (Figure 1). A short-wave trough first wrapped up cyclonically and induced surface cyclogenesis to the north of Rafael (Figure 2(d, e)). Meanwhile, a broad downstream trough (labelled D) elongated along the European coast. On 18 October, Rafael interacted with an upstream trough (U on Figure 2(e)) and became an extratropical cyclone (Avila, 2013). The upstream trough later wrapped cyclonically around Rafael which reintensified on 19 October (Figure 2(e,f)). The downstream trough elongated further and induced strong southerly flow at lower levels over eastern Spain and southern France, which again transported moisture from over the sea towards the surrounding topography (not shown). Intense orographic precipitation up to 223 mm (24 h) 1 was locally recorded on 19 October in the Pyrenees and caused flooding on the Spanish and French sides of the mountain range, while moderate orographic precipitation below 100 mm (24 h) 1 was recorded in the French Massif Central (Ducrocq et al., 2013, electronic supplement). This precipitation was a target of HyMeX IOP14 (Figure 1). The trough later evolved into a cut-off and intense rainfall was recorded in eastern Spain, southeastern France, Corsica then Sardinia. The intense rainfall was the target of HyMeX IOP15. Q. J. R. Meteorol. Soc. 141: (2015)

4 970 F. Pantillon et al. Hurricane Sandy first recurved eastward on 27 October along the North American coast before it turned westward again on 29 October and made a devastating landfall in New Jersey on 30 October (Figure 1). Its unusual westward turn resulted from the combination of a broad trough upstream (labelled U on Figure 2(g)) and a ridge downstream (Figure 2(h)). The broad trough was rather stationary and was not part of a Rossby wave train (Figure 1). Sandy was located between two upper-level jets on 28 October, one along the broad upstream trough and the other along a downstream cut-off low (Figure 2(g)). Sandy reintensified on 29 October while the upstream trough wrapped up cyclonically (Figure 2(g, h)). While Sandy was declared extratropical by 2100UTC (Blakeet al., 2013), Galarneau et al. (2013) showed that Sandy reintensified as an extratropical cyclone via the process of warm seclusion. The jet along the upstream trough split around Sandy and formed a long subtropical jet across the North Atlantic. The remaining polar jet was located along the ridge downstream of Sandy. Meanwhile, a downstream trough developed over the eastern North Atlantic (labelled D). It broadened around the British Isles on 1 November (Figure 2(i)). At that time, a shortwave trough farther downstream induced strong southwesterly flow at lower levels over the northwestern Mediterranean, which once again transported moisture from over the sea towards the surrounding topography (not shown). Intense convection was reported over northeastern Italy on 1 November (Ducrocq et al., 2013, electronic supplement) and was a target of HyMeX IOP 18 (Figure 1). Large-scale orographic precipitation was later recorded over northern Italy from 3 to 5 November when the broad downstream trough reached the Mediterranean and was the target of HyMeX IOP Numerical experiments The evolution of the midlatitude flow described above indicates that Leslie, Rafael and Sandy interacted with the midlatitude flow over the North Atlantic upstream of episodes of intense rainfall over the Mediterranean. Numerical experiments were performed to assess the local impact of the hurricanes on the midlatitude flow and to investigate their possible remote impact on the Mediterranean during the episodes of intense rainfall. The Meso-NH research model (Lafore et al., 1998) was integrated for 5 days in a domain that encompassed the North Atlantic and the Mediterranean. Initial fields were taken from operational analyses of the ECMWF and were interpolated on a polar stereographic horizontal grid and 72 vertical levels. The grid mesh was 16 km in the domain centre and matched the grid mesh of operational ECMWF analyses and forecasts in autumn The vertical levels followed the terrain and were separated by 60 m near the surface, and by 600 m at the model top at approximately 30 km altitude. ECMWF analyses were also used as lateral boundaries. The domain was designed to be large enough to ensure that the evolution of the midlatitude flow over the Mediterranean was not influenced by the lateral boundaries during the model runs. For this purpose, the polar stereographic projection used for Leslie (Figure 2(a c)) differs slightly from the projection used for Rafael and Sandy (Figure 2(d i)). Meso-NH is dedicated to the study of atmospheric mesoscale processes at space- and time-scales ranging from turbulent to convective scales. It is a grid-point Eulerian model and solves primitive equations with the anelastic approximation of Durran (1989). The advection is computed with a fourth-order centred scheme for the momentum and with the piecewise parabolic method (Colella and Woodward, 1984) for the other variables. The time differencing is computed with a forward scheme for thermodynamic variables and with a leap-frog scheme for the wind. Various parametrizations were needed to describe subgrid-scale physical processes. Mass-flux schemes were used to describe deep convection (Bechtold et al., 2001) as well as shallow convection and thermals (Pergaud et al., 2009), while a 1.5-order Table 1. Names of Meso-NH control and filtered simulations, with corresponding initialisation time for each hurricane. Hurricane Initialisation time Control Filtered Leslie 0000 UTC 10 September LES10 NOLES Rafael 1200 UTC 16 October RAF16 NORAF Sandy 0000 UTC 28 October SAN28 NOSAN closure scheme was used to describe turbulence (Cuxart et al., 2000). Clouds were described with a microphysical scheme for mixed-phase clouds (Pinty and Jabouille, 1998) and a stastistical scheme for subgrid clouds (Chaboureau and Bechtold, 2005). Finally, a scheme was included to describe air sea surface fluxes (Belamari, 2005). The same parametrizations were used in Meso- NH simulations of the interactions of a Rossby wave train with hurricane Helene (2006) during extratropical transition (Pantillon et al., 2013a). Two numerical experiments were run for each of the selected hurricanes Leslie, Rafael and Sandy (Table 1). A control simulation was first initialized from the ECMWF analysis, then a second simulation was initialized from a modified analysis in which the hurricane had been filtered out. The initialization time was chosen as early as required to ensure that the hurricane was still isolated from the midlatitude flow, and as late as possible to obtain a good forecast of the episode of intense rainfall in the Mediterranean. The best compromise between these two constraints was found prior to the onset of extratropical transition, before the hurricane interacted with its new baroclinic environment. As no unique definition exists for the onset of extratropical transition (e.g. Kofron et al., 2010), the choice of the initalization time was based on an examination of the wind, temperature, moisture and PV fields at lower and upper levels. Different initialization times were tested to find the best compromise between a good control simulation and a good filtering out of the hurricane. The model was initialized at 0000 UTC on 10 September for Leslie, 1200 UTC on 16 October for Rafael and 0000 UTC on 28 October for Sandy (Figure 2(a, d, g), respectively, and Table 1). Hurricane anomalies were computed in four steps, according to Kurihara et al. (1993). In a first step, a low-pass Barnes filter was applied to extract the large-scale environmental fields. In a second step, the circulation centre was defined at the top of the boundary layer. In a third step, the hurricane radius was defined in 24 directions with a threshold on the tangential wind. In a fourth and last step, a cylindrical filter was applied within the hurricane radius in each of the 24 directions to extract the background fields. The remaining fields were considered as the hurricane anomalies. Anomalies in wind, temperature, water vapour and mean-sealevel pressure (MSLP) for each hurricane were computed. This filtering method took into account the asymmetry of both the hurricane and its environment. It was implemented in Meso- NH pre-processing tools for the filtering out of tropical cyclones (Nuissier et al., 2005) and was successfully applied to filter out hurricane Helene (2006) at the onset of extratropical transition (Pantillon et al., 2013a). The same method was applied by Klein et al. (2002) to filter out tropical cyclones prior to extratropical transition. Alternatively, Grams et al. (2011, 2013b) used a PV inversion technique to remove the PV anomaly associated with tropical cyclones before they interacted with the midlatitude flow. The extracted anomalies varied significantly between the hurricanes. Leslie had the deepest pressure anomaly (Table 2) and an eyewall-like structure existed in the precipitable water anomaly (Figure 3(a)). Rafael was the smallest hurricane and the corresponding anomaly in precipitable water was spatially limited (Figure 3(b)). The deep pressure anomaly resulted in a strong pressure gradient and in the highest wind anomaly (Table 2). Sandy was the largest hurricane; its anomalies in MSLP, wind and precipitable water had a broad spatial extent (Figure 3(c)). However, the pressure and wind anomalies were weaker than those of Leslie and Rafael (Table 2). The filtering method

5 Remote Impact of North Atlantic Hurricanes on the Mediterranean 971 Table 2. Maximum amplitude of extracted anomalies in MSLP, 850 hpa wind speed and precipitable water for the filtered hurricanes. Hurricane MSLP Wind speed Precip. water (hpa) (m s 1 ) (mm) Leslie Rafael Sandy underestimated the actual amplitude of the anomalies, because the large size and the very asymmetric structure of Sandy blurred the distinction between the hurricane and its environment. The remnants that were not filtered out did not reintensify into a new hurricane but possibly enhanced the baroclinic zone (not shown). The total impact of Sandy on the midlatitude flow may therefore have been underestimated. 4. Results 4.1. Evolution of tropical cyclones The LES10 simulation was initialised while Leslie was still located over warm waters on 10 September, with a sea surface temperature (SST) above 26 C (Figure 4(a)). LES10 captured the acceleration of Leslie on 11 September and its brief track over Newfoundland. LES10 also captured the deepening of Leslie from 1200 UTC on 10 September onward, though it had a positive bias of about 5 hpa in minimum MSLP compared to the analysis (Figure 5(a)). LES10 then captured the abrupt increase in MSLP on 11 September. LES10 was faster than the ECMWF analysis on 12 September when Leslie slowed down (Figure 4(a)). LES10 correctly simulated the reintensification of Leslie as an extratropical cyclone on that day (Figure 5(a)). The reintensification occurred during the cyclonic wrapping of the upstream trough around Leslie (Figure 2(b)). The RAF16 simulation was initialised while Rafael was still located over warm waters at 1200 UTC on 16 October with an SST above 26 C (Figure 4(b)). The track of Rafael in RAF10 was close to the ECMWF analysis during the first 2 days of simulation (Figure 4(b)). RAF16 then captured the counterclockwise loop in the track of Rafael from 19 to 21 October. The loop occurred during the cyclonic wrap-up of the upstream trough (U on Figure 2(e, f)). RAF16 also captured the quick reintensification of Rafael as an extratropical cyclone during the first half of its loop on 19 October (Figure 5(b)). Rafael then filled from 20 October onward, while moving slowly southeastward (Figure 4(b)). It eventually dissipated on 26 October close to the coast of Portugal (not shown). Sandy had just left the warm waters with a SST above 26 C when the SAN28 simulation was initialised on 28 October (Figure 4(c)). The SST decreased gradually along the track of Sandy over the Gulf Stream until late on 29 October. During that time, SAN28 captured the northwestward turn of Sandy with a 6 h delay compared to the analysis. The reintensification of Sandy with respect to its weakening prior to 28 October (not shown) was also captured with a 6 h delay by SAN28 (Figure 5(c)). The reintensification occurred during the cyclonic wrap-up of the upstream trough (U on Figure 2(g, h)). The 6 h delay persisted during the quick filling of Sandy on 30 October (Figure 5(c)), followed by its landfall in New Jersey (Figure 4(c)). Sandy dissipated afterwards and could not be clearly located on 1 November (Figure 2(i)). These results for the evolution of the hurricanes show that control Meso-NH simulations captured the track of the hurricanes over the western North Atlantic and their reintensification as extratropical cyclones during the cyclonic wrap-up of an (a) (b) (c) 30 m s 1 30 m s 1 30 m s 1 Figure 3. Filtering out of hurricanes at the initialisation of Meso-NH simulations.difference in precipitable water (shading,mm),mslp (contours every 10 hpa) and 850 hpa wind (vectors over 10 m s 1 ) between original and filtered analysis, for hurricanes (a) Leslie,(b)Rafael and (c) Sandy. (a) (b) (c) Figure 4. Track of hurricanes (a) Leslie,(b)Rafael and (c) Sandy in ECMWF analyses (solid black curves) and in Meso-NH control simulations (dashed curves), with dates and circles at 0000 UTC. Sea surface temperature (shading, C) is shown at the initialisation of Meso-NH control simulations.

6 972 F. Pantillon et al. (a) (b) (c) Figure 5. Evolution of the MSLP minimum of hurricanes (a) Leslie, (b)rafael and (c) Sandy in ECMWF analyses (solid black curves) and in Meso-NH control simulations (dashed curves), with dates and circles at 0000 UTC. upper-level trough. They suggest that the control simulations are reliable and can be used as references for the impact of hurricanes on the midlatitude flow Local impact on the midlatitude circulation The local impact of hurricanes on the midlatitude circulation is estimated from the difference in the upper-level PV between control and filtered simulations. The negative advection of upperlevel PV by the divergent wind is further used as a diagnostic of the local impact on the midlatitude circulation. The negative PV anomaly and the divergent outflow are a result of diabatic heating in the hurricane core and along the enhanced baroclinic zone. The negative advection of upper-level PV by the divergent outflow of the cyclone impedes the forward progression of an upstream trough and promotes the phasing of the cyclone with the extratropical flow (e.g. Archambault et al., 2013). In the context of extratropical transition, this diagnostic has been used to show the contribution of the cyclone to the amplification of the midlatitude flow. It has first been applied in idealized simulations (Riemer et al., 2008; Riemer and Jones, 2010) and more recently in composites of predecessor rain events associated with tropical cyclones over the North Atlantic (Moore et al., 2013) and in a climatology of the extratropical flow response to tropical cyclones over the western North Pacific (Archambault et al., 2013). Following Archambault et al. (2013), the strongest negative advection of upper-level PV by the divergent wind referred to as the maximum interaction was searched for in the vicinity of the hurricanes. The advection of upper-level PV by the divergent wind was then averaged in a 1500 km box centred on the point of maximum interaction in both control and filtered simulations (Figure 6) to distinguish between the impact of the hurricanes and the evolution of the midlatitude flow without the hurricanes. The time of maximum interaction was further chosen to illustrate the local impact of the hurricanes on the midlatitude flow (Figure 7). The 330 K isentropic level was used for consistency with Figure 2. Leslie was about to reintensify at 1800 UTC on 11 September (Figure 5(a)). Strong negative PV advection by the divergent wind occurred in LES10 (Figure 6(a)) along the eastern flank of the upstream trough (Figure 7(a)). Negative PV advection occurred earlier and was weaker in NOLES (Figures 6(a) and 7(d)). Comparing the position of the upstream trough between LES10 and NOLES shows that Leslie impeded its forward progression. Such an impact of a hurricane on the upstream trough has previously been noticed during extratropical transition (e.g. Agusti-Panareda et al., 2004; Pantillon et al., 2013a). The reintensification of Leslie is typical of a favourable coupling between the storm and the extratropical flow during extratropical transition (Klein et al., 2002). A new low also intensified in NOLES (Figure 7(d)), near the location of Leslie in LES10 (Figure 7(a)). The new low deepened explosively to 968 hpa at 1200 UTC on 12 September in NOLES, close to the MSLP of Leslie in LES10 (not shown). Using the terminology of Klein et al. (2002), this explosive deepening characterizes the favourable contribution of the midlatitudes to the reintensification of Leslie. Rafael was approaching a large surface low to the northwest at 0000 UTC on 19 October (Figure 7(b)). Harr et al. (2000) (a) (b) (c) Figure 6. Evolution of the advection of potential vorticity by the divergent wind for hurricanes (a) Leslie, (b)rafael and (c) Sandy in Meso-NH filtered (solid grey curves) and control simulations (dashed curves), with dates and circles at 0000 UTC. The advection of potential vorticity by the divergent wind is computed on the 330 K isentropic level and averaged in a 1500 km box centred on the point of maximum interaction (see text for details).

7 Remote Impact of North Atlantic Hurricanes on the Mediterranean 973 (a) (b) (c) 80 m s 1 80 m s 1 80 m s 1 (d) (e) (f) 80 m s 1 80 m s 1 80 m s 1 Figure 7. Local impact on the upper levels by hurricanes (a, d) Leslie, (b,e)rafael and (c, f) Sandy: comparison between (a c) Meso-NH control simulations and (d f) simulations without hurricanes. Potential vorticity (shading, 1 pvu = 10 6 Km 2 kg 1 s 1 ), wind (vectors over 30 m s 1 ) and negative advection of potential vorticity by the divergent wind (bold black contours below 10 pvu day 1 ) on the 330 K isentropic level, and MSLP (thin contours below 1000 hpa every 10 hpa). The black dot in (a c) marks the point of maximum interaction. characterized such a synoptic pattern as favourable for hurricane reintensification during extratropical transition. Rafael reintensified on 19 October (Figure 5(b)) and its impact on the upstream trough was similar to that of Leslie: comparing RAF16 and NORAF shows that strong negative PV advection by the divergent wind occurred from 0000 UTC on 18 October onwards when Rafael was present (Figure 6(b)) along the eastern flank of the upstream trough (Figure 7(b)). The strong negative PV advection impeded the forward progression of the upstream trough (Figure 7(b, e)). Rafael further delayed the wrap-up of the upstream trough. The upstream trough was wrapping around Rafael in RAF16 on 19 October (Figure 7(b)), whereas it wrapped earlier around the large surface low in NORAF (Figure 7(e)). Sandy was reintensifying at 1200 UTC on 29 October (Figure 5(c)), while the broad upstream trough was wrapping up cyclonically (Figure 7(c)). Very strong negative PV advection by the divergent wind occurred in SAN28 (Figure 6(c)) in a large area along the eastern flank of the upstream trough (Figure 7(c); the domain size is twice as large as in Figure 7(a, b)). Negative PV advection also occurred in NOSAN but was not as strong as in SAN28 (Figure 6(c)) and was restrained to a smaller area at the tip of the trough (Figure 7(f)). Comparing SAN28 and NOSAN shows that Sandy like Leslie and Rafael impeded the forward progression of the upstream trough (Figure 7(c, f)). The impeded forward progression of the trough may be attributed to the outflow of the enhanced baroclinic zone, rather than the outflow of the hurricane core, because of the large distance between the centre of Sandy and the region of strong negative PV advection (Figure 7(c)). This distance was larger in the case of Sandy than in the cases of Leslie and Rafael (Figure 7(a, b)). Because of its large size (Figure 3(c)), Sandy enhanced the baroclinic zone over a large area downstream of the circulation centre (not shown). As a result, Sandy had a stronger local impact on the midlatitude flow than Leslie and Rafael (Figure 6) Downstream propagation As discussed above, the hurricanes locally modified the midlatitude circulation to different degrees (Figure 7). With varying degrees and impacts, local modifications quickly propagated downstream across the North Atlantic and eventually impacted the Mediterranean. The downstream propagation is investigated here on the basis of the difference in upper-level PV between Meso-NH control and filtered simulations (Figure 8). Several studies have used Hovmöller plots of the difference between control and filtered simulations to show the impact of cyclones on the midlatitude circulation during extratropical transition (e.g. Riemer et al., 2008; Reynolds et al., 2009; Anwender et al., 2010; Grams et al., 2013a). The interpretation of latitudeaveraged differences in such Hovmöller plots was found to be ambiguous here. A sequence of maps every 24 h was preferred. The impeded forward progression of the trough upstream of Leslie (Figure 7(a, d)) is depicted by a negative difference in PV between LES10 and NOLES on 12 September (between U and H on Figure 8(a)). The impact of Leslie propagated downstream along the upper-level jet and reached the downstream trough over Europe on 13 September (D on Figure 8(b)). The downstream trough was deeper when Leslie was present than without the storm. The deeper trough is depicted by a strong positive difference in PV between LES10 and NOLES along its western flank. The trough evolved into a cut-off low over Italy on 14 September, which was still deeper when Leslie was present than without the storm (D on Figure 8(c)).

8 974 F. Pantillon et al. (a) (b) (c) 60 m s 1 60 m s 1 60 m s 1 (d) (e) (f) 60 m s 1 60 m s 1 60 m s 1 (g) (h) (i) 60 m s 1 60 m s 1 60 m s 1 Figure 8. Propagation of the impact on the upper levels of hurricanes (a c) Leslie, (d f) Rafael and (g i) Sandy. Difference in potential vorticity (shading, 1 pvu =10 6 Km 2 kg 1 s 1 ) between Meso-NH control simulations and simulations without hurricanes, and potential vorticity (contour at 2 pvu) and wind (vectors over 30 m s 1 ) in the control simulations, all on the 330 K isentropic level. When relevant, labels mark the position of the hurricane (H) and the upstream (U) and downstream (D) troughs in the control simulations. This result shows that Leslie had a remote impact on the synoptic conditions over the Mediterranean. Locally impeding the forward progression of the upstream trough over the North Atlantic on 11 September resulted in deepening the cut-off low over Italy 3 days later. Leslie modifed a pre-existing Rossby wave train and amplified wave breaking downstream. It is worth noting that a cut-off low also formed in the simulation that did not include Leslie. The impeded forward progression of the trough upstream of Rafael (Figure 7(b, e)) is also depicted by a negative difference in PV between RAF16 and NORAF on 18 October (between U and H on Figure 8(d)). The impact of Rafael propagated downstream and reached the downstream trough along the European and African coasts on 19 October (D on Figure 8(e)). The downstream trough was deeper and more elongated when Rafael was present than without the storm. The deeper and more elongated trough is depicted by a positive difference in PV between RAF16 and NORAF along its western flank and at its tip. The difference in PV persisted during the evolution of the trough into a cut-off low on 20 October (D on Figure 8(f)). This result shows that the remote impact of Rafael on the synoptic conditions over the Mediterranean was similar to that of Leslie one month earlier. Rafael impeded the forward progression of the upstream trough locally over the North Atlantic, which resulted in deepening of the downstream trough over the Mediterranean. Like Leslie, Rafael modified a pre-existing Rossby wave train and amplified wave breaking downstream. Again, the downstream trough also elongated and evolved into a cut-off low in the simulation that did not include Rafael. The impeded forward progression of the trough upstream of Sandy (Figure 7(c, f)) is depicted by a large negative difference in PV between SAN28 and NOSAN from the northeastern US to Greenland on 30 October (connecting U with D on Figure 8(g)). The impact of Sandy quickly propagated downstream and reached the downstream trough over the eastern North Atlantic on 31 October (D on Figure 8(h)). Differences eventually propagated over the Mediterranean, where a short-wave trough had formed on 1 November (Figure 8(i)). These differences were weak in comparison with those downstream of Leslie and Rafael. Sandy also differed from Leslie and Rafael because of the presence of a subtropical jet downstream (Figure 8(g i)). The advection of low-pv air by the divergent outflow of Sandy locally modified the PV gradient along the subtropical jet (Figure 7(c, f)). Differences in PV over the central North Atlantic show that the impact of Sandy propagated downstream along the subtropical jet (Figure 8(g i)). The junction of both subtropical and polar jets makes the impact of Sandy on the Mediterranean complex Remote impact on the Mediterranean Quantitative prediction of intense rainfall is a challenge over the Mediterranean, because it requires the correct forecast of finescale processes such as convection, turbulence and microphysics, and of their nonlinear interactions with larger-scale processes (Ducrocq et al., 2013). Therefore, the simulations with a lead range of several days investigated here are not expected to predict intense rainfall with high accuracy. The intense rainfall downstream of Leslie and Rafael is discussed to highlight the consequences of modifications in the synoptic conditions over the Mediterranean (Figure 8(c, e)). The intense rainfall downstream of Sandy is not discussed, because differences in the synoptic conditions were weak between the control and the filtered simulations (Figure 8(i)). In the cases of Leslie and Rafael, an upper-level PV anomaly induced low-level moisture flux over the sea toward the surrounding topography, where orographic lifting enhanced

9 Remote Impact of North Atlantic Hurricanes on the Mediterranean 975 (a) (d) (b) (e) (c) (f) Figure 9. Synoptic conditions and precipitation over the Mediterranean downstream of hurricanes (a c) Leslie and (d f) Rafael, compared between (a, d) observations and ECMWF analysis, (b, e) Meso-NH control simulations and (c, f) Meso-NH filtered simulations. Accumulated precipitation (shading, mm), average 330 K potential vorticity (contours every 2 pvu), average convergence of Q-vectors (black contours), and average 850 hpa moisture flux (vectors over 100 g kg 1 ms 1 ). Observations of precipitation are displayed only over land and in the HyMeX target area (see text for details). The average convergence of Q-vectors is displayed at 500 hpa every Pa 1 s 3 in (a c) and at 850 hpa every Pa 1 s 3 in (d f). the precipitation. These common ingredients for heavy precipitation over the Mediterranean (Nuissier et al., 2011) are compared between analysis, control and filtered simulations during the actual rainfall events (Figure 9). The 24 h time-averaged spatially smoothed upper-level PV and low-level moisture flux are displayed along with observations and forecasts of 24 h accumulated precipitation. The quasi-geostrophic forcing for ascent is also displayed on Figure 9 as the convergence of Q-vectors (Hoskins et al., 1978), averaged over 24 h and smoothed spatially. The topography of the northwestern Mediterranean in the Meso-NH simulations is displayed with the names of relevant mountain ranges on Figure 10.

10 976 F. Pantillon et al. Figure 10. Topography of the northwestern Mediterranean. Height (shading, m) in the Meso-NH simulations and names of relevant mountain ranges. Downstream of Leslie, the forming cut-off low induced strong cyclonic moisture flux at lower levels centred on southern Italy on 14 September (Figure 9(a)). Intense precipitation occurred over southern and central Italy and locally reached more than 300 mm (24 h) 1 over central Italy. The low-level moisture flux overlapped with quasi-geostrophic forcing for ascent at mid levels to trigger and sustain the precipitation in these areas. The overlap with the mid-level forcing was allowed by the location of the cut-off low directly above the low-level moisture flux. The steep topography of the Apennines (Figure 10) suggests that orographic lifting also enhanced the precipitation. Precipitation was forecast too far east in the control simulation LES10, because the forming cut-off low was located too far east (Figure 9(b)). The simulated precipitation was more moderate than the observations (Figure 9(a)). Strong low-level moisture flux triggered precipitation over southern and northeastern Greece, where it locally reached 100 mm (24 h) 1 (Figure 9(b)). Quasigeostrophic forcing for ascent at mid levels did not overlap with the strong low-level moisture flux and contributed to only moderate precipitation below 100 mm (24 h) 1 over the sea. The cut-off low formed earlier and was shallower in the filtered simulation NOLES (Figure 9(c)) than in the control simulation LES10 (Figure 9(b)). However, quasi-geostrophic forcing for ascent at mid levels better overlapped with strong moisture flux to sustain intense precipitation over northern Greece which locally reached more than 300 mm (24 h) 1 (Figure 9(c)). The comparison between LES10 and NOLES shows that the cut-off low was deeper and formed later when Leslie was present than without the storm. This modified the location and intensity of rainfall. However, rainfall was also forecast and was even more intense when Leslie was filtered out. Downstream of Rafael, the elongated trough induced strong and persistent moisture flux at lower levels toward eastern Spain and southern France on 19 October (Figure 9(d)). Quasigeostrophic forcing for ascent at mid levels was found along the eastern flank of the elongated trough (not shown). It did not overlap with the low-level moisture flux, because the elongated trough was located too far westward. Instead, the moisture flux overlapped with quasi-geostrophic forcing for ascent at lower levels to sustain moderate precipitation below 100 mm (24 h) 1 over western France and over the Massif Central, as well as intense precipitation which locally reached more than 200 mm (24 h) 1 over the western Pyrenees. The steep orography of the Pyrenees and the Massif Central (Figure 10) suggests that orographic lifting enhanced the precipitation. The elongated trough and the precipitation were well forecast in the control simulation RAF16 (Figure 9(e)). Strong low-level moisture flux again overlapped with quasi-geostrophic forcing for ascent at lower levels to sustain moderate precipitation over western France and intense precipitation which reached 150 mm (24 h) 1 over the western Pyrenees. Precipitation was overestimated over the eastern Pyrenees and the Massif Central, where intense precipitation over 150 mm (24 h) 1 was locally predicted. The trough was broader and more shallow in the filtered simulation NORAF (Figure 9(f)) than in the control simulation RAF16 (Figure 9(e)). It induced strong moisture flux at lower levels toward southern France rather than toward eastern Spain, which shifted the location of precipitation eastward (Figure 9(f)). In particular, strong moisture flux overlapped with quasigeostrophic forcing for ascent at lower levels and with orographic forcing over the southern Massif Central, where intense precipitation up to 300 mm (24 h) 1 was locally forecast. The comparison between RAF16 and NORAF shows that the upper-level trough was deeper and more elongated when Rafael was present than without the storm. This modified the location and intensity of rainfall. However, as for Leslie one month earlier, rainfall was also predicted and was even more intense when Rafael was filtered out. 5. Conclusion This study has investigated the impact of tropical cyclones over the North Atlantic on the Mediterranean during episodes of intense rainfall in autumn Three tropical cyclones were selected for their possible impact during HyMeX SOP1. Mediumrange simulations of hurricanes Leslie, Rafael and Sandy were performed with the Meso-NH model in a domain encompassing the North Atlantic and the Mediterranean. To assess the impact of the hurricanes on the extratropical flow, control simulations were compared to simulations in which the hurricanes were filtered out from the initial conditions. Control simulations correctly captured the tracks of the hurricanes and their reintensification during the cyclonic wrap-up of a trough upstream. Leslie and Rafael shared a similar impact on the midlatitude flow. They locally impeded the forward progression of an upstream trough and this local impact quickly propagated downstream along the polar jet. As a consequence, a cut-off low and an elongated trough were deeper downstream of Leslie and Rafael, respectively. The local modification of a pre-existing Rossby wave train therefore amplified wave breaking downstream. This remote impact modified the location and intensity of rainfall over the Mediterranean. Sandy had a more complex impact on the midlatitude flow. Like Leslie and Rafael, Sandy locally impeded the forward progression of an upstream trough, but in addition modified the PV gradient along a subtropical jet downstream. The local impact of Sandy

11 Remote Impact of North Atlantic Hurricanes on the Mediterranean 977 propagated downstream along both the polar and the subtropical jet. This resulted in a weak remote impact on the Mediterranean. These results show that the impact of North Atlantic hurricanes on the Mediterranean in autumn 2012 was not constrained by their size. Sandy was the largest hurricane and it strongly impacted the midlatitude flow locally, but its remote impact on the Mediterranean was weak. The pattern of the large-scale flow has been suggested as a constraint on the efficiency of downstream baroclinic development during extratropical transition over the western North Pacific (Cordeira and Bosart, 2010). Here, the strong ridge over the North Atlantic might have hindered the propagation of the amplification in the midlatitude flow downstream of Sandy. The chain of events that modified the Rossby wave breaking downstream of Leslie and Rafael was similar to that noted downstream of hurricanes Irene (1999) (Agusti-Panareda et al., 2004) and Helene (2006) (Pantillon et al., 2013a) over the North Atlantic. In all four cases, the hurricane impeded the forward progression of the upstream trough and amplified Rossby wave breaking downstream. The similarity between these four cases, among the few case-studies of the remote impact of hurricanes, suggests that this chain of events may be common during the interaction of hurricanes with the midlatitude flow over the North Atlantic. This chain of events is a perturbation of a mature Rossby wave train that also breaks without the hurricane present. It does not necessarily lead to more intense rainfall, as illustrated by the simulations of Leslie and Rafael which predicted more intense precipitation when the hurricanes were filtered out. The perturbation of a mature Rossby wave train by tropical cyclones over the North Atlantic documented here may be considered a relatively weak remote impact in comparison with the triggering of a Rossby wave train in idealised simulations or with the systematic amplification in the midlatitude flow during the recurvature of tropical cyclones over the western North Pacific (Harr and Dea, 2009; Cordeira and Bosart, 2010; Archambault et al., 2013). The apparent differences between the two oceanic basins may be related to the strength of the jet stream or to the likelihood for recurving tropical cyclones to interact with a pre-existing Rossby wave train. A climatological comparison of recurving tropical cyclones in the western North Atlantic versus the western North Pacific is necessary to better understand these differences. Finally, the impact of Nadine on the Mediterranean could not be assessed because a reliable control simulation was not obtained for the hurricane and the midlatitude flow. Past case-studies already showed low predictability during and downstream of extratropical transition (Harr et al., 2008; Anwender et al., 2008). Initial conditions can constrain the phasing of the cyclone with an upstream trough and can be important sources of forecast uncertainty in the midlatitude flow (Reynolds et al., 2009; Anwender et al., 2010; Keller et al., 2011; Pantillon et al., 2013b). Ensemble forecasts are therefore more appropriate than deterministic forecasts to investigate those situations which are marked by low predictability. The impact of Nadine on the Mediterranean will be assessed in future work with an ensemble perspective. Acknowledgements The authors thank Juan Escobar for his support in running Meso-NH. 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