1. Introduction
Recurving tropical cyclones (TCs) undergoing extratropical transition (ET) are known to affect the extratropical circulation by altering the amplification, the propagation, and the breaking of midlatitude Rossby wave packets (RWPs; see Jones et al. 2003; Wirth et al. 2018; Keller et al. 2019). However, the impact of ET on the waveguide exhibits a very large case-to-case variability (e.g., McTaggart-Cowan et al. 2001, 2004; Agustí-Panareda et al. 2005; Agustí-Panareda 2008; Harr and Dea 2009; Keller et al. 2014; Pantillon et al. 2016). Both the extratropical reintensification of the recurving TC and the evolution of RWPs downstream of ET were found to be very dependent on the relative position (i.e., the “phasing”) between the TC itself and the upper-level positive PV anomalies (troughs) propagating along the midlatitude waveguide (Klein et al. 2000; Harr et al. 2000; Klein et al. 2002; Ritchie and Elsberry 2007; Pantillon et al. 2013; Grams et al. 2013a; Riemer and Jones 2014). The present study aims to understand the dynamical processes that relate phasing during ET to downstream RWP amplification and atmospheric blocking.
In general, the presence of an upper-level trough upstream of the recurving TC favors a strong impact of ET on the downstream flow (Archambault et al. 2015; Torn and Hakim 2015; Quinting and Jones 2016). Sensitivity studies highlighted a positive correlation between the time spent by the TC ahead of such a trough and the strength of the downstream impact (Scheck et al. 2011a,b; Grams et al. 2013a; Riemer and Jones 2014). This is most likely due to an enhanced potential for reintensification of the transitioning TC as an extratropical cyclone. Ahead of a trough, the storm may be located in a region of positive vorticity advection and forcing for ascent in the vicinity of a low-level baroclinic zone, a setup conducive to Petterssen–Smebye Type-B cyclogenesis (Klein et al. 2002; Ritchie and Elsberry 2003). The mutual reinforcing of upper-level PV anomalies and lower-level temperature anomalies was also summarized in the “phase locking” paradigm of baroclinic instability in the seminal work by Hoskins et al. (1985): this perspective will be adopted in the present study to physically explain the importance of phasing.
Another aspect highlighted in previous research is the importance of latent heat release in shaping the evolution of ET and its downstream impact. Moist and unstable tropical air masses, which surround recurving TCs, ascend in the vicinity of the jet stream, forming clouds and precipitation and leading to strong latent heat release (Grams et al. 2013b; Grams and Archambault 2016). The associated patterns of potential vorticity (PV) creation and destruction can influence the large-scale flow by enhancing low-level cyclonic circulations (Stoelinga 1996; Čampa and Wernli 2012; Binder et al. 2016; Crezee et al. 2017) and upper-level anticyclonic circulations (Hoskins et al. 1985; Davis et al. 1993; Pomroy and Thorpe 2000; Joos and Wernli 2012). This second effect is most evident during ET, as it can significantly contribute to the genesis of a negative PV anomaly at the waveguide (ridge) downstream of the TC. The building of such a ridge is shaped by the action of the cyclonic and anticyclonic circulations of the transitioning TC and by the irrotational, low-PV outflow due to strong and persistent latent heat release (Atallah and Bosart 2003; Riemer et al. 2008; Riemer and Jones 2010; Hodyss and Hendricks 2010; Grams et al. 2013b; Archambault et al. 2013, 2015).
A second effect of latent heat release during ET is the thinning and the meridional elongation of the trough upstream of the transitioning TC and the hindering of its eastward propagation (Pantillon et al. 2013; Riemer and Jones 2014; Archambault et al. 2015). Variations in trough speed can be explained by the amplification of the downstream ridge, which opposes the eastward propagation of the trough itself as it propagates against the background flow. As a result, the transitioning TC can remain ahead of a decelerating trough for a longer time, favoring phase locking and enhancing ridge building and downstream impact. In addition, latent heat release in clouds can diminish tropospheric static stability and increase the Rossby penetration depth, facilitating the onset and increasing the strength of phase locking (Hoskins et al. 1985). The simultaneous, two-way interaction of the circulations induced by latent heat release, upper-level PV anomalies, and lower-level temperature anomalies during ET has also been described as a “synergistic interaction” (Bosart and Lackmann 1995; Quinting and Jones 2016). A first objective of the current study is the systematical investigation of this interaction and of its role in modulating the downstream impact of ET.
As discussed previously, ET can lead to RWP amplification, and blocking anticyclones are a typical atmospheric feature occurring in meridionally amplified flow (e.g., Rex 1950; Altenhoff et al. 2008; Masato et al. 2012). A documented example is the upper-level warm anomaly associated with Hurricane Katrina (2005), which evolved into a block over the North Atlantic despite the missing reintensification of the storm as an extratropical cyclone during ET (McTaggart-Cowan et al. 2007a,b). Atmospheric blocking can also pave the way to extreme events: Barton et al. (2016) documented how a series of three TCs (Florence, Helene, and Leslie), recurving in the North Atlantic between September and October 2000, led to the continuous invigoration of atmospheric blocks over the North Atlantic and to repeated Rossby wave breaking over western Europe associated with extreme flooding over southern Switzerland and northern Italy. It has been hypothesized that a link exists between ET in the western North Pacific and enhanced blocking activity over western North America during the months of August, September, and October (Small et al. 2014). The second objective of this study is testing this hypothesis, via a targeted assessment of the relationship between ET and blocking frequency anomalies in the North Pacific.
In synthesis, the present study proposes a climatological investigation of phasing in the context of recurving TCs interacting with upper-level troughs, with the aim to understand how small differences in phasing can result in big differences in RWP amplitude and atmospheric blocking downstream of ET. This analysis will be carried out for a subset of TCs that occurred between 1979 and 2013 in the western North Pacific. A feature-based approach is adopted: the analysis will be carried out in a frame of reference moving with the upstream troughs, with the aim to characterize their degree of phasing with the TC and to highlight the synergistic interaction between adiabatic and diabatic processes during ET. The employed trough tracking, together with other relevant diagnostics, is discussed in section 2. A description of TC–trough interactions in terms of interplay between adiabatic and diabatic processes and of their effect on atmospheric blocking constitutes the main results of this study (section 3). Finally, conclusions are summarized in section 4.
2. Data and methodology
a. Data
ECMWF interim reanalysis (ERA-Interim; Dee et al. 2011) data interpolated on isentropic levels, at 1° × 1° horizontal resolution and 6-hourly temporal resolution, are employed to characterize and track upper-level troughs. The IBTrACS dataset (Knapp et al. 2010) provides information about recurving TCs in the western North Pacific between 1979 and 2013: this basin is chosen as it features the highest absolute number of recurving TCs among all basins (e.g., Knaff 2009). Recurvature is defined as a change in the direction of a northward-moving TC at the westernmost point of its track, where the TC stops heading westward and starts moving eastward: intensity at recurvature time must be at least 17 m s−1 and the storm classified as extratropical at the end of its life cycle (as in Archambault et al. 2013). In contrast to Archambault et al. (2013), TCs that undergo recurvature above 45°N are not included, whereas TCs that track in a loop are included (as in Riboldi et al. 2018).
The 215 TCs recurving during the months of August, September, and October (ASO) are analyzed. The vertically averaged negative PV advection by the irrotational wind in the upper troposphere (345–355-K isentropic layer) is then employed to quantify and locate the interaction of TCs with the waveguide, similarly to Archambault et al. (2015). The time of minimum PV advection by the irrotational wind, searched in a 30° × 15° search box north of the TC, is defined as the maximum interaction time 
b. Trough tracking
Thresholds adopted to define trough objects.
It is assumed that the closest trough in the vicinity of the TC at 
Example of trough tracking for TC Nock-ten, between 1200 UTC 25 Oct and 0000 UTC 27 Oct 2004, every 6 h (format dd/hh). Arrows depict nondivergent wind after TC removal, and trough objects are highlighted (blue contour, dark gray shading). The light gray box depicts the region of CVA averaging at each time step and therefore contains the position of the trough center of mass (blue filled dot). Box is centered (a),(b),(f),(g) at the expected position of the trough center of mass (black dot) and (c)–(e) at the location of maximum interaction (red star). The two orange contours depict isentropically averaged (315–355 K) PV (only 2 and 3 PVU), while light blue lines in (d) indicate the −5, −10, and −15 PVU day−1 contours of PV advection by the irrotational component of the wind at 350 K at 
Citation: Monthly Weather Review 147, 2; 10.1175/MWR-D-18-0271.1
Away from 
An upper-level trough could be identified and tracked for 213 out of 215 recurving TCs: the result of this procedure is 213 time series of trough CM positions. The zonal component of trough CM speed c is obtained using forward finite differences in longitude across the considered time steps, followed by a 48-h moving average. For 
c. Downstream impact diagnostics
d. Atmospheric blocking diagnostics
A block is identified as a long-lived, slow-moving, negative PV anomaly in the upper troposphere that corresponds to a vast, coherent area of anticyclonic flow through the troposphere. To diagnose it, vertically averaged (500–150 hPa) PV anomalies with respect to the climatological PV distribution of each month are computed in ERA-Interim. These anomalies are then smoothed employing a 2-day running mean. A blocked region is then identified as a closed region of negative PV anomaly lower than −1.3 PV units (PVU; 
If 
The statistical significance of 
3. Results
a. Revisiting the downstream impact of ET
The interaction of recurving TCs with upper-level troughs during ASO is associated with a significant increase in the amplitude of RWPs over the North Pacific in the 5 days following 
Distribution of standardized anomalies with respect to 
Citation: Monthly Weather Review 147, 2; 10.1175/MWR-D-18-0271.1
Highly amplified RWPs often precede the onset of atmospheric blocking (Masato et al. 2012; Grams and Archambault 2016). Given this, and in light of previous studies suggesting a connection between western North Pacific ET and blocking (Small et al. 2014; Archambault et al. 2014), we explore whether statistical links exist between the TC–trough interactions belonging to ALL and the frequency of downstream blocking. Climatologically, blocking occurs 5%–10% of the time during ASO at the end of the Pacific storm track, south of Alaska and over the Bering Sea (black dashed lines in Fig. 3). However, the probability of atmospheric blocking at the eastern edge of the Pacific storm track is 2 times higher than climatology when TC recurvature has occurred over the western North Pacific during the previous 1–5 days (Figs. 3b–d). Blocking is mostly enhanced over the Gulf of Alaska and western Canada (i.e., in the vicinity and to the east of areas of already high climatological 
Lagged composites of statistically significant values of 
Citation: Monthly Weather Review 147, 2; 10.1175/MWR-D-18-0271.1
The employed diagnostics confirm previous results about the impact of transitioning TCs on RWPs and highlight the presence of atmospheric blocking downstream of ET. In the following, both these features will be investigated in detail from a phase locking perspective.
b. Stratification of downstream impact
A stratification of the 195 TC–trough interactions in ALL with respect to the variation in the zonal speed of the trough CM is performed. Trough deceleration is evaluated as the difference in trough speed right after and before 
(a) Distribution of slowdown metric S for the considered set of 195 TC–trough interactions. The dashed indigo and orange lines indicate the locations of the lower and upper quartiles of the distribution, respectively. (b) Time series of zonal trough speed (in m s−1) distributions for the DECEL (indigo) and ACCEL (orange) subsets, relative to 
Citation: Monthly Weather Review 147, 2; 10.1175/MWR-D-18-0271.1
As discussed in the introduction, previous work has shown that the variability in downstream impact is mostly related to the phasing of the transitioning TC with an upstream trough. Indeed, the presence of trough deceleration appears to modulate the amplitude of downstream RWPs (Fig. 5). DECEL cases feature stronger RWP amplitude anomalies after 
As in Fig. 2, but for N = 49 TCs in the (a) DECEL and (b) ACCEL subsets. Empty dots refer to anomalies that are significantly different from climatology at the 95% confidence level, while filled dots highlight significance at the 99% confidence level.
Citation: Monthly Weather Review 147, 2; 10.1175/MWR-D-18-0271.1
c. Composite analysis
Before TC–trough interaction, the TC moves northeastward and approaches an upper-level trough embedded in the jet stream, represented by the positive PV anomaly 10°–20° upstream (Figs. 6a,b,d,e). No significant differences between the two subsets are visible before 
Lagged composites for the (left) DECEL and (middle) ACCEL subsets at times −2, −1, …, +2, +3 days with respect to 
Citation: Monthly Weather Review 147, 2; 10.1175/MWR-D-18-0271.1
A Hovmoeller diagram of upper-level meridional wind provides a compact depiction of phasing and RWP amplification in each subset (Fig. 7). In DECEL, the mean TC track remains ahead of the trough CM (without overtaking it) in the 3.5 days between 
Hovmoeller diagram of 345-K meridional wind for the N = 49 TCs in the (a) DECEL and (b) ACCEL subsets, averaged for each interaction in a 30° latitudinal band centered at the location of maximum interaction at 
Citation: Monthly Weather Review 147, 2; 10.1175/MWR-D-18-0271.1
The mean tracks of TCs involved in DECEL and ACCEL interactions are clearly differentiated from each other when observed from the perspective of the trough (Fig. 8). Even if all the considered TCs recurve with respect to the position of maximum interaction (Fig. 8a), most ACCEL TCs do not recurve in a frame of reference that is moving with the trough CM and head westward after 
Tracks of the considered 195 TCs with respect to (a) the location of maximum interaction and (b) the trough CM (both located at the origin of the relative latitude/longitude axes), with DECEL (indigo), ALL (gray), and ACCEL (orange) tracks highlighted. Mean tracks between 
Citation: Monthly Weather Review 147, 2; 10.1175/MWR-D-18-0271.1
d. Contribution of the large-scale flow
Differences in forcing for ascent and phase locking highlight that DECEL troughs feature an environment more conducive to downstream cyclogenesis than ACCEL troughs: these favorable conditions of the large-scale flow act to magnify the downstream impact of DECEL TC–trough interactions.
As already performed in the context of ET by Riemer and Jones (2010), Grams et al. (2013a), and Riemer et al. (2014), forcing for ascent is evaluated for the DECEL and ACCEL subsets by inverting the divergence of the Q vector on pressure layers between 900 and 100 hPa to obtain the vertical velocity ω [Fig. 9; see Davies (2015) for a review]. A partitioning of 
(a),(b) Trough CM-centered (blue dot) composites of quasigeostrophic vertical velocity at 700 hPa, averaged between 
Citation: Monthly Weather Review 147, 2; 10.1175/MWR-D-18-0271.1
The performed analysis shows that ascending motions are overall stronger during DECEL interactions than in ACCEL (Figs. 9a,b). However, DECEL interactions feature more favorable conditions for baroclinic growth and subsequent ridge building than ACCEL because of the high values of 
The deformation of the low-level potential temperature gradient due to 
Trough CM-centered (blue dot) composites of vertically averaged (315–355 K) PV anomaly (shaded), 850-hPa potential temperature anomaly (every 1 K; positive values in red, negative in blue), PV at 350 K (bold brown contours), and surface-level pressure (hPa; thin black contours) at (a),(b) 
Citation: Monthly Weather Review 147, 2; 10.1175/MWR-D-18-0271.1
On the other hand, ACCEL troughs do not exhibit a strong baroclinic interaction between the trough and the low-level potential temperature anomaly (Figs. 10b,d). The upstream trough is positively tilted and is associated with a negative low-level temperature anomaly, while the positive potential temperature anomaly is weaker and remains in the vicinity of the TCs (Fig. 10f). Such an anomaly gets advected eastward in the warm sector of a broad low pressure area located directly downstream of the trough. An analogous evolution, featuring the eastward displacement of the warm, moist air mass related to the TC, was observed for TC–jet stream interactions not leading to Rossby wave initiation (Riboldi et al. 2018). The relevance of phase locking can also explain why RWP amplification during ET is more difficult in a low-waviness configuration, where only troughs with small amplitude are present (Torn and Hakim 2015; Riboldi et al. 2018).
The favorable conditions to baroclinic development can be linked to the observed evolution of TC intensity in the two subsets (Fig. 11). TCs in DECEL weaken less rapidly than TCs in ACCEL, even if they do not exhibit a clear reintensification signature. The higher intensity may be related to the jet crossing observed in the composite. Both the southward elongation of the upstream trough and the strong ridge downstream of the TC promote a northward motion of midlatitude cyclones and have been related to jet crossing and deepening of extratropical cyclones (Coronel et al. 2015). Conversely, ACCEL cases feature overall less deep TCs that remain at the equatorward side of the jet stream and weaken steadily with time. The presence of a positively tilted trough upstream of the TC has been shown not to be conducive to posttransition reintensification (Ritchie and Elsberry 2003).
Time series of mean (filled dots; connected by bold lines) and standard deviation (vertical bars) of central SLP, relative to 
Citation: Monthly Weather Review 147, 2; 10.1175/MWR-D-18-0271.1
e. Contribution of diabatic processes
After having examined how large-scale forcing can lead to phase locking during ET, the role of diabatic processes is now considered.
The poleward advection of warm and moist air lads to heavy precipitation and strong latent heat release that can contribute synergistically to increase the amplitude of RWPs downstream of ET (Bosart and Lackmann 1995; Grams and Archambault 2016). This happens mainly in two ways: 1) by directly inducing a negative PV anomaly at the waveguide, as low-PV air is advected poleward in the direction of the jet stream, and 2) by indirectly inducing the deceleration of the upstream upper-level trough, which helps to establish phase locking with lower-level temperature anomalies.
Recurving TCs featuring sustained negative PV advection by the irrotational wind along the eastern side of the trough were associated with strong downstream impact [strong interaction cases; see Archambault et al. (2013, 2015)]. On the other hand, the opposite held in cases of weak PV advection by the irrotational wind (weak interaction cases). Strong and weak interactions are more likely to be part of the DECEL and ACCEL subsets, respectively. The detailed lists of TC–trough interactions belonging to each subset can be found in Tables S1 and S2 [the lists of strong and weak interactions are taken from Archambault et al. (2013)]. Furthermore, DECEL troughs are also associated with lower values of negative PV advection by the irrotational wind than in ACCEL, when averaging in time and space around 
Distributions of PV advection by the irrotational wind for the DECEL (indigo), ALL (black), and ACCEL (orange) subsets, averaged in the isentropic layer 345–355 K between 
Citation: Monthly Weather Review 147, 2; 10.1175/MWR-D-18-0271.1
A more refined PV advection budget is performed to better relate trough deceleration to the action of diabatic processes. After partitioning the wind field at 350 K in its irrotational and nondivergent components, PV advection is averaged in a 15° × 15° box centered at the position of the trough CM at each time step. This solution is preferred to the choice of a geographically fixed box, as it better highlights the evolution of PV advection during TC–trough interaction and avoids that rapidly moving troughs cross and exit the area during time averaging. The difference between the two subsets is particularly large in the time steps preceding 
(a)–(c) Time series of the distributions of averaged PV advection by the (a) irrotational, (b) nondivergent component of the wind, and (c) total wind relative to 
Citation: Monthly Weather Review 147, 2; 10.1175/MWR-D-18-0271.1
Negative PV advection by the irrotational wind is more pronounced in DECEL than in ACCEL (Figs. 13d,g): this is due to the stronger irrotational wind related to latent heat release in clouds (not shown), as hinted by the accumulated precipitation to the north of the transitioning TC. PV advection by the remaining, nondivergent component of the wind field is similar between the subsets before 
f. Far-downstream effects: Atmospheric blocking
It has been shown that the presence of TC–trough interactions is associated with an increased likelihood over the North Pacific (see section 3a). However, this response is also strongly modulated by the occurrence of phase locking.
A large part of the observed increase in blocking frequency over the eastern North Pacific for the ALL subset (Fig. 3) is related to the DECEL subset. The presence of high-amplitude RWPs over the region makes atmospheric blocking up to 3 times more frequent than climatology in the 1–5 days following 
As in Fig. 3, but for the N = 49 recurving TCs in the DECEL subset.
Citation: Monthly Weather Review 147, 2; 10.1175/MWR-D-18-0271.1
As in Fig. 3, but for the N = 49 recurving TCs in the ACCEL subset.
Citation: Monthly Weather Review 147, 2; 10.1175/MWR-D-18-0271.1
The planetary wave pattern strongly influences where the genesis of atmospheric blocking occurs. It still needs to be assessed whether significant differences in planetary-scale flow, which could be related to the different observed blocking patterns, exist between ACCEL and DECEL. For instance, the presence of significantly positive 
g. Final remarks
The presence of track bifurcation with respect to the upstream trough (Fig. 8b) corroborates from a climatological point of view the hypotheses of Scheck et al. (2011b), Grams et al. (2013a), and Riemer and Jones (2014) regarding the presence of hyperbolic stagnation points and track bifurcation during ET. In summary, TCs that are recurving not only in a geographical sense, but also in a trough-relative frame of reference, are related to a significant downstream impact on the midlatitude flow.
It is not yet clear whether trough deceleration is mostly due to the described adiabatic or diabatic processes. The upstream trough in DECEL, which is more meridionally extended than in ACCEL already before
A detailed analysis of the impact of the mean state on atmospheric blocking downstream of ET lies outside the scope of the current study. Low-frequency modes as ENSO, the Madden–Julian oscillation (MJO), or the Pacific–North American pattern may also influence the mean circulation pattern over the North Pacific (see, e.g., Cordeira and Bosart 2010; Henderson et al. 2016), and their effect also needs to be evaluated. However, it can be concluded from the current study that some types of ET in the western North Pacific are significantly more related than others to blocking over the western portion of the North American continent.
4. Conclusions and outlook
A set of 195 TC–trough interactions in the western North Pacific during ASO between 1979 and 2013 has been analyzed to understand how characteristics of the midlatitude flow modulate the downstream impact of recurving TCs. Confirming previous results, such interactions lead to significantly enhanced RWP amplitude over the North Pacific in the 2–5 days following ET. Additionally, TC recurvature in the western North Pacific is significantly correlated in the same time frame with positive deviations in blocking frequency over the central and eastern North Pacific.
We have developed and applied a trough-tracking algorithm to study changes in the zonal phase speed of troughs during the interaction with recurving TCs. Specifically, a substantial deceleration of troughs indicates phase locking. Most of the troughs featured such a deceleration: however, 1/4 of the considered troughs (the ACCEL subset; 49 cases) did not decelerate during the interaction with a recurving TC. This subset of troughs was compared with the one featuring the strongest deceleration (the DECEL subset; also 49 cases) to understand the differences in phasing, downstream impact, and blocking activity.
DECEL cases featured troughs that remained upstream of the TC during the interaction, exhibited phase locking, and were associated with a higher downstream RWP amplitude than average. On the other hand, ACCEL TCs already were overtaken by the approaching trough 24 h after 
We conclude that to a first order, phase locking between the TC and the upper-level trough can explain the observed variability in downstream impact of ET. Two contributions to downstream impact are highlighted in this study. The “adiabatic” contribution is based on the classical concept of interplay between an upper-level PV anomaly (the trough) and a lower-level potential temperature anomaly (related to the transitioning TC), favored by the presence of strong cyclogenetic forcing for ascent. The “diabatic” contribution highlights the role of the irrotational outflow from diabatic processes in enhancing ridge building during ET and in inducing trough deceleration, which supports the subsequent phase locking.
The different downstream impact is closely related to the occurrence of atmospheric blocking over the North Pacific. DECEL cases featured blocking even more frequently than the average TC–trough interaction, while the frequency of atmospheric blocking downstream of ACCEL cases was not significantly different from climatology. Given that latent heat release and irrotational outflow during TC–trough interaction were stronger in DECEL than in ACCEL, the described differences in blocking activity further support hypotheses relative to the importance of ET (and more generally of upstream diabatic processes) in modulating atmospheric blocking activity (Small et al. 2014; Pfahl et al. 2015).
The limitations of the present study are discussed in the following, together with possible future directions. The employed RWP amplitude metric may give an underestimation of the meridional amplitude of the midlatitude flow in configurations of Rossby wave breaking and does not take into account the semigeostrophic nature of Rossby waves (see Wolf and Wirth 2015, 2017). Sensitivity studies are still needed to understand in detail the synergistic interplay between the adiabatic and diabatic processes: for instance, the framework of Teubler and Riemer (2016) would be useful to understand the role of divergent, diabatic, and barotropic components in the generation of the PV anomaly. The eddy kinetic energy framework (e.g., Keller et al. 2014; Quinting and Jones 2016; Keller 2017) can be employed to complement this view and to assess whether the presence of trough deceleration depends also on the structure of the waveguide at 
In conclusion, the current study confirms from a climatological perspective that trough deceleration during ET is actually related to an enhanced downstream impact, due to the occurrence of phase locking and to the synergistic interaction between adiabatic and diabatic processes.
Acknowledgments
The suggestions of three anonymous reviewers were very useful to improve the quality and the readability of the manuscript. We are grateful to Heini Wernli and Ron McTaggart-Cowan for discussing an earlier version of the manuscript and for the support in writing and revising it. We thank Julian Quinting for providing the algorithm of the Zimin et al. (2006) RWP diagnostics, Maxi Boettcher for the Q-vector inversion algorithm, and Michael Sprenger for the technical support. We also thank the Atmospheric Dynamics group at ETH Zurich for discussing technical issues and supporting in the interpretation of the results. The work of JR is supported by the ETH Zurich Foundation, in collaboration with Coop (ETH Research Grant 1014-1). The contribution of CMG was supported by the Swiss National Science Foundation (SNSF) Grant PZ00P2_148177/1 and completed while he holds a Young Investigator Group grant by the German Helmholtz Association (VH-NG-1243). MR acknowledges funding from the subproject “A4: Evolution and predictability of storm structure during extratropical transition of tropical cyclones” of the Transregional Collaborative Research Center SFB/TRR 165 “Waves to Weather” program funded by the German Science Foundation (DFG).
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