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Quarterly Journal of the Royal Meteorological Society

Q. J. R. Meteorol. Soc. 140: 1945–1957, July 2014 B DOI:10.1002/qj.2263

Centennial trends in Northern Hemisphere winter storm tracks over the twentieth century Bolan Gan* and Lixin Wu Physical Oceanography Laboratory/Qingdao Collaborative Innovation Center of Marine Science and Technology, Ocean University of China, Qingdao, China *Correspondence to: B. Gan, Physical Oceanography Laboratory, Ocean University of China, 5 Yushan Road, Qingdao 266003, P. R. China. E-mail: [email protected]

In this study changes in Northern Hemisphere winter storm tracks during the twentieth century are investigated based on the individual 56 ensemble members of the twentieth century re-analysis dataset. It is found that the twentieth century trends in storm-track activities exhibit large discrepancies between the upper and lower troposphere. In the upper troposphere, a substantial intensification is identified at the poleward and downstream regions of the North Pacific and North Atlantic storm-track activities, indicating a large northeastward expansion of storm tracks in the late twentieth century. However, in the lower troposphere the synoptic eddy activities, especially in terms of the eddy kinetic energy and meridional eddy heat flux, tend to be significantly weakened over the high latitudes of central-western North Pacific and the upstream regions of the North Atlantic storm tracks. Further inspections find that such strengthening (weakening) of storm tracks in the upper (lower) troposphere are attributed mainly to an increase (decrease) of baroclinic instability, which is predominantly determined by the meridional temperature gradient changes. Moreover, from a local energetic perspective, the baroclinic generation and barotropic damping of the synoptic eddies are found to be substantially enhanced at the upstream and downstream regions of the two storm tracks in the upper troposphere, respectively, while in the lower troposphere the baroclinic energy conversion to eddies is generally decreased. Key Words:

storm tracks; global warming; centennial trend

Received 25 April 2013; Revised 23 September 2013; Accepted 25 September 2013; Published online in Wiley Online Library 28 November 2013

1. Introduction Storm tracks, characterized by the intense activities of synopticscale baroclinic eddies (Blackmon et al., 1977), play a critical role in the global climate system. In particular, the synoptic disturbances migrating along storm tracks not only influence the local weather in association with precipitation, cloudiness and winds, but also the climate by transporting large amounts of heat, moisture and momentum poleward, and by interacting with the large-scale mean flow (e.g. Trenberth and Hurrell, 1994; Chang et al., 2002; Kug et al., 2010). Moreover, storm tracks can influence the ocean by modulating the upper-level jet streams and transporting the mean westerly momentum downward to maintain the surface westerlies, which can drive a nearby warm ocean current (e.g. Lau and Holopainen, 1984; Nakamura et al., 2004). Therefore, any systematic changes in the intensity, frequency and geographical positions of storm tracks will lead to considerable changes in the extratropical weather and climate. Observational evidence has indicated that the wintertime storm tracks over the North Pacific and North Atlantic exhibit a distinct interdecadal variability, with an intensification trend since the 1960s and a sharp transition around the mid-1970s (e.g. Geng and c 2013 Royal Meteorological Society 

Sugi, 2001; Chang and Fu, 2002; Nakamura et al., 2002; Lee et al., 2012), although this trend might be overestimated in re-analysis data (Chang, 2007). The interdecadal variability in the Northern Hemisphere (NH) storm tracks are proposed to be linked with dominant modes of the extratropical climate variability, including the Arctic Oscillation (AO), North Atlantic Oscillation (NAO), Pacific Decadal Oscillation (PDO), Pacific–North American (PNA) pattern (e.g. Chang and Fu, 2002; Rivi`ere and Orlanski, 2007; Pinto et al., 2011; Lee et al., 2012). The North Atlantic storm tracks are well associated with the AO and NAO, with increased intensity and a poleward shift during strong positive AO–NAO anomaly winters (e.g. Rivi`ere and Orlanski, 2007; Nie et al., 2008); the North Pacific storm tracks are related to the PDO (e.g. Chang and Fu, 2002) and the PNA, with a poleward (equatorward) shift in the negative (positive) PNA phase (e.g. Pinto et al., 2011). However, the close relationship between the NH storm-track activities and the PDO–NAO seems to disappear after the early 1980s (Lee et al., 2012). In addition, decadal variability of the Atlantic meridional overturning circulation (AMOC) is found to have significant influence on the atmospheric dynamics associated with storm tracks, such that in cold seasons an equatorward shift and decrease of the Atlantic storm-track activity is identified in response to the AMOC-induced North Atlantic warming,

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resembling the Atlantic Multidecadal Oscillation (AMO) (e.g. Msadek et al., 2011; Gastineau and Frankignoul, 2012). As a prominent part in the mid-latitude climate dynamics, storm tracks are expected to be modulated by global warming. In several climate-model simulations forced by increasing greenhouse gases concentration, the storm-track activities generally tend to be intensified and shifted poleward in the upper troposphere (e.g. Hall et al., 1994; Kushner et al., 2001; Yin, 2005; Wu et al., 2011), although the intensification might be controversial (e.g. O’Gorman and Schneider, 2008). However, the dynamics underlying such a shift of storm tracks in response to global warming still remains elusive. Wu et al. (2011) proposed that in a warmer climate the enhancement of planetary and atmospheric energy imbalance associated with changes in the baroclinic instability is responsible for the intensification and poleward shift of storm tracks. Hall et al. (1994), however, found that the total zonal mean poleward energy transport is virtually unchanged in the greenhouse warming simulation. In a Lagrangian framework, in which individual cyclones are identified and tracked, modelling studies present varied results of the changes in extratropical cyclone activities under global warming. Several models have predicted a general decrease in the total number of winter cyclones on the hemispheric scale (e.g. Geng and Sugi, 2003; Lambert and Fyfe, 2006; Bengtsson et al., 2009; Catto et al., 2011; Mizuta et al., 2011). Furthermore, some studies highlight a significant increase in the number of intense cyclones or associated extreme events (e.g. Lambert and Fyfe, 2006; Bengtsson et al., 2009; Mizuta et al., 2011), but this appears to be model-dependent and limited to specific regions (e.g. Geng and Sugi, 2003; Bengtsson et al., 2006; Pinto et al., 2007; Eichler et al., 2013). The decrease in the total number of cyclones is attributable to the reduced baroclinicity in the lower troposphere as a consequence of a weakened meridional temperature gradient, whereas the increase in intense cyclones might be related to the enhanced release of latent heat associated with the diabatic heating. It is likely that these two competing processes contribute to the discrepant behaviours of storm tracks among the models, implying much more complicated dynamics of storm tracks in response to global warming. Given the substantial increase of greenhouse gases concentration during the twentieth century, it is conceivable that the NH storm tracks may exhibit significant long-term changes. Indeed an upward trend in the NH winter storm-track activities during the past half century has been demonstrated. However, this trend may be influenced by decadal climate variations. It is therefore necessary to assess at least the centennial time-scale trends in storm tracks. Such long-term trends, if significant, may have some indications of global warming influences. Recent studies have identified a significant upward trend in European extreme winds (Donat et al., 2011; Br¨onnimann et al., 2012) and extratropical cyclone activities (Wang et al., 2012) since 1871, based on the newly available twentieth century re-analysis dataset (Compo et al., 2011). For the current study this re-analysis dataset is used to investigate the twentieth century trends in the NH winter storm tracks. Furthermore, from a local energetic perspective, changes in the eddy–mean-flow interaction are investigated for the sake of dynamical consistency. The article is arranged as follows: section 2 describes the twentieth century re-analysis dataset and the methods to diagnose trends in storm-track activities; section 3 presents the twentieth century trends in the NH winter storm tracks; followed by the changes in the tropospheric baroclinicity and eddy–mean-flow interaction in section 4; concluding remarks and discussions are presented in section 5. 2. Data and methods To investigate the centennial trends in storm tracks, we take advantage of the newly developed (Version 2) twentieth century re-analysis dataset (20CRv2), which prescribes observed c 2013 Royal Meteorological Society 

monthly sea-surface temperature (SST) and sea-ice distributions as boundary conditions for the atmosphere (Compo et al., 2011). It assimilates only synoptic surface-pressure observations based on an ensemble Kalman filter method, which optimally combines observations and estimates of the background state, generating an ensemble with 56 members on a horizontal resolution of T62 and a vertical resolution of 28 hybrid sigma-pressure levels. Thus the 20CRv2 provides two datasets: the ensemble-mean fields with a horizontal resolution of 2◦ × 2◦ and 24 pressure levels (available at http://www.esrl. noaa.gov/psd/data/gridded/data.20thC ReanV2.html), and some limited fields in each of 56 ensemble members (available at http://portal.nersc.gov/project/20C Reanalysis/). Intercomparisons over the second half century of the 20CRv2 with other re-analysis datasets indicate that it is generally of high quality and reliable for assessing trends in the NH winter storm tracks (e.g. Compo et al., 2011; Donat et al., 2011; Wang et al., 2012). Note that considering possible data scarcity prior to 1900, the estimated long-term trends throughout the twentieth century are restricted to the period of 1900–2008. According to the literature, storm tracks can be diagnosed based on two basic approaches: one is the Lagrangian approach, which identifies and tracks extratropical cyclones and then produces cyclone statistics such as density, mean intensity and mean growth rate (see a review by Ulbrich et al., 2009); the other is the Eulerian approach, which applies a bandpass-filter to the daily time series at individual grid points and thus extracts subweekly perturbations associated with the synoptic-scale eddies (Blackmon et al., 1977). Surface cyclone statistics can provide the complementary information about storm attributes, and they more closely relate to surface weather and extreme events. The methodologies of cyclone detection and tracking, however, are diverse in the literature. Compared with the cyclone statistics, the bandpass-filtered eddy statistics are more straightforward to compute and relate more to how storm tracks interact with the large-scale circulation (Chang, 2009). In the lower troposphere the spatial pattern of the bandpass-filtered eddy statistics also marks regions of high cyclone frequency and intensity (e.g. Chang et al., 2002; Pinto et al., 2007). Therefore, changes in the lowertropospheric storm tracks in the Eulerian framework probably provide some indication of surface cyclone feature changes. In this study the wintertime storm-track activities are represented by various transient eddy statistics based on the Eulerian approach: (i) standard deviation of filtered geopotential

height ( z z ); (ii) eddy kinetic energy (EKE); (iii) meridional eddy flux of westerly momentum (u v ); and (iv) meridional eddy heat flux (v T  ) averaged from December to February (DJF). All these statistics are derived from the bandpass-filtered daily data in which the variability on synoptic time-scales of 2–8 days is retained (Blackmon et al., 1977). The linear trends are calculated based on a least-square regression, with statistical significance assessed using a Student’s t-test. Note that in this study the overbar represents averaging over the individual winter months, and the prime denotes the deviation of the filtered variable relative to the mean. To examine whether the ensemble-mean daily fields are suitable for analyzing the long-term changes in winter storm tracks, we compared the time series of area-averaged stormtrack indicators in the NH, North Pacific and North Atlantic, as derived from the ensemble-mean fields and the ensemble average of 56 corresponding series obtained from estimating each of the 56 ensemble members. For all storm-track indicators except u v , as shown in Figure 1, the ensemble-mean-derived series (red curve) show much stronger trends than the ensembleaverage series (black curve), particularly in the NH and North Pacific. Specifically, these two curves are close to each other from the middle of the 1940s, from when sufficient observations are available to constrain individual ensemble members (see figure 3 in Compo et al., 2011). However, before the early 1940s they are greatly deflected, especially in the North Pacific, probably due to exiguous observations assimilated into the 20CRv2. Further Q. J. R. Meteorol. Soc. 140: 1945–1957 (2014)

Centennial Trends in Northern Hemisphere Winter Storm Tracks

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 Figure 1. Time series of area-averaged z z (m) at 200 mb, eddy kinetic energy (EKE; m2 s−2 ) at 250 mb, u v (m2 s−2 ) at 250 mb, v T  (m s−1 K) at 850 mb in the ◦ Northern Hemisphere (10–70 N), North Pacific (10–70◦ N, 130◦ E to 110◦ W) and North Atlantic (10–70◦ N, 100◦ W to 20◦ E), as derived from the ensemble-mean fields (red curve) and the ensemble average (black curve) of 56 corresponding series obtained from analyzing each of 56 ensemble members. Shading indicates the spread between the maximum and minimum value of 56 ensemble members of the 20CRv2.

inspection finds that eddy statistics, derived from the ensemblemean fields, exhibit an unrealistic intensification trend over the North Pacific, with 60–90% increases in maximum intensity (not shown). Additionally, the ensemble spreads are also wider in the first half of the twentieth century, indicating larger uncertainty in the storm-track indicators (see shading in Figure 1). Compared with the uncertainty of storm tracks in the North Pacific, there is much less uncertainty in the North Atlantic, which is primarily attributed to the higher density of observations. It is worthwhile noting that for u v the ensemble-mean-derived series and the ensemble-average series almost match with each other throughout the twentieth century, but for v T  the former displays an upward trend rather than the downward trend shown in the latter. In general, considering the unrealistically strong trends in storm-track activities derived from the ensemble-mean output (see figures 12 and 13 in Compo et al., 2011; also see Figure 1), the daily fields in the individual 56 ensemble members seem more appropriate for investigating centennial trends in winter storm tracks. In fact, recent studies have pointed out that ensemblemean outputs are not suitable for studying extremes such as cyclone and storm activities (Br¨onnimann et al., 2012; Wang et al., 2012). Thus, in the following sections all trend estimates, unless stated otherwise, are derived from the ensemble-average stormtrack indicators obtained from estimating each of 56 ensemble members. Note that in this article the long-term trends in storm tracks are presented in the form of trends multiplying length of samples (i.e. annual trend × 108 winters). c 2013 Royal Meteorological Society 

Furthermore, given the ‘symbiotic’ relationship between storm tracks and mean flow (e.g. Cai and Mak, 1990; Lau and Nath, 1991), changes in the eddy–mean-flow interaction are investigated by analyzing local energetics in terms of the barotropic and baroclinic energy conversion and the transient eddy feedback on the mean state. This diagnostic analysis is conducted in a quasi-geostrophic framework based on the study of Cai et al. (2007) and also used by Lee et al. (2012) to study the interdecadal changes in NH winter storm-track activities. 3. Centennial trends in storm tracks As shown in Figure 2, the twentieth century  trends in the NH winter storm-track activities as represented by z z exhibit large discrepancies between the upper and lower troposphere, which is most discernible over the North Pacific region. In the upper troposphere, there are significant upward trends coinciding with the climatological NH storm tracks in the period of 1900–2008. In particular, the North Atlantic storm tracks appear to largely expand northeastward, as indicated from the substantial increases (∼23%) in intensity at the downstream regions, i.e. over Iceland and the Baltic Sea (Figure 2(a)). In the lower troposphere, however, storm tracks over eastern Russia, northern Canada and North Pacific, except regions off the east coast of Japan, display significant downward trends, with at most a 20–35% decrease in intensity (Figure 2(c)). At the entrance regions of the North Atlantic storm tracks, however, there is a weakening Q. J. R. Meteorol. Soc. 140: 1945–1957 (2014)

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 Figure 2. Twentieth century trends (colours) in standard deviation of the 2–8 days bandpass-filtered geopotential height ( z z ) at (a) 200 mb, (b) 500 mb and (c) 1000 mb, with the corresponding climatology (contours) in the boreal winter (DJF; December–February). Trends and climatology are derived from the ensemble-average statistic obtained from estimating each of 56 ensemble members in the period of 1900–2008. Contour intervals are 10 m. Stippling indicates regions of trends exceeding the 95% confidence level.

 of z z accompanied by a strengthening to the northeast. In the middle troposphere, high-latitude storm tracks tend to be more weakened in intensity as compared with those in the lower troposphere (Figure 2(b)). Here, because of the unavailability of the geopotential height at 250 and 850 mb in each ensemble  member, the pressure levels chosen for investigation of z z in the upper and lower troposphere are different from other eddy statistics. Furthermore, giventhe identified changes in storm-track

activities in terms of z z , it is conceivable that the EKE, meridional heat flux (v T  ) and meridional flux of westerly momentum (u v ) associated with the synoptic eddies embedded in storm tracks generally may have undergone coherent changes throughout the twentieth century. As shown in Figure 3(a) and (b), the EKE in the upper and lower troposphere apparently exhibits opposite trends, especially over the North Pacific,  consistent with the results of z z . At 850 mb, the EKE shows a significant weakening over the North Pacific basin and the entrance regions of the North Atlantic storm tracks, with ∼42% decrease in maximum intensity over the northern part of the North Pacific (Figure 3(a)). In sharp contrast, the EKE at 250 mb displays a general poleward and downstream intensification with a maximum increase of ∼26% occurring at 50◦ N, 40◦ W, which indicates a significant northeastward expansion (Figure 3(b)). c 2013 Royal Meteorological Society 

Storm-track activities mostly weakened in the lower troposphere but strengthened in the upper troposphere, and this is also manifested in the meridional eddy heat flux (v T  ) and meridional eddy flux of westerly momentum (u v ), respectively. For v T  at 850 mb, there are significant downward trends over the high latitudes of the central-western North Pacific and the upstream regions of the North Atlantic storm tracks, in spite of upward trends occurring over the Sea of Japan and areas around 50◦ N, 40◦ W (Figure 3(c)). It is also noticeable that v T  has a tendency to intensify over the southern United States and Gulf of Mexico. For u v at 250 mb, the intensification in the dipole-like structure over the North Pacific is likely to result in a stronger convergence of momentum fluxes into storm-track axes along ∼ 45◦ N. The intensified transient momentum flux also can be seen over the United States. Over the North Atlantic there seems to be an increase on the poleward flank of the climatology of transient momentum flux, but a decrease on the equatorward flank, although in general these changes are insignificant. 4. Changes in eddy–mean-flow interaction Due to the capability of transient eddies migrating along storm tracks to transport heat and momentum fluxes, which is of critical importance to the atmospheric circulation, storm tracks Q. J. R. Meteorol. Soc. 140: 1945–1957 (2014)

Centennial Trends in Northern Hemisphere Winter Storm Tracks

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Figure 3. Same as Figure 2 but for eddy kinetic energy (EKE) at (a) 850 mb, (b) EKE at 250 mb, (c) meridional eddy heat flux (v T  ) at 850 mb and (d) meridional eddy flux of westerly momentum (u v ) at 250 mb. Contour intervals are 5 m2 s−2 for (a), 20 m2 s−2 for (b), 2 ms−1 K for (c) and 5 m2 s−2 for (d).

are also sites of intense eddy–mean-flow interaction. For better understanding of long-term changes in storm tracks it is essential to investigate changes in the eddy–mean-flow interaction in terms of the local energetics, including specific barotropic and baroclinic energy conversion, the tropospheric baroclinicity and transient eddy feedback on the mean state, during the twentieth century. 4.1.

Barotropic energy conversion

Following Cai et al. (2007), the local barotropic energy conversion (BTEC) from mean kinetic energy (MKE) to EKE can be derived from → − →− BTEC(MKE → EKE) = C0 E · D      ∂u ∂v ∂v ∂u 1 + (−u v ) = C0 (v 2 − u 2 ) − + , (1) 2 ∂x ∂y ∂x ∂y where C0 = Pg0 , P0 is the mean sea-level pressure (1000 mb), g is the acceleration of gravity and (u, v) are the zonal and − →  meridional winds, respectively. In particular, E = 12 (v 2 − → − → 2 − u ) i + (−u v ) j measures the transient eddy  properties,  − → − → − → and terms in D = ∂∂ ux − ∂∂ yv i + ∂∂ xv + ∂∂ uy j represent the stretching and shearing deformation of the background mean c 2013 Royal Meteorological Society 

flow, respectively. Positive (negative) values indicate barotropic energy conversions from the mean flow (eddies) to eddies (mean flow). Barotropic damping is of critical importance for the dissipation of eddy energy along the central and downstream regions of the North Pacific and North Atlantic storm tracks via losing kinetic energy to the mean flow, whereas eddies gain kinetic energy from the mean flow at the genesis regions of two storm tracks (Cai et al., 2007). This can be clearly inferred from the comparison of the climatological BTEC (Figure 4(a)) with the corresponding EKE (Figure 3(b)). For the long-term changes in the BTEC, however, substantial enhancement of barotropic damping is detected at the downstream regions of the North Pacific and North Atlantic storm tracks in the upper troposphere (Figure 4(a)), consistent with the identified downstream intensification of the two storm tracks (Figure 3(b)). Note that over southern North America there are significant increases of kinetic energy obtained from the mean flow, which may lead to the intensified storm-track activities therein, as detected in the previous section. At the entrance regions of the North Pacific storm tracks where eddies extract kinetic energy, the barotropic conversion centre tends to sharpen and shift slightly southward. To further investigate the relative contributions of transient eddy properties and the background deformation fields to the BTEC, the energy conversion is decomposed into the stretching and shearing deformation terms. Specifically, the Q. J. R. Meteorol. Soc. 140: 1945–1957 (2014)

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Figure 4. Twentieth century trends (colours) in (a) barotropic energy conversion from mean kinetic energy (MKE) to EKE based on Eq. (1) at 250 mb, and baroclinic energy conversion from mean available potential energy (MAPE) to eddy available potential energy (EAPE) based on Eq. (2) at (b) 250 mb and (c) 850 mb, with the corresponding climatology (contours) in 1900–2008 December–February (DJF). Units are W m−2 . Stippling indicates regions of trends exceeding the 95% confidence level. Note that all field quantities are derived from the daily fields of individual 56 ensemble members, except the air temperature used in (b), which is derived from the ensemble-mean daily fields because of the unavailability of individual ensemble members.

stretching (shearing) deformation is defined as the product of − → − → x(y) components of E and D . Cai et al. (2007) further points out that for a non-divergent background flow, the stretching deformation is attributed primarily to the stationary wave parts of the background flow and is closely related to the meridionally/zonally elongated eddies from the perspective of eddy orientations. The shearing deformation, which is related to the horizontally tilted eddies, also can be decomposed into and the associated zonally uniform basic flow C0 (−u v ) ∂∂[u] y  ∗  ∗ ∂ v ∂ u stationary waves C0 (−u v ) ∂ x + ∂ y in which [] denotes the ,

zonal mean operator and * stands for the deviation from the zonal mean flow. Comparison of Figure 4(a) with Figure 5(a) and (b) indicates that the most dominant contribution to the enhanced barotropic conversion over the eastern North Pacific, southern North America and North Atlantic comes from the stretching deformation term associated with the stationary waves. However, changes in the barotropic conversion centre over the western North Pacific are attributable to the shearing deformation term, in which changes in the stationary wave portion overwhelms the zonal mean portion, as seen in comparison of Figure 5(c) and (d). In particular, the more meridionally elongated eddies in the exit regions of the North Pacific storm tracks mainly c 2013 Royal Meteorological Society 

lead to the intensified barotropic damping of eddies therein, − → as indicated from trends in the x-components of E (not shown). For the North Atlantic storm tracks, however, the enhanced barotropic conversion contributed from the increased degree of meridional elongation of eddies is comparable to that from changes in the stationary-wave stretching deformation flow. 4.2.

Tropospheric baroclinicity

To look further into the long-term changes in the baroclinic generation of the synoptic eddies associated with storm tracks, we investigate the tropospheric baroclinicity, which has been documented to be critical for generating the baroclinic eddies and mid-latitude cyclogenesis through converting available potential energy of time-mean flow to eddy kinetic energy, and thus crucial for the organization of storm tracks (see a review by Chang et al., 2002). As a useful estimation of the linear baroclinic instability, the maximum Eady growth rate has been utilized in a range of studies on storm tracks under climate change (e.g. Yin, 2005; Mizuta et al., 2011; Wu et al., 2011). Here, following Lindzen and Farrell (1980), the maximum Eady growth rate is calculated as σ = 0.31gN −1 T −1 |∂ T /∂ y|, where T is temperature and N is the Q. J. R. Meteorol. Soc. 140: 1945–1957 (2014)

Centennial Trends in Northern Hemisphere Winter Storm Tracks

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Figure 5. Twentieth century trends (colours) in barotropic energy conversion at 250 mb associated with (a) stretching deformation and (b) shearing deformation, which is further decomposed into the associated (c) zonal mean components and (d) stationary wave components of the background flow, with the corresponding climatology (contours) in 1900–2008 December–February. Units are W m−2 . Stippling indicates regions of trends exceeding the 95% confidence level.

Brunt–Vaisala frequency N 2 = gT −1 (dT /dz + d ) in which d is the dry adiabatic lapse rate. As shown in Figure 6, changes in the tropospheric baroclinicity have been primarily responsible for the corresponding trends in storm-track activities during boreal winter since 1900. In the upper troposphere, the baroclinic instability tends to substantially increase over the storm-track domains (Figure 6(a)). In particular, the baroclinicity over the North Atlantic shows significant poleward enhancement, indicating a large expansion of baroclinic zones. This is clearly associated with the intensification in the upper-tropospheric storm tracks, because more available potential energy contained in the larger baroclinic zones is likely to be tapped into the baroclinic eddies. Note that because of the unavailability of the air temperature at 250 mb in each ensemble member, the maximum Eady growth rate at 250 mb is derived from the ensemble-mean daily fields, in which the twentieth century trend is likely to be overestimated. In the lower troposphere, the baroclinic instability is significantly decreased over the Sea of Okhotsk and central-eastern North Pacific, despite a substantial increase detected over the Sea of Japan (Figure 6(b)). At the entrance regions of the North Atlantic storm tracks, the baroclinic instability is also significantly decreased, whereas there is an upward trend over the Gulf of Mexico. Further comparison of Figure 3 with Figure 6(b) reveals that changes in the low-tropospheric baroclinicity generally conform to the corresponding changes in storm-track activities. Given some resemblances between the changing characteristics of σ and |∂ T /∂ y| (not shown), it is speculated that the strengthening (weakening) of the baroclinicity are attributable mainly to the increase (decrease) of the meridional temperature gradient. This can be further demonstrated by decomposing changes in the baroclinic instability σ into the changes c 2013 Royal Meteorological Society 

caused by the static stability N and the meridional temperature gradient |∂ T /∂ y|, i.e. σ ≈ 0.31g (N −1 )T −1 |∂ T /∂ y| + 0.31gN −1 (T −1 |∂ T /∂ y|). For explicit illustration, each component is calculated at 850 mb and shown in Figure 6(c) and (d), as well as the corresponding changes of σ shown in Figure 6(b). It is evident that the effect of the meridional temperature gradient is dominant over that of the static stability. Also note that over the western North Pacific the weakened baroclinicity contributed from the decreased meridional temperature gradient is partially compensated by the contribution from the decreased static stability. 4.3.

Baroclinic energy conversion

Given the fact that the efficiency of eddies’ ability to tap into the baroclinicity of time-mean flow can be influenced by many factors, such as diabatic heating, eddy structure and jet streams (e.g. Chang, 2001; Nakamura et al., 2002), more specific baroclinic energy conversion needs to be investigated. The local baroclinic energy conversion (BCEC) from mean available potential energy (MAPE) to eddy available potential energy (EAPE) and from EAPE to EKE are defined as follows,   ∂T ∂T + v T  , (2) BCEC(MAPE → EAPE) = −C1 u T  ∂x ∂y R

Cv

where C1 = ( PP0 ) Cp (− ddp )−1 ( PP0 ) Cp Rg , and R,  and Cp (Cv ) are the gas constant for dry air, potential temperature, specific heat capacity of dry air at the constant pressure (volume), respectively. More details are available in Cai et al. (2007). Compared with the barotropic energetics, the baroclinic energy conversion from MAPE to EAPE is more closely tied to the Q. J. R. Meteorol. Soc. 140: 1945–1957 (2014)

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Figure 6. Twentieth century trends (colours) in the maximum Eady growth rate σ at (a) 250 mb and (b) 850 mb, with the corresponding climatology (contours) in 1900–2008 December–February (DJF). Changes in (b) σ attributed to the changes in (c) the static stability N and (d) the meridional temperature gradient |∂ T /∂ y|. Contour intervals are 0.1 day−1 for (a) and 0.2 day−1 for (b). Stippling indicates regions of trends exceeding the 95% confidence level. Note that the maximum Eady growth rate at 250 mb and at 850 mb is derived from the ensemble-mean daily fields and the daily fields of the individual 56 ensemble members, respectively.

baroclinic instability and maximizes at the entrance regions of two storm tracks, which provides a major source for the growth of transient eddies and thus is crucial for the development of storm tracks (as seen in the contours of Figure 4(b) and (c)). During the twentieth century, the maximum potential energy conversion from the mean state was significantly increased along the southern flank of the corresponding climatological centre in the North Pacific and coincided with the climatology in the North Atlantic, indicative of the enhanced development of two storm tracks (Figure 4(b)). In sharp contrast, as seen in Figure 4(c), baroclinic energy conversion to transient eddies generally decreases in the lower troposphere, being primarily responsible for the identified c 2013 Royal Meteorological Society 

weakening of storm-track activities. A further inspection finds that these changes in the BCEC are attributed mainly to changes in the term −C0 v T  ∂∂Ty . To better understand changes in the BCEC associated with the meridional temperature contrast, we investigated the twentieth and stationary century trends in its zonal mean part −C0 v T  ∂∂[T] y ∗

wave part −C0 v T  ∂∂Ty , as shown in Figure 7. In the upper troposphere, it is mainly the enhanced stationary waves, and to a lesser extent, the intensified zonally uniform basic flow that contribute to a substantial increase of baroclinic energy conversion to eddies embedded in storm tracks (as seen by Q. J. R. Meteorol. Soc. 140: 1945–1957 (2014)

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Figure 7. Twentieth century trends (colours) in potential energy conversion from the mean flow to transient eddies due to the meridional temperature contrast ∗

associated with (a) zonal mean flow −C0 v T  ∂∂[T] and (b) stationary waves −C0 v T  ∂∂Ty at 250 mb, with the corresponding climatology (contours) in 1900–2008 y

December–February (DJF). ((c) and (d)) Same as in (a) and (b), respectively, but for 850 mb. Units are W m−2 . Stippling indicates regions of trends exceeding the 95% confidence level.

comparison of Figure 7(a) with (b)). In the lower troposphere, however, the decreased zonally symmetric part of the time-mean flow appears to dominate over the changes in stationary waves, which contributes to a decrease of potential energy conversion from the mean state and thus leads to the weakening of two storm tracks (as seen by comparison of Figure 7(c) with (d)). 4.4.

Eddy feedback on mean state

We now proceed to investigate the feedback of transient eddies on the quasi-stationary flow and the mean state of − →  temperature by diagnosing the synoptic-scale E = 12 (v 2 − → − →  − u 2 ) i + (−u v ) j , the eddy-induced geopotential height and temperature tendency. Specifically, Trenberth (1986) pointed out − → that the E approximately directs to the wave energy propagation relative to the mean flow. The divergence and cyclonic curvature − → of E indicate the eddy-induced acceleration of the westerly and southerly mean flow, respectively. Lau and Nath (1991) c 2013 Royal Meteorological Society 

documented that although the geopotential tendency induced by eddy vorticity is opposite to that induced by eddy heat flux in the upper troposphere, the net eddy forcing tends to reinforce the local mean height changes. This reinforcing effect is same as the effect exerted by the stand-alone forcing of barotropic transient eddy. In addition, the baroclinic temperature tendency is determined primarily by the convergence of eddy heat flux and is predominant in the lower troposphere (e.g. Lee et al., 2012). Considering these facts, following Cai et al. (2007), the geopotential height and temperature tendency induced by the convergence of transient eddy vorticity and heat flux, respectively, are calculated as

∂h f0 − → = ∇ −2 [−∇·( V ζ  )] (geopotential height tendency) ∂t g and ∂T − → = −∇·( V T  ) (temperature tendency), ∂t where ∇ −2 is the inverse Laplacian operation. Q. J. R. Meteorol. Soc. 140: 1945–1957 (2014)

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Figure 8. (a) Twentieth century trends (colours) in the eddy-induced temperature tendency at 850 mb, with the corresponding climatology (contours) in 1900–2008 − → December–February (DJF). (b) Climatological distributions of the eddy-induced geopotential height tendency (colours, in m day−1 ) and E (vectors, in m2 s−2 ) at −1 for (a) with the zero line omitted. 250 mb in December–February (DJF). (c) Same as in (b), but for the twentieth century trends. Contour intervals are 0.2 K day Stippling and thick arrows indicate trends exceeding the 95% confidence level.

Figure 8(a) shows north–south dipole structures of the temperature tendency over eastern coasts, in which polarity is opposite to the mean state of temperature. Therefore, the transient eddies basically act to hinder the low-tropospheric temperature changes by locally reducing the meridional temperature gradient. During the twentieth century, the centennial changes in the temperature tendency are most discernible over the eastern region of North America, with a significant reduction (31%) of the positive pole over eastern Canada associated with the weakened storm tracks. This clearly indicates a weakening of the eddy-induced damping effect on the local temperature field. However, over the western North Pacific the spatial distribution of changes in the temperature tendency is complex, with significant increases over the Kuroshio–Oyashio Extension accompanied by decreases to the north and west. As for the transient eddy feedback on the quasi-stationary flow, it is evident that eddies act to maintain the upper-tropospheric circulation by enhancing the Aleutian low and the north–south gradient of the geopotential height over the North Atlantic, as seen in Figure 8(b). In addition, at the downstream regions − → of two storm tracks there is a divergence of E , implying a westerly acceleration of mean flow, which thus is favourable for the corresponding geopotential height tendency. During the twentieth century, this eddy-induced feedback is substantially intensified in the mid-latitude North Pacific and North Atlantic, as seen from the significant increase of geopotential height tendency in Figure 8(c), which is associated with the enhanced storm tracks in the upper troposphere. Furthermore, there c 2013 Royal Meteorological Society 

− → is a strong divergence and cyclonic curvature of E at the upstream regions of the North Pacific and North Atlantic storm tracks, indicative of eastward and northward wind acceleration, respectively (Figure 8(c)). This configuration of zonal and meridional wind changes is conducive to the corresponding changes in the geopotential height tendency. In general, the intensified storm tracks in the upper troposphere throughout the twentieth century play a critical role in reinforcing changes in the mean atmospheric circulation. 5. Summary and discussion In this study the centennial trends in the NH winter storm tracks during the twentieth century are investigated based on the individual 56 ensemble members of the twentieth century re-analysis dataset. It is found that the twentieth century trends in winter storm-track activities exhibit large discrepancies between the upper and lower troposphere. In the upper troposphere, a substantial intensification is identified at the poleward and downstream regions of the North Pacific and North Atlantic storm-track activities, in terms of the bandpass-filtered EKE 

and z z , indicating a large northeastward expansion of storm tracks in the late twentieth century. Such intensification is also manifested in the meridional eddy flux of westerly momentum (u v ) over the North Pacific and United States. However, in the lower troposphere the synoptic eddy activities, especially in terms of the EKE and meridional eddy heat flux (v T  ), tend to be significantly weakened over the high-latitudes of Q. J. R. Meteorol. Soc. 140: 1945–1957 (2014)

Centennial Trends in Northern Hemisphere Winter Storm Tracks the central-western North Pacific and the upstream regions of the North Atlantic storm tracks. In fact, the structure of v T  bears a great resemblance to the spatial distribution of the linear baroclinic instability measured by the Eady growth rate, as seen from comparison of Figure 3(c) with Figure 4(c). As for the extratropical cyclone activity, it is mainly affected by the baroclinicity in the lower troposphere. The identified weakening of v T  , as mentioned above, thus suggests a probable weakening of extratropical cyclone activity therein. This also seems to be reflected in the downward trends of cyclone activity index identified by Wang et al. (2012) in the twentieth century re-analysis. Changes in the NH winter storm tracks are mainly attributed to the corresponding changes in the baroclinic instability associated with the tropospheric thermal structure. The baroclinic zones in the upper troposphere tend to intensify and expand poleward, which is predominantly determined by the meridional temperature-gradient changes and is most discernible over the North Atlantic. This is primarily responsible for the intensification in the upper-tropospheric storm tracks, because more available potential energy contained in the larger baroclinic zones is likely to be tapped into the baroclinic eddies. In the lower troposphere, the baroclinic instability is decreased significantly over the centraleastern North Pacific and upstream regions of the North Atlantic storm tracks, resulting in the weakening of storm-track activities therein. The consistency between changes in the baroclinicity and storm tracks in the upper and lower troposphere further indicate a local influence of baroclinic instability on storm tracks, rather than the sensitivity of eddy activities to either upper or lower tropospheric baroclinicity suggested by previous studies (e.g. Lunkeit et al., 1998; Wu et al., 2011). To further look into the long-term changes in storm tracks, we investigated changes in the eddy–mean-flow interaction during the twentieth century. From a local energetic perspective, it was found that in the upper troposphere, the barotropic damping of eddies is substantially enhanced at the downstream regions of the North Pacific and North Atlantic storm tracks, conforming to the identified downstream intensification of two storm tracks. The baroclinic generation of eddies is also significantly increased along the North Pacific storm track axis and entrance regions of the North Atlantic storm tracks, indicative of the enhanced development of the two storm tracks. In sharp contrast, this baroclinic conversion is generally decreased over the upstream regions of the two storm tracks in the lower troposphere. Consequently, for the transient eddy feedback on the quasi-stationary flow and the mean state of temperature, it was found that the intensified storm tracks in the upper troposphere play a critical role in reinforcing changes in the mean atmospheric circulation, and the eddy-induced damping effect on the temperature field over the eastern region of North America tended to be weakened. The 20CRv2 dataset is likely to suffer from temporal inhomogeneity, especially in the pre-1940 period due to the much lower density of observations available for assimilation (see Figure 1). For understanding to what extent the inhomogeneity issue may be related to the detected trends in the NH winter storm tracks, we used the penalized maximal F (PMF) test in the RHtestsV3 software package (Wang and Feng, 2010; Wang et al., 2012) to test the temporal homogeneity of the ensemble-average time series at some grid points for each transient eddy statistic. The grid points were selected based on the remarkable changes of eddy statistics in the North Pacific–North Atlantic region shown in Figures 2 and 3. The PMF test is based on a common-trend two-phase regression model for detecting undocumented sudden changes in the mean without an accompanying trend change (Wang, 2008). The transient eddy statistics for the North Atlantic and centralwestern North Pacific are found to be generally homogeneous since the early 1900s, as inferred from a few significant changepoints detected by the PMF test (see Table 1). The eddy c 2013 Royal Meteorological Society 

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statistics for the mid-latitude eastern North Pacific seem to haveinhomogeneity issues, however, particularly as seen from

the z z and EKE data in the lower troposphere listed in Table 1, which may lead to an overestimate of the real climate trend. Recent studies have also pointed out that the 20CRv2 data seem to be homogeneous in the North Atlantic–European region since the late nineteenth century, whereas in the other Northern Hemisphere regions homogeneities are found since 1949 (Br¨onnimann et al., 2012; Wang et al., 2012). In general, the centennial trends of storm tracks identified in the North Atlantic and central-western North Pacific seem less affected by the data inhomogeneities, whereas in the mid-latitude eastern North Pacific the detected trends are probably overestimated. In the lower troposphere, much of the mid-latitude precipitation during the cold season is found to accompany passages of cyclones and to occur south of the cyclone centres (e.g. Chang and Song, 2006). The anomaly patterns of the bandpass-filtered statistics also are spatially well correlated with precipitation anomalies (e.g. Chang, 2009). Given these facts, precipitation over the high latitudes of the central-western North Pacific and North America may be expected to be lowerthan-normal during the twentieth century, since the EKE and v T  are weakened therein. Additionally, the weakening of the eddy-induced damping effect on the temperature field over the eastern region of North America may contribute to the dipolelike distribution of long-term trends of temperature therein, as revealed by Kumar et al. (2013). In the upper troposphere, the intensified storm tracks may enhance the polar-front jet and thus the westerly wind through the increase of westerly momentum transport from the Subtropics. Moreover, it has been demonstrated that the transient eddy feedback associated with storm tracks’ response to SST anomalies in the western boundary current regions plays a critical role in the mid-latitude air–sea coupling (e.g. Peng and Whitaker, 1999). Consequently, the enhancement of storm tracks identified here is likely to indicate a strengthening of the mid-latitude air–sea coupling. In fact, the air–sea coupling over the North Pacific has been found to have intensified in the twentieth century, which is attributed to global warming (Gan and Wu, 2012). In this study the results of linear baroclinic instability and energy conversion are based on the dry-atmosphere dynamics. However, the development of storm-track activities is also affected in the presence of moisture. Recent studies suggest that the differential heat supply across the oceanic frontal zones with pronounced SST gradient is essential for maintaining the nearsurface baroclinicity and, in turn, to sustain the development of storm tracks (e.g. Nakamura et al., 2004; Sampe et al., 2010). In addition, latent heat release can enhance the eddy growth by increasing the baroclinic energy conversion (e.g. Hayashi and Golder, 1981). Changes in the moisture convergence induced by the transient eddies also are indicative of changes in the hydrological cycle (evaporation minus precipitation) in the extratropics (Seager et al., 2007). Therefore, the role of moisture in the long-term changes in storm tracks and the associated eddy moisture flux changes need to be further investigated. The significant long-term changes in the NH winter storm tracks during the twentieth century may indicate a fingerprint of global warming. In fact, several climate model simulations under the enhanced greenhouse gas forcing have documented a poleward shift and downstream intensification of the wintertime storm tracks measured as eddy variance and covariance statistics, consistent with the identified centennial trend in the uppertropospheric storm tracks (e.g. Yin, 2005; Wu et al., 2011; Chang et al., 2012). These studies, which focus on changes in the upper-tropospheric storm tracks, however, do not provide much information about the lower-tropospheric storm tracks. Wu et al. (2011) show a poleward intensification of the meridional eddy heat and moisture flux at 700 mb under global warming. However, in the NH the wintertime storm tracks, as measured by the 2–6 days mean sea-level pressure variance, is found Q. J. R. Meteorol. Soc. 140: 1945–1957 (2014)

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Table 1. Temporal inhomogeneity (fourth column) of the ensemble-average time series at the selected grid point (second column) for the specific eddy statistic (first column), as identified by the penalized maximal F (PMF) test conducted at the 5% confidence level. Eddy statistic  z z 200 mb

EKE 250 mb

u v

 z z

250 mb

1000 mb

EKE 850 mb

v T 

850 mb

Grid point

Original trenda

Change point

Adjusted trenda

44◦ N, 138◦ E 32◦ N, 172◦ E 52◦ N, 166◦ W 36◦ N, 84◦ W 54◦ N, 52◦ W 60◦ N, 22◦ W 46◦ N, 138◦ E 50◦ N, 178◦ W 50◦ N, 134◦ W 40◦ N, 92◦ W 52◦ N, 42◦ W 62◦ N, 4◦ E 42◦ N, 154◦ E 34◦ N, 166◦ E 48◦ N, 174◦ W 46◦ N, 112◦ W 42◦ N, 90◦ W

15.374 (1.000) 12.258 (1.000) 9.967 (0.998) 12.751 (1.000) 14.192 (0.999) 19.274 (0.999) 21.281 (1.000) 28.519 (1.000) 31.381 (1.000) 35.098 (1.000) 36.943 (1.000) 33.781 (1.000) −11.029 (0.999) 10.725 (1.000) −13.223 (1.000) 11.831 (0.942) 11.805 (0.971)

None None 1940 1950 None None None None None None None None None None None None None

N/A N/A −3.834 (0.907) −1.339 (0.680) N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A

46◦ N, 148◦ E 64◦ N, 172◦ E 48◦ N, 152◦ W 48◦ N, 74◦ W 56◦ N, 62◦ W 48◦ N, 150◦ E 58◦ N, 178◦ E 46◦ N, 158◦ W 46◦ N, 68◦ W 44◦ N, 134◦ E 52◦ N, 148◦ E 64◦ N, 172◦ E 34◦ N, 88◦ W 44◦ N, 70◦ W

−5.549 (0.984) −9.613 (0.999) −9.706 (0.993) −6.795 (0.997) 6.269 (0.996) −9.123 (1.000) −11.501 (1.000) −10.706 (0.998) −4.979 (0.998) −2.038 (0.998) −2.355 (1.000) −3.091 (1.000) 2.311 (0.990) −2.038 (0.957)

None None 1912 None None None None 1914 None None None None None 1953

N/A N/A −0.403 (0.560) N/A N/A N/A N/A −4.158 (0.945) N/A N/A N/A N/A N/A −4.686 (1.000)

a

The third column lists the 1900–2007 trends estimated from the raw ensemble-average time series. The fifth column lists the 1900–2007 trends estimated from the mean-adjusted version of the corresponding time series, and includes accounting for the identified change point (fourth column). Trend is presented in the form of (annual trend × 108 winters). Values in parentheses are (1 - α ) in which α is the confidence level for the trend. Details in the mean-adjustments method are available in Wang and Feng (2010).

generally to be weakened in a warmer climate simulated by CMIP5 models (Harvey et al., 2013). This is consistent overall with the present result, except over the northwestern North Pacific in which the intermodel spread of storm-track response is large. In addition, many modelling studies indicate a decrease in extratropical cyclone frequency, but a regional increase in cyclone intensity or the number of intense cyclones in a warmer climate (e.g. Pinto et al., 2007; Bengtsson et al., 2009; Eichler et al., 2013). The case where cyclone frequency changes outweigh intensity changes would resemble the centennial trend identified in the lower-tropospheric storm tracks. Acknowledgement This work is supported by China National Global Change Major Research Project (2013CB956201), China National Science Foundation Key Project (41130859), and the Research Project of Chinese Ministry of Education (113041A). We are grateful to Dr Sun-Seon Lee for providing us with a program for solving the geopotential height tendency. We also appreciate two anonymous reviewers for their suggestions to improve the manuscript substantially. References Bengtsson L, Hodges KI, Roeckner E. 2006. Storm tracks and climate change. J. Clim. 19: 3518–3543. Bengtsson L, Hodges KI, Keenlyside N. 2009. Will extratropical storms intensify in a warmer climate? J. Clim. 22: 2276–2301. Blackmon ML, Wallace JM, Lau NC, Mullen SL. 1977. An observational study of the northern hemisphere wintertime circulation. J. Atmos. Sci. 34: 1040–1053. Br¨onnimann S, Martius O, von Waldow H, Welker C, Luterbacher J, Compo GP, Sardeshmukh PD, Usbeck T. 2012. Extreme winds at northern c 2013 Royal Meteorological Society 

mid-latitudes since 1871. Meteorol. Z. 21: 13–27, doi: 10.1127/09412948/2012/0337. Cai M, Mak M. 1990. Symbiotic relation between planetary and synoptic scale waves. J. Atmos. Sci. 47: 2953–2968. Cai M, Yang S, Van den Dool HM, Kousky VE. 2007. Dynamical implications of the orientation of atmospheric eddies: a local energetics perspective. Tellus 59A: 127–140. Catto JL, Shaffrey LC, Hodges KI. 2011. Northern Hemisphere extratropical cyclones in a warming climate in the hiGEM high-resolution climate model. J. Clim. 24: 5336–5352. Chang EKM. 2001. GCM and observational diagnoses of the seasonal and interannual variations of the Pacific storm track during the cool season. J. Atmos. Sci. 58: 1784–1800. Chang EKM. 2007. Assessing the increasing trend in northern hemisphere winter storm track activity using surface ship observations and a statistical storm track model. J. Clim. 20: 5607–5628. Chang EKM. 2009. Are band-pass variance statistics useful measures of storm track activity? Re-examining storm track variability associated with the NAO using multiple storm track measures. Clim. Dyn. 33: 277–296. Chang EKM, Fu Y. 2002. Interdecadal variations in northern hemisphere winter storm track intensity. J. Clim. 15: 642–658. Chang EKM, Song S. 2006. The seasonal cycles in the distribution of precipitation around cyclones in the western north Pacific and Atlantic. J. Atmos. Sci. 63: 815–839. Chang EKM, Lee S, Swanson KL. 2002. Storm track dynamics. J. Clim. 15: 2163–2183. Chang EKM, Guo Y, Xia X. 2012. CMIP5 multimodel ensemble projection of storm track change under global warming. J. Geophys. Res. 117: D23118, doi: 10.1029/2012JD018578. Compo GP, Whitaker JS, Sardeshmukh PD, Matsui N, Allan RJ, Yin X, Gleason BE Jr, Vose RS, Rutledge G, Bessemoulin P, Br¨onnimann S, Brunet M, Crouthamel RI, Grant AN, Groisman PY, Jones PD, Kruk MC, ˜ Ross TF, Trigo RM, Kruger AC, Marshall GJ, Maugeri M, Mok HY, Nordli A, Wang XLL, Woodruff SD, Worley SJ. 2011. The twentieth century reanalysis project. Q. J. R. Meteorol. Soc. 137: 1–28, doi: 10.1002/qj.776. Donat MG, Renggli D, Wild S, Alexander LV, Leckebusch GC, Ulbrich U. 2011. Reanalysis suggests long-term upward trends in European storminess since 1871. Geophys. Res. Lett. 38: L14703, doi: 10.1029/2011GL047995. Q. J. R. Meteorol. Soc. 140: 1945–1957 (2014)

Centennial Trends in Northern Hemisphere Winter Storm Tracks Eichler TP, Gaggini N, Pan Z. 2013. Impacts of global warming on Northern Hemisphere winter storm tracks in the CMIP5 model suite. J. Geophys. Res. Atmos. 118: 3919–3932, doi: 10.1002/jgrd.50286. Gan B, Wu L. 2012. Modulation of atmospheric response to north Pacific SST anomalies under global warming: a statistical assessment. J. Clim. 25: 6554–6566. Gastineau G, Frankignoul C. 2012. Cold-season atmospheric response to the natural variability of the Atlantic meridional overturning circulation.Clim. Dyn. 39: 37–57, doi: 10.1007/s00382-011-1109-y. Geng Q, Sugi M. 2001. Variability of the North Atlantic cyclone activity in winter analyzed from NCEP–NCAR reanalysis data. J. Clim. 14: 3863–3873. Geng Q, Sugi M. 2003. Possible change of extratropical cyclone activity due to enhanced greenhouse gases and sulfate aerosols – study with a high-resolution AGCM. J. Clim. 16: 2262–2274. Hall NMJ, Hoskins BJ, Valdes PJ, Senior CA. 1994. Storm tracks in a highresolution GCM with doubled carbon dioxide. Q. J. R. Meteorol. Soc. 120: 1209–1230. Harvey BJ, Shaffrey LC, Woollings TJ. 2013. Equator-to-pole temperature differences and the extra-tropical storm track responses of the CMIP5 climate models. Clim. Dyn. In press, doi: 10.1007/s00382-013-1883-9. Hayashi Y, Golder DG. 1981. The effects of condensational heating on midlatitude transient waves in their mature stage: control experiments with a GFDL GCM. J. Atmos. Sci. 38: 2532–2539. Kug JS, Jin FF, Par JH, Ren HL, Kang IS. 2010. A general rule for synoptic-eddy feedback onto low-frequency flow. Clim. Dyn. 35: 1011–1026. Kumar S, Merwade V, Kinter J III, Niyogi D. 2013. Evaluation of temperature and precipitation trends and long-term persistence in CMIP5 20th century climate simulations. J. Clim. 26: 4168–4185. Kushner PJ, Held IM, Delworth TL. 2001. Southern Hemisphere atmospheric circulation response to global warming. J. Clim. 14: 2238–2249. Lambert SJ, Fyfe JC. 2006. Changes in winter cyclones frequencies and strengths simulated in enhanced greenhouse warming experiments: Results from the models participating in the IPCC diagnostic exercise. Clim. Dyn. 26: 713–728. Lau NC, Holopainen EO. 1984. Transient eddy forcing of the time-mean flow as identified by geopotential tendencies. J. Atmos. Sci. 41: 313–328. Lau NC, Nath MJ. 1991. Variability of the baroclinic and barotropic transient eddy forcing associated with monthly changes in the midlatitude storm tracks. J. Atmos. Sci. 48: 2589–2613. Lee SS, Lee JY, Wang B, Ha KJ, Heo KY, Jin FF, Straus DM, Shukla J. 2012. Interdecadal changes in the storm track activity over the North Pacific and North Atlantic. Clim. Dyn. 39: 313–327. Lindzen RS, Farrell BF. 1980. A simple approximation result for maximum growth rate of baroclinic instabilities. J. Atmos. Sci. 37: 1648–1654. Lunkeit F, Fraedrich K, Bauer SE. 1998. Storm tracks in a warmer climate: sensitivity studies with a simplified global circulation model.Clim. Dyn. 14: 813–826. Mizuta R, Matsueda M, Endo H, Yukimoto S. 2011. Future change in extratropical cyclones associated with change in the upper troposphere. J. Clim. 24: 6456–6470. Msadek R, Frankignoul C, Li LZX. 2011. Mechanisms of the atmospheric response to North Atlantic multidecadal variability: a model study. Clim. Dyn. 36: 1255–1276.

c 2013 Royal Meteorological Society 

1957

Nakamura H, Izumi T, Sampe T. 2002. Interannual and decadal modulations recently observed in the Pacific storm track activity and East Asian winter monsoon. J. Clim. 15: 1855–1874. Nakamura H, Sampe T, Tanimoto Y, Shimpo A. 2004. Observed associations among storm tracks, jet streams and midlatitude oceanic fronts, Earth’s climate: the ocean–atmosphere interaction. In Wang C, Xie S-P, Carton JA(eds) Geophysical Monograph 147: 329–346. American Geophysical Union: Washington, DC. Nie J, Wang P, Yang W, Tan B. 2008. Northern hemisphere storm tracks in strong AO anomaly winters. Atmos. Sci. Lett. 9: 153–159. O’Gorman PA, Schneider T. 2008. Energy of midlatitude transient eddies in idealized simulations of changed climates. J. Clim. 21: 5797–5806. Peng S, Whitaker JS. 1999. Mechanisms determining the atmospheric response to midlatitude SST anomalies. J. Clim. 12: 1393–1408. Pinto JG, Ulbrich U, Leckebusch GC, Spangehl T, Reyers M, Zacharias S. 2007. Changes in storm track and cyclone activity in three SRES ensemble experiments with the ECHAM5/MPIOM1 GCM. Clim. Dyn. 29: 195–210. Pinto JG, Reyers M, Ulbrich U. 2011. The variable link between PNA and NAO in observations and in multi-century CGCM simulations. Clim. Dyn. 36: 337–354. Rivi`ere G, Orlanski I. 2007. Characteristics of the Atlantic stormtrack eddy activity and its relation with the North Atlantic oscillation. J. Atmos. Sci. 64: 241–266. Sampe T, Nakamura H, Goto A, Ohfuchi W. 2010. Significance of a midlatitude SST frontal zone in the formation of a storm track and an eddy-driven westerly jet. J. Clim. 23: 1793–1814. Seager R, Ting M, Held I, Kushnir Y, Lu J, Vecchi G, Huang HP, Harnik N, Leetmaa A, Lau NC, Li C, Velez J, Naik N. 2007. Model projections of an imminent transition to a more arid climate in southwestern North America. Science 316: 1181–1184. Trenberth KE. 1986. An assessment of the impact of transient eddies on the zonal flow during a blocking episode using localized Eliassen–Palm flux diagnostics. J. Atmos. Sci. 43: 2070–2087. Trenberth KE, Hurrell JW. 1994. Decadal atmosphere–ocean variations in the Pacific Clim. Dyn. 9: 303–319. Ulbrich U, Leckebusch GC, Pinto JG. 2009. Extra-tropical cyclones in the present and future climate: A review. Theor. Appl. Climatol. 96: 117–131. Wang XLL. 2008. Penalized maximal F test for detecting undocumented mean shift without trend change. J. Atmos. Oceanic Technol. 25: 368–384, doi: 10.1175/2007/JTECHA982.1. Wang XLL, Feng Y. 2010. RHtestsV3 User Manual. Climate Research Division, Science and Technology Branch, Environment Canada, Toronto, http://etccdi.pacificclimate.org/software.shtml (accessed 13 September 2012). Wang XLL, Feng Y, Compo GP, Swail VR, Zwiers FW, Allan RJ, Sardeshmukh PD. 2012. Trends and low frequency variability of extratropical cyclone activity in the ensemble of twentieth century reanalysis. Clim. Dyn. 40: 2775–2800. Wu Y, Ting M, Seager R, Huang HP, Cane MA. 2011. Changes in storm tracks and energy transports in a warmer climate simulated by the GFDL CM2.1 model. Clim. Dyn. 37: 53–72. Yin H. 2005. A consistent poleward shift of the storm tracks in simulations of 21st century climate. Geophys. Res. Lett. 32: L18701, doi: 10.1029/2005GL023684.

Q. J. R. Meteorol. Soc. 140: 1945–1957 (2014)