Clim Dyn (2013) 40:1057–1070
DOI 10.1007/s00382-013-1687-y
Response of Northern Hemisphere storm tracks to Indian-western
Pacific Ocean warming in atmospheric general circulation models
Cuijiao Chu • Xiu-Qun Yang • Xuejuan Ren
Tianjun Zhou
•
Received: 17 February 2011 / Accepted: 30 January 2013 / Published online: 13 February 2013
Ó Springer-Verlag Berlin Heidelberg 2013
Abstract With 40 years integration output of two
atmospheric general circulation models (GAMIL/IAP and
HadAM3/UKMO) forced with identical prescribed seasonally-varying sea surface temperature, this study
examines the effect of the observed Indian-western Pacific
Ocean (IWP) warming on the Northern Hemisphere storm
tracks. Both models indicate that the observed IWP
warming tends to cause both the North Pacific storm track
(NPST) and the North Atlantic storm track (NAST) to
move northward. Such a consistent effect on the two
storm tracks is closely associated with the changes in the
low-level atmospheric baroclinicity, high-level jet stream
and upper-level geopotential height. The IWP warming
can excite a wavelike circum-global teleconnection in the
geopotential height that gives rise to an anticyclonic
anomaly over the midlatitude North Pacific and a positive-phase NAO anomaly over the North Atlantic. These
geopotential height anomalies tend to enhance upper-level
zonal westerly winds north of the climatological jet axes
and increase low-level baroclinicity and eddy growth
rates, thus favoring transient eddy more active north of
the climatological storm track axes, responsible for the
northward shift of the both storm tracks. The IWP
warming-induced northward shift of the NAST is quite
C. Chu X.-Q. Yang (&) X. Ren
Institute for Climate and Global Change Research,
School of Atmospheric Sciences, Nanjing University,
Nanjing 210093, China
e-mail: [email protected]
T. Zhou
State Key Laboratory of Numerical Modeling for Atmospheric
Sciences and Geophysical Fluid Dynamics, Institute of
Atmospheric Physics, Chinese Academy of Sciences,
Beijing, China
similar to the observed, suggesting that the IWP warming
can be one of the key factors to cause decadal northward
shift of the NAST since the 1980s. However, the IWP
warming-induced northward shift of the NPST is completely opposite to the observed, implying that the
observed southward shift of the NPST since the 1980s
would be primarily attributed to other reasons, although
the IWP warming can have a cancelling effect against
those reasons.
Keywords Northern Hemisphere storm tracks Indian-western Pacific Ocean warming Baroclinicity Eddy growth rate
1 Introduction
The storm tracks play a large role in transporting heat,
momentum and water vapor horizontally and vertically,
thereby influencing the large-scale atmospheric circulation.
Midlatitude weather and climate during the cool seasons
are closely related to the changes in the location and
intensity of the storm tracks (Chang 2001; Chang et al.
2002). The Northern Hemisphere storm tracks (NHSTs)
exhibit a remarkable seasonal cycle. The North Atlantic
storm track (NAST) is the strongest during midwinter when
the meridional temperature gradient across the storm track
is the largest (Chang and Zurita-Gotor 2007), while the
North Pacific storm track (NPST) peaks in late autumn and
early spring but weakens significantly in midwinter (Nakamura 1992). The so-called midwinter suppression of the
NPST is a striking phenomenon discussed in many previous studies (e.g., Nakamura 1992; Chang 2001; Nakamura
and Sampe 2002; Chang and Zurita-Gotor 2007; Penny
et al. 2010), and has been attributed to multi-contributions,
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such as various local dynamical mechanisms (Nakamura
and Sampe 2002), the role of diabatic heating over tropical
regions (Chang and Guo 2007) and the upstream seeding
(Penny et al. 2010). Therefore, the NPST midwinter suppression can serve as a benchmark for evaluating the performance of climate models (Christoph et al. 1997; Zhang
and Held 1999; Deng and Mak 2005; Chang and ZuritaGotor 2007).
A number of recent studies have suggested that the
NHSTs have changed in the second half of the twentieth
century (Simmonds and Keay 2000; Norris 2000; Gulev
et al. 2001; McCabe et al. 2001; Zhang et al. 2004; Ulbrich
et al. 2009; Lee et al. 2012). Norris (2000) showed that the
NPST in summer moves equatorward and intensified
between 1952 and 1995. McCabe et al. (2001) found that
there has been a significant decrease in midlatitude cyclone
activity and an increase in high-latitude cyclone frequency,
suggesting a poleward shift of the storm track, with storm
intensity increasing over the North Pacific and North
Atlantic. Mesquita et al. (2008) demonstrated a detectable
upward-trend of mean intensity and lifetime for the storms
over the North Pacific during summer of 1948–2002. Significant increasing trends over the North Pacific were also
found in eddy meridional velocity variance at 300 hPa and
other statistics (Chang and Fu 2002; Paciorek et al. 2002;
Lee et al. 2012). Superimposed on these long-term changes, decadal-scale variability has occurred in particular
geographic regions, such as North Pacific Ocean (e.g.,
Zhang et al. 2004). The North Pacific midwinter storm
track activity was significantly stronger from the late 1980s
to early 1990s than from the early to mid-1980s (Nakamura
et al. 2002).
The mechanisms responsible for the decadal-to-interdecadal changes of the NHSTs especially the NPST
remain unknown (Chang 2001). Higher sea surface temperatures (SST) at mid and high latitudes may lead to an
intensification of extratropical cyclones. Through performing zonal wavenumber-1 SST anomalies to a zonally
uniform background SST field experiment, Inatsu et al.
(2003) suggested that the strengthening of the SST gradients may favor active extratropical cyclones. In addition
to midlatitude oceanic forcing, tropical ocean has fundamental impact on the storm track variability (Raible and
Blender 2004; Orlanski 2005; Ren et al. 2008). However,
the relationship between the NPST and some SST indices
like the Pacific Decadal Oscillation (PDO) and El NiñoSouthern Oscillation (ENSO) shows interdecadal changes.
The storm track activity is highly (weak) correlated with
the PDO (ENSO) before 1980, whereas the relationship
has weakened (strengthened) dramatically since the early
1980s (Lee et al. 2012). Since 1977 tropical SSTs have
increased by approximately 0.4 K in the Indian Oceanwestern Pacific (IWP) relative to the period of 1950–1976
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(Webster et al. 1999; Saji et al. 1999; Hoerling et al.
2001; Deser and Phillips 2006). The IWP warming has
been regarded as one mechanism for the interdecadal
variability of East Asian summer monsoon (Gong and Ho
2002; Zhou et al. 2008, 2009a; Li et al. 2010; Zhao et al.
2011). And even, the IWP warming has been considered a
main contributor to the recent positive trend of the North
Atlantic Oscillation (NAO) (Hurrell 1995; Hurrell et al.
2004), and thus can affect North Atlantic climate (Bader
and Latif 2003; Selten et al. 2004; Hoerling et al. 2001,
2004; Bader and Latif 2005; King et al. 2010). For
example, an ensemble simulation by King et al. (2010)
suggested that the tropical SST warming can contribute
up to 30 % of the NAO trend. It has also been hypothesized that the North Atlantic response is mainly eddydriven via a circum-global pattern along the South Asian
and North Atlantic Jets (Hoerling et al. 2001) associated
with changes along the local storm track (SanchezGomez
et al. 2008), while the Atlantic storm track trend found in
reanalysis data is closely related to the NAO trend (Geng
and Sugi 2001). However, whether the IWP warming has
some responsibility for the long-term variability of the
NPST as well as the NAST is still unclear.
The main purpose of this study is to evaluate the performance of two climate models in simulating the NHSTs,
and to examine the possible effect of the IWP warming on
the long-term changes of both NPST and NAST, through
analyzing the output of a European Union Framework 6
project ‘‘understanding the dynamics of the coupled climate system’’ (DYNAMITE). Coordinated by the
DYNAMITE project, several Atmospheric General Circulation Models (AGCMs) were forced by an identical idealized SST pattern mimicking observed decadal changes
representative of the observational IWP warming and
cooling. These coordinated experiments have been performed, for understanding the dynamics of the coupled
climate system, as well as the impacts on climate of
the IWP basin-scale warming (SanchezGomez et al.
2008; Zhou et al. 2009a; Hodson et al. 2010). More details
about DYNAMITE project can be found at http://
dynamite.nersc.no/. Only two AGCMs with 4 times daily
outputs archived, HadAM3 and GAMIL, are analyzed here.
We first assess the performances of these two models in
simulating the climatological features of the NHSTs, and
then investigate the simulated responses of the NPST and
NAST to the IWP warming. The remainder of the paper is
organized as follows. The model experiments and analysis
methods are introduced in Sect. 2. The performances of
two AGCMs in simulating the climatological feature of the
NHSTs are examined in Sect. 3. The responses of the
NPST and NAST to the IWP basin-scale warming are
presented in Sect. 4. Summary and discussion are provided
in Sect. 5.
Response of Northern Hemisphere storm tracks
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Fig. 1 Distributions of monthly sea surface temperature anomalies (°C) for a June, b July, c August, d December, e January, and f February that
are specified in the AGCMs for the Indian-western Pacific warming experiments
2 Model experiments and analysis methods
The outputs of two AGCMs involved in the DYNAMITE
project are analyzed. The two models are the GAMIL
model developed at the State Key Laboratory of Numerical
Modeling for Atmospheric Sciences and Geophysical Fluid
Dynamics of Institute of Atmospheric Physics (LASG/IAP)
in China, and the HadAM3 model developed at the Hadley
Centre for Climate Prediction and Research/Met Office in
United Kingdom. The GAMIL model is a grid-point
atmospheric model with a horizontal resolution of 2.8° in
latitude by 2.8° in longitude and 26 vertical levels, and the
convection scheme of Zhang and McFarlane (1995) is
employed in this model. A detailed description of the
model can be found in Li et al. (2007, 2008). The HadAM3
is a hydrostatic, grid point atmospheric model with a horizontal resolution of 2.5° in latitude by 3.75° in longitude
and 19 vertical levels and with an Eulerian advection
scheme and a full set of parameterizations (Pope et al.
2000). The convection scheme in HadAM3 is adopted from
Gregory and Rowntree (1990) with the addition of convective downdrafts (Gregory and Allen 1991). Both models
have been widely used in twentieth century climate simulation (Li et al. 2007; Zhou et al. 2009b; Scaife et al. 2009),
and in Asian monsoon studies (Zhou et al. 2009c;
Kucharski et al. 2009).
In the DYNAMITE project, three experiments of 40-year
length were performed: a control experiment (CNTL) in
which the AGCMs were forced with climatological SST
and sea ice concentration for 1961–1990 that were taken
from the HadISST dataset (Rayner et al. 2003), and two
sensitivity experiments in which everything is the same as
in the control experiment except for the SSTs that were
specified in the IWP domain (roughly bounded by 30°E–
160°E, 35°S–25°N) are different. In the two experiments,
an idealized SST pattern representative of the IWP basin-
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scale warming (denoted by IOP) and cooling (denoted by
ION), respectively, was specified in the AGCMs. The SST
forcing pattern for the IWP warming or cooling experiments
was derived from the monthly mean trends in SST between
1951 and 1999. The full SST forcing pattern for IOP (ION)
was generated by adding (subtracting) scaled anomalies
(IO0 ) to (from) the SST climatology over the IWP region:
IOP ¼ Clim þ 23:5IO0 ;
ð1Þ
ION ¼ Clim 24:5IO0 :
ð2Þ
For IOP, the IO0 anomaly is scaled by the number of years
between 1999 and the midpoint of the period used to create
the climatology (1961–1990), hence, 23.5 years. For ION,
using 1951, we obtain 24.5 years. These two SST patterns
reflect the changes of SST in the IWP between 1951 and
1999 (Zhou et al. 2009a). Figure 1 illustrates the SST
anomalies for the summer and winter months used to drive
the AGCMs in the IWP warming experiment. These
anomalies qualitatively coincide with the observed SST
trends in the tropical Indian Ocean and far western Pacific
for 1951–1999. Each experiment was integrated for
40 years of which the first 10 years were discarded as
being the spinup stage. The analysis was done for each
month and each season. The seasonal mean was computed
by averaging 3-month periods (say, December-January–
February (DJF) for winter, and June–July–August (JJA) for
summer) in each year. Then the climatological seasonal
means were calculated by averaging seasonal means each
year over the period of last 30 years of model experiments.
Assuming that each year is statistically independent, this is
equivalent for the anomaly experiments to an ensemble
mean with 30 realizations (SanchezGomez et al. 2008;
Zhou et al. 2009a). To verify the model performance, the
observed atmospheric fields are taken from the European
Centre for Medium-Range Weather Forecasts (ECMWF)
Reanalysis (ERA-40) dataset (Uppala et al. 2005). The data
has a horizontal resolution of 2.58 9 2.58 in longitude and
latitude and covers the period from September 1, 1957 to
August 31, 2002. The 6-hourly and monthly mean fields
are used in the present analysis.
To extract the storm tracks associated with migratory
synoptic-scale disturbances at periods of 2.5–6 days, a
bandpass-filtered technique (Murakami 1979) is applied to
the 6-hourly geopotential height (z) at the 500 hPa. The
storm track is measured in this study with the standard
deviation of 2.5–6 days bandpass-filtered geopotential
height (z) at 500 hPa. The midlatitude weather systems
associated with the storm tracks are believed to have their
origin in processes encapsulated in the theory of baroclinic
instability (Hoskins and Valdes 1990). A suitable measure
of the baroclinicity is provided by the eddy growth rate
maximum:
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rBI ¼ 0:31f oV ozN 1 ;
ð3Þ
where f is the Coriolis parameter, V is the time-mean
horizontal wind fields, and N is the Brunt-Väisälä frequency. Lindzen and Farrell (1980) have shown that this
formula provides an accurate estimate of the growth rate
maximum in a range of baroclinic instability problems. In
this study, we have calculated the eddy growth rate maximum and try to link it with the storm track change.
3 Performance of the models in simulating NHSTs
The climatological distributions of the observed and simulated NHSTs during winter are shown in Fig. 2. In
the observation (Fig. 2a), the NHSTs are confined to the
midlatitude North Atlantic and North Pacific, and the
NAST with maximum center value exceeding 70 gpm is
obviously stronger than the NPST with maximum center
value exceeding 60 gpm. In comparison, the two models
have reasonably reproduced the primary features of NHSTs
in the location and in the track orientation (Fig. 2b, c). In
particular, the HadAM3 model gives the most realistic
simulation of two storm tracks either in their locations or in
Fig. 2 Climatological distributions of the SD (gpm) of 2.5–6 days
bandpass-filtered geopotential height at 500 hPa over Northern
Hemisphere in winter for a the ERA-40 reanalysis data
(1961–1999), and the control runs of b GAMIL and c HadAM3
models. The axes of the NHSTs are indicated by the bold lines
Response of Northern Hemisphere storm tracks
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their intensities, although the simulated NAST is slightly
weaker than the observed by about 10 gpm (Fig. 2c). This
good agreement between ERA-40 and HadAM3 model
provides us with confidence that the HadAM3 model can
produce a good representation of the NHSTs. Relatively,
the GAMIL model has a large systematic bias in simulating
the NHST intensity (Fig. 2b). The winter NHSTs in GAMIL model (Fig. 2b) is obviously weaker than that in the
reanalysis roughly by 20 gpm, with maximum intensity
about 50 gpm over the North Pacific and North Atlantic.
Also, the axis of the NPST near the dateline shifts
northwestward.
The observed and simulated seasonal evolutions of the
NPST and NAST are compared in Fig. 3 in which the
storm track is illustrated as a month-latitude plot, averaged
over 140°E–140°W for the NPST, and over 80°W–20°W
for the NAST, respectively. In the reanalysis, the main
NPST axis is located near 50°N in September (Fig. 3a). It
moves southward slowly from October to December, and
stays steadily near 40°N in January and February. Then it
withdraws northward to 50°N in spring and finally back to
50°N in July. The NAST is characterized by a similar
seasonal northward March and a southward retreat in axis
(Fig. 3d). Both models reasonably capture the major features of the seasonal evolution of the axes of both storm
tracks (Fig. 3b, c, e, and f). For the seasonal evolution of
intensity, the NPST is significantly strong in late autumn
and early spring but noticeably weak in midwinter, indicating a substantial midwinter suppression phenomenon in
the NPST (Fig. 3a). This observed feature is successfully
simulated in the GAMIL model (Fig. 3b), but not obvious
in the HadAM3 model (Fig. 3c). Thus, in comparing the
models with the observation, the phenomenon of the
midwinter suppression of NPST is better simulated by
GAMIL than HadAM3. Differently, the NAST peaks during winter (December-January) in the observation (Fig. 3d)
as indicated in Chang and Zurita-Gotor (2007). Both
models have simulated a strongest center for the NAST
around winter during the seasonal cycle. However, the
strongest NAST happened 1 month earlier (November–
Fig. 3 Climatological latitude-month distributions of a–c the NPST
averaged between 140°E–140°W and d–f the NAST averaged
between 80°W–20°W at 500 hPa for a, d the ERA-40 reanalysis
data (1961–1999), and the control runs of b, e GAMIL and c,
f HadAM3 models. The storm track is measured with the SD (gpm) of
2.5–6 days bandpass-filtered geopotential height at 500 hPa. The
solid lines with cross marks represent the location of the NPST/NAST
centers in every month
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December) in the GAMIL model than in the observation,
while that 1 month delay in the HadAM3 model.
The eddy growth rate maximum, rBI, is a dynamics
measure of baroclinicity and storm track activities. The
growth rate is usually calculated in the lower level of the
atmosphere because the baroclinic development primarily
occurs in the lower troposphere (Lunkeit et al. 1998). In
Fig. 4a, the rBI over the North Pacific in the reanalysis
displays a single-peak structure. It reaches its maximum in
midwinter and the lowest value during midsummer. The
above seasonal evolvements of observed rBI are well
simulated in both models (Fig. 4b, c), except that the
simulated rBI in both models is stronger in midwinter but
slightly weaker in midsummer. Further analysis indicates
that the bias of simulated stronger rBI in midwinter is
dominated by the bias of simulated smaller Brunt-Väisälä
frequency (N) in both models, while the simulated vertical
wind shear (oV=oz) is close to that of the ERA-40 (figures
not shown). Over the North Atlantic, the HadAM3 model
exhibits a realistic seasonal evolution in the growth rate
(Fig. 4f); however, the GAMIL model simulated a stronger
eddy growth rate maximum, as shown in Fig. 4e. The
strong eddy growth rate maximum over the North Atlantic
in GAMIL model also arises from the weaker Brunt-Väisälä frequency rather than from the vertical shear, as over
the North Pacific.
We further examined the zonal wind speed at 250 hPa
over the midlatitude associated with the two storm tracks.
Over the North Pacific and North Atlantic, the jet stream
strongly influences the weather and climate locally as well
as in the downstream regions (Yang et al. 2002; Li and
Wang 2003; Jhun and Lee 2004; Ren et al. 2008). In
Fig. 5a, d, the zonal wind speed also shows a single-peak
structure, the same as the observed eddy growth rate
maximum. Both models reasonably capture the feature.
However, there is a slightly weaker speed bias over the
North Pacific (Fig. 5b) and a stronger speed bias over the
North Atlantic (Fig. 5e) in the GAMIL model.
Fig. 4 Climatological latitude-month distributions of the eddy
growth rate maximum (day-1) between 850 and 700 hPa over the
North Pacific a–c averaged between 120°E–180°E and over the North
Atlantic d–f averaged between 90°W–50°W for a, d the ERA-40
reanalysis data (1961–1999), and the control runs of b, e the GAMIL
and c, f HadAM3 models
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Response of Northern Hemisphere storm tracks
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Fig. 5 Same as in Fig. 4, but for the zonal wind speed (ms-1) at 250 hPa over the North Pacific a–c averaged between 110°E–180°E and over
the North Atlantic d–f averaged between 100°W–50°W
4 Response of the NHSTs to the IWP warming
The midlatitude climate variations are closely related to
two types of forcing: the external atmospheric forcing such
as SST anomalies and/or land surface process, and the
internal dynamic processes operating within the atmosphere itself such as synoptic scale transient eddy or
blocking in the mid-high latitudes (Hoskins and Pearce
1983). One of the aims of our study is to investigate the
response of the NHSTs and associated atmospheric circulation to the IWP warming. To identify the IWP warming
effect, we use the difference between two sensitivity
experiments (IOP minus ION) to indicate the response to
the IWP warming. In this section, the difference fields in
the NHSTs, rBI, and large-scale circulation are presented
as a major focus.
Previous analyses have suggested that the Northern
Hemisphere storm tracks in winter had undergone decadal
variations (Graham and Diaz 2001; Chang and Fu 2002;
Lee et al. 2012). Figure 6a presents the observed spatial
distributions of decadal difference of the standard deviation
of 2.5–6 days bandpass-filtered geopotential height during
winter between 1980–1999 and 1961–1979. The striking
feature as seen from the figure is that this decadal difference is characterized by a meridional dipole structure in the
transient eddy (TE) activity anomalies in the midlatitudes.
Over the North Pacific, the TE activity exhibits a large
enhancement south of the climatological NPST axis but a
slight decrease north of it. This character favors a decadal
southward shift of the NPST since the 1980s. However, an
opposite situation occurred for the NAST. Over the North
Atlantic, the TE activity exhibits an obvious enhancement
along and north of the climatological NAST axis but a
slight decrease south of it, favoring a decadal northward
shift of the NAST.
The simulated spatial distributions of difference of the
standard deviation of 2.5–6 days bandpass-filtered geopotential height during winter between IOP and ION experiments are shown in Fig. 6b, c. It is evident that both
models demonstrate significant response in the TE activities and storm tracks. Over the North Atlantic, since the
winter is the timing for the strongest storm track, the
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Fig. 6 Distributions of the differences of the SD (contours, gpm) of
2.5–6 days bandpass-filtered geopotential height at 500 hPa over
Northern Hemisphere in winter between 1980–1999 and 1961–1979
for a the ERA-40 reanalysis data, and between the IWP warming and
cooling experiments for b the GAMIL and c HadAM3 models,
respectively. Shaded regions are statistically significant at 10 % level
according to the student’s t test. The climatological axes of the
NHSTs are indicated by the bold lines
response of the wintertime storm track to the IWP warming
is the most significant, with an increased TE activity north
of the climatological NAST axis but a reduced TE activity
south of it for both models. Such kind of response is
especially large in the HadAM3 model (Fig. 6c), and
agrees well with the observed decadal change. Over the
North Pacific, despite the midwinter suppression of the
NPST, a significant response pattern of TE anomalies
similar to those over the North Atlantic is well simulated in
the HadAM3 model. However, the wintertime response
over the midlatitude North Pacific in the GAMIL model
appears to be rather weak, this may be because the simulated climatological NPST is weak and because there is a
midwinter suppression phenomenon uniquely in the NPST.
To further examine the effect of IWP warming on the
NHSTs, Fig. 7 presents the seasonal evolution of the
NHST response in comparison with the observed decadal
change. It is clearly seen in Fig. 7a, d that in the observation the NPST and NAST have opposite decadal change
trend throughout the major seasons when storm tracks are
climatologically prominent as shown in Fig. 3, that is,
since the 1980s, the NPST appears to shift southward while
the NAST shifts northward. In the simulations, the IWP
warming yields the NAST change with a northward shift in
both models (Fig. 7e, f) that is quite similar to the observed
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(Fig. 7d). The IWP warming also induces the NPST change
with a northward shift in both models (Fig. 7b, c). However, this result is completely opposite to the observed
(Fig. 7a).
The activeness of the storm track in the midlatitudes can
be closely related to the strong baroclinicity of the timemean flow (Chang 2001). To gain insight into the response
of the NHSTs in the two models, we further examine the
seasonal evolution of the eddy growth rate maximum (rBI)
change (Fig. 8) as a response to the IWP warming. For
comparison, the observed decadal change of this variable is
also shown in Fig. 8. It can be seen in Fig. 8a, d that the
observed eddy growth rate is intensified over most of the
North Pacific south of 40°N and weakened north of 40°N
after the 1980s (Fig. 8a), and intensified over the North
Atlantic north of nearly 40°N and weakened south of 40°N
(Fig. 8d). This decadal change in the eddy growth rate is
fundamentally consistent with the observed NHST change
shown in Fig. 7a, d. The simulated differences in rBI
between the IWP warming and cooling experiments are
overall consistent among models and among two sectors.
The eddy growth rate tends to be increased north of 40°N
and deceased south of it over either the North Atlantic
sector or the North Pacific sector. The effect of the IWP
warming on the eddy growth rate maximum is dynamically
associated with the response of both storm tracks and can
be determined by the large scale zonal wind response.
The value of rBI is mainly determined by the vertical
wind shear according to the formula (3). The change of rBI
over the North Pacific is highly related to the high-level
wind fields (Ren et al. 2008). The observed patterns of
decadal changes in 250 hPa zonal wind over the North
Pacific and the North Atlantic shown in Fig. 9a, d resemble
the corresponding rBI patterns shown in Fig. 8a, d,
respectively. Similar to the responses of the seasonal
evolution of rBI, the simulated responses of mean flow to
the IWP warming in two models are also quite consistent.
In the GAMIL model, the weakening of the westerly jet
stream is found over the Northwest Pacific south of 40°N
(Fig. 9b), which is congruent with the decreased response
of rBI (Fig. 8b). In the HadAM3 model, the strengthened
westerly jet stream is found over the North Pacific in the
region north of nearly 37°N and the reduced one equatorward of 35°N (Fig. 9c). The mean flow responses in the
HadAM3 model bear a good agreement with the corresponding changes of rBI in Fig. 8c. Similarly, the North
Atlantic responses of high-level wind fields in Fig. 9e, f are
also consistent with their corresponding changes of rBI for
the IWP warming.
The large scale zonal wind change is determined by the
geopotential height change that can be generated by direct
external forcing like the IWP warming (Hoskins and Pearce 1983) and/or by the mean flow-TE interaction (Ren
Response of Northern Hemisphere storm tracks
1065
Fig. 7 Latitude-month distributions of the differences (shaded, gpm)
of a–c the NPST averaged between 140°E–140°W and d–f the NAST
averaged between 80°W–20°W at 500 hPa between 1980–1999 and
1961–1979 for a, d the ERA-40 reanalysis data, and between the IWP
warming and cooling experiments for b, e the GAMIL and c,
f HadAM3 models. The solid lines with cross marks represent the
climatological location of the NPST/NAST centers in every month
et al. 2008; Xiang and Yang 2012). Figure 10 displays the
spatial distributions of the observed decadal difference
between 1980–1999 and 1961–1979 and the simulated
differences between the IWP warming and cooling experiments for the two models of the geopotential height at
250 hPa during winter. It can be seen in Fig. 10b, c that the
IWP warming excites a wavelike circum-global teleconnection pattern. Over the North Pacific, the IWP warming
gives rise to an anomalous positive geopotential height
north of about 35°N in the GAMIL model (Fig. 10b) and
over most of the North Pacific in the HadAM3 model
(Fig. 10c), thus a decreased westerly wind along 30°N.
This anomaly pattern is dramatically different from the
observed decadal change over the North Pacific (Fig. 10a)
where a significant negative geopotential height anomaly
occurred over Aleutian region with an increased westerly
wind along 30°N. On the other hand, the geopotential
height anomaly that is characterized by a dominant positive
North Atlantic Oscillation (NAO) phase concurrent with
the IWP warming emerges in two models, which is consistent with the observed. Such an anomaly pattern in the
geopotental height field favors an enhanced westerly wind
along 50°N as shown in Fig. 9d–f that is substantially
associated with the low-level baroclinicity (eddy growth
rate) change and eventually with the northward shift of the
NAST.
5 Summary and discussion
Two AGCMs (GAMIL/IAP and HadAM3/UKMO)
involved in a European Union DYNAMITE project were
integrated for 40 years with identical prescribed sea surface temperature. With the two models, three parallel
experiments were carried out in which a control run was
forced with seasonally-varying climatological SST and
two sensitivity runs were forced with seasonally-varying
climatological SSTs plus anomalous SSTs representing
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C. Chu et al.
Fig. 8 Same as in Fig. 7, but for the eddy growth rate maximum
(day-1) over North Pacific a–c averaged between 120°E–180°E and
over North Atlantic d–f averaged between 90°W–50°W. The solid
lines with cross marks represent the climatological location of the
eddy growth rate maximum centers in every month
observed Indian-western Pacific Ocean warming and
cooling, respectively. With the last 30 years output of the
control run, we firstly evaluate the performances of the two
AGCMs in simulating the climatological features of the
NHSTs. Then, with the last 30 years output of the sensitivity runs and the difference between the warming and
cooling experiments, we examine the effect of the observed
IWP warming on the NHSTs.
It is demonstrated that the GAMIL and HadAM3 models
are capable of reasonably simulating the major climatological features of the NHSTs and associated low-level
baroclinicity (indicated by the eddy growth rate maximum)
and high-level jet (zonal wind at 250 hPa) as well as their
seasonal evolutions in location and intensity such as the
midwinter suppression of the NPST and the largest intensity occurred in winter of the NAST. Overall, the HadAM3
model exhibits a better performance in capturing the climatological intensity of the NHSTs but a worse performance in reproducing the midwinter suppression
phenomenon in the NPST. The main discrepancy of the
GAMIL model is that the simulated intensity of the NHSTs
is weaker than the observed; however, this model exhibits a
successful performance in reproducing the midwinter
suppression of the NPST.
As inferred from ERA-40 reanalysis, the NHSTs experienced a significant decadal change around the end of the
1970s in which the NPST and NAST have opposite change
trend throughout the major seasons when storm tracks are
climatologically prominent. Since the 1980s, the NPST
appears to shift southward while the NAST shifts northward. However, the sensitivity experiments by both models
indicate that the observed Indian-western Pacific Ocean
warming tends to cause both the NPST and the NAST to
move northward.
The consistent effect of the IWP warming on the two
storm tracks of Northern Hemisphere is closely associated
with the changes in the low-level atmospheric baroclinicity
indicated by the eddy growth rate maximum, the high-level
jet stream (zonal wind at 250 hPa) and the upper-level
geopotential height. The IWP warming can excite a
wavelike circum-global teleconnection in the geopotential
height that gives rise to an anticyclonic anomaly over the
midlatitude North Pacific and a positive-phase NAO
anomaly over the North Atlantic. These geopotential height
123
Response of Northern Hemisphere storm tracks
1067
Fig. 9 Same as in Fig. 8, but for the zonal wind (shaded, ms-1) at 250 hPa
anomalies tend to enhance upper-level zonal westerly
winds north of the climatological jet axes and increase lowlevel baroclinicity and eddy growth rates, thus favoring
transient eddy more active north of the climatological
storm track axes. This is responsible for the northward shift
of the both storm tracks.
Obviously, the simulated NAST change is quite similar
to the observed decadal change, suggesting that the IWP
warming can be one of the key factors to cause decadal
northward shift of the NAST since the 1980s. However, the
simulated NPST change is completely opposite to the
observed. This implies that the observed southward shift of
the NPST since the 1980s would be primarily attributed to
other reasons, although the IWP warming can have a
cancelling effect against those reasons. One of those reasons would be the local decadal SST change associated
with the Pacific Decadal Oscillation (PDO), the strongest
signature on decadal-to-interdecadal time scales in the
midlatitude North Pacific air-sea interaction system
(Mantua et al. 1997). Around the end of the 1970s, the
North Pacific experienced a regime shift in which the
midlatitude North Pacific became cooled, and the Aleutian
Low and associated high-level jet moved southward, corresponding to a warm PDO phase (Zhu and Yang 2003;
Zhu et al. 2008a, b). This co-varying feature in both ocean
and atmosphere involves an unstable midlatitude air-sea
interaction (Fang et al. 2006; Fang and Yang 2011).
Whether or not PDO is one of the major reasons for the
observed southward shift of the NPST needs to be further
investigated. It is still an open question how the midlatitude
North Pacific cooling affects the NPST. It maybe largely
swamped by the strong internal variability of the atmosphere and by the oceanic front-related SST change in the
midlatitude North Pacific. The complexity needs to be
examined further and will be the focus of future research.
The other issue is that the models used here have considerable systematic biases in simulating the storm tracks,
especially for the GAMIL model. Lots of reasons would be
responsible for those biases. Previous studies have shown
that the adequate representation of the mean circulation and
then storm tracks is highly influenced by the physical
parameterizations, dynamical cores as well as resolution
used in the model (Mcguffie and Henderson-Sellers 2001;
Carril et al. 2002; Greeves et al. 2007). Other reasons are
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C. Chu et al.
DYNAMITE project partners for providing the model data. The ERA40 data are obtained from the ECMWF Data Server.
References
Fig. 10 Same as in Fig. 6, but for the geopotential height (contour,
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possibly related to how the model reflects the effect of
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storm tracks.
Acknowledgments This work was jointly supported by the National
Natural Science Foundation of China under Grants 41275068 and
40730953, the 973 program under Grant 2010CB428504, the National
Public Benefit Research Foundation of China under Grant
GYHY200806004, and the Jiangsu Natural Science Foundation under
Grant BK2008027. Special thanks are given to the two anonymous
reviewers for their insightful criticism and suggestions that led to
significant improvement of the manuscript. We thank the
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