Response of Northern Hemisphere storm tracks to Indian-westernPacific 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

� 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 sea-

sonally-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 posi-

tive-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

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 com-

pletely 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 (Na-

kamura 1992). The so-called midwinter suppression of the

NPST is a striking phenomenon discussed in many previ-

ous 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,

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: xqyang@nju.edu.cn

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

123

Clim Dyn (2013) 40:1057–1070

DOI 10.1007/s00382-013-1687-y

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 sup-

pression can serve as a benchmark for evaluating the per-

formance of climate models (Christoph et al. 1997; Zhang

and Held 1999; Deng and Mak 2005; Chang and Zurita-

Gotor 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. Sig-

nificant 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 chan-

ges, 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-inter-

decadal changes of the NHSTs especially the NPST

remain unknown (Chang 2001). Higher sea surface tem-

peratures (SST) at mid and high latitudes may lead to an

intensification of extratropical cyclones. Through per-

forming zonal wavenumber-1 SST anomalies to a zonally

uniform background SST field experiment, Inatsu et al.

(2003) suggested that the strengthening of the SST gra-

dients may favor active extratropical cyclones. In addition

to midlatitude oceanic forcing, tropical ocean has funda-

mental 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 Nino-

Southern 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 Ocean-

western Pacific (IWP) relative to the period of 1950–1976

(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 hypoth-

esized that the North Atlantic response is mainly eddy-

driven 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 per-

formance 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 cli-

mate system’’ (DYNAMITE). Coordinated by the

DYNAMITE project, several Atmospheric General Circu-

lation Models (AGCMs) were forced by an identical ide-

alized SST pattern mimicking observed decadal changes

representative of the observational IWP warming and

cooling. These coordinated experiments have been per-

formed, 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.

1058 C. Chu et al.

123

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 hor-

izontal 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 con-

vective downdrafts (Gregory and Allen 1991). Both models

have been widely used in twentieth century climate simu-

lation (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-

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

Response of Northern Hemisphere storm tracks 1059

123

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 Climþ 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:

rBI ¼ 0:31f oV�oz

�� ��N�1; ð3Þ

where f is the Coriolis parameter, V is the time-mean

horizontal wind fields, and N is the Brunt-Vaisala fre-

quency. 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 max-

imum 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 sim-

ulated 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

1060 C. Chu et al.

123

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 GA-

MIL 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 fea-

tures 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, indi-

cating 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 dur-

ing 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

Response of Northern Hemisphere storm tracks 1061

123

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-Vaisala

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-Vai-

sala 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

1062 C. Chu et al.

123

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 atmo-

sphere 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 circu-

lation 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 differ-

ence 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 geopo-

tential height during winter between IOP and ION exper-

iments are shown in Fig. 6b, c. It is evident that both

models demonstrate significant response in the TE activi-

ties and storm tracks. Over the North Atlantic, since the

winter is the timing for the strongest storm track, the

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

Response of Northern Hemisphere storm tracks 1063

123

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 simu-

lated 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 obser-

vation 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

(Fig. 7d). The IWP warming also induces the NPST change

with a northward shift in both models (Fig. 7b, c). How-

ever, 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 time-

mean 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 equator-

ward of 35�N (Fig. 9c). The mean flow responses in the

HadAM3 model bear a good agreement with the corre-

sponding 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 Pe-

arce 1983) and/or by the mean flow-TE interaction (Ren

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

1064 C. Chu et al.

123

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 exper-

iments 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 telecon-

nection 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 con-

sistent 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 sur-

face 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

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

Response of Northern Hemisphere storm tracks 1065

123

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 sensi-

tivity 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 climato-

logical 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 inten-

sity occurred in winter of the NAST. Overall, the HadAM3

model exhibits a better performance in capturing the cli-

matological intensity of the NHSTs but a worse perfor-

mance 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 expe-

rienced 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 north-

ward. 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

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

1066 C. Chu et al.

123

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. 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 rea-

sons 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, cor-

responding 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 atmo-

sphere 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 con-

siderable 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

Fig. 9 Same as in Fig. 8, but for the zonal wind (shaded, ms-1) at 250 hPa

Response of Northern Hemisphere storm tracks 1067

123

possibly related to how the model reflects the effect of

land-sea contrast and topography over Asian-Pacific region

on the atmospheric circulation. For example, the accurate

representation of the thermal and dynamical influences of

the Tibetan Plateau on the circulation, energy, and water

cycles of the climate system is profoundly important not

only locally but also remotely over the North Pacific (Wu

et al. 2007; Son et al. 2009). The last reason might be

related to the realism of the decadal changes of storm track

in the reanalysis data (Bengtsson et al. 2004; Chang 2005,

2007). While changes in mean flow found in ERA-40

reanalysis (also NCEP-NCAR reanalysis) in the Northern

Hemisphere are generally considered to be quite reliable,

this does not mean that the storm track trend is also reli-

able. The storm track changes found in ERA-40 and

NCEP-NCAR reanalysis are not entirely consistent with

the mean flow trend over the Pacific (Chang 2007; Chang

and Fu 2003). All of those reasons can be referred in future

studies to improve the AGCM for a realistic simulation of

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

DYNAMITE project partners for providing the model data. The ERA-

40 data are obtained from the ECMWF Data Server.

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