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Role of the Tibetan Plateau on the Annual Variation of Mean Atmospheric Circulationand Storm-Track Activity*
SUN-SEON LEE,1 JUNE-YI LEE,# KYUNG-JA HA,1 BIN WANG,# AKIO KITOH,@YOSHIYUKI KAJIKAWA,& AND MANABU ABE**#

1Division of Earth Environmental System, Pusan National University, Busan, South Korea
International Pacific Research Center, and Department of Meteorology, University of Hawaii, Honolulu, Hawaii@
Meteorological Research Institute, Tsukuba, Japan&
Advanced Institute for Computational Science, RIKEN, Kobe, Japan**
Arctic Environment Research Center, National Institute of Polar Research, Tokyo, Japan(Manuscript received 30 March 2012, in final form 18 January 2013)
ABSTRACTThis study reexamines how the Tibetan Plateau (TP) modulates the annual variation of atmospheric circulation and storm-track activity based on the Meteorological Research Institute’s atmosphere–ocean coupled model experiments with a progressive TP uplift from 0% to 100% of the present height. Three majorroles of the TP on atmospheric circulation and storm-track activity are identified. First, consistent witha previous finding, the TP tends to intensify the upper-level jet and enhance baroclinicity in the North Pacific
Ocean but significantly weaken storm-track activity over the TP, East Asia, and the western North Pacificduring the cold season. Second, the TP amplifies stationary waves that are closely linked to transient eddies. In
particular, the TP enhances the Siberian high and the Aleutian low, which together contribute to thestrengthening of the East Asian winter monsoon circulation and the weakening of storm-track activity. Third,
the TP significantly modulates the subseasonal variability of the Pacific storm-track (PST) activity. In particular, the TP tends to suppress PST activity during midwinter despite the fact that it strengthens baroclinicity
along the Pacific jet. The midwinter suppression of PST activity, which is well reproduced in a control run witha realistic TP, gradually disappears as the TP height decreases. Major factors for the midwinter suppression of
the PST associated with the TP include the 1) destructive effect of an excessively strong jet leading to an
inefficiency of barotropic energy conversion, 2) reduction of baroclinicity over the northern part of the TP,
and 3) subseasonally varying SST change and resulting moist static energy.

1. Introduction
Large topographic features such as the Tibetan Plateau(TP) and the Rocky Mountains exert significant impacts
on weather and global climate. The TP is one of the major
topographic features in the world. Many previous studies
examined the mechanical and thermodynamical effectsof the TP on the variability of global climates (Liu et al.
2012; Wu et al. 2012a,b). It is noted that the thermal* School of Ocean and Earth Science and Technology Publication Number 8824 and International Pacific Research Center
Publication Number 943.

Corresponding author address: Dr. June-Yi Lee, University of
Hawaii/International Pacific Research Center, POST Bldg., Room
403B, 1680 East West Road, Honolulu, HI 96822.
E-mail: juneyi@hawaii.edu
DOI: 10.1175/JCLI-D-12-00213.1
Ó 2013 American Meteorological Society

forcing by the TP on the Asian monsoon climate exhibitsa weakening trend (Liu et al. 2012). Unlike summer,
when the thermal forcing of the TP is dominant, themechanical effect of the TP becomes more important
than the thermal effect in winter. The retarding and deflecting effects of the TP as a large obstacle generate an
asymmetric dipole structure of wind with an anticyclone
to the north and a cyclone to the south of the TP (Wuet al. 2007). The northward deflection of the westerly flow
along the northern TP and the resulting enhancement of
the meridional temperature gradient contribute to a favorable condition for the growth of baroclinic eddies(Sauliere et al. 2012). Broccoli and Manabe (1992) notedthat large-amplitude stationary waves develop owing to
the presence of the TP and these waves influence themidlatitude climate.
In addition to such important factors as backgroundflow, land–sea contrast, and diabatic heating, topography

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also affects storm-track activity. Using a simple dry balance model, Lee and Mak (1996) examined the role ofNorthern Hemisphere (NH) orography in the maintenance of storm tracks in winter. They suggested thatzonal flow interacting with orography could reinforce
the localized baroclinicity in the western part of the
Pacific and the Atlantic Oceans. Son et al. (2009) discussed that a prominent topographic feature modifies
the downstream storm-track intensity and such an effect is highly dependent on the structure of the background flow. In regard to the subseasonal variability ofstorm-track activity, it was suggested that the midwintersuppression of the Pacific storm-track (PST) activity iscaused by a midwinter minimum Eady growth rate overmidlatitude Asia (Penny et al. 2010). Park et al. (2010)
showed that a decrease in storm-track activity is associated with a weakening of downstream eddy development
and a suppression of baroclinic energy conversion overeastern Eurasia in the presence of the central Asian
mountains.
While recent research has pointed out a potential
importance of stationary waves and the associated eddyseeding effect on North Pacific transient eddies (Pennyet al. 2010; Park et al. 2010), dynamical linkage between
stationary waves and transient eddies is still not wellunderstood. Effects of topography-forced stationary
waves on downstream transient eddy activity should be
examined further to better understand the storm-track
dynamics. In this study, the effects of orography on theseasonal mean, subseasonal variability, and the annual
cycle of the mean flow and transient eddy activity are
explored in depth based on a TP height control experiment with an atmosphere–ocean GCM (AOGCM). In
particular, the role of the TP on the midwinter suppression of the PST is focused.
Section 2 describes the model and the experimental
design. In section 3, changes in the time-mean flow by
TP-induced stationary waves during the cold season are
discussed. In particular, upper- and low-level circulations,surface pressure systems, and temperature changes arecompared. Section 4 is devoted to an examination of the
impact of TP on the subseasonal variability of stormtrack activity. Possible factors controlling the subseasonal variability of PST activity are discussed in
section 5. Specifically, jet intensity, barotropic energetics,
baroclinicity, and sea surface temperature (SST) effects
are investigated. A summary and discussion follow insection 6.

2. Model and experiment
A numerical model used in this study is the Meteorological Research Institute (MRI) Coupled

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Atmosphere–Ocean General Circulation Model, version
2.3.2 (MRI-CGCM 2.3.2; Yukimoto et al. 2001, 2006).
The atmospheric component of the model has a T42
resolution (about 2.88 3 2.88) and 30 vertical levels withthe top at 0.4 hPa. The distribution of vegetation types isfixed. The oceanic component of the model is a Bryan–
Cox-type ocean GCM (OGCM). The OGCM has a
2.58 3 28 longitude–latitude resolution and 23 verticallayers. Near the equator, between 48S and 48N, the latitudinal grid spacing is set to 0.58 in order to maintaina good resolution for the equatorial oceanic waves. The
atmosphere–ocean coupling interval is 24 h and a flux
adjustment was performed at the interface.
To examine the role of the TP on the time-mean flow
and transient eddy activity, five different TP heights
were used: 0% (TP00), 25% (TP25), 50% (TP50), 75%(TP75), and 100% (TP100, control run). The TP100 experiment has a realistic TP height. The TP height is zeroin the TP00 experiment. In all model integrations, theland–sea configurations as well as the initial conditions
are the same. Except for the TP, a realistic orography
was used, which differs from Abe et al. (2003, 2004) andKitoh (2004, 2007); the latter studies show the effect of
global mountain changes using the MRI-CGCM, versions
1 and 2. In addition, MRI-CGCM 2.3.2 has more realistic
mean fields of SST, precipitation, and circulation withrealistic global mountains (not shown) than the older
versions used in previous studies. Thus, it is anticipatedthat the new version provides a more reliable estimationof the role of the TP on storm-track activity than the older
ones since the correction of inherent bias in the meanstate of a model is critical for improving simulations ofvariability (Lee et al. 2010). All runs are executed for 50
years and the last 35-yr data are used for the analysis in
this study. Figure 1 shows the TP height condition in fiveexperiments used in this study.

3. Changes in seasonal-mean structureFirst, the effect of topography in controlling the position and amplitude of the jet stream is examined since
storm-track activity is closely related to the backgroundflow. Figure 2 presents the 300-hPa zonal wind averaged
over the cold season [November–March (NDJFM)] fromthe National Centers for Environmental Prediction(NCEP)/U.S. Department of Energy (DOE) Atmospheric Model Intercomparison Project (AMIP)-II reanalysis (Reanalysis-2; Kanamitsu et al. 2002) and thefive experiments. The 500- and 850-hPa zonal winds
averaged over the latitude band 358–408N are also
given in Figs. 2g and 2h; the cores of the Pacific and theAtlantic jet streams are located in this latitude band.
As the TP height increases, with an exception of TP00,

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FIG. 1. (a)–(d) Maps of the topography for the TP simulations (m, MSL). (e),(f) Lat–height and lon–height, respectively. Here, TP100 indicates that the ratio of TP height in the experiment to that of the real TP is 1.0 (100%) andso forth.

the upper-tropospheric Pacific jet core intensity during
the cold season increases while the zonal wind over the
TP weakens. A similar feature is also found in Figs. 2g
and 2h. The TP00 experiment yields a result that isslightly inconsistent with other experiments; while thezonal wind over the TP is weaker, the Pacific jet core isstronger than TP25 (Figs. 2f–h). The Pacific jet coreduring the cold season in TP00 is stronger than its counterpart in TP25 because of a stronger jet in December andJanuary (not shown).
Compared with Reanalysis-2, the core of the Pacificjet is shifted slightly eastward to approximately 1608E
in all the experiments. The latitudinal location of the
Pacific jet core, however, is not affected by the TP. It isnoted that the southward shift of the Pacific jet as it
passes the TP becomes more pronounced with the increase of the TP elevation. Meanwhile, the Atlantic jet
exhibits no clear tendency with the TP height, implyingthat the Atlantic jet is not significantly influenced by the
TP. The southwest–northeast tilt of the Atlantic jet in
Reanalysis-2 is much less in all the experiments. Figures
2g and 2h indicate that the TP effect on the jet stream islarge to the west of the date line. In particular, the zonal
wind difference between TP75 and TP50 is outstanding
at approximately 1608E.
Since transient eddy activity is strongly linked withstationary waves (e.g., Branstator 1992; Chang 2009),
the characteristics of the stationary eddy component

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FIG. 2. The averaged 300-hPa zonal wind (m s21) during NDJFM obtained from (a) Reanalysis-2, (b) TP100,(c) TP75, (d) TP50, (e) TP25, and (f) TP00. The zonal wind intensities at (g) 500 and (h) 850 hPa are averaged overthe latitude band 358–408N.

(deviation from the zonal mean) in the surface pressure
system are examined in Reanalysis-2 and the TP experiments (Fig. 3). The dominant features in the surfacepressure system in winter are the Siberian high and the
Aleutian low, which together control the structure andvariability of the East Asian winter monsoon (EAWM)
system (Wang et al. 2010; Lee et al. 2013). Nakamura

et al. (2002) showed that the weakening of the Siberianhigh and the EAWM is related to the midwinter weakening of the PST. The Siberian high over the Asian continent and the Aleutian low over the North Pacific arefaithfully reproduced by MRI-CGCM 2.3.2 (Fig. 3b). It isnoted that both the Siberian high and the Aleutian low areenhanced with the height of the TP, while their center

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FIG. 3. The stationary eddy component (departure from the zonal mean; hPa) of SLP during NDJFM
obtained from the (a) Reanalysis-2, (b) TP100, (c) TP75, (d) TP50, (e) TP25, and (f) TP00 experiments.

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FIG. 4. The 300-hPa streamfunction of stationary waves (departure from the zonal mean; 106 m2 s21) averaged
over NDJFM derived from (a) Reanalysis-2 and (b) TP100. The differences of the 300-hPa streamfunction averaged
over NDJFM between (c) TP100 and TP75, (d) TP100 and TP50, (e) TP100 and TP25, and (f) TP100 and TP00. Thick
contours in (a),(b) correspond to 15 3 106 m2 s21 (solid contour) and 215 3 106 m2 s21 (dashed contour). Shading in(c)–(f) denotes a 95% confidence level.

positions remain nearly the same in all the experiments.
This is in accordance with Kitoh (2002), who showed thatboth the Siberian high and the Aleutian low are more
intense in a present-day orography experiment than ina no-mountain experiment. The similarity of the sea levelpressure (SLP) changes in the TP experiment and theglobal mountain experiment indicates that the TP alone
can exert a significant influence on the NH atmosphericcirculation by intensifying the Siberian high and the
Aleutian low. As in Fig. 2, the Aleutian low, downstreamof the TP, becomes slightly stronger in TP00 than in
TP25, while the magnitude of the Siberian high remains
the same in the two experiments.
Although orographic forcing generates weaker winterstationary waves than does the diabatic heating (Heldet al. 2002; Chang 2009), the former plays an importantrole in the development of transient eddies. To examine
the effect of the TP on the stationary waves, the 300-hPa
streamfunction in the NH is calculated (Fig. 4). The simulated stationary wave pattern in TP100 is comparable tothe observed counterpart except for a slight northeastward shift of a weaker negative core over the western
North Pacific (Figs. 4a and 4b). A comparison between
TP100 and other experiments reveals that substantial

differences are found over the TP region and the western North Pacific, which indicates that the TP plays
a significant role in amplifying stationary waves over the
TP and its downstream region, that is, the western North
Pacific. However, the amplitude change of the stationary waves with the TP height in January (not shown) andthe cold season, in general, in the MRI-CGCM is smaller
than that in Park et al. (2010, 2011) and Chang and Lin(2011). Difference in the physics and dynamics of themodels causes somewhat different sensitivities of stationary waves to the TP elevation. In addition, Fig. 4
suggests that the TP should not influence in any substantial way stationary waves over North America, contrary to the result of Held et al. (2002). This discrepancy
may also be related to the physical and dynamical differences of the models and indicate that our understanding of how orography affects stationary waves isstill insufficient.
We further examine the TP effect on the EAWM bycomparing circulation and temperature in the TP100
and TP25 experiments. Figures 5a and 5b show the lowlevel wind and surface temperature during December–
January (DJ) in TP100 and TP25, respectively. Whilethe northwesterly wind over the northeastern part of the5276

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vice versa. A negative temperature anomaly is dominant
not only over the TP, but also over East Asia in the
TP100-minus-TP25 difference as a result of the northerly
anomaly. Meanwhile, the warm temperature anomaly
over the Sea of Okhotsk seems related to a stronger
southeasterly or easterly wind associated with a deeper
Aleutian low over the North Pacific in TP100 relative toTP25 as shown in Fig. 3. The strengthening of the EAWM
and the Siberian high in the presence of the TP leads to an
intensification of the midwinter minimum of the PST.
This issue will be revisited in section 5.
The results presented in Figs. 6a and 6b also show aclear dependence of circulation over East Asia on theelevation of the TP. In the annual cycles of zonal andmeridional winds over the region (308–508N, 1058–
1458E), TP100-minus-TP25 difference is larger in winterthan in summer. This implies that the TP exerts stronger
influence on the East Asian circulation over the region(308–508N, 1058–1458E) during winter than summer in
MRI-CGCM 2.3.2. The zonal wind becomes stronger,
particularly in winter, when the TP height decreases,
indicating that a high-topographic feature weakens thedownstream zonal wind. In a realistic TP condition, there
is a stronger northerly wind in winter and a stronger
southerly wind in summer than in other simulations. The
seasonal variation of the meridional wind is clearly seen
even when the TP altitude is zero.

4. Changes in storm-track activity
FIG. 5. The 850-hPa wind [vector, reference vector below(c); m s21], 850-hPa northerly wind (blue dotted contour; interval
1 m s21), and surface temperature (shading; K) averaged over DJ inthe (a) TP100 and (b) TP25 experiments. (c) Difference in 850-hPa
wind (vector) and surface temperature (contour; interval 1 K) averaged over DJ between TP100 and TP25. Gray contours in (a),(b)indicate mountains and the contour levels are 1000 and 1500 m.
Shading in (c) denotes that surface temperature difference is significant at a 95% confidence level. Wind vectors significant at a 95%
confidence level are shown in (c).

Asian continent, a crucial component of the EAWM, isprominent in the TP100 experiment, the zonal flow
pattern dominates the circulation field over northeast
Asia including China, the Korean Peninsula, and Japanin the TP25 experiment. In particular, the northerly wind
(blue dotted contours in Fig. 5a) extends farther to thesouth of the Korean Peninsula and Japan in the TP100
experiment. The northerly flow in TP25, on the other
hand, is confined to the north of the Korean Peninsula
and the Sea of Okhotsk. The northerly anomaly, henceforth, is a distinct difference between TP100 and TP25
(Fig. 5c). The anomalous cold advection by stronger
northerly wind is responsible for a stronger EAWM anda. NDJFM-mean change
In this section, we examine changes in transient synoptic eddy activity by the presence of the TP during theentire cold season. In the present study, storm-track activity is defined in terms of the root-mean-square (RMS)
values of 2–8-day bandpass-filtered meridional wind at
300 hPa. Figure 7 shows the observed and simulated
storm-track activity during NDJFM. The coupled modelwith a realistic TP (Fig. 7b) is capable of simulating theobserved features of the climatological storm-track
activities, particularly over Asia and the Pacific. On
the other hand, the model tends to underestimate theAtlantic storm-track (AST) activity. In the observations,the PST is weaker than the AST during the entire coldseason (e.g., Lee et al. 2011, 2012b). Figures 7c–f showthat storm-track activity tends to weaken over the TP
area, East Asia, and the western North Pacific during thecold season as the TP height increases. The location of the
maximum PST activity is not affected by the TP height.

b. Subseasonal variation
In the annual cycle, PST activity exhibits double peaks
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FIG. 6. The annual cycle of 850-hPa (a) zonal and (b) meridional wind (m s21) over the region (308–508N, 1058–1458E).

a single peak in midwinter (e.g., Nakamura 1992; Leeet al. 2011). Since the TP effect on the jet stream andstorm-track activity is larger over the North Pacific thanover the North Atlantic, the subseasonal variability of
PST activity is focused (Fig. 8). The MRI-CGCM iscapable of reproducing the double peaks and the midwinter minimum in the annual cycle of PST activity witha realistic TP as shown in Fig. 8a, although the midwintersuppression of PST activity is slightly underestimatedcompared to Reanalysis-2. It is noted that the TP plays acrucial role on the subseasonal variation of PST activity.
The midwinter suppression of PST activity diminishes
as the TP height decreases and subsequently TP25 exhibits a single peak in midwinter. The midwinter suppression of PST activity, however, appears again in
TP00. This may be related to a stronger Pacific jet in
TP00 than in TP25 as shown in Fig. 2; an excessively
strong jet hinders baroclinic eddy growth (Nakamura

FIG. 7. The 300-hPa storm-track activity (m s21) averaged over NDJFM derived from (a) Reanalysis-2 and(b) TP100. The difference of the storm-track activity averaged over NDJFM between (c) TP100 and TP75, (d) TP100
and TP50, (e) TP100 and TP25, and (f) TP100 and TP00. Shading in (c)–(f) denotes that the values are significant at
a 95% confidence level.

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FIG. 8. The annual cycle of (a) 300-hPa storm-track activity (m s21) over (358–558N, 1408E–1408W) in Reanalysis-2
and the TP experiments. The time–latitude cross section of the 300-hPa storm-track activity (shading) and zonal wind(contour; m s21) are shown for (b) TP100 and (c) TP25. Storm-track activity was averaged over 1408E–1408W andzonal wind was averaged over 1208E–1708W in (b),(c).

1992). Interestingly, TP00 shows a stronger peak in
December.
The time–latitude cross section of the 300-hPa PST
activity and zonal wind further illustrates a distinct difference between TP100 and TP25 (Figs. 8b and 8c).
TP100 shows double peaks of PST activity along witha stronger Pacific jet than TP25. The peak in March isstronger than that in November. Observations also show
similar characteristics (Fig. 2a in Lee et al. 2011). Figure 9
further shows transitions of the PST and the jet from
November to March in TP100 and TP25. It is noted thatboth experiments simulate a southward shift of the Pacificjet and the PST during midwinter. Although the latitudinal movement of the jet from November to March issimilar in both the experiments, change in PST activity is
considerably different between the two.

5. Factors for the midwinter minimum of the PSTThis section describes how the TP affects the midwinter minimum PST activity by comparing TP100 with

TP25. We identify three major factors for the midwinterPST minimum induced by the TP.

a. Changes in the jet, eddy structure, and local
energetics
It is found that the TP modulation of the seasonalvariation of the Pacific jet along with eddy structure andlocal energetics contribute to the midwinter suppression. Figure 9 indicates that the TP tends to intensify the
Pacific jet in January resulting in a weakening of PST
activity, despite a strong baroclinicity over the North
Pacific. Similar to the hindering effect of an excessively
strong jet in Nakamura (1992), storm-track activity
tends to intensify linearly as the Pacific jet increases up to
about 40 m s21 but weakens after that in the experiments(Fig. 10). When the jet intensity is stronger than 40 m s21,
a negative correlation of 20.36 is found between thejet speed and the PST activity in January in TP100,
and the negative relationship becomes either weak in
January (correlation 5 20.18) or positive in March(correlation 5 0.13) in TP25. Figure 10 also suggests

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FIG. 9. The 300-hPa storm-track activity (shading; m s21) and zonal wind (contour; m s21) in (left) TP100 and(right) TP25 for (a),(f) November; (b),(g) January; (c),(h) March; (d),(i) difference between January and November;and (e),(j) difference between March and January. Shading in (d),(e),(i),(j) signifies that the differences are significant at a 95% confidence level. The top color scale refers to shading in (a)–(c) and (f)–(h).

that a narrower range of the amplitude variation of the
Pacific jet in TP25 is more favorable for the PST intensification than a wider range in TP100.
The destructive effect of the excessively strong jet isrelated to an inefficient barotropic energy conversion
from the mean flow to eddies in TP100. Lee et al. (2011)
demonstrated that barotropic energy conversion associated with a shearing deformation of the backgroundflow is essential for a PST initiation. Although the jet isstronger in TP100 than in TP25, barotropic energy conversion from the mean flow to transient eddies (positive)
is much weaker, particularly during January (Fig. 11).
This indicates that the barotropic governor effect,

which is associated with meridional shear (James 1987),
is strong during midwinter in TP100. The strong barotropic governor effect may contribute to the inefficiency of barotropic energy conversion. On the other
hand, Chen et al. (2007) addressed that the barotropic
governor reduces eddy amplitude not by simply increasing the inefficiency of barotropic energy conversion, but by decreasing baroclinic production. Further
investigation is needed to better understand these dynamic processes. The inefficiency of barotropic energy
conversion is attributable to a change in the eddy structure in the presence of the TP. As the westerly jetstrengthens, eddy structure tends to become less efficient

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FIG. 10. Scatter diagram of the PST activity and the Pacific jet intensity during (left) January and (right) Marchin (a),(d) TP100, (b),(e) TP25, and (c),(f) TP00. The storm-track activity and the jet are averaged over (358–508N,
1408E–1408W) and (358–508N, 1208E–1708W), respectively.

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FIG. 11. The kinetic energy conversion (W m22) at 300 hPa related to the shearing deformation in (a)–(c) TP100 and(d)–(f) TP25 during (top) November, (middle) January, and (bottom) March in the NH.

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FIG. 12. The 700–850-hPa maximum Eady growth rate (day21) in (left) TP100 and (right) TP25: (a),(f) November;(b),(g) January; (c),(h) March; (d),(i) difference between January and November; and (e),(j) difference between
March and January. Shading in (d),(e),(i),(j) denotes that the differences are significant at a 95% confidence level.

in converting the mean flow energy into eddy energy(Chang 2001; Nakamura et al. 2002). In addition to an
inefficient barotropic conversion, a weak baroclinic generation is found during midwinter in TP100 (not shown).
It is also shown in the experiments that the eddy structure
in TP100 is less anisotropic with a smaller axial tilt overEast Asia and the western North Pacific than that in
TP25, leading to a weakening of PST activity in January(not shown).

b. Changes in baroclinicity over the northern partof TP
Baroclinicity is among the major factors for transient
eddy development. The Eady growth rate (Lindzen
and Farrell 1980) is used in this study as a measureof baroclinicity in the atmosphere. Since storm-track
activity is closely related to the low-level Eady growthrate (e.g., Hoskins and Valdes 1990; Nakamura andShimpo 2004; Penny et al. 2010; Lee et al. 2011), we
compare the 700–850-hPa maximum Eady growth ratein the TP100 and TP25 experiments. Figure 12 presents
a 700–850-hPa maximum Eady growth rate in November, January, and March in TP100 and TP25. Over thewestern North Pacific, the seasonal variation of baroclinicity is similar between the two experiments exhibiting a maximum in January. Differences in baroclinicity
between the two experiments, however, are notable over
the TP area. In TP100, the baroclinicity over the northernpart of the TP is minimum in January but maximum inMarch. On the other hand, TP25 shows a gradual increase15 JULY 2013

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of baroclinicity from November to March over the southern part of the TP.
The Eady growth rate is dependent on static stability
and vertical wind shear. Using reanalysis data, Pennyet al. (2010) suggested that the near-surface static stability over the Eurasian continent could exert significant
control on the midwinter suppression of PST activity.
Based on an atmospheric GCM result, however, Parket al. (2010) addressed that there are minimal changes ofnear-surface static stability with the TP height. Therefore,
we examine changes in static stability and vertical wind
shear with the TP height. The annual cycle of baroclinicity and its related components, that is, static stability
and vertical wind shear, in TP100 are compared withthose in TP25. Area averages over the region (408–608N,
808–1208E) where large differences are found, are used
for the comparison. Following Penny et al. (2010), theEady growth rate can be expressed in terms of static
stability and vertical wind shear as follows:
Ds Dl DN
’
,
2
s
l
N
where s indicates the maximum Eady growth rate, lrepresents vertical wind shear, and N is static stability.
An overbar denotes the annual mean and D implies
departure from the annual mean. The temporal evolution of static stability, vertical wind shear, and the Eadygrowth rate over the northern part of the TP is shown in
Fig. 13. There is a distinct seasonality of static stability
with a maximum in December or January in a realistic
TP height condition, suggesting a strong TP effect onstabilizing the atmosphere. Vertical wind shear showsa minimum in January, which is somewhat different
from the observational result in Penny et al. (2010). A
strong static stability and a weak wind shear in January
results in a minimum Eady growth rate, consequently
reducing baroclinicity over the northern part of the TP;
this leads to the midwinter suppression of the PST in
TP100. In the TP25 experiment, however, static stability
does not change much over the northern part of the TP
although a maximum is seen in midwinter. Vertical wind
shear also exhibits insignificant changes during the coldseason. As a result, only a slight decrease in the Eadygrowth rate is seen between November and January, andhence, baroclinicity cannot contribute in any substantial
way to the midwinter suppression of PST activity.

c. The effect of SST change
An examination of the effect of SST change, as a result of circulation change in the presence of the TP,on the subseasonal variability of storm-track activity,
differences in SST, low-level circulation, and moist static

FIG. 13. The annual cycle of near-surface Eady growth rates
along with contributions from static stability and wind shear averaged over the region (408–608N, 808–1208E) in the 700–850-hPa
layer in (a) TP100 and (b) TP25.

energy between TP100 and TP25 is shown in Fig. 14. It
appears that the SST change from the TP contributes to
the PST peak in March. Figure 14 indicates that thenortherly anomaly from the enhanced Siberian high andthe Aleutian low causes a negative SST anomaly over
the North Pacific including the coastal seas off the
Korean Peninsula and Japan, as discussed in Seager
et al. (2002). Kitoh (2002) addressed that an SST decrease to the east of Japan in the CGCM in the presentday orography run is much smaller than that in theatmosphere/mixed layer ocean coupled model, because
ocean circulation in the former tends to reduce SSTchange.
It is noted that the change in the easterly trade winds issignificant in the central and western equatorial Pacific in
the mountain uplift experiment using the MRI-CGCM(Kitoh 2007). With high mountains, warmer water appears in the western Pacific and eastern Indian Ocean

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FIG. 14. Difference in (left) SST (contour; K) and 850-hPa wind [vector, reference vector below (c); m s21], and(right) moist static energy (kJ kg21) between TP100 and TP25 in (top) November, (middle) January, and (bottom)
March. Shading in (a)–(c) indicates that the SST difference is significant at a 95% confidence level. Only vectors
significant at a 95% confidence level are shown in (a)–(c).

because of stronger easterly trade winds in the westernequatorial Pacific and southwesterly winds in the Indian
Ocean, respectively. In our experiment, the TP100-minusTP25 difference exhibits warm SST anomaly along withanomalous westerly winds in the Indian Ocean and thewestern equatorial Pacific. The anomalous westerly in the
Indian Ocean and the western equatorial Pacific is more
prominent in March than in November or January. Withan SST increase, particularly in March, moist static energy
increases significantly over the entire western Pacific(Fig. 14). The enhanced moist static energy may playsome role in strengthening the storm-track activity over
the North Pacific, given a weaker baroclinicity over theregion in March than in November or January.

6. Summary and discussion
This study investigated the role of the TP on modulating the seasonal mean and the annual variation of
atmospheric circulation and transient eddy activity byconducting a numerical experiment with various TP
heights using MRI-CGCM 2.3.2. Specific changes in thewinter-mean climate, storm-track activity, and its subseasonal variability by the TP are explored.

It is demonstrated that the annual variation of atmospheric circulation and storm-track activities are significantly modulated by the TP. The shape and intensity ofzonal wind meandering during the cold season in theupper troposphere over the TP and the Pacific are affected by the presence of the TP. Analysis of stationary
eddies reveals that the Siberian high and the Aleutian
low are enhanced with the TP, and subsequently strong
northwesterly winds along the eastern boundary of the
Asian continent strengthen the EAWM in TP100. In the
TP25 experiment, however, the circulation pattern overEast Asia is characterized by the dominance of zonal
wind and northerly wind restricted to the northeasternpart of the Asian continent and to the Sea of Okhotsk.
This indicates that the TP is responsible for the distinct
annual cycle of meridional wind over East Asia. Liu
and Yin (2002) showed that there was no significant
northerly wind in north China under the ‘‘no topography’’ condition and suggested that the influence of the
TP is more significant on the EAWM than the summer
monsoon.
It is noted that the MRI-CGCM is capable of reproducing the midwinter suppression of PST activity andthe double peaks in the annual cycle with a realistic TP.

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The midwinter suppression of PST activity becomes lesspronounced as the TP height decreases, and thus the
TP25 experiment simulates a single peak in the annual
cycle with a maximum storm-track activity in midwinter.
This implies that the TP plays an important role ingenerating the midwinter suppression of PST activity. In
this study, it is found that the TP tends to modulate theannual cycle of the PST in three different ways. First, theTP tends to modulate the seasonal variation of the Pacific jet, which contributes to the midwinter suppressionof PST activity as a barotropic governor. In TP100, thePacific jet in January is significantly intensified by theTP, and consequently results in a weakening of the PST
despite a strong baroclinicity over the Pacific. The destructive effect of an excessively fast jet as a result of the
TP is responsible for a structural change of eddies, whichlead to an inefficiency of barotropic energy conversion
from the mean flow to eddies. Second, the TP tends tomodulate the annual cycle of baroclinicity over thenorthern part of the TP, which contributes to the initiation of the annual cycle of transient eddies. A weakbaroclinicity over the northern part of the TP in January
in TP100 may lead to a weakening of storm-track activities particularly over the northern part of the TP and
East Asia. Third, the considerably warmer SST over the
Indian Ocean and the western Pacific with a stronger
westerly anomaly in TP100 than in TP25 may enhancemoist static energy and contribute to the intensificationof the PST in March.
Storm-track activity is linked to several factors suchas diabatic heating (Chang 2001) and teleconnection
patterns (Nie et al. 2008; Pinto et al. 2011), in addition
to the structure of background flow (Nakamura 1992;
Nakamura and Sampe 2002; Lee and Kim 2003). Therefore, more studies are needed to examine how the TP
alters the effect of these factors on the midwinter minimum of PST activity; such studies may help better understand the subseasonal variability of the storm-track
activities.
How the Siberian high, the EAWM, and storm-track
activities will be changed under anthropogenic global
warming (e.g., Wu et al. 2011; Lee and Wang 2013) is
an important question in a climate projection. An investigation under different external forcings, including
mountain uplift, should give us good insight into themechanism and historical/future change of the prominent NH winter climate system.
Acknowledgments. This work was supported by the
Global Research Laboratory (GRL) grant of the National Research Foundation (NRF) funded by the Korean
Government (Ministry of Education, Science and Technology; 2011-0021927). JYL and BW were supported by

the Japan Agency for Marine-Earth Science and Technology (JAMSTEC), NASA (NNX07AG53G), and NOAA
(NA17RJ1230) through their sponsorship of research
activities at the International Pacific Research Center(IPRC). The authors sincerely thank anonymous reviewers and Professor Kwang-Yul Kim for their valuable comments that helped to improve our manuscript.
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