We thank Chang and Lin for their thoughtful and

constructive comments on our study (Park et al. 2010).

In Park et al. (2010), we did not explicitly state that the

topography-forced stationary waves are the direct cause

for the reduced downstream transient eddy kinetic energy

(EKE). The response of stationary waves to topography

may saturate even with a relatively small mountain (Cook

and Held 1992); furthermore, their magnitudes are much

smaller than thermally forced stationary waves (Chang

2009; Held et al. 2002). Instead, we suggest that quasistationary waves generated by the central Asian mountains may strongly affect North Pacific storminess by

changing the year-to-year variability of westerly winds

over the eastern Eurasian continent. Observational analyses

indicate that the midwinter suppression of North

Pacific storminess does not occur every year. Some years

experience stronger and more meridionally confined

zonal winds over the western North Pacific, leading to

stronger midwinter suppression (Harnik and Chang

2004; Nakamura and Sampe 2002)

Chiang, John C. H.

Park, Hyo-Seok

Son, Seok-Woo

English

Caltech Authors - Main

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HYO-SEOK PARK
Department of Environmental Science and Engineering, California Institute of Technology, Pasadena, California
JOHN C. H. CHIANG
Department of Geography, and Center for Atmospheric Sciences, University of California, Berkeley, Berkeley, California
SEOK-WOO SON
Department of Atmospheric and Ocean Sciences, McGill University, Montreal, Quebec, Canada
(Manuscript received 11 April 2011, in final form 9 August 2011)
We thank Chang and Lin for their thoughtful and
constructive comments on our study (Park et al. 2010).
In Park et al. (2010), we did not explicitly state that the
topography-forced stationary waves are the direct cause
for the reduced downstream transient eddy kinetic energy
(EKE). The response of stationary waves to topography
may saturate evenwith a relatively smallmountain (Cook
and Held 1992); furthermore, their magnitudes are much
smaller than thermally forced stationary waves (Chang
2009; Held et al. 2002). Instead, we suggest that quasi-
stationary waves generated by the central Asian moun-
tains may strongly affect North Pacific storminess by
changing the year-to-year variability of westerly winds
over the eastern Eurasian continent. Observational anal-
yses indicate that the midwinter suppression of North
Pacific storminess does not occur every year. Some years
experience stronger and more meridionally confined
zonal winds over the western North Pacific, leading to
stronger midwinter suppression (Harnik and Chang
2004; Nakamura and Sampe 2002).
In our atmospheric general circulationmodel (AGCM)
analyses, the interannual variability of westerly winds and
storminess over the North Pacific decrease substantially
in the absence of the central Asian mountains; a year
with strong midwinter suppression over the North Pacific
occurs rarely in this simulation. Moreover, it is still un-
clear why the presence of the central Asian mountains
strengthens the interannual variability of westerly jets and
storminess over the western North Pacific. We believe
understanding the cause of this strong interannual var-
iability is key to understanding the mechanisms for the
midwinter suppression. Indeed, fundamental questions
still remainwith regard to the dynamics of quasi-stationary
waves, such as howmountains affect the diabatic heating
field (Held et al. 2002) and the convergence of eddy mo-
mentum fluxes (Chang 2009).
We share the concern of Chang and Lin (2011) that the
AGCM integration time (18 yr in our study) may be in-
sufficient to accurately capture the quantitative response
of downstream storminess. Also, we agree with Chang
and Lin (2011) that a multimodel ensemble approach will
be required to better quantify the impact of the moun-
tains on downstream storminess. Along these lines, we
tested the robustness of our results using the global at-
mospheric model, version 2.1 (AM2.1), developed at
the Geophysical Fluid Dynamics Laboratory (GFDL;
Anderson et al. 2004). This version of AM2.1 uses a
finite-volume dynamical core (Lin 2004) with 2.58 3 2.08
horizontal resolution (M45) and 24 vertical levels (L24).
Seasonally varying insolation and climatological sea surface
temperatures (SSTs) are prescribed in themodel. The SSTs
are from 50 yr of monthly mean Reynolds reconstructed
historical SST analysis, spanning from 1950 to 2000 (Smith
et al. 1996). We ran the model for 60 yr, and the last 54 yr
are used for the analysis, tripling the integration period of
our previous work.Unlike the previous paper, for which an
8-day high-pass filter was used, we used a 10-day high-pass
filter to define transient eddies. This method slightly re-
duces uncertainty by increasing the spectral band by 2 days
and is widely used for defining synoptic-scale transients.
Overall results are virtually identical with those from the
8-day high-pass filteringmethod used in Park et al. (2010).
Corresponding author address: Hyo-Seok Park, California In-
stitute of Technology, MC 100-23, Pasadena, CA 91125.
E-mail: hyo@caltech.edu
2804 JOURNAL OF THE ATMOSPHER IC SC IENCES VOLUME 68
DOI: 10.1175/JAS-D-11-096.1
 2011 American Meteorological Society
We found that the sensitivity of downstream stormi-
ness to the presence of the central Asian mountains in
AM2.1 is slightly weaker than what we found in Commu-
nity Climate Model 3.10 (CCM3; Kiehl et al. 1998). In
CCM3, the removal of the Altai-Sayan Mountains and the
northern part of the Tibetan Plateau [i.e., the M50 exper-
iment in Park et al. (2010)] increased downstream stormi-
ness by 20%–30%. On the other hand, we had to remove
the entire centralAsianmountains (hereafter referred to as
the MN05 experiment) to get a comparable response of
downstream storminess in AM2.1. However, the response
of downstream storminess in AM2.1 is still substantially
stronger than what Chang and Lin (2011) suggest.
Figure 1a shows the difference in the 10-day high-pass
filtered EKE between the MN100 and MN05 experi-
ments (MN100minusMN05) duringmidwinter (from 15
December to 14February). TransientEKE is reducedover
a wide range of midlatitudes in the presence of the central
Asian mountains. In general, the magnitude of the EKE
reduction over the North Pacific is around 20%, which is
a little bit smaller than what Park et al. (2010) found, but it
reaches up to 30% in some areas sporadically. Consistent
with a transient EKE response, the standard deviation of
the 10-day high-pass filtered geopotential height
ﬃﬃﬃﬃﬃﬃﬃ
Z92
p
decreases over a wide range of midlatitudes (Fig. 1b). The
magnitude of the response is around 15%–25%, which can
substantially deepen the midwinter suppression signal.
Figure 2a shows the wintertime stationary waves, de-
fined by the 300-hPa eddy streamfunction, simulated by
AM2.1. The climatological mean amplitude of stationary
waves simulated by AM2.1 is about 10% weaker than
what CCM3 simulates. The anomalously strong station-
ary waves over the North Atlantic Ocean, appearing in
CCM3 (Park et al. 2010) and in an old version of the
GFDLmodel (Held et al. 2002), appearmuted inAM2.1.
In particular, strong positive and negative dipoles near
508N over North America, which appear in CCM3 (Park
et al. 2010) and in the previous version of the GFDL
model (Held et al. 2002), are substantially weakened.
Figure 2b shows stationary waves forced by the central
Asian mountains, calculated by the difference between
MN100 and MN05. Overall, the magnitude of the re-
sponse is smaller than what CCM3 simulated in Park
et al. (2010), but larger than what Chang and Lin (2011)
found.
As we mentioned earlier, midwinter suppression does
not occur every year. Thus, the climatologically aver-
aged impact of the central Asian mountains on down-
stream storminess can substantially vary depending on
the model used and integration period chosen. We plan
to further analyze our AM2.1 simulations to better un-
derstand why the central Asian mountains enhance the
interannual variability of North Pacific storminess in the
context of quasi-stationary waves.
Acknowledgments. We thank Yohai Kaspi for read-
ing this manuscript and providing constructive com-
ments.
FIG. 1. Anomalous (a) 10-day high-pass filtered transient EKE
(shading, m2 s22) at 300 hPa, calculated from the differences be-
tween MN100 and MN05 (MN100 2 MN05). The contour lines
indicate the climatological mean transient EKE. (b) As in (a), but
for geopotential height.
FIG. 2. (a) The 300-hPa eddy streamfunction for MN100.
(b) Anomalous eddy streamfunction calculated from the differences
betweenMN100 andMN05. The contour interval is 33 106 m2 s21.
NOVEMBER 2011 CORRES PONDENCE 2805
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2806 JOURNAL OF THE ATMOSPHER IC SC IENCES VOLUME 68


Comments on "The Role of the Central Asian Mountains on the Midwinter Suppression of North Pacific Storminess" - Reply

A ‘‘vertically Lagrangian’’ finite-volume dynamical core for global models.

Comments on ‘‘The role of the central Asian mountains on the midwinter suppression of North Pacific storminess.’’

Diabatic and orographic forcing of northern winter stationary waves and storm tracks.

Northern winter stationary waves: Theory and modeling.

Reconstruction of historical sea surface temperatures using empirical orthogonal functions.

The effects of variations in jet width on the growth of baroclinic waves: Implications for midwinter Pacific storm-track variability.

The National Center for Atmospheric Research Community Climate Model:

The new GFDL global atmosphere and land model AM2-LM2: Evaluation with prescribed SST simulations.

The role of the central Asian mountains on the midwinter suppression of North Pacific storminess.

The stationary response to largescale orography in a general circulation model and a linear model.

Trapping of synoptic-scale disturbances into the North-Pacific subtropical jet core in midwinter.Geophys.

http://authors.library.caltech.edu/28522/1/Park2011p16511J_Atmos_Sci.pdf

Comments on "The Role of the Central Asian Mountains on the Midwinter Suppression of North Pacific Storminess" - Reply

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