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JOURNAL OF METEOROLOGICAL RESEARCH
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A New Circulation Index to Describe Variations in Winter
Temperature in Southwest China
SHEN Lelin1∗ (
Ï
í
), CHEN Longxun2 (
), JIN Qihua2 (
), and
ZHU Yanfeng
3
ý)
(
1 National Climate Center, Laboratory for Climate Studies, Beijing 100081
2 Chinese Academy of Meteorological Sciences, Beijing 100081
3 National Meteorological Information Center, Beijing 100081
(Received April 30, 2014; in final form September 28, 2014)
ABSTRACT
A new circulation index (ISW ) that can realistically describe winter temperature variations over Southwest
China is defined based on analysis of the NCEP/NCAR reanalysis data (version 1) and the observations at
585 stations in China. The study period is from January 1961 to February 2011. The relationship between
ISW and general circulation patterns in East Asia is also analyzed. Results show that ISW successfully
captures the variations in winter temperature over Southwest China. High ISW values correspond to the
intensified Mongolian high, the weakened Aleutian low, increases in the strength of the Middle East westerly
jet stream over the south of the Tibetan Plateau (TP), and decreases in the strength of the subtropical
westerly jet over the north of the TP. Meanwhile, the East Asian trough deepens and extends southwestward,
making it easier for the cold air mass from the north to intrude Southwest China along the trough. These
circulation patterns lead to a decrease in winter temperature over Southwest China (and vice versa). In
addition to the East Asian winter monsoon, the two westerly jets that dominate the upper level circulation
over East Asia also exert important influences on winter temperature in Southwest China, especially the
Middle East westerly jet to the south of the TP.
Key words: winter temperature, Southwest China, westerly jet, Tibetan Plateau (TP)
Citation: Shen Lelin, Chen Longxun, Jin Qihua, et al., 2015: A new circulation index to describe variations
in winter temperature in Southwest China. J. Meteor. Res., 29(2), 228–236, doi: 10.1007/s13351-015-4104-0.
1. Introduction
The worst blizzard in the past 50 years struck
South China in the winter of 2008, devastating large
areas of this region. Great concerns have been raised
about the mechanism for the anomalous winter climate and interannual climate variation, which poses a
challenging research topic for government-sponsored
researchers and meteorologists. The weather and
climate in monsoon regions are directly affected by
anomalous circulation patterns of East Asian winter
monsoon, which is the dominant weather system in
the Northern Hemisphere during the winter. Various
monsoon indices have been defined to investigate characteristics of the East Asian winter monsoon (EAWM)
and their impacts on regional weather and climate.
Most of these indices were defined based on certain
characteristics of the EAWM circulation, such as sea
level pressure (Guo, 1994; Shi, 1996; Gong et al.,
2001), the East Asian trough (Sun and Li, 1997), subtropical surface meridional wind (Ji and Sun, 1997;
Chen et al., 2000), the East Asian jet stream at 300
hPa (Jhun and Lee, 2004), and zonal wind speed (u)
(Wang and Jiang, 2004). Gao (2007) summarized four
EAWM indices that can reflect the basic characteristics of the EAWM circulation, and showed that all
the four indices can correctly describe anomalies of
the EAWM circulation. Specifically, in strong (weak)
EAWM years, both the Siberian high and the upperlevel sub-tropical westerly jet are stronger (weaker)
than normal, and the Aleutian low and the East Asian
trough are deeper (shallower) than normal. Such ano-
Supported by the National Natural Science Foundation of China (41375089).
author: [email protected].
©The Chinese Meteorological Society and Springer-Verlag Berlin Heidelberg 2015
∗ Corresponding
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SHEN Lelin, CHEN Longxun, JIN Qihua, et al.
malous conditions are favorable (unfavorable) for
stronger northwesterly winds and lower temperatures
over the subtropical region of East Asia.
Wang and Chen (2010) analyzed the definitions
of 18 existing EAWM strength indices and selected
six among them for further analysis. Their results
showed that all the indices could describe the changes
in the EAWM system components, but almost each
individual index demonstrated a weak performance in
describing the surface air temperature over large areas to the east of the Sichuan Basin, which might be
related to topographic effects. Chen and Sun (2001)
and Zhu (2008) also revealed the poor correlation between the EAWM index and surface air temperature
in Southwest China; they attributed this poor correlation to complex terrain and large local-scale variations in temperature in Southwest China. Jiang and Li
(2010) also indicated that the current EAWM indices
cannot well reflect the influence of cold air on the temperature changes in Southwest China. This might be
because climate in Southwest China is highly variable
as Southwest China covers a broad area of the Tibetan
Plateau (TP) as well as the vast area to the east of the
TP. Since Southwest China is a region with the most
abundant water resources in China, the anomaous climate in this area affects water resources not only in
local areas but also downstream areas (Zhou et al.,
2009). Consequently, research on temperature changes
in Southwest China is very important.
Several previous studies have investigated the
mechanisms of temperature changes in winter in
Southwest China, and found that there was a significant correlation between the Middle East jet stream
and surface air temperatures in Southwest China (Wen
et al., 2009; Ni et al., 2010a, b). Jiang and Li
(2011) studied the temperature variations by dividing
Southwest China into eastern and western sub-regions.
They found that the zonal migration of the western
Pacific subtropical high, accompanied by anomalous
vertical motion and regional winds, was closely related to the variability in surface air temperatures in
Southwest China. Anomalous snow cover on the TP
might also affect China’s winter climate (Chen et al.,
1996; Dong and Yu, 1997). Although great progress
229
has been achieved in studies of the EAWM index and
its relationship with temperature over East China, a
number of questions about winter temperature change
and variation in Southwest China are not addressed
yet. Therefore, in this study, we define a circulation
index, ISW , to reflect winter temperature in Southwest China, and analyze its relationship with the background circulation in East Asia.
2. Data and methods
The data used in this study are long-term, highquality observations of monthly mean surface air temperature at 585 meteorological stations, which are chosen from 740 meteorological stations in China. The
data are provided by the National Meteorological Information Center of the China Meteorological Administration. The data cover the period 1961–2011.
Datasets of the NCEP/NCAR reanalysis (version 1.0)
for the period from January 1961 to February 2012
are also used. The data have a horizontal resolution
of 2.5◦ ×2.5◦ , including wind field, geopotential height
at 500 hPa, and sea level pressure. In this paper, winter refers to the period from December of one specific
year to February of the next year. For example, the
winter of 1961 refers to the period from December 1961
to February 1962. In this study, we focus on the period 1961–2011 (winter); and the mean climate state
is also for the period 1961–2011.
Several methods, including the empirical orthogonal function (EOF) and correlation analyses, are applied in this study. A 9-yr Gaussian type filter is used
to retain the signals of 10 yr and longer timescales in
interdecadal variability analyses. The student t-test is
used to assess the statistical significance of the results
obtained. As shown in Fig. 1, the non-shaded areas in
the study domain (23◦ –39◦ N, 90◦ –105◦ E) is located
in Southwest China.
3. The circulation index (ISW ) representing the
winter temperature in Southwest China
3.1 The definition of ISW
Based on the existing EAWM indices, we are able
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to describe anomalous patterns of the EAWM circulations. We chose four indices and analyzed their relationship with winter temperatures in China, respectively (Fig. 1). The four indices include: the index of
low-level meridional wind in the east coast of Chinese
mainland (IJi ), the index of land-sea pressure (IGong ),
the index of the variation of the upper-level East Asian
jet stream (IJhun ), and the index of the variation of
winter temperatures in most of East China (IZhu ).
Figure 1 shows the simultaneous correlation coefficients between these four indices and winter temperatures in China at 585 stations. A common feature is that there are no significant relationships between each index and temperature changes in Southwest China. This area has a complex terrain, with a
maximum height above 4500 m and a minimum height
below 500 m, which is very different from the terrain
in East China.
Ding (1990) pointed out the three major paths
of cold waves that influence China. According to the
operational forecast experiences, the circulation sys-
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tems influencing winter monsoon outbreaks often exist in the middle troposphere. Accordingly, the simultaneous correlations between winter temperatures
over Southwest China and winter mean zonal wind
fields (10◦ –80◦ N, 40◦ –160◦ E) at 500 hPa during 1961–
2011 are calculated in this study (figure omitted). The
results show a significant “negative-positive-negative”
(“– + –”) correlation pattern corresponding to three
significant correlation regions in high latitudes (65◦ –
80◦ N), middle latitudes (39◦ –51◦ N), and low latitudes
(15◦ –29◦ N), respectively. The absolute values of correlations in middle and low latitudes are larger than 0.6,
implying that the zonal wind speed in these regions
is closely related to winter temperatures in Southwest
China.
According to the above analyses, we performed
EOF analyses on anomalies of 500-hPa winter zonal
winds in the East Asian region (10◦ –80◦ N, 40◦ –160◦ E)
and found that the first two modes account for 22.8%
and 19.2% of the total variation, respectively. From
Fig. 2, we can see that the first mode also shows a
Fig. 1. Concurrent correlation coefficients of four indices and China’s winter temperature during 1961–2011: (a) IJi ,
(b) IGong , (c) IJhun , and (d) IZhu . Correlation coefficients at or below the 0.05 significance level are shaded.
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SHEN Lelin, CHEN Longxun, JIN Qihua, et al.
clear “+ – +” pattern in high, middle, and low latitudes. Out-of-phase variation in zonal winds exists
between south of 35◦ N and north of 35◦ N in East Asia.
The correlation coefficient between the time series of
the first mode and winter temperatures in Southwest
China is –0.54, which passes the 0.001 significance test
and indicates a strong relationship in-between. Therefore, we chose the first mode from the EOF analyses
and discussed its relationship with winter temperatures in Southwest China.
According to the first mode’s characteristics, we
defined a circulation index to reflect winter temperatures over Southwest China. It is expressed as:
ISW = U 500(18◦ −29◦ N,
80◦ −120◦ E)
−U 500(39◦ −50◦ N,
80◦ −120◦ E) ,
(1)
where U 500(18◦ −29◦ N, 80◦ −120◦ E) represents 500-hPa
zonal wind averaged over the region (18◦ –29◦ N, 80◦ –
120◦ E), and U 500(39◦ −50◦ N,80◦ −120◦ E) represents that
over the region (39◦ –50◦ N, 80◦ –120◦ E; the boxed regions in Fig. 2). Figure 3 shows the temporal vari-
231
ation of the normalized index ISW during the period
1961–2011 with a solid-square line. There is a simultaneous negative correlation between ISW and winter
temperatures in Southwest China with the value of
–0.78, which passes the 0.001 significance test. Positive ISW values correspond to a stronger zonal wind
shear in the middle troposphere between low and middle latitudes in East Asia, a stronger zonal flow to
the south of 29◦ N, and lower winter temperatures in
Southwest China. Negative ISW values are accompanied by higher winter temperatures in Southwest
China. To compare the temporal variation in winter
temperature in Southwest China, Fig. 3 shows two
curves, and ISW is multiplied by –1.0. We can see
from Fig. 3 that ISW exhibits an increasing trend after the late 1960s, reaches a peak value in the 1980s,
and continuously decreases after the early 1990s. The
index decrease occurs twice, one in the early 1980s and
the other in the early 1990s, and the downward trend
is more significant in the latter period. The nine-point
running mean of winter temperatures in Southwest
Fig. 2. The spatial pattern of the first EOF mode for the winter mean 500-hPa zonal wind field (10◦ –80◦ N, 40◦ –160◦ E)
during 1961–2011.
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Fig. 3. A normalized time series of winter temperatures in Southwest China (the hollow-circle and dashed lines show
the long-term tendency of the nine-point running mean) and negative ISW (the solid-square and solid lines show the
long-term tendency of the nine-point running mean).
China demonstrates two obvious warming periods after the 1980s, and the interannual variability shown
by these two lines is well superposed.
3.2 Correlations between I SW and winter temperatures in China
Figure 4 displays simultaneous correlation coefficients of ISW values and winter temperatures at 585
stations over China. Apparently significant correlations occur over most areas of Southwest China (including the eastern part of the TP, Yunnan Province,
and some parts of the continent extending eastward
to 110◦ E) and north of Heilongjiang Province. Compared with Fig. 1, ISW in this section can be used
Fig. 4. Concurrent correlation coefficients of ISW and
winter temperatures at 585 stations in China (the shading
areas are significant at the 99.9% level).
to reflect the winter temperatures in Southwest China
and in several small areas of Heilongjiang Province
(these areas are not covered by the EAWM indices).
The simultaneous correlation coeffcients of ISW and
the four EAWM indices (IJi , IGong , IJhun , and IZhu )
mentioned above are –0.28, 0.15, 0.35, and 0.22, respectively. Only the correlation between ISW and
IJhun is significant. As a result, it is concluded that,
in addition to the East Asian winter monsoon, there
are other factors that have important effects on winter temperatures in Southwest China; these factors are
further discussed in the following sections.
4. Correlation between ISW and East Asian
atmospheric circulation
Previous studies have revealed several circulation
systems that have significant effects on winter climate
in East Asia, including the Siberian high (Gong and
Wang, 2002), the Aleutian low (Yang et al., 2005),
the 500-hPa trough (Chan and Li, 2004), and the subtropical westerly jet (Hanawa et al., 1988). In this
paper, the simultaneous correlations between ISW and
sea level pressure, the geopotential height at 500 hPa,
zonal winds at 200 hPa, and wind fields at 850 hPa
are analyzed, respectively (Fig. 5).
Figure 5a shows simultaneous correlation coefficients between ISW and winter mean sea level pres-
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SHEN Lelin, CHEN Longxun, JIN Qihua, et al.
233
Fig. 5. Concurrent correlation coefficients of ISW and (a) sea level pressure, (b) 500-hPa geopotential height, (c)
200-hPa zonal wind, and (d) 850-hPa wind, respectively (correlations at the 0.05 significance level are shaded).
sure. A positive high correlation center occurs in East
China, where the Mongolian high-pressure system is
located. Another positive high correlation center is located at the equatorial zone in the West Pacific. There
are two negative high correlation centers: one is located in northwestern Europe and the other is over
the Northwest Pacific (the active position of the Aleutian low). In high (low) ISW years, strong (weak) cold
Mongolian high pressures and weak (strong) Aleutian
low pressures lead to low (high) winter temperatures
in Southwest China. Similar results are found through
the analysis of composite anomalies for strong and
weak ISW values (figure omitted). It is inferred that
the Mongolian high and Aleutian low are highly correlated with winter temperatures in Southwest China.
Figure 5b shows simultaneous correlation coefficients between ISW and winter mean geopotential
heights at 500 hPa. The maximum positive correlation
appears in the north of Mongolia with the center over
Baikal, while the maximum negative correlation appears in the south of the TP with a zonally elongated
shape that extends to Northwest Pacific. The other
area of negative correlation is located in northwestern Europe. This correlation pattern implies that the
anomalous winter temperatures in Southwest China
can result in a meridional circulation that prevails at
500 hPa. The composite maps (figures omitted) of the
anomalies of the 500-hPa geopotential height fields in
high and low ISW years indicate that all of the anomalous patterns in high ISW years are clearly different
from those in low ISW years. High ISW years are characterized by a stronger anomalous trough in northwestern Europe, a stronger anomalous ridge in Baikal,
and a deeper East Asian trough that extends southwestward to reach northern Bay of Bengal. Southwest
China is controlled by northwesterly flow behind this
deepened trough, along which cold air mass can be
transported from high latitude, leaving this area to be
anomalously cold, and vice versa.
In the winter mean zonal wind field at 200 hPa,
there are two westerly jets in East Asia: one in the
vicinity of 20◦ N (relatively strong and stable) and the
other near 50◦ N (Zhang and Wang, 1987). Figure 5c
shows that ISW is positively correlated with the jet
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JOURNAL OF METEOROLOGICAL RESEARCH
stream near 20◦ N in the south of the TP and negatively correlated with the jet stream near 50◦ N in the
north of the TP. Some studies have shown that the
position and intensity of the Middle East jet stream,
which is the source of the jet in the vicinity of 20◦ N,
were closely related to winter temperatures in Southwest China (Wen et al., 2009; Ni et al., 2010a; Qu
et al., 2012). The correlation coefficient between the
index of the Middle East jet stream defined by Yang
et al. (2004) and ISW is 0.36, at the 0.05 significance
level. These results imply that the 200-hPa jet streams
in both the north and the south of the TP have effects
on winter temperatures over Southwest China.
Figure 5d shows simultaneous correlation coefficients between ISW and the wind field at 850 hPa.
Significant correlations occur in the region with an
anomalous anticyclone centering on Baikal, in the region of Siberia with anomalous westerly winds, and in
the West Pacific region with anomalous northeasterly
winds. Analyses of the composite anomalies of the
850-hPa wind field for high and low ISW years (figure omitted) reveal that high ISW often corresponds to
anomalous anticyclonic circulation near Baikal. Meanwhile, anomalous northerly winds emerge in the vicinity of Iceland, and anomalous northeasterly winds are
found in the area from eastern Baikal to Southwest
China, and throughout Northeast and North China.
Such circulation patterns cause cold winters in Southwest China, and vice versa. Significant correlations
also exist in the areas between south of the TP and
north of India. A previous study has revealed that the
trough of the southern TP was somewhat related to
temperatures in Southwest China (Wen et al., 2009).
Based on the above analyses, high ISW values are
characterized by a strong cold Mongolian high, a weak
Aleutian low, a strong Middle East jet stream in the
south of the TP, a weak subtropical jet stream in the
north of the TP, and a deepened East Asian trough.
The atmospheric circulations corresponding to weak
ISW years are opposite to those in strong ISW years.
5. Summary
In this study, we defined a circulation index that
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successfully describes winter temperatures over Southwest China.
(1) Based on EOF analyses on the anomalies of
500-hPa winter zonal winds in East Asian region (10◦ –
80◦ N, 40◦ –160◦ E), we defined a circulation index, ISW ,
which successfully reflects winter temperatures over
Southwest China. The correlation coefficient between
the index and surface temperature in Southwest China
can be up to –0.78. Interdecadal changes in ISW exhibit an increasing trend after the late 1960s, reach
its peak value in the 1980s, and decrease continuously
since the 1980s.
(2) ISW reveals that variations in winter temperatures in Southwest China are closely related to the
intensities of the Mongolian high and Aleutian low.
Positive ISW values correspond to strong Mongolian
high, weak Aleutian low, and low winter temperatures
over Southwest China, and vice versa.
(3) Two westerly jet streams at 200 hPa have
certain effects on winter temperatures in Southwest
China. In high ISW years, the Middle East jet stream
in the south of the TP is strong, whereas the subtropical jet stream in the north of the TP is weak at
the upper level. At the lower level, the anomalous
northerly wind emerges in the vicinity of Iceland, and
the anomalous northeasterly wind controls large areas
from eastern Baikal to Southwest China and throughout Northeast and North China. Such a circulation
pattern favors the transport of cold air mass from
Iceland to Chinese mainland. As the deepened East
Asian trough extends southward, cold air mass can
reach south of the TP, leading to low temperatures in
Southwest China, and vice versa. In addition to the
influence of the East Asian winter monsoon, the two
westerly jets over East Asia also exert important influences on changes in winter temperatures over Southwest China. Effects of the westerly jet to the south
of the TP are especially strong. Further study is required to determine how the two jet streams affect
winter temperature in detail and how they interact
with each other.
Acknowledgments. The authors would like to
thank Dr. Gao Hui and Dr. Chen Junming for their
helpful comments and suggestions.
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SHEN Lelin, CHEN Longxun, JIN Qihua, et al.
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