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STUDIES ON INTERDECADAL CLIMATE VARIATION IN CHINA *

LI Chongyin1,  HE Jinhai2  and  ZHU Jinghong3

1.  LASG, Institute of Atmospheric Physics, CAS, Beijing 100029, China

2.  Nanjing Institute of Meteorology, Nanjing 210044, China

3.  Department of Atmospheric Sciences, Peking University, Beijing 100871, China

ABSTRACT

Interdecadal climate variability is also paid more attentions by Chinese scientists, a lot of studies in relation to interdecadal variation have been completed in recent years. In this paper, an introduction in outline for interdecadal climate variation research is appeared. The content includes the feature of interdecadal climate variability in China, global warming and interdecadal temperature variability, the NAO/NPO and interdecadal climate variation in China, the interdecadal mode of SSTA in the North Pacific and its climate impact, and abrupt change feature of the climate.

I.  INTRODUCTION

In 1990s, the researches on interdecadal variation originally focused on the oceanic state, because the oceanic variability is thought a slower process and its interdecadal feature is more evident. Some studies have shown that the sea surface temperature (SST) variation in the North Atlantic Ocean has clear interdecadal character and it is related with the NAO (Bacon and Carter, 1993; Hurrell, 1995). In the Pacific Ocean, interdecadal variation of the ENSO has been studied (Wang, 1995; Qian et al., 1998) and the EOF analysis of SST in the North Pacific still showed an interdecadal variation feature. In the EOF analyses, the part of main EOF component, which is similar to the ENSO variation, was regarded as the representative of interannual variation (Tanimoto et al., 1993); the surplus part of main EOF components, which is similar to the ENSO mode, was regarded as the interdecadal variation and named “ENSO-like mode” (Zhang et al., 1997) or the Pacific Decadal Oscillation (PDO) (Muntua et al., 1997). The studies also indicated the existence of interdecadal variation in the North Pacific with other data analyses (Trenberth, 1994; Li and Liao, 1996; Li, 1998) and it was still clear in the thermocline variation (Zhang and Levitus, 1997). Naturally, the interdecadal variability of the North Pacific SST and its impact on the climate became important parts of the international CLIVER (A Study of Climate Variability and Predictability) program (WMO et al., 1995). The studies also indicated that the interdecadal climate variation is still shown in the atmospheric circulation variability (Li and Li, 1999; Li, 2000).

In fact interdecadal climate variation in China and its jump feature were studied early. The drought and flood variations in China, long-term variation of the summer rainfall and surface temperature in China had been studied and some interesting results were found (Wang and Zhao, 1979; Wang et al., 1981; Wang, 1990; Jiang et al., 1999). Some studies and major results in recent years will be shown in this paper in broad outline. Since the space is limited, there may be some omissions in this paper, which are hard to avoid.

 

II.  FEATURE OF INTERDECADAL CLIMATE VARIATION IN CHINA

Some studies have shown that there is a clear interdecadal variability of summer rainfall over eastern China during the second half of 20th century (Zhao, 1999; Wang, 2001). Power spectrum analyses for summer rainfall showed a significant peak at 26.7 a. A decreasing trend in precipitation variations has been found based on the observations since 1951, and it seems end in the 1980s. A weak increasing trend was observed in the 1990s, but their characteristics were different for different areas in China. It is shown that summer rainfall over the eastern China, especially over North China, is above normal during 1950s. It was a little bit more than normal over North of the Huaihe River and drought occurred along lower-middle reaches of Yangtze River Basin and South China during 1970s. The floods occurring along Yangtze River Basin and droughts were predominant in South and North China during 1980s. In 1990s, summer rainfall was above normal along the Yangtze River Basin and South China. At the same time, North China was still facing a prolonged drought period.

The characteristics of interdecadal variability of annual precipitation are similar to those of summer rainfall over the eastern China. There have been five drought spells since 1880. The first one was from the end of 19th century to the beginning of 20th century, then from the second half of 1920s to the beginning of 1930s, for the period of whole 1940s and 1960s, and from the end of 1970s to the beginning of 1980s. No linear trend occurred during the period from 1880 to 1999. Power spectrum analysis shows a significant peak around 30-year period. A 20-40 year periodicity is predominated for the whole series. The anomalies relative to the period of 1880-1999 for 6-flood and 5-drought spells are shown in Table 1.

 

Table 1.  The Precipitation Anomalies Averaged for 35 Stations Relative to the Period of 1880-1999 during 6-Flood and 5-Drought Periods (Wang, 2001)

No.

flood

drought

period

anomalies (mm)

period

anomalies (mm)

1

1881-1885

1888-1892

42.2

72.0

1899-1902

-109.6

2

1911-1915

1918-1922

94.5

65.2

1925-1929

-75.5

3

1931-1935

45.7

1942-1946

-19.9

4

1950-1954

90.0

1963-1968

65.1

5

1972-1976

62.2

1978-1982

-30.4

6

1990-1994

43.2

 

 

It is quite different of precipitation variation for eastern China and western China. There was no linear trend during 1880-1999 and the 20-40 year oscillation was predominated for the whole series of the eastern China. On the contrary, the increasing trend of precipitation in West China was very noticeable in the second half century, especially during the last 30 years. It also shows tremendous drought in 1920s-1930s in West China.

A decadal-centennial variability is studied based on rainfall coded levels data on 25 stations over eastern part of China for the period of 1470-1999. The power spectrum analyses of 530-year rainfall data demonstrate that 80 year oscillation exists in some areas of the eastern part of China and reaches 95% significant level (Zhu and Wang, 2001). The 80 year oscillation component of summer rainfall in North China even explains 27% variance in low frequency bands. This component of summer rainfall over North China, lower-middle reaches of Yangtze River valley and South China shows that the phase of this component over North China is precisely consistent with that over South China and out of phase to that along lower-middle reaches of Yangtze River valley.

The East Asian Summer Monsoon (EASM) is one of the important factors, which control summer rainfall over the east part of China. The rainfall in North China is also sensitive to the intensity of EASM. The subtropical high and ITCZ move usually to the north if the summer monsoon is strong and active, then the precipitation will above normal over North and South China. On the opposite, the subtropical high and ITCZ are located southerly when the summer monsoon is weaker, droughts will be found over North and South China, and the flood occurs along the middle and lower reaches of the Yangtze River. It is illustrated that 80 year oscillation of EASM related to this component of summer rainfall over the eastern part of China is very clear although the index of EASM is not long enough.

An examination of the correlations between the SSTA over the China off-sea area, the ridge line of subtropical high over the western Pacific, on one side, and on the another the equatorial eastern-central Pacific SSTA in the preceding winter and spring was completed (Li and He, 2000). The result showed that the relation between the EASM and SSTA in the equatorial eastern-central Pacific displays an evident interdecadal change, with the higher correlation after 1976 than that before 1976. Interesting to note that the negative correlation coefficient between the index of East-Asian summer meridional cell and zonal Walker cell index over the equatorial Pacific is greater than –0.77 in 1976-1993 with 99.9% significance level, much higher than –0.41 in 1958-1975, although there is a stable significant relation between the Walker cell and SSTA in the equatorial eastern-central Pacific. Therefore, there is a clear interdecadal variation of the relation between the EASM and SSTA in the equatorial eastern-central Pacific. After 1976 the warmer equatorial eastern-central Pacific led to a weaker zonal Walker cell (the weaker westerly wind in the upper troposphere and the weaker easterlies in the lower one), but didn't enhance the East-Asian summer meridional cell. The weaker summer monsoon and the shifting southward of the subtropical high enhanced the summer rainfall over the low-mid Yangtze River basin.

III.  GLOBAL WARMING AND INTERDECADAL TEMPERATURE VARIABILITY

The annual mean temperature anomaly series of ten regions in China are studied and obtained for the period of 1880 to 1999, which are determined relative to the normal of 1961-1990 (Wang et al., 1998). The temperature series of China are averaged over ten regions considering the regional weights (figure omitted). It is indicated that the warming in twentieth century started in 1920 and was interrupted in the 1950s and 1960s. Positive anomalies over China during 1920s and 1940s are noticeable. Temperature increased persistently since the end of 1960s. Linear trend for the period of 1880-1999 is 0.62°C/100a, a little greater than that of the globe (0.60°C/100a). 1998 was the warmest year in China since 1880. Studies of the relationship between temperature and precipitation indicated no consistent correlation.

On the basis of Multi-Taper spectral analysis (Jiang et al., 2001), the examination analysis of monthly mean temperature time series in the Northern Hemisphere and Southern Hemisphere from 1856 to 1998 showed that the warming trend played a dominant role in mean temperature variability in the Northern and Southern Hemispheres during the last 150 years. However, there is the significant interdacadal variation with the periods about 40 and 60-70 years, which superimposed on a linear warming trend for Northern Hemisphere mean temperature (Fig.1). This situation leads to diminish the linear warming rate with its significance and stability, as opposed to that in the Southern Hemisphere, especially in summer. Moreover, in comparison of surface temperature on the land to the sea, interdacadal variations detected in the latter are more remarkable than those in the former, as contrasted to the linear warming rate. Meanwhile, in terms of the GCM results from the HadCM2 model, preliminary analysis implied the interdacadal variation may be the inherent oscillation of the ocean and atmosphere system, but warming trends are not related natural variability.

 

 

 

Fig.1.  MTM spectral estimation of (a) Southern and (b) Northern  Hemisphere mean surface temperature time series (MTM spectrum based on original series (solid line), MTM spectrum based on time series with trend subtracted off (dotted lines) and the 95% (dot-dashed lines) and 99% (dashed lines) confidence limits based on robust red noise fit to spectrum).

 

IV.  NAO/NPO AND INTERDECADAL CLIMATE VARIATION IN CHINA

Recent years, some studies have indicated that the interdecadal variation of the NAO showed to be a rising trend (Hurrell, 1995; Jones, et al., 1997). Through data analyses, it is very evident that both the NAO index and NPO index all occurred variation suddenly in the 1960s, their common characteristics were represented by the abnormal rising of the amplitude, the amplitude after the 1960s was about 2-3 times as much as that before the 1960s (Li and Li, 1999). In order to expose further interannual variation feature of the NAO and NPO, the wavelet analyses, which have been used widely to study the features of different time-scale climate variations and their relationship between each other, were completed. The wavelet analyses for temporal variations of the NAO and NPO are respectively shown that interdecadal variations of the NAO and NPO are represented very clear (figure omitted). At first, the amplitudes of the NAO and NPO increased abnormally since 1960s. The second, it is very evident that the interannual variations with 3-4 year period were fundamental both for the NAO and for the NPO before the 1960s, but the decadal variations with 8-15 year period were fundamental since the 1960s. Therefore, both the NAO and NPO occurred anomalous variations, which were not only represented in the amplitude increasing but also in the changing of period for the dominant mode from 3-4 years to 8-15 years. In other words, the increasing of amplitude since the 1960s is not only represented in the NAO but also in the NPO. Therefore, this kind of interdecadal variation does not only exist in the North Atlantic region, it seems to be a global feature.

The climate jump in China in 1960s is indicated in some studies (Yamamoti, 1986; Yan, et al., 1990). It still showed clearly in the summer (June-August) precipitation anomaly (%) in Huabei region, there were mainly positive precipitation anomalies (on the more side) before 1964 but mainly negative precipitation anomalies (on the few side) from and after 1964, the averaged summer precipitation changed suddenly from the stage above normal to the stage below normal (Li, 1992). The surface air temperature anomaly in winter (December-February) in Sichuan was changed into the cold period since 1962, because there were mainly negative temperature anomalies after 1962, even though positive anomalies during the shorter time were in existence (Li and Li, 2000). Obviously, it is very evident that a climate jump occurred in the 1960s and the interdecadal climate variation in China was demarcated in the 1960s. These results can suggest that the interdecadal variations of the NAO and NPO are closely related to the climate jump in the 1960s. Although it is difficult to say that the climate jump in 1960s, or interdecadal climate variation in China, resulted from atmospheric circulation variation, particularly from the interdecadal variation of the NAO and NPO, because the mechanism of interdecadal climate variation has not been understood very well; At least, the above-mentioned analyses can suggest that the climate jump in the 1960s, or interdecadal climate variation in China, is closely related to interdecadal variation of the NAO and the NPO.

The influence of the NAO variation on the East-Asian monsoon and climate is studied preliminarily (Wu and Huang, 1999) and indicated that the Siberia cold high will be affected by the NAO variation at first, then anomalous Siberia cold high can affect the cold waves (winter monsoon) and the climate (including summer rainfall) in East Asia, because strong (weak) East-Asian winter monsoon is closely related to strong (weak) Siberia cold high and strong (weak) NAO index.

Based on the climate jump in the 1960s and its relationship to the anomalies of the NAO and NPO, it can be suggested that the atmospheric circulation anomaly, which is represented by the NAO and NPO, is also an possible important factor to cause interdecadal climate variation in China. Although we are aimed at interdecadal time-scale climate variation, the relationship between the circulation feature and the climate pattern is very clear and it may be same to the case in which the short-term climate or weather variation feature in any region is corresponding to a certain atmospheric circulation pattern. Therefore, the analysis study of the atmospheric circulation anomalies can be also regarded as an important way to understand interdecadal climate variation and the climate jump.

 

V.  INTERDECADAL MODE OF SSTA IN THE NORTH PACIFIC AND ITS IMPACT

In order to understand interdecadal variation of the SSTA in the North Pacific Ocean (it means the Pacific Ocean north of 10oS latitude in this study), the interdecadal mode of the North Pacific SST and its evolution feature are investigated further by using the Hadley Center data (1900-1997) but different from EOF analysis. The spectrum analyses of the SSTA in the North Pacific showed that two common main spectrum peaks can be found, one is about 7-10 years period and another is about 25-35 years period. The wavelet analysis results of the SSTA in the North Pacific are also shown that the 7-10 years and 25-35 years are two fundamental periods (Xian and Li, 2003). Therefore, the variations with 7-10 years and 25-35 years periods can be regarded as two major interdecadal modes although there are still other periods.

In order to show the pattern of two interdecadal modes of the SSTA variation in the North Pacific, the band-pass filtering of the SST in the North Pacific with 7-10 year filter and 25-35 year filter is respectively completed and the patterns of the two modes are obtained (Xian and Li, 2003). In Fig.2, the basic situations in positive phase and negative phase of the 7-10 years mode are shown. For the positive phase, there is positive SSTA in the area of 30-50oN latitudes and west of 140oW; but negative SSTA in the southern area of 30oN latitude and along the coast of North America. For the negative phase, there is positive SSTA in the southern area of 30oN latitude and along the coast of North America; but negative SSTA in the area of 30-50oN latitudes and west of 140oW. The pattern of the 25-35 years mode is similar with that of the 7-10 years mode, for the positive (negative) phase, there is positive (negative) SSTA in the area of 30-50oN latitudes and west of 140oW; but negative (positive) SSTA in the southern area of 30oN latitude and along the coast of North America (figure omitted). Although the pattern of 7-10 years mode is similar with that of 25-35 years mode, we do not want to compose them into one. They exist independently in the spectrum analysis results, so that it is unsuitable to compose them into one artificially.

 

 

Fig.2.  The patterns of 7-10 years mode of the North Pacific SST for positive phase (above) and negative phase (below).

The fundamental patterns of interdecadal mode of the North Pacific SSTA are different from the “ENSO-like mode”, although they also showed the signal is stronger in the mid-latitude than in the tropics. Because the basic character of the ENSO mode should be as follows: There is positive (negative) SSTA within the limited scope of the equatorial eastern Pacific and the maximum SSTA nearby the equator; There is a band-type negative (positive) SSTA in southwest-northeast direction from the equatorial western Pacific to the northeastern Pacific but positive (negative) SSTA in the northwestern Pacific. However, the pattern of interdecadal mode of the North Pacific SSTA has consistent symbol SSTA in the equatorial Pacific, the western Pacific and the northeastern Pacific, so that the feature of ENSO mode is not clear there.

For temporal evolution of 7-10 year mode or 25-35 years mode, the data analyses showed that positive (or negative) SSTA center will arise in proper order in the northeastern Pacific, the subtropical eastern Pacific, the subtropical western Pacific and go back to the northwestern Pacific (figure omitted). In other words, the 7-10 year mode and the 25-35 year mode of intrdecadal variation of the North Pacific SST have quite similar pattern and activity. The clockwise rotation along the Pacific Ocean basin can be regarded as one of the evolution features of interdecadal mode. Combining the oscillation feature of interdecadal mode discussed before, it can be suggested that appearing oscillation in northwest-southeast direction and the clockwise rotation along the Pacific Ocean basin are the common evolution feature of interdecadal mode of the North Pacific SSTA .

The composite analyses of sea level pressure (SLP) corresponding to 25-35 year mode of the North Pacific SSTA showed that during winter when the positive phase of the North Pacific SSTA 25-35 year mode appears, positive SLP anomalies are found north of 30°N in the Pacific region, with a maximum of 4 hPa located near the Aleutian Islands, indicating a weak Aleutian low in that period (Fig.3a). Positive anomalies are also found over the Siberian and the North Atlantic Ocean indicating a strengthened Siberian high and a weak Icelandic low; furthermore, negative anomalies with smaller amplitude appear on the North American continent, suggesting a weak North American high in that period. During winters when the negative phase of the 25-35 year mode is present, an opposite SLP anomaly pattern over the North Pacific emerges. There are negative anomalies over the North Pacific, centered at the Aleutian Islands with a maximum of 4 hPa, smaller negative anomalies over the North Atlantic, positive anomalies over most parts of the North American continent, and weak positive anomalies over the North of Eurasia Continent (Fig.3b). The SLP anomalies over the North Pacific are most directly affected by SSTA. Therefore when SSTA changes its polarity from positive to negative, the sign of SLP anomalies is also changed correspondingly. 

During winter when the positive (negative) phase of the North Pacific SSTA 7-10 year mode appears, the anomalous patterns of the SLP field are similar to that of 25-35 year mode positive (negative) phase. This means that corresponding to the positive or negative phase of 7-10 year mode or 25-35 year mode of the North Pacific SSTA, global sea level pressure field generally has identical responses. In other words, similar anomalous SLP patterns correspond to similar SSTA distribution in the North Pacific, which fully reveals the significant impacts of interdecadal SSTA modes (variation) in the North Pacific on the atmospheric circulation and climate.

The anomalous patterns of 500 hPa height and 1000 hPa wind field in wintertime corresponding to positive / negative phases for the 25-35 year mode and 7-10 year mode are analyzed. It can be shown that anomalous patterns of the SLP and 500 hPa fields are very similar each other, the anomalous wind field is coordinated to anomalous SLP field systematically.  Furthermore, comparing the spatial signatures of the response fields of the SLP and 500 hPa height in the extratropical North Pacific and tropical Pacific, we find a positive correlation (response) in the extratropical region and a negative correlation (response) in the tropical region, i.e., positive (negative) SLP anomaly in the extratropical region results from positive (negative) SSTA, but negative (positive) SLP anomaly in the tropical region results from positive (negative) SSTA .

 

Fig.3.  The SLP anomalies (hPa) in winter corresponding to the positive (a) and negative (b) phases of SSTA 25-35 year mode in the North Pacific.

The anomalous field of annual global land precipitation is analyzed corresponding to the two phases of each interdecadal mode, in order to understand the relationship between climate anomaly and the interdecadal variability of SST in the North Pacific. The results show clearly that the global precipitation pattern is closely related with interdecadal modes of the North Pacific SSTA. During year when the positive phase of SSTA 25-35 year mode in the North Pacific occurs, there is more precipitation in eastern and southeastern Asia, but less precipitation in southern of the North America. In Australia, there is more precipitation in the east, and less in the west. During years when the negative phase of SSTA 25-35 year mode in the North Pacific appears, there is less precipitation in eastern and southeastern Asia, but more precipitation in southern of the North America. In Australia there is less precipitation in the east, and more in the west (Fig.4).

A similar precipitation anomalous field is also found corresponding to the North Pacific SSTA 7-10 year mode. During year when the positive phase is present, there is more precipitation in eastern China; for the North America, there is less precipitation in the east and south, and more in the west. There is less precipitation in central and southern South America, more precipitation in eastern Australia and central Africa (figure omitted). During year when the negative phase appears, there is less precipitation in eastern China, more precipitation in eastern /southern of the North America, less in northwestern of the North America, and more in central and southern of the South America and less in central Africa.

 

Fig.4.  Annual precipitation anomalies corresponding to the positive (left) and negative (right) phases of the North Pacific SSTA 25-35 year mode. Top: East Asia; middle: North America; bottom: Australia (shadow represents positive anomaly).

 

VI.  CLIMATE ABRUPT CHANGE

 

The climate variation, particularly the long-term climate variation (change) usually shows an abrupt change feature. Some data analyses have indicated that there are evident climate (temperature and precipitation) abrupt changes in China during 1920s and during 1960s (Yan et al.,1990;. Yan, 1992; Ye and Yan, 1993).  A statistical test method named Mann-Kendall Rank is available to determine climate abrupt change. As an example, by using the Mann-Kendall test result of the drought index in eastern China during 1887-1986, it is shown that an abrupt change was at 1922, which represents a point of intersection between the curve C1 and C2 (figure omitted). This means that a climate abrupt change occurred at about 1922 in eastern China, from the relative moist period to relative drought period.

 

Using the Mann-Kendall Rank, the abrupt change of summer rainfall over the north China and the low-mid reaches of Yangtze River Basin is also found around the mid-1970s (Li and He, 2000). This abrupt change was related with the anomaly of East-Asian summer monsoon (EASM). The stronger EASM leads to more summer rainfall amount in the North China, as contrasted to the low-mid reaches of Yangtze River Basin before 1976. And the opposite situations were observed after 1976. The analysis still showed that the EASM anomaly is closely correlated to SSTA in the North Pacific, which affected the interannual variation of summer rainfall in the North China before the mid-1970s. After the mid-1970s, the EASM anomaly closely relates to SSTA in the equatorial eastern-central Pacific instead of the North Pacific SSTA and the impact of the equatorial eastern-central Pacific SSTA on summer rainfall over the low-mid Yangtze River Basin was enhanced. Furthermore, the analyses still showed that the air-sea temperature difference over the North Pacific displays a significant interdecadal change (Fig.5). Before the mid-1970s, there was greater air-sea temperature difference over the North Pacific and means the SSTA have stronger impact upon the atmosphere, the relation of the north Pacific SSTA with EASM circulation was enhanced. While after the mid-1970s, the air-sea temperature differences were lower and the effect of SSTA on the atmosphere is insignificant, there was the weaker relation between the North Pacific SSTA and the ESAM.

 

 

Fig.5. Time series of 9 point running mean air-sea temperature difference over the North Pacific

(a) SST-T925hPa (solid line); (b) SST-T1000hPa~850hPa (dashed line); (c) SST-T850hPa(dot-dashed line).

VII.  CONCLUSSION

(1) The climate (precipitation and temperature) variability in China shows interdecadal feature, 20-40 year, around 10 year and 60-80 year are the major periods. These interdecadal climate variations are closed with the SSTA in the Pacific.

(2) The warming trend plays a dominant role in mean temperature variability in the Northern and Southern Hemispheres during the last 150 years. However, the significant interdacadal variation was superimposed on a linear warming trend of mean temperature, particularly, in the Northern Hemisphere.

(3) The temporal variations of the NAO and NPO very clearly showed that the amplitudes of these two oscillations increased suddenly in the 1960s and their main period of interannual variations changed from 3-4 years to 8-15 years. These evident variations of two oscillations in the 1960s represented fundamental anomaly of the atmospheric circulation in the 1960s. The climate jump occurring in the 1960s was related to the anomalies of the NAO and NPO, it can be suggested that the atmospheric circulation anomaly, which is represented by the NAO and NPO, is also a possible important factor to cause interdecadal climate variation in China and the atmospheric circulation anomalies can be also regarded as an important way to understand interdecadal climate variation and the climate jump.

(4) Interdecadal variation of the North Pacific SST has two fundamental modes, i.e., the 7-10 year mode and the 25-35 year mode. These two interdecadal modes have similar pattern, particularly their fundamental patterns for positive phase and for negative phase. Whichever in positive phase or negative phase, the pattern of intrdecadal mode is different from the “ENSO-like mode”. From the analyses, it can be suggested that appearing oscillation in northwest-southeast direction and the clockwise rotation along the Pacific Ocean basin are the common evolution feature of interdecadal mode of the North Pacific SST.

(5) Corresponding to positive and negative phases of interdecadal mode of the SSTA in the North Pacific, the anomalous patterns of atmospheric circulation / climate are very different. This means the impact of interdecadal mode of SSTA in the North Pacific on atmospheric circulation / climate is very clear. Global SLP field has similar responses to same phases of the 25-35 year mode and the 7-10 year mode of SSTA in the North Pacific. During winter when the positive (negative) phase of the interdecadal modes in the North Pacific appears, there are positive SLP anomalies over the North Pacific and its ambient region, centered on the Aleutian Islands. Negative (positive) anomalies are found over the northern Eurasian Continent and the North Atlantic Ocean. Negative (positive) anomalies prevail over the North American Continent. Responses of SLP to the SSTA, are positive for the extratropical atmosphere and negative for the tropical atmosphere; a positive (negative) SSTA will lead to a positive (negative) SLP anomaly in the extratropical region, and a negative (positive) SLP anomaly in the tropical region. The 500 hPa height anomaly patterns are similar with the SLP anomaly patterns, which implies the response of the extratropical atmosphere to the interdecadal  modes of SSTA in the North Pacific exhibits a barotropical structure, but  the response of the tropical atmosphere shows a baroclinic structures. The global 1000 hPa wind field has an analogous response corresponding to the two interdecadal modes of SSTA in the North Pacific. The anomalous wind field has a systematic structure and is well coherent with the anomalous SLP field.

(6) The impact of the interdecadal mode of SSTA in the North Pacific on regional annual mean precipitation is not neglectable. During year when the positive (negative) phase of the interdecadal modes appears, some regions have less or more precipitation. For example, during positive (negative) phase period, there is more (less) precipitation over east China, less (more) over southern of the North America, and a band of increased precipitation in the east (west) of Australia.

(7) The climate variation, particularly the long-term climate variation (change) usually shows an abrupt change feature. The Mann-Kendall Rank analyses showed that in the last century 3 abrupt changes occurred respectively in 1920-1925, 1960-1965 and the mid-1970s.

 

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* This study is partly supported by the National Natural Science Foundation of China (Grant No.40233033) and Chinese Academy of Sciences (KZCX2-203 and ZKCX2-SW-210).


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