Summertime relationships between the Asian–Pacific Oscillation (APO) and climate anomalies over Asia, the North Pacific, and North America are examined on an interdecadal time scale. The values of APO were low from the 1880s to the mid-1910s and high from the 1920s to the 1940s. When the APO was higher, tropospheric temperatures were higher over Asia and lower over the Pacific and North America. From the low-APO decades to the high-APO decades, both upper-tropospheric highs and lower-tropospheric low pressure systems strengthened over South Asia and weakened over North America. As a result, anomalous southerly–southwesterly flow prevailed over the Asian monsoon region, meaning stronger moisture transport over Asia. On the contrary, the weakened upper-tropospheric high and lower-tropospheric low over North America caused anomalous sinking motion over the region. As a result, rainfall generally enhanced over the Asian monsoon regions and decreased over North America.
A number of studies have addressed the relationships in summertime atmospheric circulation between Asia and North America, mostly on intraseasonal-to-interannual time scales (e.g., Barnston and Livezey 1987; Nitta 1987; Lau 1992; Rodwell and Hoskins 2001; Wang et al. 2001; Lau and Weng 2002; Ding and Wang 2005; Zhang et al. 2005). El Niño–Southern Oscillation, the Arctic Oscillation, the Pacific decadal oscillation, and the Asia–Pacific–America teleconnection, among others, have been applied to explain the links of summertime rainfall between different regions in the Northern Hemisphere (Lau 1992; Ding and Wang 2005; Li et al. 2005; Zhang et al. 2005).
Recently, Zhao et al. (2007) addressed a summertime zonal teleconnection pattern of the tropospheric temperatures between Asia and the North Pacific and referred to it as the Asian–Pacific Oscillation (APO). The APO also measures the variability of a seesaw pattern of temperatures between the eastern and western hemispheres (Zhao et al. 2010). On an interannual time scale, it is closely linked to the variations of Asian monsoon rainfall, the South Asian high, the extratropical westerly jet stream over East Asia, the tropical easterly jet stream over southern Asia, the North Pacific subtropical high, and the upper-tropospheric ridge over North America. It is also closely linked to extratropical and tropical Pacific sea surface temperature (SST) anomalies. More recently, Liu et al. (2011) revealed a close relationship between APO and East Asian rainfall on an interdecadal time scale, providing a further insight on the long-term variability of the East Asian summer monsoon system.
In spite of the previous effort, several issues have not been well addressed. For example, the interdecadal variations of teleconnection patterns across the Asia–Pacific–America sector and their associations with temperature and precipitation anomalies have not been fully understood. It is known that a close relationship exists between the Pacific decadal oscillation and the interdecadal variability of North American rainfall (McCabe et al. 2004). It is also known that the extratropical North Pacific SST anomalies are linked to the APO (Zhao et al. 2010, 2011). Then, how are the climate anomalies over North America related to APO on an interdecadal time scale?
In this study, we use data for the past 100 years to examine the features of APO-related atmospheric circulation and rainfall anomalies over Asia, the North Pacific, and North America on an interdecadal time scale. We first describe the features of datasets in section 2, and then discuss the interdecadal variability of APO in section 3 and its relationship with climate anomalies in section 4. A summary of the results obtained is presented in section 5.
We apply the monthly mean data of the twentieth-century reanalysis V2 products, in a horizontal resolution of 2° × 2° latitude and longitude, for 1871–2008 (Compo et al. 2011), the global monthly SST from the Hadley Centre Sea Ice and Sea Surface Temperature, in a resolution of 1° × 1°, for 1871–2002 (Rayner et al. 2003), and the global land monthly precipitation from the Climatic Research Unit (CRU) analysis, in a resolution of 0.5° × 0.5°, for 1901–2002 (New et al. 2000). The monthly total rainfall at the Nanjing station of China [World Meteorological Organization (WMO) identification number 58238), which is at 32°03′N, 118°47′E and is 679 m above sea level, for 1905–2002, is also used to assess the robustness of results obtained from the CRU data. The monthly station data, archived at the National Meteorological Information Centre of China, have three missing records for 1938, 1939, and 1945, which are linearly interpolated in the analysis. The statistical significance of linear correlation and composite differences is assessed by the Student’s t test.
3. Interdecadal variability of APO
We first perform an area-weighting empirical orthogonal function (EOF) analysis of the anomalies of summer [May–September (MJJAS)] upper-tropospheric (300–200 hPa) temperature deviation for the Northern Hemisphere in 1871–2008. Here, is defined as , where T is air temperature, and is the zonal mean of T. The most dominant mode (EOF1) accounts for 25% of the total variance. As seen from Fig. 1a, EOF1 exhibits an out-of-phase variation between Asia and the extratropical North Pacific. Positive values appear mainly over Eurasia and North Africa, and negative values occur mainly over the extratropics of the North Pacific, North America, and the North Atlantic. Comparing the pattern in Fig. 1a with the APO-related features shown by Zhao et al. (2007) indicates that the structure of this Northern Hemispheric extratropical teleconnection is similar to the structure associated with APO. The time series of EOF1 in Fig. 1b is highly correlated with the APO index defined by Zhao et al. (2007) (R = 0.88 for 1871–2008). Following Zhao et al. (2010), we use standardized time series of EOF1 in Fig. 1b to measure APO.
Figure 1b shows an apparent interdecadal variation, but no linear trend, consistent with the insignificant trends of upper-tropospheric temperature over the extratropics of Eurasia, the Pacific, and North America (figures not shown). Low-APO values were observed from the 1880s to the mid-1910s and from the late 1970s to the 1990s, with the lowest values in 1901–1910 (Fig. 1b). On the contrary, high-APO values were found from the 1920s to the 1940s and from the late 1950s to the early 1970s, with the highest values in the 1930s.
The APO is highly correlated to the Pacific SST (Zhao et al. 2010), and the strong APO–SST relationship may explain the reliability of APO variability in the early period (also see Liu et al. 2011). The correlations between APO and SST are significantly positive (negative) over the extratropical North Pacific (tropical eastern Pacific) in 1871–1950. The result is consistent with the feature for 1958–2001 (Zhao et al. 2010), and the stable APO–SST relationship implies reliable APO variability in the early period.
Clearly, the APO index exhibited a large variation prior to 1950. Indeed, large variability also occurred in the summer rainfall over Asia and North America (see section 4). To examine the anomalies of atmospheric circulation and rainfall associated with the interdecadal variability of APO, we select 1884–1913 as a low-APO period and 1921–1950 as a high-APO period for a comparative analysis.
4. Atmospheric circulation and rainfall anomalies associated with APO variability
Figure 2a shows that, corresponding to the high values of APO, significant positive anomalies of tropospheric occur over Asia, and significant negative anomalies appear over the Pacific and North America. In Fig. 2b, significant positive anomalies cover the midlatitudes of Asia and significant negative anomalies extend from the extratropical North Pacific to North America. Meanwhile, weak anomalies are found over the tropical Indian Ocean. These features are similar to the features found on interannual time scale for the recent decades (see Zhao et al. 2010). The above result suggests a warmer troposphere over Asia and a cooler troposphere over the extratropical North Pacific and North America, and thus a larger tropospheric temperature gradient between Asia and its adjacent oceans, in the high-APO decades.
Associated with the anomalies of tropospheric temperature are significant changes in the deviation of tropospheric geopotential height (). As seen from the difference in composite pattern along 35°N (Fig. 3a), significant positive and negative anomalies appear over Asia at the upper troposphere and the lower troposphere, respectively. A feature of opposite signs is observed over both the Pacific and North America, where significant negative and positive values are found at the upper layers and the lower layers, respectively. At 150 hPa, large-scale positive (negative) anomalies cover Asia (the North Pacific and North America) between 20° and 50°N (figures not shown). At 850 hPa (Fig. 3b), there are large-scale significant negative anomalies of over Asia south of 50°N, with a central value of −10 m in the South Asian monsoon region. On the other hand, large-scale significant positive anomalies are observed over the central-eastern North Pacific and North America, with a maximum value of 20 m.
In summer, a large-scale high pressure system (i.e., the South Asian high) prevails at the upper troposphere over tropical–subtropical Asia and a large-scale low pressure system appears at the lower troposphere with a center over northern India (figures not shown). Meanwhile, there are also a relatively weaker high pressure system (the Mexican high) at the upper troposphere and a low pressure system at the lower troposphere over North America. During the high-APO index decades, at the upper troposphere, the positive anomalies of over Asia and the negative anomalies over North America apparently indicate a strengthened South Asian high and a weakened Mexican high. Similarly, at the lower troposphere, the negative anomalies of over Asia and the positive anomalies over North America indicate a strengthened low system over Asia and a weakened low system over North America. That is, the monsoon trough over South Asia is stronger in the high-APO index decades.
It is seen from Fig. 3b that anomalous westerly winds prevail over South Asia and anomalous southerly or southwesterly winds prevail over southern China corresponding to the anomalous low over Asia. These anomalous southwesterly or southerly winds indicate stronger Asian monsoon circulation in the high-APO index decades. On the other hand, the East Asian summer monsoon circulation weakened pronouncedly from the 1960s to the 1990s (Wang 2001; Jiang and Wang 2005), corresponding to the decadal decrease of APO index (see Fig. 1b).
The regressed MJJAS rainfall against APO during 1901–2002 shows significant positive and negative anomalies of rainfall over North China and the Yangtze River Valley, respectively (figure not shown), consistent with the result for 1958–2001 (Zhao et al. 2007). This consistency implies a stable relationship between APO and the rainfall over eastern China during 1901–2002. On an interdecadal time scale, the pattern of regressed rainfall against the 9-yr running mean of APO exhibits significant positive anomalies over India, the Indo-China Peninsula, and southern China between 15° and 30°N (Fig. 4a). Particularly, a highly significant correlation of 0.64 is obtained between APO and the rainfall over South Asia (75°–100°E, 20°–30°N), indicating more monsoon rainfall over South Asia in the high-APO index decades. As strengthened summer monsoon circulation over South Asia is often accompanied by less summer rainfall over the Yangtze River basin of China (e.g., Wang 2001; Zhao et al. 2007), a significant negative correlation appears between APO and the Yangtze River basin rainfall (Fig. 4a). For example, a strong correlation of −0.71 is found between APO and the area-average rainfall over 115°–120°E, 29°–33°N. In addition, a large correlation coefficient of −0.65 is obtained between APO and the rainfall over Nanjing station in eastern China for 1905–2002 (Fig. 5a), further supporting the result from the analysis of the CRU data. These relationships between APO and Asian monsoon rainfall on an interdecadal time scale are similar to those found on interannual time scale (Zhao et al. 2007).
During the high-APO index decades, anomalous anticyclonic circulation appears over the high latitudes of Asia (Fig. 3b). Correspondingly, anomalous southeasterly or southwesterly winds are observed over the mid-high latitudes of East Asia, strengthening local moisture transport. Over West Asia, however, anomalous northwesterly or northeasterly winds occur, weakening moisture transport. Accordingly, there is generally more rainfall over the mid-high latitudes of East Asia (with exceptions in scattered areas) and less rainfall over West Asia (Fig. 4a). The correlation coefficient is 0.30 (significant at the 99% confidence level) between APO and the rainfall over the mid- to high latitudes of East Asia (95°–140°E, 40°–60°N) and −0.54 (significant at the 99.9% confidence level) between APO and the rainfall over West Asia (60°–75°E, 30°–60°N).
Over North America, anomalous sinking motion appears in the troposphere (figures not shown), associated with a weakened upper-tropospheric Mexican high and an anomalous anticyclonic circulation pattern underneath (Fig. 3). This change in vertical motion leads to a decrease in local rainfall, with a significant negative correlation of 0.40 (significant at the 99.9% confidence level) between APO and the rainfall over 130°–80°W, 30°–55°N for 1901–2002. Figure 4b reveals large-scale negative anomalies of rainfall over most of North America between 30° and 60°N in the high-APO index decades, and Fig. 5b exhibits an out-of-phase relationship between APO and the rainfall over North America on an interdecadal time scale. The low-APO values in the 1900s and the 1980s–1990s are accompanied by more rainfall, and the high-APO values in the 1920s–1930s are accompanied by less rainfall, with an APO–rainfall correlation coefficient of −0.67 for 1901–2002. The interannual variability of APO measured by the raw APO index minus its 9-yr running mean also has a significant correlation with the interannual variability of rainfall over North America (R = −0.29 for 1901–2002, significant at the 99% confidence level).
The summertime Asian–Pacific Oscillation (APO) index exhibits an interdecadal variation. Low-APO values occurred from the 1880s to the mid-1910s, and high-APO values appeared from the 1920s to the 1940s. The troposphere was warmer over Asia and cooler over the North Pacific and North America in the high-index decades than in the low-index decades. Associated with this variation of APO, significant signals were observed in the summertime tropospheric atmospheric circulation over the Asia–Pacific–America sector. From the low-index decades to the high-index decades, the upper-tropospheric South Asian high strengthened, with an intensified lower-tropospheric low pressure system over Asia. Anomalous southerly–southwesterly winds prevailed over the Asian monsoon region, leading to strong northward transport of moisture and enhanced rainfall over the Asian monsoon region. On the other hand, the upper-tropospheric Mexican high weakened, with a diminished lower-tropospheric low pressure system over North America. As a result, anomalous sinking motion and reduced rainfall were observed.
Moreover, the largest variability of APO over the past 100 years occurred prior to 1950, accompanied by the largest fluctuations in rainfall over Asia and North America. This relationship between APO and rainfall, which may be physically explained, may also support the reliability of the APO variability in the twentieth-century reanalysis for the early period prior to 1950.
Previous studies have shown that the interdecadal variability of summer rainfall and drought over North America is closely associated with the annual Atlantic multidecadal oscillation and the Pacific decadal oscillation (McCabe et al. 2004; Sutton and Hodson 2005). The current study further reveals a significant relationship between APO and the rainfall over North America, especially the possible contribution of APO to rainfall variation, on an interdecadal time scale. This analysis also suggests the importance of further investigations into the teleconnection patterns of atmospheric circulation over Asia, the North Pacific, and North America for improved understanding of the relationships between Asian and North American climate anomalies.
We thank the NOAA/Earth System Research Laboratory for providing the 20th Century Reanalysis V2 data products (online at http://www.esrl.noaa.gov/psd/data/20thC_Rean). Support for the Twentieth Century Reanalysis Project dataset is provided by the U.S. Department of Energy, Office of Science Innovative and Novel Computational Impact on Theory and Experiment (DOE INCITE) program, and Office of Biological and Environmental Research (BER), and by the National Oceanic and Atmospheric Administration/Climate Program Office. We also thank the Climatic Research Unit, University of East Anglia, United Kingdom, for providing the precipitation analysis dataset on the Internet. This work was jointly sponsored by the National Key Basic Research Project of China (2009CB421404) and the National Natural Science Foundation of China (40921003 and 40890053).