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XU Han-Lie, FENG Juan, SUN Cheng. Impact of Preceding Summer North Atlantic Oscillation on Early Autumn Precipitation over Central China. Atmospheric and Oceanic Science Letters, 2013, 6(6): 417-422  
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Impact of Preceding Summer North Atlantic Oscillation on Early Autumn Precipitation over Central China
XU Han-Lie1, 2, FENG Juan1, SUN Cheng1
1 State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China
2 University of Chinese Academy of Sciences, Beijing 100049, China
Corresponding author: XU Han-Lie, xuhanlie@mail.iap.ac.cn
Abstract

This study examined the impact of the preceding boreal summer (June-August) North Atlantic Oscillation (NAO) on early autumn (September) rainfall over Central China (RCC). The results show that a significant positive correlation exists between the preceding summer NAO and the early autumn RCC on the interannual timescale. In order to understand the physical mechanism between them, the role of ocean was investigated. It was found that the strong summer NAO can induce a tripole sea surface temperature anomaly (SSTA) in the North Atlantic; this SSTA pattern can persist until early autumn. The diagnostic analysis showed that the tripole SSTA pattern excites a downstream Atlantic-Eurasian (AEA) teleconnection, which contributes to an increase in RCC. The circulation anomalies related to SSTA caused by the weak NAO are opposite, so the RCC is less than normal. The results imply that the preceding summer NAO may be regarded as a forecast factor for the early autumn RCC.

Keyword: North Atlantic Oscillation; early autumn rainfall; tripole sea surface temperature anomaly; Atlantic-Eurasian teleconnection; atmospheric-oceanic coupled bridge
1 Introduction

The North Atlantic Oscillation (NAO) is one of the most important patterns in the Northern Hemisphere, and it represents a large-scale seesaw in the atmospheric mass between the subtropical high and polar low ( Walker and Bliss, 1932; Bjerknes, 1964; Wallace and Gutzler, 1981; Barnston and Livezey, 1987; Li and Wang, 2003). Its unique characteristics and impact on weather, climate, and the ecological system have attracted a lot of attention from the meteorologists ( Hurrell, 1995; Osborn et al., 1999; Marshall et al., 2001b).

Some Chinese researchers also analyzed the impact of NAO on the climate in China ( Wu et al., 2009, 2012; Li et al., 2013; Xu et al., 2012). They found that interannual and interdecadal variations of the East Asian winter monsoon (EAWM) are closely related to the winter NAO activities ( Wu and Huang, 1999; Li and Li, 2000; Li et al., 2002; Li and Wang, 2003). Yang and Zhang (2008) further found that the summer NAO has a significant impact on the rainfall over Xinjiang district. Xu et al. (2012) pointed out that the winter NAO has an asymmetric impact on the rainfall in Southwest China. In addition to its significant impact on simultaneous climate, the NAO can also influence the climate in the next month or next season. Wu et al. (2009) found that the preceding spring NAO can induce a tripole sea surface temperature anomaly (SSTA) pattern in North Atlantic that persists into the ensuing summer and excites a downstream teleconnection, called the Atlantic-Eurasian (AEA) teleconnection ( Li et al., 2013). The AEA teleconnection can impact the East Asian summer monsoon (EASM) and precipitation over related area. Li et al. ( 2011a, b, 2013) used the theory of “atmospheric-oceanic coupled bridge” to explain the physical mechanism of the impact of preceding spring NAO on EASM and pointed out that the AEA teleconnection is the key linkage between North Atlantic SSTA pattern and East Asian climate. The “atmospheric-oceanic coupled bridge” theory ( Li et al., 2011a, b, 2013) plays an important role in clarifying the physical mechanism of the relationship between the NAO and EASM, moreover, this theory establishes the physics foundation to explain the relationship between the NAO and the climate anomaly in East Asia.

Although several researches were conducted to investigate the relationship between the NAO and the climate in China, most of them focused on the climate in winter and summer, and less attention was paid to the climate in autumn. China is a major agricultural country; autumn climate has an important impact on its agricultural production. Therefore, in this study, we focused on the relationship between the preceding summer NAO and the early autumn precipitation in China and, using the theory of “atmospheric-oceanic coupled bridge”, tried to explain the physical mechanism of this relationship.

2 Data resources

The main datasets employed in the study include the following: (1) monthly circulation data, gridded at a 2.5°× 2.5° resolution, taken from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) ( Kalnay et al., 1996); (2) monthly station precipitation dataset over China, collected from the Chinese Meteorology Administration; (3) monthly SST data, gridded at a resolution of 2.5°×2.5° and collected from the improved Extended Reconstructed SST Version 3 (ERSST V3) ( Smith et al., 2008); and (4) the NAO index (NAOI), defined as the difference in the normalized monthly sea level pressure (SLP) regionally zonal averaged over the North Atlantic sector from 80°W to 30°E between 35°N and 65°N ( Li and Wang, 2003), computed using the NCEP/NCAR reanalysis data (available at http://ljp.lasg.ac.cn/dct/page/65610). This NAOI is able to capture well large-scale circulation features of the NAO and is essentially a measure of the intensity of zonal winds across the central North Atlantic ( Kingston et al., 2006; Lee and Ouarda, 2010; Lee et al., 2012). All data used in this study spanned the period from 1951 to 2009. June, July, and August were considered as summer, and September as early autumn.

3 Results
3.1 Relationship between the preceding summer NAO and the early autumn rainfall

Figure 1a represents the correlation distribution between the preceding summer (June-August) NAOI and the autumn (September-November) rainfall over China. A significant correlation was observed over Central China. However, if the correlation coefficients are calculated using the preceding summer NAOI and the rainfall in September, October, and November separately, the significant positive correlation is observed only in September (Fig. 1b), but not in October and November (figures not shown). Therefore, it can be inferred that the significant positive correlation between the preceding summer NAOI and the autumn rainfall over Central China (RCC) is mainly contributed by the significant positive correlation between the preceding summer NAOI and the early autumn (September) RCC.

Correlation coefficients of 13 stations (in the area of 28-33°N, 108.5-117.5°E) were significant at the 0.05 level, as shown in Fig. 1b; therefore, the average rainfall of these 13 stations in September was used as an index of the early autumn RCC (RI) in this study. The correlation coefficient between the RI and the preceding summer NAOI was 0.46 for the period 1951-2009, which was statistically significant at the 0.01 level, based on a two-tailed Student’s t test.

Figure 1c displays the correlation pattern between the summer NAOI and the wind and relative humidity field in the early autumn in the low troposphere. When the summer NAO is strong, positive relative humidity anomalies are observed over Central China, together with the convergence of the southward cold air and northward warm air from the Northwest Pacific at the mid and low reaches of Yangtze River, which contributes to the formation and maintenance of a quasistationary front, leading to more rainfall in that area. Figure 1d shows the corresponding vertical motion and relative humidity within the scope of Central China. Positive relative humility anomalies are accompanied with ascending anomalies. Therefore, it is evident that a significant positive correlation exists between the preceding summer NAO and the RCC on the interannual timescale. When the NAO in the preceding summer is characterized by a positive (negative) anomaly, the RCC is more (less) than normal.

Figure 1 Correlation distribution between the preceding summer (June-August) NAOI (the North Atlantic Oscillation index) and (a) autumn (September-November) rainfall, (b) early autumn (September) rainfall, (c) wind field at 850 hPa (vector) and relative humidity at 700 hPa (shaded), and (d) zonal mean vertical movement (shaded) and relative humidity (contour) over Central China (110-115°E) for 1951-2009. The shaded areas indicate significance at the 0.1 and 0.05 levels, from light to dark. The contour interval is 0.1.

3.2 AEA teleconnection associated with the early autumn RCC

Figure 2 displays the anomalous global circulation structure accompanying RI. At the low level of troposphere, a significant feature over East Asia is that a positive anomalous subtropical high along with an anomalous anticyclone, which enhanced southwesterly winds on its northwest flank, prevails from South China to the middle and lower reaches of the Yangtze River and southern Japan (Fig. 2a). This anomalous pattern contributes to the transport of warm and wet air from the Northwest Pacific to Central and South China. To the northwest of the anomalous subtropical high, negative geopotential height anomalies, and an anomalous cyclone control North and Northwest China, implying that the strengthened cold air can reach the middle and lower reaches of the Yangtze River. A similar circulation feature can be observed at 500 hPa over East Asia (Fig. 2b).

From Fig. 2, it can be emphasized that a barotropic teleconnection pattern controls the mid and high latitudes from North Atlantic to the Okhotsk Sea, which is similar to the structure presented by Wu et al. (2009). This kind of structure is also consistent with the AEA teleconnection proposed by Li et al. (2013), but in early autumn. Two positive anomalies associated with the AEA tele connection exist in the Ural Mountains and the Sea of Okhotsk, with a negative anomaly within them. The cold air moves south downward along the trough between two blocking highs and meets the warm and wet air from the Northwest Pacific in middle reaches of the Yangtze River, leading to more RCC. Some previous studies have shown that the summer AEA teleconnection has important impact on EASM ( Wu et al., 2009; Li et al., 2011a, b, 2013). From the above analysis, it can be concluded that the AEA teleconnection also exists in early autumn, contributing to the rainfall variation over Central China.

Figure 2 Regression distributions of early autumn geopotential height (contours, units: gpm) and winds (arrows, units: m s-1) with an index of the early autumn rainfall over Central China (RCC) (RI) at (a) 850 hPa, (b) 500 hPa, and (c) 250 hPa, respectively. The shaded areas indicate geopotential heights statistically significant at the 0.1 level.

3.3 Physical mechanisms

It is not yet certain how the preceding summer NAO affects the early autumn RCC. Because the atmosphere (NAO) lacks the mechanisms to generate predictable variations, potential predictability of such fluctuations can arise only from coupled mechanisms that involve low boundary forcing such as SST ( Shukla, 1998). Here we used the theory of “atmospheric-oceanic coupled bridge” ( Li et al., 2011a, ( b, 2013) to explain the possible mechanism.

Figure 3 shows the SST correlation distribution between the RI and the summer NAOI from spring to autumn. From Figs. 3a-d, we can see that a tripole SSTA pattern in the North Atlantic from June to September corresponds to the RI. A similar set of tripole SSTA is significant in the correlation patterns corresponding to the preceding summer NAOI and persists from summer to autumn, and it is more significant in September (Fig. 3f). In mid and high latitudes, the main aspect of the air-sea interaction is that atmosphere impacts SST ( Frankignoul and Hasselmann, 1977; Frankignoul, 1985; Zhao and Li, 2010). Many studies found that the winter NAO pattern can force SST and trigger a tripole SSTA in the North Atlantic ( Cayan, 1992a, b; Deser and Blackmon, 1993; Deser and Timlin, 1997; Visbeck et al., 1998; Watanabe and Nitta, 1999; Seager et al., 2000; Marshall et al., 2001a). Wu et al. (2009) further found that the spring NAO can also induce a tripole SSTA in the North Atlantic, which has a close relationship with the EASM. As evident from Fig. 3, the tripole pattern, accompanied by the simultaneous NAO, is also observed in boreal summer. The tripole SSTA in North Atlantic Ocean may act as a linkage between the preceding summer NAO and the early autumn RCC. The preceding summer NAO can induce a tripole SSTA in summer, which persists into the early autumn. Furthermore, such a pattern has a close relationship with the early autumn RCC.

In order to investigate the relationship between the early autumn tripole SSTA in North Atlantic and RCC, an index of tripole SSTA (SSTI) is required. For this purpose, we defined three areas (red boxes in Fig. 3f)— N-area, C-area, and S-area—from north to south. The SSTI is defined as follows:

SSTI = SSTC-(SSTN+SSTS)/2.

The SSTC, SSTN, and SSTS are the September-normalized area-averaged SST in the three areas, respectively. This index can reflect the variation of the tripole SSTA in North Atlantic, and its variation is closely linked to the variability of the NAO, with a correlation coefficient of 0.43, at the 99% confidence level. The correlation coefficient between SSTI and RI is also significant, with a value of 0.33 at the 95% confidence level.

Previous studies have shown that the SSTA can impact the rainfall over some regions ( Feng and Li, 2011; Zhang et al., 2012). Does the tripole SSTA in early autumn can also impact the rainfall in Central China through the AEA teleconnection? Figure 4 illustrates the 250 hPa geopotential height in September associated with the SSTI. There is a significant teleconnection across the mid and high latitudes of North Atlantic and Eurasian continent, which is similar to the AEA teleconnection. This result shows that the early autumn tripole SSTA pattern in North Atlantic triggers an AEA teleconnection and impacts the RCC.

Figure 3 (a-d) Correlation distribution between North Atlantic SST from June to September and RI; shaded areas indicate correlation coefficients significant at 0.1, 0.05, and 0.02 levels, from light to dark. (e-h) The same as the preceding part, except for the SST from summer (June-Autumn) to autumn and the summer NAOI.

Figure 4 Regression distribution of early autumn geopotential height (contours, units: gpm) associated with an index of tripole SST anomaly (SSTI) at 250 hPa. The shaded areas indicate geopotential heights statistically significant at the 0.1 level.

4 Summary

This study investigated the relationship between the preceding summer NAO and the early autumn RCC, and using the theory of “atmospheric-oceanic coupled bridge”, attempted to explain the involved physical mechanism.

A significant positive correlation was observed between the summer NAO and the early autumn RCC. The results of this study suggest that summers with a strong (weak) NAO are often followed by more (less) RCC. A positive anomalous NAO in summer can induce a tripole SSTA pattern in the North Atlantic, which sustains from summer to early autumn due to the persistence of ocean memory effects. The tripole SSTA pattern in September can excite a downstream AEA teleconnection in subpolar Eurasian region, inducing two positive geopotential height anomalies in the west of the Ural Mountains and the area of Sea of Okhotsk and Yakutsk, along with a negative anomaly between them. The cold air moves south downward along the trough between two blocking highs and meets the warm and wet air from the North Pacific in middle reaches of the Yangtze River. This contributes to the formation and sustenance of a quasistationary front along the Yangtze River, leading to more RCC.

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