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HAN Jin-Ping, LIU Ge, XIN Yu-Fei. A Dipole Pattern of Summer Precipitation over Mid-high Latitude Asia and Related Snow Cover Anomalies in the Preceding Spring. Atmospheric and Oceanic Science Letters, 2014, 7(4): 364-368  
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A Dipole Pattern of Summer Precipitation over Mid-high Latitude Asia and Related Snow Cover Anomalies in the Preceding Spring
HAN Jin-Ping, LIU Ge*, XIN Yu-Fei
Chinese Academy of Meteorological Sciences, Beijing 100081, China
Abstract

A dipole pattern of summer precipitation over the mid-high latitudes of Asia, which is characterized by opposing summer precipitation variations between the Mongolian and Northeast China (MNC) region and the West Siberian Plain (WSP), is found to be clear and stable on both interdecadal and interannual scales during 1981- 2011. Spring snow cover anomalies over a small region within the WSP and the Heilongjiang River (HR) region are closely related to the variation of this dipole mode during the subsequent summer, and they can therefore be considered as forecasting factors. Our statistical results imply a potential process explaining the relationship between the spring snow anomalies and the summer rainfall dipole. Corresponding to the snow anomalies, Rossby waves propagate along a path from the WSP region, via the Mongolian Plateau, to the Stanovoy Range during summer. At the same time, Rossby-wave energy divergences and convergences along this path maintain and reinforce an anomalous cyclone and anticyclone pairing over the Asian continent, which is significantly linked to opposite summer precipitation anomalies between the MNC and WSP regions. Numerical experiments are need-ed to further confirm the above conjecture and demonstrate the detailed physical mechanisms linking the spring snow cover anomalies and summer precipitation dipole.

Keyword: precipitation; snow cover; climatic anomaly; dipole

Received 11 February 2014; revised 7 April 2014; accepted 21 April 2014; published 16 July 2014

Citation: Han, J.-P., G. Liu, and Y.-F. Xin, 2014: A dip-ole pattern of summer precipitation over mid-high latitude Asia and related snow cover anomalies in the preceding spring, Atmos. Oceanic Sci. Lett., 7, 364-368, doi: 10.3878/j.issn.1674-2834.14.0012.

1 Introduction

Accompanied by corresponding variations in atmospheric circulation, precipitation over a region is often not isolated but closely related to that over another region, showing a type of pattern. Dipole-like variations in precipitation have been extensively observed in different areas all over the world.

One of the notable characteristics of cool-season precipitation variability in western North America is the nor-th-south dipole pattern on interannual-to-decadal timesca-les ( Dettinger et al., 1998). A north-south dipole structure in annual precipitation with opposing variations between the central United States and eastern Canada has also been identified, primarily at a decadal scale of 10-16 years ( Jutla et al., 2006). In northern Eurasia, summer precipitation between eastern and western Siberia shows an out-of- phase relationship, which constitutes an east-west dipole pattern ( Fukutomi et al., 2003, 2004). As the second leading mode, a dipole pattern of monsoon precipitation from northeast to southwest is also dominant over the Pan- Asian region ( Gao and Wang, 2012). For precipitation in China, several dipole patterns have also been revealed. For example, Han and Zhang (2009) indicated that a dipole pattern with precipitation decreasing (increasing) to the north (south) of the Yangtze River should be considered as a prominent mode in eastern China during the period 1958-2001. Tang and Lin (2007) found that a seesaw pattern of summer precipitation frequently appears over Northwest and North China.

Recently, Sun and Wang (2012) revealed a summer North Atlantic Oscillation (NAO) related dipole pattern in summer precipitation, with opposing variations between central East Asia and northern East Asia. In our research, we further study the variation of the dipole pattern on various time scales, especially on the interannual scale. The related atmospheric circulations and preceding factors in spring snow cover are also investigated.

2 Data and methods

The data used in the present study include: the Climate Prediction Center (CPC) Merged Analysis of Precipitation for the period 1981-2010 ( CMAP; Xie and Arkin, 1997); the monthly mean geopotential height and U- and V-wind data obtained from the National Centers for Environmental Prediction (NCEP) reanalysis dataset ( Kalnay et al., 1996); and the 20th century reanalysis monthly percentage of snow coverage at 192 × 94 Gaussian grids obtained from the National Oceanic and Atmospheric Administration-Cooperative Institute for Research in Environmental Sciences (NOAA-CIRES) Climate Diagnostics Center, Boulder, Colorado ( Compo et al., 2011).

A wave-activity flux defined by Takaya and Nakamura (1997, 2001) is used in the present study to diagnose the stationary wave propagation. For the detailed calculation of the wave activity flux, readers are referred to Takaya and Nakamura (1997, 2001) and Bueh and Nakamura (2007).

Other methods used include empirical orthogonal function (EOF), correlation, regression, and Lanczos filtering ( Duchon, 1979). Statistical significance is assessed using the Student's t-test.

3 Results
3.1 A dipole of summer precipitation over mid-high latitude Asia

For the period 1981-2011, an EOF analysis is performed to explore the dominant pattern of summer precipitation variation over the mid-high latitudes of Asia (Fig. 1a). The first EOF mode reflects a pattern with an opposing variation in normalized summer precipitation between the Mongolian and Northeast China (MNC) region (i.e., the region to the southeast of Lake Baikal) and the high-latitude region of Asia. There are two centers in the latter region: one is in the West Siberian Plain (WSP), and another in the Stanovoy Range. This result is consistent with the NAO-related dipole pattern in summer precipitation during 1979-2009 ( Sun and Wang, 2012), albeit with slightly different centers. Our result further demonstrates that the dipole revealed by Sun and Wang (2012) is a dominant pattern in summer precipitation over the mid-high latitudes of Asia.

The regionally averaged precipitation over the MNC region (40-52°N, 109-130°E) is defined as the PMNC index. The correlation between the summer PMNC index and summer precipitation field (Fig. 1b) further shows

Figure 1 (a) The first EOF mode (×0.01) of normalized summer precipitation over the mid-high latitudes of Asia, in which positive (negative) values greater (smaller) than 3 (-3) are shaded. (b) Correlation of summer precipitation field with the summer precipitation regionally averaged over the Mongolian and Northeast China (MNC) region (40-52°N, 109-130°E). (c) is the same as (b), except for the summer precipitation is regionally averaged over the West Siberian Plain (WSP) region (61-73°N, 67-100°E). In (b) and (c), the shaded areas denote correlation significant at the 95% confidence level. In (a), (b), and (c), black boxes indicate the MNC and WSP regions.

that the variation of summer precipitation over the MNC region primarily corresponds to the opposing variation over the WSP region. In turn, the summer precipitation over the WSP region (61-73°N, 67-100°E), which can be referred to as the PWSP index, is also negatively correlated with the summer precipitation over the MNC region. Namely, the summer precipitation over the MNC region and that over the WSP region comprise a dipole pattern over the mid-high latitudes of Asia.

The correlation coefficient between the summer PMNC and PWSP indices is -0.62, significant at the 99.9% confidence level (Fig. 2a). The high correlation may be partly due to their simultaneous interdecadal abrupt change around the late 1990s (Fig. 2a). After removing the variations with periods longer than 10 years by using Lanczos filtering, the correlation coefficient between the two interannual indices is still as high as -0.57, significant at the 99.8% confidence level. To summarize, the dipole pattern with opposite variations in summer precipitation between the MNC region and the WSP region is clear and steady on both the interannual and interdecadal scales.

The difference between the PMNC and PWSP indices (the former minus the latter) can be used to reflect the variation of the summer precipitation dipole (hereafter the difference is called the dipole index). Next, we investigate atmospheric circulations associated with the dipole.

3.2 Circulation anomalies associated with the dipole

The 200-hPa geopotential height anomalies regressed by the original dipole index (Fig. 3a) shows a negative- positive-negative wave-like pattern from Europe, via the WSP region, to the Mongolian Plateau. A similar anomalous pattern also appears at the middle and low levels of the troposphere (not shown), showing a quasi-barotropic structure.

Accompanying the negative geopotential height anom-alies, an anomalous low-level (850-hPa) cyclone dominates over the MNC region and induces anomalous ascent and eventually enhances precipitation in situ (Fig. 3a).

Figure 2 (a) Normalized time series of the summer precipitation regionally averaged over MNC region (solid line) and that over the West Siberian Plain (WSP) region (dash line) for 1981-2011; (b) is the same as (a), except it shows the time series after removing the variations with periods longer than 10 years by using Lanczos filtering.

Figure 3 (a) Summer 200-hPa geopotential height (gpm, contours) and 850-hPa wind (m s-1, arrows) anomalies regressed by the summer precipitation dipole, in which the shaded areas denote geopotential height anomalies significant at the 95% confidence level. (b) is the same as (a), except it shows the results regressed by the interannual variation of the summer precipitation dipole.

Corresponding to positive geopotential height anomalies, an anomalous low-level (850-hPa) anticyclone appears over the WSP region, which may reduce precipitation over there (Fig. 3a). The anomalous cyclone and anticyclone pairing over the Asian continent matches the summer precipitation anomalies, and should be considered as a crucial reason for resulting in the dipole.

The regression (Fig. 3b) by the interannual dipole index (obtained by Lanczos filtering) resembles Fig. 3a. This implies that similar circulation anomalies give rise to the summer precipitation dipole on the interannual scale.

3.3 Preceding snow cover anomalies associated with the dipole

Preceding factors in snow cover are explored to forecast the dipole pattern of summer precipitation over the mid-high latitudes of Asia. The correlation between the summer dipole index and snow cover during the preceding spring indicates several significant correlation areas appearing over Eurasia, labeled A-F in Fig. 4. Snow co-ver anomalies over the areas A-F during the preceding spring are all highly related to the original dipole index; however, not all of them are significantly correlated with the interannual variation of the dipole index. On the interannual scale, the spring snow cover anomalies regionally averaged over a small region (59-66°N, 77-99°E; i.e., region B) within the WSP and Northeast China, especially the Heilongjiang River (HR) region (47-56°N, 120-134°E; i.e., region F) maintain high correlations with the summer dipole index (0.48 and -0.66, respectively), and are even higher than those with the original dipole index (0.38 and -0.45, respectively). Furthermore, a step-wise regression indicates that the spring snow cover ano-malies over the small WSP and HR regions are the optimal factors for predicting the interannual variation of the summer dipole index.

In summary, the spring snow cover anomalies over the small WSP and HR regions (i.e., regions B and F) are

Figure 4 Correlation of preceding spring snow cover with the summer precipitation dipole index. The shaded areas denote correlation significant at the 95% confidence level. The significant correlation areas are labeled from A to F (see red boxes).

significantly related to the summer dipole index, both on the interdecadal and interannual scales. Thus, they can reflect well the variation of the summer dipole and be considered as important preceding factors.

To further reveal the effects of spring snow cover ano-malies over the small WSP and HR regions (i.e., regions B and F, respectively), their associated summer wave activity fluxes are analyzed. Four positive years ( 1983, 1993, 1996, and 1998) with snow cover anomalies higher than 0.5 standard deviation ( σ) over the small WSP region, and four negative years ( 1995, 1997, 1999, and 2005) with snow cover anomalies lower than -0.5 σ over the same region, are chosen to make composite differences (positive minus negative). As shown in Fig. 5a, cor-responding to the positive anomaly of snow cover over the small WSP region, a strong convergence of wave activity flux (purple shading) can be seen around the same region during summer. This convergence suggests that Rossby wave energy is accumulated over the WSP region and acts to maintain and reinforce positive geopotential height anomalies, and therefore reduce precipitation over this area. The Rossby waves propagate southeastward from the WSP region to the Mongolian Plateau, and then turn northeastward to the Stanovoy Range. Meanwhile, along the path of Rossby wave propagation, new wave- energy divergences and convergences originate over dow-n-stream areas and lead to negative geopotential height anomalies over the MNC region and, accordingly, more precipitation in situ.

Similarly, based on positive ( 1986, 1991, 1995, 2001, 2004, 2006, and 2007) and negative ( 1990, 1993, 1998, 2003, 2005, and 2008) years with snow cover anomalies over the HR region, composite differences (negative minus positive) are also calculated (Fig. 5b). The anomalous geopotential height pattern, the path of Rossby wave pro-pagation, and the divergences and convergences of wave energy are generally in good agreement with Fig. 5a. Note that a distinct difference between Figs. 5a and 5b is that the convergence (purple shading) over the HR region is considerably stronger in Fig. 5b. This implies that the convergence over the HR region may to some extent be modulated by snow cover anomalies in situ. Furthermore, the convergence over the HR region possibly plays a role in inducing upstream Rossby wave activities and associated geopotential height anomalies.

Clearly, the geopotential height anomalies in Fig. 5 are similar to those in Fig. 3, signifying that the dipole-related

Figure 5 (a) Composite differences of 200-hPa geopotential heights (gpm, contours) and wave activity fluxes (m2 s-2, arrows) anomalies between positive and negative snow cover anomaly years (positive minus negative). Here, the positive (negative) years have snow cover anomalies higher (lower) than 0.5 standard deviation σ (-0.5 σ) over a small region (59-66°N, 77-99°E; i.e., region B in Fig. 4) within the West Siberian Plain. Green (purple) shading marks the regions where the divergences of wave activity fluxes are higher (lower) than 5×10-7 (-5×10-7) m s-2. (b) is the same as (a), except it shows the composite differences between positive and negative years (negative minus positive) with snow cover anomalies over the Heilongjiang River (HR) region (47-56°N, 120-134°E; i.e., region F in Fig. 4).

anomalous circulation pattern is closely linked with spring snow cover anomalies over the small WSP and HR reg-ions. However, the exact mechanism that the spring snow cover anomalies influence the rainfall dipole pattern shou-ld be further investigated through numerical experiments.

4 Summary and discussion

In the present paper, a primary pattern of summer precipitation over the mid-high latitudes of Asia is explored. Results show that the summer precipitation over the MNC region is significantly oppositely correlated with that over the WSP region, which resembles the summer NAO-rela-ted dipole pattern in summer precipitation ( Sun and Wang, 2012). Further analyses indicate that the opposing variations of summer precipitation between the two regions are clear and steady on the interdecadal and interannual scales, and constitute a dipole pattern over mid- high latitude Asia.

On the interdecadal and interannual scales, the summer precipitation dipole is closely linked to an anomalous cyclone and anticyclone pairing over the Asian continent. Specifically, more precipitation over the MNC region is attributed to an anomalous low-level (850-hPa) cyclone in situ, while less precipitation over the WSP region is the result of an anomalous low-level (850-hPa) anticyclone there, and vice versa.

Corresponding to the spring snow cover anomalies over the small WSP region and the HR region, Rossby waves propagate along a path from the WSP region, via the Mongolian Plateau, to the Stanovoy Range, and mean-while result in the wave-energy divergences and convergences along this path. Finally, the wave-energy diverge-nces and convergences maintain and strengthen the anom-alous circulation pattern over the Asian continent, and thus lead to the variation of the summer precipitation dipole. Therefore, we speculate that the spring snow cover anomalies over the small WSP and HR regions may to some extent play a role in modulating the variation of the summer precipitation dipole, and can therefore be used as forecasting factors.

The memory of anomalous winter-spring snow resides in the wetness of the underlying soil as snow melts during the spring and summer seasons, which implies that preceding snow anomalies can exert a lingering effect on summer precipitation through the bridge of soil moisture anomalies ( Shukla and Mooley, 1987; Bamzai and Shukla, 1999). In the present study, the detailed processes by which spring snow cover anomalies influence the sum-mer precipitation dipole, especially the bridge effect of soil moisture, remain unclear, and should thus be further investigated. Numerical experiments are also needed to further confirm and demonstrate the mechanisms by which the spring snow cover anomalies influence the dipole pattern of summer precipitation over the mid-high latitudes of Asia. Moreover, other factors, such as sea surface temperature, sea ice concentration, and circulation anomalies, which are proven predictors of climate anomalies over northeast China ( Fan and Wang, 2010; Wu et al., 2011), may also be involved in the variation of the summer precipitation dipole. Therefore, to improve the ability of short-term climate forecasting, further discussion is needed in future research on how these factors, together with spring snow cover anomalies, modulate the variation of the dipole.

Acknowledgements. We acknowledge the joint support of the National Natural Science Foundation of China (Grant No. 41375090), the Basic Research Fund of the Chinese Academy of Meteorological Sciences (Grant No. 2013Z002), and the International Cooperation and Exchange of the Ministry of Science and Technology of China (Grant No. 2009DFA21430).

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