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GUAN Zhao-Yong, Jin Da-Chao. Two Opposite Extreme Events in Seasonal Mean Winter Rainfall over East China during the Past Three Decades. Atmospheric and Oceanic Science Letters, 2013, 6(5): 240-247   
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Received:2013-01-08 Accepted:2013-03-14
Two Opposite Extreme Events in Seasonal Mean Winter Rainfall over East China during the Past Three Decades
GUAN Zhao-Yong, Jin Da-Chao
Key Laboratory of Meteorological Disaster of Ministry of Education/School of Atmospheric Science, Nanjing University of Information Science and Technology, Nanjing 210044, China Key Laboratory of Meteorological Disaster of Ministry of Education/School of Atmospheric Science, Nanjing University of Information Science and Technology, Nanjing 210044, China
Abstract

In this study, the extremes of winter seasonal mean precipitation have been investigated by using daily precipitation data from 91 stations in East China, the National Centers for Environmental Prediction/the National Center for Atmospheric Research (NCEP/NCAR) monthly reanalysis, and sea surface temperature data from the Hadley Centre for 1979-2007. The largest anomalous rainfall amount was observed in regions south of the Yangtze River. In the most recent three decades, extreme events in the seasonal mean winter precipitation occurred in 1985 and 1997. Because it was influenced mainly by a La Niñ a event, the precipitation in 1985 showed a deficit following a stronger winter monsoon. The rainfall amount in 1997 was influenced by El Niñ o and was significantly larger than normal with a weaker winter monsoon. Both the circulation anomalies and wave energy dispersions during the winters of 1985 and 1997 differed significantly. In 1985, the North Atlantic Oscillation anomalously excited the Eurasian-Pacific teleconnection and circumglobal teleconnection phenomena. Consequently, Rossby wave energy propagated along the north and south branches of the westerlies, strengthening the East Asian trough along with a stronger winter monsoon, which facilitated the wintertime dry extreme in East China. In 1997, however, Rossby wave energy propagated from low latitudes northeastward into the southern part of China, resulting in a weaker winter monsoon and the wettest winter. The results of this study will be helpful for future monitoring and prediction of extreme winter rainfall events in East China.

Keyword: extreme seasonal mean rainfall; winter monsoon; ENSO event; Rossby waves; East China
1 Introduction

Increasing attention has been paid to precipitation extremes and their substantial influences on global warming (Easterling et al., 2000). Many studies have reported that the precipitation extremes in recent decades show increasing trends in both intensity and occurrence in the global domain (e.g.,Manton et al., 2001). In the region of China, particularly in the eastern area, such precipitation extremes have been investigated frequently in the Chinese climate community (e.g.,Huang et al., 2006; Guan et al ., 2011). The droughts and floods that occur during the boreal summer generally have huge influences on both the economy and human activities; interannual variability of winter precipitation in the eastern part of China is also very large. The extremes of precipitation in the boreal winter frequently result in either a lengthy wintertime drought or heavy snowfall in East China, thus causing agriculture disasters related to the rainfall deficit or transportation hindrances related to the heavy snowfall (Wen et al., 2009; Bao et al., 2010; Wu et al., 2011).

The winter monsoon prevails in the boreal winter in East China, and the precipitation extremes are evidently affected by the East Asian winter monsoon (EAWM). The winter precipitation in East Asia has been reported to exhibit notable interannual variability (Zhou, 2011). When the EAWM is strong in a given year, the precipitation amount is less in East Asia because the anomalous southwest wind is weaker in the strong EAWM years. This activity prevents the water vapor from being transported from both the Bay of Bengal and the South China Sea northward to East Asia, thus weakening the convergence of water vapor in the region. In weak EAWM years, opposite scenarios are observed (Wang et al., 2000; Zhang and Sumi, 2002; Zhou and Wu, 2010; Zhou, 2011).

The EAWM is influenced by many factors. In the tropics, the Maritime Continental region (Ramage, 1968) plays a very important role in influencing EAWM (Chang et al., 2006). In the boreal winter, a strong convection zone persists over the Maritime Continent (MC; Ramage, 1975; Chang et al., 2006). In addition, convection in this region (Chang et al., 1979; Lau and Chang, 1987) can be enhanced by a cold wave, which affects El Niñ o events (Li, 1989,1990). The altered El Niñ o-Southern Oscillation (ENSO) conversely influences EAWM via anticyclonic circulation east of Philippines (Wang et al., 2000; Zhang and Sumi, 2002; Zhou and Wu, 2010) and affects convections in the MC region via the Walker circulation. Thus, ENSO has an impact on the EAWM via meridional circulation around the meridian of 110° E. On the contrary, the sea surface temperature anomalies (SSTAs) in the South China Sea are apparently related to winter precipitation in southeastern China (Zhou et al., 2010).

In the mid-high latitudes, many factors influence the winter climate of China. The Arctic Oscillation (AO,Thompson and Wallace, 1998), which is closely related to North Atlantic Oscillation (NAO), can affect the EAWM by disturbing the Siberian High, thereby influencing the winter climate of East Asia (Gong et al., 2001; Wu and Wang, 2002). Moreover, the East Asian jet stream (EAJS) and the related effects as a Rossby waveguide also influence the winter climate of East Asia (Yang et al., 2002; Wen et al., 2009).

These previous studies have revealed that the many factors influencing the winter climate of East Asia include precipitation anomalies. However, the mechanism of these factors that influences extreme events of winter precipitation in East China requires further clarification. By analyzing the two extreme events of 1985 and 1997, the anomalous winter precipitation extremes in East China are investigated in the present study. Our results may provide a better understanding of the formation processes of

winter droughts and flooding in this region to enable more accurate prediction.

2 Data

The research area in the present study includes six provinces of Fujian, Jiangxi, Anhui, Zhejiang, Jiangsu, and Shandong in addition to the city of Shanghai. The daily precipitation data were obtained from 91 stations in East China (23-38° N, 114-123° E) (Fig. 1a), the National Centers for Environmental Prediction/the National Center for Atmospheric Research (NCEP/NCAR) monthly reanalysis (Kalnay et al., 1996) at a spatial resolution of 2.5° × 2.5° , and SST data from the Meteorological Office Hadley Centre (Rayner et al., 2003) at a spatial resolution of 1° × 1° . Annual winter precipitation was determined by preprocessing using its daily value over the winter season from 1 December to 28 February of the following year. The length of the time series was 29 from 1979 to 2007. Term “ anomaly” is defined as the departure of the December-January-February (DJF) mean (sum) of a quantity in a particular year from the mean climatology of that quantity.

Figure 1 Variations in December-January-February (DJF) precipitation. (a) The mean climatology of winter precipitation in East China is indicated by shading; root mean square values of precipitation anomalies (mm) are represented by contours. Scattered red thick dots denote locations of stations. (b) The time series of DJF precipitation anomalies (mm) of East China is represented by bars; green and red indicate positive and negative values, respectively. Dashed horizontal lines represent values at the level of one standard deviation of rainfall anomalies. (c) Precipitation anomalies (mm) in 1985 are indicated by shading; percentages (%) of precipitation anomalies to the DJF total mean climatology of precipitation are represented by contours. (d) Precipitation anomalies (mm) in 1997 are indicated by shading; percentages (%) of precipitation anomalies to the DJF total mean climatology of precipitation are represented by contours.

3 Temporal and spatial variations in winter rainfall

The wintertime precipitation in East China is spatially distributed in a non-uniform pattern (Fig. 1a). The maximum precipitation occurs over Jiangxi, Fujian, Anhui, and Jiangsu provinces with a value of more than 280 mm, whereas the minimum precipitation is received in northern Shandong with a value of less than 20 mm. The winter precipitation in East China decreases south and north of 29° N.

Annual changes in winter precipitation are relatively higher in southern East China, as determined from the distributions of the root mean squared (RMS) value of the winter precipitation (Fig. 1a). Larger RMS values are observed in southern Jiangxi, with a maximum value of more than 100 mm, while smaller values are measured in northern Shandong, with a minimum of less than 20 mm. Strong interannual variations in precipitation are also evident in the time series of winter precipitation anomalies averaged over East China (Fig. 1b). Anomalous precipitation of more than one standard deviation appeared in 1985, 1989, 1995, 1997, 1998, 2002, and 2004. Extreme events include the driest and wettest winters in 1985 and 1997, respectively. The regional mean wintertime precipitation anomaly in 1985 was -68.7 mm, with an anomalous percentage of -45.8% to the total precipitation of mean winter climatology. However, the anomalous precipitation in 1997 amounted to 168.7 mm, showing a 112.5% increase in mean climatology of precipitation.

In the extremely dry year of 1985, the precipitation anomalies were negative in the entire East China region

(Fig. 1c). The driest areas were in Jiangxi and Anhui provinces. In Jiangxi, the largest anomalous precipitation amounted to -100 mm, which was approximately -40% less than normal. The largest ratio of anomalous precipitation to its mean climatology in Anhui province was above -60%. In contrast, a high amount of rainfall was received in all parts of East China in 1997, with a maximum value of precipitation anomaly of 300 mm recorded in Jiangxi (Fig. 1d). The precipitation anomaly percentage exceeded 90% in the entire East China region and approximately 200% in Shandong.

4 Extreme events and related circulation anomalies

The winter precipitation in East China was anomalously low in 1985 and anomalously high in 1997. These extreme events are related to the EAWM variations. In 1985, the strong winter monsoon was witnessed whereas it was weak in winter of 1997 (Wang and Jiang, 2004). In the following, why and how these two extreme events occurred are to be discussed.

4.1 Vapor transfer

Water vapor transfer plays a crucial role in the East Asian monsoon system (Zhang and Sumi, 2002). In 1985, an anomalous cyclonic circulation occupied the Kuroshio area in the northwestern Pacific in the lower troposphere (Fig. 2a), persisted over eastern China, and brought drier and colder air southward to southern China. The EAWM was reinforced in this region, and the total water vapor, obtained through vertical integration from the Earth’ s surface up to 300 hPa, diverged in East China to induce a rainfall deficit. The intensified northerly winds over the Kuroshio region could have induced the anomalous wind-driven offshore sea flow along with higher than normal evaporation in the oceanic region to produce colder SSTs. A convergence center in the lower troposphere was observed east of the Philippines (15° N, 135° E), where the SSTAs were positive. This northwest-southeast contrast in SSTAs, illustrated in Fig. 2a, may have facilitated formation of a vertical circulation that slanted from East China southeastward to east of the Philippines to cause stronger downward motion of air over mainland China and create in rainfall deficit.

Figure 2 Divergent component of anomalous water vapor fluxes integrated vertically from the Earth’ s surface up to 300 hPa (arrows, units: kg m-1 s-1), the anomalous circulation at 850 hPa (streamlines, units: mm s-1), and the sea surface temperature anomalies (shading, units: ° C) in (a) 1985 and (b) 1997.

However, the scenario in 1997 appears to be opposite that in 1985. At 850 hPa, water vapor was anomalously transported from the South China Sea northeastward to East China (Fig. 2b). The vertically integrated vapor fluxes showed divergent flows emanating outward from the region east of the Philippines, and a corresponding an anomalous anticyclonic circulation was observed east of the Philippines. On the contrary, an anomalous cyclonic circulation appeared in East China, weakening the persisting EAWM and facilitating more rainfall in the region. However, the SSTAs were positive along the coastline of mainland China due to both the warmer air flow and the weakened EAWM and were negative in the southeastern flank of the anomalous anticyclonic circulation east of the Philippines. This northwest-southeast contrast in the thermal state of the sea surface may have induced the anomalous vertical circulation slanted in the northwest-southeast direction with the ascending branch over East

China to intensify precipitation.

4.2 Anomalous vertical circulations and tropical forcing

The extremely dry and wet winter events in East China are closely associated with the vertical circulation anomalies and thermal forcings in the tropical region. In the winter of 1985, anomalous divergent winds at 850 hPa (Fig. 3a) showed a convergence center over the Sahara due to an anomalous ascent of air created by Ekman pumping (Fig. 3e,Sun et al., 2008). Correspondingly, an anomalous divergence center was observed at 200 hPa, just over the Sahara region (Fig. 3c). Divergence in the upper troposphere was the vorticity source, which excited the Rossby wave train that propagated eastward along the Asian jet stream (Watanabe, 2004). Interestingly, it was determined that at 200 hPa, several divergence and convergence centers alternatively appeared in zone along the westerly at approximately 27.5° N from the Sahara eastward to the Northwest Pacific via East China. These centers of disturbances may have affected the climate varia-tions in the Sahara, the Arabian Peninsula, the Indian sub-continent, and East Asia (Figs. 3e and 3c). In association with these divergence and convergence centers, an anomalous descending branch in eastern China, was responsible for the suppressed rainfall in East China (Fig. 3e).

Figure 3 Left column shows 1985 anomalous velocity potential (shaded contours, 106 m2 s-1) and superimposed divergent wind (vectors, m s-1) at (a) 850 hPa, (c) 200 hPa, and (e) the zonal-vertical section of circulation anomalies averaged in the meridional region over (25° N-37.5° N) (streamlines) along with the zonal component of anomalous divergent winds in m s-1(shaded contours). Right column shows 1997 anomalous velocity potential (shaded contours, 106 m2 s-1) and superimposed divergent wind (vectors, m s-1) at (b) 850 hPa, (d) 200 hPa, and (f) the meridional-vertical section of anomalous circulation averaged over (115° E-122.5° E) along with the meridional component of anomalous divergent winds in m s-1(shaded contours).

The 1997 scenarios differed significantly from those in 1985. A dipole pattern of velocity potential was surprisingly observed in both the lower and upper troposphere in the tropical region (Fig. 3d). This dipole structure included zonal-vertical circulation, indicating an anomalous ascending branch over the western Pacific. In the East Asia sector, an anomalous meridional vertical circulation known as local Hadley circulation formed with the downdraft branch in the Indonesia region and the updraft branch over East China (Fig. 3f). This well-defined anomalous vertical circulation could have been a response of the atmosphere to the anomalous thermal forcing in the MC in association with the SSTAs in the northwestern Pacific. As a consequence, anomalous anti-Hadley circulation in East Asia facilitated heavy rainfall in East China in the winter of 1997 (Fig. 3f).

The winter climate of East China is readily affected by SSTAs including the components related to ENSO and local variability independent of ENSO (Wang et al., 2000; Zhang and Sumi, 2002; Wen et al., 2009; Zhou et al., 2010; Zhou and Wu, 2010; Zhou, 2011). In El Niñ o (La Niñ a) years, an anomalous anti-cyclone (cyclone) is usually generated in the lower troposphere over the western Pacific (East of the Philippines,Wang et al., 2000; Zhang and Sumi, 2002 Guan and Li, 2008), which plays a crucial role in the attenuation (reinforcement) of the continental anti-cyclone and results in anomalous south (north) winds. This mechanism regulates the EAWM is regulated and hence influences precipitation in East China.

In the winter of 1997, a strong El Niñ o event occurred. As previously mentioned, an anomalous anticyclone (Fig. 2b) was generated in the area east of the Philippines, which favored an anomalous ascent of air. The thermal state in the Indonesia region was also influenced by El Niñ o; hence, an anomalous anti-Hadley circulation (Fig. 3f) dominated in East Asia. These changes in circulation were in responsible for the heavy precipitation in East China in the winter of 1997.

On the contrary, a La Niñ a event occurred in the extremely dry winter of 1985, which induced anomalous cyclonic circulation over the northwestern Pacific. However, a more northward shift of the cyclonic circulation in 1985 than that of the anticyclonic circulation in 1997 resulted in divergent wind that appeared to be more zonal around the northern edge of the tropics; therefore, zonal rather than meridional vertical circulation formed in East Asia (Fig. 3e).

4.3 Wave propagations

The disturbances along the westerly jet stream significantly influence winter monsoon anomalies. In 1985, the circulation anomalies over Eurasia at different isobaric levels appeared to be similar; positive anomalies of geopotential height and sea level pressure (SLP) were detected in higher latitudes, and negative anomalies were observed in the southeastern flank of the positive anomalies. These anomalous circulations indicate an equivalent barotropic structure in the middle and upper troposphere (Figs. 4a, 4c, and 4e). Interestingly, some centers of anomalous geopotential height at both 500 hPa and 200 hPa were distributed with a sign sequence of “ negative-positive-negative” from the Ural-Siberia Plateau to East Asia via the Siberian Plateau (Figs. 4c and 4e) and displayed a wave structure in middle latitudes. Similar structures were also observed in SLP anomalies (Fig. 4a), although the amplitude was significantly smaller. This wave appeared to be similar to the Eurasian-Pacific (EUP) teleconnection in the positive phase (Wallace and Gutzler, 1981). The occurrence of the EUP teleconnection in 1985 implies that the trough in region west of the Urals has deepened, the ridge over Lake Baikal has been reinforced, and the East Asian trough has been intensified. The positive anomalous geopotential heights east of Lake Baikal together with the negative anomalous geopotential heights in East Asia resulted in a strengthening of the north wind between the two regions, which induced a stronger than normal EAWM (Gong et al., 2001; Wu and Wang, 2002).

The Takaya-Nakamura (TN) wave activity flux (Takaya and Nakamura, 2001) observed in the winter of 1985 (Figs. 4a, 4c, and 4e) suggests that the disturbance energy was transported from the Greenwich Meridian (0° E) eastward to East Asia. Interestingly, two paths along which the Rossby wave energy propagated were found on the south and north side of the Tibetan Plateau (Figs. 4c and 4e), respectively, which were obviously waveguide-related. During the boreal winter, the westerly is usually separated into two braches due to the topographic forcing of the Tibetan Plateau. One branch is located on the north side of the Tibetan Plateau, showing a jet stream structure in the middle and upper troposphere; the other is located just on the south side of the plateau and is known as the south branch of the westerly. Both branches of the westerly are sufficiently strong to act as waveguides. As a result, the wave activity fluxes from both the northern and southern waveguides converge around Japan, intensifying the disturbance and hence deepening the East Asian trough.

The anomalous disturbances in the extremely wet year of 1997 differed significantly from those in the extremely dry year of 1985. Over Siberia and northeastern Asia, oppositely signed anomalies were observed in SLP and geopotential heights in 1997 (Figs. 4b, 4d, and 4f), which is opposite that of the 1985 scenario. In 1997, the wave structure in mid-high latitudes appeared to be an EUP teleconnection in the negative phase in which both the Mongolian high pressure system and the East Asian trough were weakened significantly. This teleconnection caused a reduction in the zonal pressure gradient over East China and thus led to the northward transports of vapor anomalously from the equatorial western Pacific to East China (Fig. 2b).

Figure 4 Left column shows 1985 wave activity fluxes with magnitudes larger than 2 m2 s-2 (vectors) with shaded contours for (a) anomalous sea level pressure (hPa), anomalous geopotential heights in dgpm (c) at 500 hPa and (e) at 200 hPa. Right column shows 1997 wave activity fluxes with magnitudes larger than 2 m2 s-2 (vectors) with shaded contours for (b) anomalous sea level pressure (hPa), anomalous geopotential heights in dgpm (d) at 500 hPa and (f) at 200 hPa.

As previously discussed, large differences in Rossby wave dispersion were observed between the extremely dry year of 1985 and the extremely wet year of 1997. In 1997, the wave energy propagated along mid-high latitude areas and was trapped in areas northwest of East China (Figs. 4b, 4d, and 4f). In 1985, however, the wave energy propagated along the Asian jet stream eastward into East Asia. This significant difference in wave energy dispersion in mid-high latitudes suggests that the behavior of the disturbances that led to the extremely dry winter differed significantly from that which led to the extremely wet winter. In lower latitudes, the Rossby waves also behaved quite differently. In 1997, some wave energy propagated from the tropics northeastward into the Yangtze River basin in East China. Several previous studies reported that the intensity change of convection in the western Pacific warm pool could strongly affect climate conditions in East Asia (e.g.,Nitta, 1987). In 1985, however, the wave activity fluxes displayed obvious eastward propagation of wave energy along the south branch of the westerly from the area west of the Tibetan Plateau into East Asia (Figs. 4a, 4c, and 4e). This result suggests that the south branch of the westerly in 1997 was a less important influence for Rossby waves in the westerly than that in 1985.

5 Conclusions and discussions

Large interannual variations occur in seasonal mean winter precipitation over East China with the largest mean climatological precipitation observed in Jiangxi, Anhui, Zhejiang, and Fujian provinces. During the study period from 1979 to 2007, two extreme events occurred in seasonal mean winter rainfall over East China; the driest event was observed in the winter of 1985, and the wettest in winter was measured in 1997 (Fig. 1).

Both the tropical signal and the disturbed westerly affected the extreme events occurring in winters of 1985 and 1997. However, their causes differed significantly. A La Niñ a event occurring in 1985 induced anomalous cyclonic circulation in the northwestern Pacific east of the Philippines, which was favorable for the north wind and vapor divergence over East China. In addition, the East Asian trough was deepened due to the convergence of the wave activity fluxes. The Rossby wave energy propagated from west Eurasia eastward to East China via the northern waveguide north of Tibetan Plateau and the southern waveguide south of the Plateau. These two waveguides in middle and upper troposphere corresponded to the two branches of the strong westerly related to the dynamic forcing of the Tibetan Plateau during the boreal winter. Disturbances from the mid-high latitudes and the tropical region facilitated intensification of the EAWM, resulting the driest winter in East Asia in 1985.

In 1997, however, an El Niñ o event occurred that led to anomalous anticyclonic circulation east of the Philippines (Wang et al., 2000) that was favorable for anomalous south winds in East China, which facilitated the northward transport of warmer and humid airflow. No stronger wave activity fluxes were transported eastward into the East Asian trough region, and the typical EUP teleconnection pattern was not explicitly observed. The south waveguide that appeared clearly in 1985 did not evidently play an important role in 1997 in the propagation of Rossby wave energy from the Mediterranean region eastward to East Asia. Instead, strong wave activity fluxes in the middle and upper troposphere were observed in lower latitudes in East Asia, which converged in the middle and lower reaches of the Yangtze River basin to intensify the anomalous cyclonic circulation over East China. Therefore, disturbances in both the tropics and middle latitudes were favorable for weakening the EAWM, resulting in the wettest winter in East China in 1997.

ENSO appears to play a crucial role in inducing anomalous changes in circulation in East Asia, which results in extreme winter precipitation events. However, the extent of ENSO’ s influence on the winter precipitation over East China remains unclear. In the analysis of SLP and other circulation anomalies in mid-latitudes, several teleconnection patterns were observed such as AO, NAO (Hurrell, 1995), circumglobal teleconnection (Branstator, 2002; Watanabe, 2004; Ding and Wang, 2005), and interhemispheric oscillation (Guan and Yamagata, 2001; Lu and Guan, 2009), which may affect the winter precipitation anomalies in East China. However, the influence of these associations on the precipitation anomalies remains unclear. Such questions required further investigation.

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