Re-Discussion on East Asian Meiyu Rainy Season

Transcription

1 ATMOSPHERIC AND OCEANIC SCIENCE LETTERS, 2013, VOL. 6, NO. 5, Re-Discussion on East Asian Meiyu Rainy Season LÜ Jun-Mei 1, JU Jian-Hua 2, 3, and TAO Shi-Yan 4 1 Chinese Academy of Meteorological Sciences, Beijing , China 2 State Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences, Beijing , China 3 Yunnan Meteorological Bureau, Kunming , China 4 Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing , China Received 4 March 2013; revised 27 April 2013; accepted 28 April 2013; published 16 September 2013 Abstract In this paper, the synoptic-climatology of Meiyu in East Asia is discussed. It is proposed that the location of the rain band of Meiyu is stable from the viewpoint of climatology, even though the active (wet) and break (dry) Meiyu are influenced by synoptic systems. The duration and the onset and retreat dates of Meiyu exhibit tremendous interannual variabilities, and thus, they are almost unpredictable in seasonal climate prediction. The Meiyu has been used as a synoptic concept and applied to the operational forecast for many decades by meteorological agencies in East Asian countries. As a result, the prediction of the onset and retreat dates of Meiyu has become an important operational work for meteorological services. This has also misled the public s and scientists attention. The northward propagation of the East Asian summer monsoon (EASM) surge associated with the intraseasonal oscillation is closely related to the active and break Meiyu. The activities and propagation of the EASM surge modulate the active/break Meiyu that cause concentrative severe precipitation processes and floods or droughts; hence, the authors suggest changing the current forecasting methodology of Meiyu. It is more meaningful from the scientific as well as application viewpoints to establish the monitoring and forecasting of the EASM surge to replace the current operational forecast of Meiyu after the seasonal progress enters the climatological Meiyu period in a year. Keywords: Meiyu rainy season, climatology, monsoon surge, intraseasonal oscillation Citation: Lü, J.-M., J.-H. Ju, and S.-Y. Tao, 2013: Re-discussion on East Asian Meiyu rainy season, Atmos. Oceanic Sci. Lett., 6, , doi: /j.issn Introduction The rainy season associated with the East Asian summer monsoon (EASM) is called Meiyu in China, Baiu in Japan, and Changma in Korea. In East Asia, more than half of the annual precipitation occurs during summer, and more than 40% of the summer precipitation occurs during the Meiyu season (Ding and Chan, 2005; Oh et al., 1997). Therefore, Meiyu is the most important supplier of water resources, but heavy rainfall due to active Meiyu often results in disasters in East Asian countries, and thus, Japan Meteorological Agency (JMA), Korea Meteoro- Corresponding author: JU Jian-Hua, jujh@cma.gov.cn logical Administration (KMA), and the meteorological departments of Yangtze-Huaihe River Basin in China have paid a significant amount of attention to the prediction of the onset and retreat dates of Meiyu as well as whether the Meiyu is wet or dry (i.e., the amount of rainfall) for many years. However, there have been a considerable number of difficulties and problems in the prediction of Meiyu. For instance, when the onset was forecasted early, Meiyu came late, and when wet Meiyu was forecasted, dry Meiyu occurred. Further, sometimes after meteorological departments declared the ending of Meiyu in the year according to the definition of the retreat of the Meiyu, Meiyu recurred just a few days later. The abovementioned problems can be attributed to the fact that the meteorological agencies of East Asian countries have applied the concept of climatology to operational forecasts for more than 50 years. The Meiyu front is the main component of the EASM system (Tao and Chen, 1987; Tao et al., 1988; Ding, 1992, 2004). The seasonal progress of the EASM includes a distinct stepwise rainy season from mid-june to mid-july over the Yangtze River Valley and the Japanese Main Islands (Meiyu/Baiu) (Tao and Chen, 1987; Saito, 1985; Ninomiya and Akiyama, 1992; Tanaka, 1992), and a slightly late rainy season from late June to late July when the rain band migrates northward to the Korean Peninsula (Changma) (Oh et al., 1997). Precipitation during the Meiyu season can result in persistent rainfall for days or weeks. Sometimes, it can also be intermittent, including frequent heavy showers and torrential rains. A number of studies on various topics related to Meiyu, such as the onset and retreat dates, the duration of Meiyu, and its intensity, which may cause either wet or dry Meiyu, and the heavy rainfall phenomena associated with the Meiyu front (Ninomiya and Mizuno, 1987; Murata, 1990; Lu et al., 2001; Chen, 2004; Wang et al., 2007), have been conducted. It is still difficult to forecast these features of Meiyu accurately at present. Further analysis can help us understand the reasons for the same. The EASM is characterized by its prominent subseasonal variation, which is the abrupt northward advance during its seasonal progress and the active/break Meiyu. The alternation between abrupt changes and steady evolution is directly associated with intraseasonal oscillations (ISO). Kang et al. (1999) suggested that the Changma cloud band is associated with the northward propagation of climatological intraseasonal oscillation (CISO). Wang

2 280 ATMOSPHERIC AND OCEANIC SCIENCE LETTERS VOL. 6 and Xu (1997) proposed that the phase-locking of ISO to the annual cycle results in a significant westward propagating CISO, which is in turn responsible for the subseasonal variation of summer monsoons. Ju et al. (2005, 2007, 2008) found that the ISO of EASM in East Asia region behaves as the northward-propagating EASM surge with a wavelike activity intensity. When the lowfrequent wave reaches its crest or trough, middle-lower reaches of the Yangtze River will occur active or break phase of precipitation correspondingly, which suggests that the activities of ISO in the East Asia monsoon region may modulate the large-scale summer precipitation over East China to some degree. The role of the northwardpropagating ISO in modulating the Meiyu has not yet been fully studied. In this study, we investigate the linkage between the activities of the EASM surge and the active/break Meiyu to propose a prospective forecasting method. 2 Data and methods When analyzing the climatological mean distribution of the Meiyu rain band, we chose the daily grid point data of precipitation (Yatagai et al., 2008) in the database of Asian Precipitation-Highly-Resolved Observational Data Integration Towards Evaluation of the Water Resources (APHRODITE), established by JMA on the basis of the data from Asian weather observatories. The data resolution is 0.25 latitude 0.25 longitude, which can describe the precipitation distribution of Korea and Japan very well, and the time ranged from 1 January 1980 to 31 December In order to get the activity features of EASM surge in the subtropical area of East Asia, a day bandpass filter based on the first-class Butterworth function (Marakami, 1979) was applied to the regional mean zonal winds at 850 hpa. The 850-hPa zonal wind is from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/ NCAR) monthly reanalysis dataset (Kalnay et al., 1996). 3 Results and discussion In order to describe the change in the features of Meiyu in a measurable way, different countries in East Asia have formulated different definitions of the onset and retreat dates as well as the intensity and duration of Meiyu. The definition criteria that China Meteorological Administration (CMA) has formulated for the Meiyu season in the Yangtze River Valley are as follows: (a) The total daily precipitation at five representative weather observatories in the middle-lower reaches of the Yangtze River equals or exceeds 10 mm. If more than half of the 10 continuous rainy days meet this condition, the first day is named the onset date of Meiyu. (b) The ridge axis of the Western Pacific subtropical high is in the range of 20 N 25 N. (c) During the Meiyu period, one or more precipitations occur, and every time, the rain must last for more than six days; in the meanwhile, the total daily rainfall amount of the five stations must be more than 25 mm. (d) The retreat date of Meiyu is defined as the next day of the last Meiyu event. The criteria of the Changma onset by KMA are as follows: (a) The duration of precipitation is more than three days, and (b) the quasi-stationary front is located over the Korean Peninsula. The retreat is defined by the following criteria: the duration of non-precipitation should exceed three days, and the quasi-stationary front should move northward from the peninsula. It is obvious that the determination of the rainy season onset and withdrawal in East Asian countries is difficult, artificial, and not unique. Figure 1 shows the duration and the onset and retreat dates of Meiyu and Changma from 1961 to 2005 according to the definitions by CMA and KMA. The onset and the withdrawal of the rainy season vary year to year significantly, indicating the high interannual variability of Meiyu and Changma due to the role of synoptic systems. Climatologically, the Meiyu begins on 19 June and ends on 8 July, whereas the Changma starts on 24 June and ends on 24 July. That is, the Meiyu season lasts for approximately 20 days, while the Changma season lasts for approximately 30 days. In order to investigate the amplitude of the interannual variation of Meiyu and Changma, the standard deviations of the Meiyu/Changma duration, the onset and retreat dates are listed in Table 1. The variation of Meiyu is evidently greater than that of Changma according to the value of the standard deviations. Furthermore, the standard deviations of the Meiyu/Changma duration and withdrawal dates are greater than those of the onset dates. It is also noted that the standard deviations of the Meiyu duration and retreat dates (13 days) account for 65% of the climatological mean Meiyu duration (20 days). This implies that the interannual variability of Meiyu has largely exceeded half of the climatological Meiyu duration and is almost unpredictable in the seasonal climate prediction. Although the rainy season characteristics, including the onset and retreat dates, the duration, the total rainfall amount, and the intensity, exhibit large interannual variations, the position of the Meiyu front is quasi-stationary. The Meiyu front often appears as a quasi-stationary front with an average lifetime of eight days. Here, we present the climatological features of the Meiyu. The climatological rain band positions of Meiyu-Baiu and Changma were derived from the 22-yr means of the total rainfall amount during the Meiyu/Baiu and Changma periods, respectively, based on the APHRO daily precipitation datasets (Yatagai et al., 2008). Figure 2 shows the climatological rain band for the Meiyu-Baiu/Changma periods over the Yangtze and Huaihe River Basins, Japan, and Korea. It can be seen that the Meiyu-Baiu rain band is mainly distributed over the middle-lower reaches of the Yangtze River with its eastern edge passing through the Japan Islands. The axis of the maximum total rainfall amount during the Meiyu period, indicating the climatological mean position of the Meiyu front, is oriented approximately in an east-west direction extending from southern Japan to central China. The rain band shifts northward to Korea with seasonal progress, and the East Asian rainy season terminates in

3 NO. 5 LÜ ET AL.: RE-DISCUSSION ON EAST ASIAN MEIYU RAINY SEASON 281 Figure 1 Duration and onset and retreat dates of (a) Meiyu and (b) Changma in the years The shaded areas denote the duration of Meiyu. The curve with the filled circles indicates the onset date and that with the unfilled circles indicates the retreat date. The dashed lines going through each curve indicate the average. Table 1 Standard deviations of Meiyu/Changma duration and onset and retreat dates during the years Standard deviation Days Meiyu onset dates 8.68 days Meiyu retreat dates days Meiyu duration days Changma onset dates 5.40 days Changma retreat dates 7.99 days Changma duration 8.47 days late July. Ding et al. (2007) demonstrated that the interannual variation of the thermal and dynamic factors of the Meiyu system is also relatively small. Meiyu should belong to a concept of climatology from this point of view, but it has been considered a concept of synoptic meteorology and implemented in operational forecast by meteorological departments in East Asian countries for 50 years or more, which has misguided the attention of the public and the decision makers. Social communities, public, and media are concerned about the onset and retreat dates of Meiyu every year when it is more important for them to realize that the climatological feature of the Meiyu provides a mean state for the interannual variability of the rainy season in East Asia. Based on the Meiyu parameters defined by CMA, three relatively wet years with the total precipitation of the Meiyu period of more than 2000 mm were chosen to study the relations between the EASM surge (ISO) and Meiyu during the period of 1961 to Figure 3 shows the impacts of the EASM surge on the changes in the average precipitation amount derived from the nine representative weather stations over the middle-lower reaches of the Yangtze River in 1969, 1996, and The dashed line denotes a day filtered curve of the regional mean 850-hPa zonal wind in the subtropical area of East Asia (22.5 N 32.5 N, E E), indicating the activities of the EASM surge in the middle-lower reaches of the Yangtze River. Precipitation is denoted by bars. Further, the open squares represent the active periods of Meiyu. When the Early Meiyu came in May 1998, the EASM surge was on the wave crest, and later in June, when the EASM surge got to the position of the wave crest again, a very typical Meiyu period occurred. Correspondently, the third wave crest of the EASM surge matched the Second Meiyu. In 1969 and 1996, the typical Meiyu period appeared just once but lasted for a longer time than usual, while the wave crest of EASM surge occurred correspondingly. In summary, from early summer to midsummer, the active Meiyu corresponds to the wave crest of the EASM surge, while the break Meiyu is consistent with the wave trough, which indicates that the EASM surge modulates the active/break Meiyu to some extent. In other words, there exists an in-phase relationship between the EASM surge and the Meiyu. The EASM surge usually begins from the tropical areas and then propagates northward to the middle-lower reaches of the Yangtze River. This implies that the EASM surges are often precursors to the active/break Meiyu. We suggest that the monitoring and forecasting of the EASM surge activities are very mean-

4 282 ATMOSPHERIC AND OCEANIC SCIENCE LETTERS VOL. 6 Figure 2 Climatology of the rain band for Meiyu-Baiu/Changma periods based on 22-yr APHRO datasets (Units: mm). The heavy dashed line outlines the maximum axis of the total rainfall amount during the Meiyu season. The shaded areas represent the total rainfall amount greater than 200 mm. Figure 3 Daily changes in the day filtered curve of the mean 850-hPa zonal wind in the typical Long Meiyu and Second Meiyu years (units: m s 1 ) as well as the mean precipitation amount of the nine stations in the middle-lower reaches of the Yangtze River during the Meiyu season (bar, units: 20 mm d 1 ). The open squares indicate the active Meiyu periods: (a) 1969, (b) 1996, and (c) 1998; the early Meiyu and the second Meiyu were recorded in ingful to the prediction of large-scale concentrative precipitation processes in the Meiyu season. 4 Discussion and concluding remarks A recent analysis (Tao and Wei, 2007) revealed that an EASM surge occurs in South China when the ISO that derived from the equatorial Indian Ocean (also named Madden-Julian Oscillation, MJO) propagates to the South China Sea. Furthermore, the summer monsoon surge meets with the cold air from northern China. Eventually, this will cause severe rainstorms along the stationary

5 NO. 5 LÜ ET AL.: RE-DISCUSSION ON EAST ASIAN MEIYU RAINY SEASON 283 Meiyu front in South China. Ju et al. (2008) examined the zonal and meridional propagating features of the atmospheric ISO (EASM surge) in the EASM region. The results show that the meridional propagation of ISO is mainly derived from the tropical region, whereas the zonal propagations include the eastward propagation generated from the Indian monsoon region and the westward propagation generated from the western Pacific. These two zonal propagations of the ISO converge at 110 E 120 E and then reinforce the northward propagation of the ISO. The ISO propagates northward continuously and affects the summer precipitation in the middle to lower reaches of the Yangtze River. Wang and Xu (1997) proposed that the strong activity of the CISO and the abrupt changes of the monsoon over East Asia are some of the prominent characteristics of the boreal summer general circulation. They have profound implications on seasonal climate predictions. We suggest that the activity and propagation of the EASM surge plays a very important role on the active/break Meiyu, although there are many factors that modulate the active/break Meiyu. This implies that the prediction of the active/break Meiyu in East Asian countries will probably depend on the detection and prediction of the EASM surge. As a concept of climatology, Meiyu provides a climatological background to the severe precipitation in the rainy season. When the Meiyu season occurs in climatology, meteorological departments should strengthen the monitoring and forecasting of the activities and propagations of the EASM surge from the tropics and focus on the monitoring and prediction of the modulating impact that the EASM surge has on the large-scale concentrative severe precipitation processes in different regions of East Asia. The mechanism through which the EASM surge governs the behavior of Meiyu remains unclear. Further, there are some other problems to be studied in more detail, such as the sources of the EASM surge, its northward propagation speed, relations between wave crest/trough and active/break Meiyu, and the methods for the prediction of Meiyu on the basis of the activities of the EASM surge. Further research will help us understand these issues. Acknowledgments. We would like to express our sincere gratitude to the two anonymous reviewers of this paper for their useful comments. This investigation was jointly supported by the National Basic Research Program of China (2012CB417205), Open Fund of State Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences (2012LASW-A02), and Basic Research Fund of Chinese Academy of Meteorological Sciences (2010Z001). References Chen, G. T. 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