The 10–30-day oscillation of winter zonal wind in the entrance region of the East Asian subtropical jet and its relationship with precipitation in southern China
Introduction
In the 1970s, Madden and Julian discovered that there is a 40–50-day oscillation in tropical circulation and defined it as the ISO, or the Madden-Julian oscillation (MJO) (Madden and Julian, 1971, Madden and Julian, 1972). Many scholars have studied the tropical ISO, and there are certain understandings of the structural characteristics and active regulars of the tropical ISO (Krishnamurti and Subrahmanyam, 1982; Murakami et al., 1984; Lau and Chan, 1986; Knutson and Weickmann, 1987; Li, 1990; Li and Zhou, 1991; Madden and Julian, 1994; Jiang et al., 2004; Li et al., 2015; Feng et al., 2015; Wang et al., 2017; Hsu et al., 2014). However, studies of the extratropical ISO began late. Some studies have shown that the ISO of different meteorological fields, such as meridional wind and zonal wind, are totally different from the ISO of geopotential height (Han et al., 2010). The ISO in the northeast Pacific is the strongest in winter and the weakest in summer, but the ISO in East Asia and the northwest Pacific is the strongest in summer and the weakest in winter. The ISO in the northwest Pacific in summer will cause anomalously persistent droughts and floods in the Yangtze-Huaihe River basin (Zhu et al., 2003; Gao et al., 2016). In recent years, continuous freezing rain and snow events occurred frequently in winter in southern China, which had significant ISO characteristics (Wu et al., 2009). A case study also shows that the persistent precipitation anomaly in southern China in winter is closely related to the intra-seasonal variation of EASWJ (Yao et al., 2014). Thus, it is of important theoretical significance to analyze intra-seasonal variation characteristics of the EASWJ in winter.
The EASWJ is a large and stable system in the upper troposphere of the subtropical frontal zone; it is a strong westerly wind belt, where the wind speed is over 30 m s−1. The horizontal length is approximately ten thousand kilometers, the horizontal width is approximately several hundred kilometers, and it is approximately several kilometers thick (Kuang and Zhang, 2006). It maintains the upper branch of the Hadley cell (i.e., the northward branch) and carries Earth’s angular momentum from the equator to subtropical regions (Holton, 2004). Thermal anomalies also have an influence on the EASWJ (Kuang et al., 2014; Guo et al., 2015). The EASWJ is strongest in winter. Generally, there is a strong divergence south of the jet entrance, which effects precipitation in southern China; therefore, it is commonly considered as one of the control systems for precipitation in China (Zhang et al., 2006).
In the past, due to the limitation of observed data, scholars have studied the EASWJ at seasonal, inter-annual and inter-decadal time scales (Yang and Webster, 1990; Liao et al., 2004; Zhang et al., 2006; Yang et al., 2000; Ren et al., 2010; Ren et al., 2011; Shi et al., 2014; Xiang and Yang, 2012; Xue and Zhang, 2017). In recent years, observed data, reanalysis data and simulation data have been very abundant, allowing sub-seasonal study to be realized. Jin et al. (2012) studied intra-seasonal characteristics, such as location and intensity, of the EASWJ in summer and noted that the feature of the position index of westerly jet not only has a synoptic scale of one week, but also has a quasi-biweekly dominant period. Other studies (Lin and Lu, 2005; Lin, 2010) showed that after the onset of the summer monsoon, there is a remarkable increase in the intra-seasonal phenomenon north of the EASWJ, which may be closely related to eddies that propagate from upstream regions in the jet; these studies also noted that the study of intra-seasonal characteristics and mutation mechanisms of the EASWJ is not deep enough. Many scholars have studied the relationship between the EASWJ and precipitation in East Asia and concluded that variations in the location and intensity of the EASWJ have an important effect on precipitation intensity and the distribution of precipitation in East Asia (Lin and Lu, 2009). However, it was found in other case studies that latent heat during the rainy period can also influence zonal wind on a sub-seasonal time scale (Yao et al., 2014; Jin et al., 2012).
In summary, the study of intra-seasonal variations in the EASWJ is insufficient. In addition, analyses have generally regarded characteristics in summer, and there has been little research about the winter EASWJ ISO. Therefore, using the ECMWF ERA-Interim 6-h reanalysis data in this paper, the ISO of zonal wind at the entrance of the EASWJ is studied, and intra-seasonal evolution characteristics, thermal propagation mechanisms, and relationships with precipitation in southern China are also studied. The rest of the paper is arranged as follows: Section 2 is for data and methods, Section 3 describes the characteristics of the intra-seasonal zonal wind, Section 4 explains the thermodynamic mechanism of the zonal wind 10–30-day oscillation, Section 5 gives the relationship between precipitation and the zonal wind 10–30-day oscillation, and Section 6 is the conclusion.
Section snippets
Data and methods
The data used in this paper are ECMWF ERA-Interim 6-h reanalysis data, which includes zonal wind, meridional wind, vertical velocity, precipitation and temperature. The horizontal resolution of the data is 1.5° × 1.5°. Observation station rainfall data are also used in the paper. The station data we used are provided by China Meteorological Administration (CMA), and the website of the data is http://data.cma.cn/data/cdcdetail/dataCode/SURF_CLI_CHN_MUL_DAY_V3.0.html
In this paper, winter is
Characteristics of the zonal wind ISO in the entrance of the EASWJ
According to observational data, the winter EASWJ is strongest at 200 hPa (Zhang et al., 2006); therefore, the zonal wind at 200 hPa is chosen for analysis. Fig. 1 shows the mean zonal wind and the standard deviation of the zonal wind for 34 winters. It is clear that the speed of the mean zonal wind center in winter is over 60 m s−1, which is located near 30°N. To focus on the intra-seasonal time scale, the seasonal cycle signal is removed first. This allows the Gaussian low-pass filter to be
The thermodynamic mechanism of the zonal wind 10–30-day oscillation
The study above shows that the zonal wind 10–30-day oscillation presents an eastward propagation, an enhancement near the entrance of the EASWJ, and that characteristics of the upper levels are consistent with those in the low levels of the vertical structure. Here, thermodynamics are used to diagnose and further explain the spatial structures and propagation mechanisms. It is found (Fig. 6) that the 10–30-day variation in zonal wind increases with height. Based on the thermal wind theorem (Eq.
The relationship between precipitation and the zonal wind 10–30-day oscillation
It can be seen from Fig. 9, Fig. 10 that diabatic heating mainly appears in the mid-troposphere, which corresponds to a strong vertical velocity (Fig. 11). In addition, vertical velocity is closely associated with rainfall. Therefore, the 10–30-day filtered precipitation is further analyzed during zonal wind oscillation events.
Both reanalysis precipitation data and the observed station precipitation data are used in this section. Observed and reanalysis precipitation data are compared. The
Conclusions
By analyzing the ISO characteristics of zonal wind near the entrance region of the EASWJ during 34 winters from 1979/1980 to 2012/2013, it is found that there is a large-value area of intra-seasonal variability to the north of the EASWJ entrance during winter. There are three active and typical dominant periods that pass the 95% reliability test in this region with the following scale cycles: less than 10 d, 10–30 d, and 30–60 d cycles. Among them, the oscillation strength of the 10–30 d time
Acknowledgments
This work was jointly supported by the National 973 Project under grants 2015CB453200, Chinese National Natural Science Foundation41775096, 41405066, and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
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