Elsevier

Atmospheric Research

Volume 258, 15 August 2021, 105611
Atmospheric Research

Effect of circulation variation associated with East Asian jet on spring rainfall over North China and Yangtze-Huaihe River Valley

https://doi.org/10.1016/j.atmosres.2021.105611Get rights and content

Highlights

  • Anomalously intensified EAWJ causes negative rainfall anomalies ineastern China (EC)

  • TP causes northerly component of EAWJ-related westerly anomalies over EC by blocking the southerly momentum flowing into EC..

  • The EAWJ-related northwesterly anomalies over EC cause cold advection anomalies over EC and transport dry air toward EC.

  • The cold advection and resultant descent anomalies reduce the rainfall in EC together with the reduced moisture anomalies.

  • Quasi-barotropic feature of these circulation anomalies suggests the external forcings of EAWJ for spring rainfall anomalies.

Abstract

The present study investigates the association of East Asian westerly jet (EAWJ) variations with spring rainfall anomalies in Northern China and Yangtze-Huaihe River Valley (NC-YHV) and the dynamics using reanalysis datasets. Based on the climatology and interannual variation in 200-hPa zonal winds, the index EAWJI is defined as the average 200-hPa zonal wind velocity over a zonal 10-degree-width belt centered around the seasonal-mean jet axis between 105°E and 145°E. Associated with anomalously strengthened EAWJ, significant negative rainfall anomalies are observed over NC-YHV. The dynamics are as follows. When the EAWJ is anomalously intensified, a quasi-barotropic Pacific-Japan-like (PJ) teleconnection along coastal China and an associated anomalous westerly flow over NC-YHV are observed. In middle-lower troposphere, Tibetan Plateau (TP) drastically reduces the anomalous southwesterly momentum transported into NC-YHV, turning the westerly anomalies into northwesterly anomalies. The anomalous northwesterly winds over NC-YHV advect cold and dry air southeastward toward NC-YHV, which induce downward motion (diabatic heating feedback is weak) and negative moisture anomalies, respectively, and thus cause reduced rainfall anomalies over NC-YHV. Anomalous winter SSTAs in western Pacific and tropical Indian Ocean associated with ENSO are sustained until spring, inducing barotropic waves that propagate northwards to cause EAWJ-associated circulation anomalies and thus bring about spring rainfall anomalies in NC-YHV. The quasi-barotropic features of the EAWJ-associated circulation anomalies and their association with the northward propagation of tropical SSTA-induced barotropic waves together suggest that EAWJ-associated circulation variations are at least partly among the external forcings responsible for spring rainfall anomalies in NC-YHV.

Introduction

A long, zonally elongated rainband extends from southern China (referring to the part of China south of 30°N in the present study) northeastwards through the East China Sea to the sea area around southern Japan in spring months (Xu et al., 2011; Okada and Yamazaki, 2012). The maximum rainfall throughout mainland China is located over the southern Yangtze River Valley, and rainfall gradually decreases northwestwards over China (Wang et al., 2002; Qian et al., 2002; Pan et al., 2013). Variations in spring rainfall drastically affect the ecological environment and socioeconomics in eastern China (referring to the part of China east of 105°E) (Sun and Yang, 2012; Yao et al., 2015; Li et al., 2019). Thus, the climatology, variation in spring rainfall in eastern China and their dynamics draw much attention from the climate research community.

Existing studies conventionally attribute the formation of persistent spring rainfall in southern China to lower-tropospheric southwesterly winds and the associated moisture transportation over southern coastal China (Tian and Yasunari, 1998; Li and Liang, 2010). The thermal and mechanical effects of the Tibetan Plateau and the thermal contrast between the land and ocean in spring may together cause southwesterly winds that form spring rainfall (Wan and Wu, 2007; Wan and Wu, 2009; Tian and Yasunari, 1998; Zhu et al., 2012). A large amount of literature exists that has concentrated on the causes of spring rainfall anomalies in eastern China. The lower-tropospheric southwesterly wind anomalies over southeastern coastal China are usually considered to be closely associated with anomalously increased rainfall in southern China (e.g., Wu et al., 2003; Zhu et al., 2014; Pan et al., 2013; Yuan et al., 2018). These rainfall anomalies can be further attributed to external forcings such as sea surface temperature anomalies (SSTAs) in the Pacific and Indian Ocean and snow cover anomalies over Siberia (Wu et al., 2003; Wu and Kirtman, 2005; Ren et al., 2017; Du et al., 2016; Yuan et al., 2019).

The East Asian subtropical westerly jet (EAWJ) is one of the most important weather systems that exist year-round over East Asia (Schiemann et al., 2009; Kuang and Zhang, 2005). Linho et al. (2008) indicates that the EAWJ plays an essential role in the formation of the climatological-mean rainband over eastern China in spring. Existing studies also suggest that the variation in the EAWJ may be associated with spring rainfall anomalies in eastern China. Huang et al. (2015) revealed that the intensification of the EAWJ can result in increased persistent rainfall events in spring in southern China, while Wu et al. (2016) and Wang et al. (2002) suggested that anomalously intensified EAWJ is associated with decreased spring rainfall amounts in southern China. Many more studies have taken the variation in EAWJ as one of the connectors between variations in spring rainfall in southern China and external climate variabilities. These external climate variabilities include the Arctic oscillation, Arctic sea ice anomalies (Wu et al., 2016), SSTAs in the central-eastern parts of the Pacific Ocean (Zhu et al., 2014; Wu and Mao, 2016; Yan et al., 2019) and Indian Ocean (Liu and Duan, 2017; Yuan et al., 2019) and anthropogenic aerosols (Deng et al., 2014). EAWJ-associated horizontal wind anomalies in the upper and lower troposphere and the resultant moisture transportation, lower- and upper-tropospheric convergence/divergence and vertical velocity anomalies are usually considered to be the causes of EAWJ-associated rainfall changes (e.g., Wu et al., 2016; Huang et al., 2015).

These studies suggested that the variation in EAWJ is essential in the rainfall variations over eastern China in spring, as it is in summer (Li and Zhang, 2014; Wang et al., 2018; Wang et al., 2019). However, it is still necessary to carry out further study to resolve the following problems. First, existing studies usually explained EAWJ-associated spring rainfall changes in eastern China involving circulation anomalies in the upper or lower troposphere. However, the circulation anomalies that exist on these levels are drastically affected by condensation heating related to rainfall anomalies (Lu and Lin, 2009; Sampe and Xie, 2010). Thus, it is difficult to determine whether EAWJ-associated circulation variations are among the large-scale environmental forcings responsible for spring rainfall anomalies in eastern China based on the previous results. Second, existing studies conventionally focused on spring rainfall anomalies over southern China. Although the climatological mean spring rainfall amount in Northern China and Yangtze-Huaihe River Valley (NC-YHV, here referring to 30°-42.5°N, 105°-120°E) is much smaller than that in southern China, rainfall variations still frequently cause severe disasters in NC-YHV in spring (Qu, 1988; Fang et al., 2014). In fact, the present study suggests that the anomalous circulation associated with leading mode of EAWJ variation drastically affects rainfall anomalies in NC-YHV, which are even stronger than the EAWJ-associated rainfall anomalies in southern China.

To resolve these problems, we conduct a systematic analysis to study the effect of the EAWJ-associated interannual variations of horizontal circulation on spring rainfall interannual anomalies in NC-YHV. The dynamical mechanism of this effect is also investigated with more attention given to isolating the large-scale environmental external forcings of these circulation variations for spring rainfall changes in NC-YHV. Here, the term “external forcing” refers to the environmental influence of large-scale circulation anomalies on rainfall variation in NC-YHV in addition to the effects of SSTA or land-air interactions on rainfall variation, which are most commonly used in existing studies. This method is similar to those of existing studies such as Sampe and Xie (2010).

The rest of the present study is organized as follows. Section 2 briefly summarizes the data and methods that will be used in the present study. Section 3 briefly summarizes the climatology and interannual variation in the circulation at the 200-hPa level in spring, based on which the index characterizing the variation in the EAWJ is defined and the influence of the EAWJ-associated circulation variation on rainfall anomalies in NC-YHV in spring is analyzed. The dynamical processes associated with these impacts will be explored in Section 4 from the perspective of the external forcing of EAWJ-associated large-scale circulation anomalies on spring rainfall anomalies in NC-YHV. A brief conclusion and discussion will be presented in Section 5.

Section snippets

Data and method

The present study uses the daily specific humidity, horizontal wind velocity, vertical wind velocity and air temperature data derived from the ERA5 reanalysis to characterize large-scale atmospheric circulation variation (Dee et al., 2011). Monthly rainfall data derived from the Global Precipitation Climatology Project (GPCP) Version 2.3 Combined Precipitation Dataset from January 1979 to July 2019 is utilized to describe rainfall anomalies in eastern China (Huffman et al., 2001). We focus on

Effect of the variation in the EAWJ on spring rainfall in eastern China

To analyze the effect of the EAWJ-associated circulation variation on spring rainfall changes in eastern China, it is necessary to define an index to characterize the variation in the EAWJ. Fig. 1 shows the climatology of the zonal wind velocity (shading) at 200 hPa level in spring. A belt of large westerly wind velocity extends eastwards toward southern Japan through the East China Sea (enclosed by the black line) through East China Sea until southern Japan, and is centered at approximately

Dynamical processes of the effect of EAWJ-associated circulation variation on spring rainfall anomalies

This section analyzes the dynamical processes responsible for the impact of EAWJ-associated horizontal circulation variations on spring rainfall anomalies in eastern China. It is well known that variations in rainfall are primarily controlled by the vertical velocity and water vapor supply changes. Variations in vertical velocity are investigated based on the thermodynamic equation, which can be described as follows:Tt¯+uTx+vTy¯Spω¯=QCp¯ABCBwhere Q denotes the diabatic heating/cooling, Sp

Conclusion and discussion

The East Asian westerly jet (EAWJ) is an important weather system over East Asia that drastically affects the climate variability in eastern China in spring. The present study investigates the effect of EAWJ-associated circulation variations on spring rainfall changes in NC-YHV. Based on the climatology and interannual variation in the 200-hPa zonal wind velocities, the key region of the EAWJ is identified as the region containing all the points that are no more than 5 latitudinal degrees away

Author statement

Shixin Wang: Conceptualization, Methodology, Writing- Original draft preparation.

Yonghui Hou: downloading ERA-interim data, graphing.

Shiyi Zhou: collecting ERA5 data.

Hongchao Zuo: Supervision.

Fen Sun: English editing.

Jing-Jia Luo:Writing- Reviewing and Editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

This study was supported by the National Natural Science Foundation of China (41805052, and 42030605), the Second Tibetan Plateau Scientific Expedition and Research Program (STEP, Grant No. 2019QZKK0103) and the Innovation and Entrepreneurship Training Plan for College Students (201810300003z).

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