Impacts of northern Tibetan Plateau on East Asian summer rainfall via modulating midlatitude transient eddies

Abstract

Roles of the Tibetan Plateau (TP) in forming and changing the seasonal Asian climate system have been widely explored. However, little is known about modulation effects of the TP on extratropical transient eddies (TEs) and subsequent synoptic responses of the East Asian rainfall. In this study, the Community Atmosphere Model version 5.1 coupled with a slab ocean model is employed to highlight the important role of the TP in regulating the upper‐tropospheric transient wave train. Comparison between sensitivity experiments with and without the TP shows that the northern TP excites a strong anomalous anticyclone, which shifts the upper‐level East Asian westerly jet northward and helps transfer barotropic and baroclinic energy from the mean flow to the synoptic TE flow. The transient wave train is primarily shifted northward by northern TP and is forced to propagate southeastward along the eastern flank of the TP until reaching eastern China. Before the strengthening of monsoonal southerlies, the TP‐modulated transient wave train cools the troposphere, which decreases the static stability over northern China. Meanwhile, the associated anomalous warm advection induces ascending motion, leading to excessive rainfall by releasing unstable energy as the southerly strengthens. Due to the southeastward propagation of the wave train, anomalous heavy rainfall subsequently appears over eastern China from north to south, which increases day‐to‐day rainfall variation in this region. Additionally, occurrence of this upper‐tropospheric transient wave train associated with low‐level southerly peak is substantially increased by northern TP.

Water vapour transport changes associated with the interdecadal decrease in the summer rainfall over Northeast Asia around the late‐1990s

“…In the following, we will concentrate on analysing the stationary water vapour transport. Considering that Northeast Asia is under the control of the mid‐latitude westerly, extratropical cyclones and transient vortex are active and have close connections with the moisture budget anomalies during individual precipitation events (Sun et al ., 2010; Xie et al ., 2012; Deng et al ., 2017). Hence, further studies are needed to demonstrate the role of the transient water vapour transport for these events, which is beyond the scope of this study.…”

Influence of the southwards shift of North American continent on North American monsoon

“…Overall, the meridional position of the North America continent plays a fundamental role in determining the size of NAM, while the Rocky Mountains act to amplify this effect. These results are generally consistent with previous studies (e.g., Xu et al ., 2009; Deng et al ., 2017; He, 2017). Moreover, the roles of other large terrains in shaping the NAM deserve further investigation in the future.…”

“…Previous studies have reported that the NAM region is bounded by the geography and topography, such as Mexican Plateau, Sierra Madre Occidental and the Rocky Mountains, which would help to shape the monsoonal circulation features via the mechanical and thermal effects (Tang and Reiter, 1984; Adams and Comrie, 1997; Gutzler et al ., 2009; Varuolo‐Clarke et al ., 2019). However, the dynamic and thermal effects of large terrains on circulation and precipitation in the adjacent areas are still being debated (e.g., Liang et al ., 2006; Deng et al ., 2017). In this study, the Rocky Mountains are moved southwards along with the continent; thus, whether or not the presence of the Rocky Mountains is the dominant factor for the NAM is not studied.…”

“…It can be coupled to ocean and land models, or be used stand‐alone (Neale et al ., 2010; Vertenstein et al ., 2011; Varuolo‐Clarke et al ., 2019). Many studies have shown that CAM5.1 can reasonably simulate the mean state of monsoon precipitation, in both stand‐alone mode (e.g., Han et al ., 2017) and coupled mode (e.g., Deng et al ., 2017). The CAM5.1 used in this study has a resolution of 1.9° (latitude) × 2.5° (longitude), and a σ – p hybrid vertical coordinate of 30 levels, with the top level at 3.643 hPa.…”

“…Moreover, the presence of large terrain, such as the Tibetan Plateau, modifies the land surface characteristics, which amplify the land–sea thermal contrast (Manabe and Terpstra, 1974; Kuo and Qian, 1982; Dirmeyer, 1998; Liang et al ., 2006; Chen and Bordoni, 2014). For example, the uplift heating of the Tibetan Plateau acts to enhance the land–sea thermal contrast, which exerts great influences on the establishment and maintenance of the circulation and thereby the formation and development of the Asian monsoon (Wu and Zhang, 1998; Sato and Kimura, 2007; Wu et al ., 2007; Tang et al ., 2013; Deng et al ., 2017). Therefore, the interactions among solar radiation, land–sea distribution, large terrain, and other drivers determine regional characteristics of monsoon (Zeng and Li, 2002).…”