An investigation of heat source effect of Tibetan Plateau on the wintertime India-Burma Trough
Introduction
Located in central Asia with an average elevation over 4500 m and an area of 2,500,000 km2, the Tibetan Plateau (TP), called the ‘Third Pole’, is the most extensive highland in the world (Kang et al., 2010). Such high elevation and area can have both dynamic (terrain obstacle) and thermal effects (heat source) on modulating the regional weather and climate over the Asia (FLohn, 1957; Yeh and Chang, 1974; Feng et al., 1998; Liu et al., 2007; Shi et al., 2017). Dynamically, the TP divides the incoming westerlies into two branches in the south and north sides of TP separately (Ye and Gao, 1979; Bothe et al., 2010). Thermodynamically, the heat source exerts force on atmosphere and influences the climate (Shen et al., 1984; Rao and Erdogan, 1989; Duan and Wu, 2008 & Duan and Wu, 2009; Lai and Gong, 2017). During past decades, influences of climate change have been found over many regions in the world, especially in semiarid and arid regions. For instances, global drylands have experienced a significant warming due to multifarious causes, which have increased during past 60 years in semiarid regions and are projected to expand in the 21st century (Huang et al., 2017; Guan et al., 2019). In many regions of China, the frequency, duration, and intensity of heat wave events have a striking increase after –1997 (Wei et al., 2020b). Over the past three decades, the TP has also experienced significant climate changes (Wang et al., 2008; Kang et al., 2010; Ma et al., 2017) and has become one of the most sensitive regions to the global warming (Yao and Zhu, 2006).
The India-Burma Trough (IBT) originates from the southern branch of the disturbed westerlies blocked by TP (Ye and Gao, 1979; Ding, 1992; Suo and Ding, 2009). The IBT exits in the winter half year (usually from October to the following May) and thus is considered as a semi-permanent low-pressure trough (Suo and Ding, 2009). The IBT has been a key system affecting the weather and climate over Southern Asia, especially in winter (Li and Zhou, 2016). In particular, it is a crucial weather system influencing the circulation over Southwestern and Southern China and regulating the process of moisture transport from the Bay of Bengal (BOB),which plays an important role in rainfall modulation over these regions (Wang et al., 2011; Li and Zhou, 2016; Shi et al., 2017; Zhang et al., 2017; Li et al., 2017). In winter, short-term heavy precipitation events over Southwestern China are associated with the notable enhancement of the IBT (Wang et al., 2009; Zhou et al., 2009; Zong et al., 2012). For example, a heavy snow event over Southern China during January and February in 2008 is attributed to the prominent augmentation of the IBT (Wang et al., 2009; Zhou et al., 2009). Meanwhile, droughts in Southwestern China in autumn, winter, and spring are closely associated with significant strengthening of the IBT (Pang and Qin, 2013; Wang et al., 2014; Wang et al., 2017; Rong et al., 2018). Therefore, it is of great significance to investigate the IBT-related climatic phenomena and influencing factors under the global warming.
Previous studies focused on the IBT mainly investigated its contribution to the circulation anomalies over Asia, suggesting that variations of IBT may be responsible for circulations far away from the TP, such as the stationary Rossby wave propagation along the jets and cold air over the plateau (Suo et al., 2008), the El Niño–Southern Oscillation (ENSO) (Li et al., 2007), Arctic Oscillation (AO), the circulation over tropical ocean and East Asian (EA) winter monsoon (Li, 2011). Although the contribution of IBT to various circulations is not negligible, studies concentrating on the influencing factors of IBT with relevant mechanism are worth conducting and quite complicated owing to combined influence of multiple-factor and modulation of various geographic features (Wang et al., 2011). On the other hand, thermal effect over the TP, with significant magnitude and scope of forcing, has been implied as a key element accounting for variation of the IBT (Ramage, 1952; Chen et al., 2000; Zhao and Chen, 2001; Liu et al., 2007; Nan and Zhao, 2011). For instance, the increasing snow accumulation can influence the air pressure over TP and winter monsoon, enhancing the IBT and increasing summer rainfall over the mid-to-lower parts of the Yangtze River basin (Chen et al., 2000). Further studies are worth investigating the relations between the thermal effect of TP and IBT, as well as corresponding physical mechanism.
Many indices have been used to characterize the thermal effect of TP. Studies showed the close correlations between the snow cover over TP and summer monsoon over EA, as well as ENSO (Wu et al., 2012; Xiao and Duan, 2016). The snow depth was employed to characterize the intensity of winter heating over TP (Zhu et al., 2008; Si and Ding, 2013). However, the snow area and snow depth over the TP varied during past decades and the area of permafrost has been undergoing extensive shrinkage due to global warming (Cheng and Wu, 2007). Other indices, such as air temperature alone and air-surface temperature, were also used to represent thermal forcing of TP (Boos and Kuang, 2010; Luo et al., 2018), but it only partially represented the thermal forcing due to diurnal variation of solar radiation and resulted in different understandings in terms of duration for the heating (Xu et al., 1990; Chen et al., 2000). Therefore, more reasonable indices need to be explored in order to fully understand the relationships between the thermal effect over TP and IBT.
In this respect, the soil temperature over TP could be employed considering its performances on characterizing the underlying forcing on the climate over alpine regions as well as EA (Wu et al., 2012; Zhang et al., 2017). However, the reliability and availability of observations may be limited owing to missing or unreleased data, sparse observations, cost for obtaining at field level from automated measurements (Finn, 2002). Datasets driven physically by models could provide opportunities for fully representing thermal forcing information over the TP (Hasfurther and Burman, 1974; Hansen et al., 1990; Sándor et al., 2017). Consequently, a multi-layer soil temperature dataset from reanalysis product will be used in this study, which also contained the characteristics of vertical structure for soil temperature. It was investigated that the soil temperature records over China differs spatially and varies with different soil depth and season (Yang and Zhang, 2016). In Canada, the study showed that the soil temperature below 5 cm has a consistent upward trend in most areas (Qian et al., 2011). In the United States, soil temperature at 10 cm was increasing in the north and northwest regions, while decreasing in the southeast (Hu and Feng, 2005; Hu and Feng, 2003). On the other hand, the influences of soil temperature at different depth on climate also vary over different regions. For instances, a good relationship between 0.8 m soil temperature in winter and spring precipitation anomaly appeared in China (Tang et al., 1987), while the spring subsurface soil temperature anomaly in the western USA can influence the summer precipitation of North America (Xue et al., 2012).
The objective of this study is to investigate the thermal effect of TP on IBT during the wintertime in China. This paper is organized as follows: Section 2 introduced the dataset, methods, and definitions of the intensity of IBT and the indices of thermal effect over TP. Section 3 analyzed and compared the statistical characteristics of thermal effect over TP and intensity of the IBT, including the trend and abruption periods. Furthermore, we examined the correlations between the IBT and thermal effect over TP and the influence mechanism behind them. Finally, Section 4 gave a conclusion of this work.
Section snippets
Data
The monthly reanalysis datasets used in this study are obtained from the Earth System Research Laboratory of National Oceanic and Atmospheric Administration (NOAA) including: (1) geopotential height, meridional wind, zonal wind, vertical speed, specific humidity, and surface pressure from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) Reanalysis dataset on Linear projection with resolution of 2.5° × 2.5° grid (Kalnay et al., 1996); (2)
Inter-annual variation of wintertime IBTI
The inter-annual variation of IBTI anomalies and periods of abruption are presented in Fig. 3. A significant increase in IBTI over 60 years is detected (Fig. 3a), which indicates a decreasing trend of IBT over theses years. On the other hand, an abrupt period for 1975–1977 both captured by M-K and moving t-test also suggests a significant weakening IBT since mid-1970s (Fig. 3b), which is considered reasonable compared with the time series for anomalies of IBTI shown in Fig. 3a. The results also
Conclusions
The IBT is formed in low levels of atmosphere and influenced by both dynamic and thermodynamic effect of the TP. The main purpose of this study is to investigate whether certain correlations between the IBT and the thermal effect of TP exist. After analyses, we reach the conclusions as follows:
- 1)
The relationships between multi-layer soil temperature of TP and IBT exhibit a significant seasonal variation. Generally, the whole year on the plateau can be divided into two stages based on intra-annual
Declaration of Competing Interest
We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
Acknowledgments
We are grateful to The National Key Research and Development Program of China (13th Five-Year Plan, 2016YFA0601504), Key International (Regional) Cooperation and Research of the National Natural Science Foundation of China (51420105014) for providing funding, and the National Natural Science Foundation of China (41605043), the Fundamental Research Funds for the Central Universities (2017B00114). Cordial thanks are extended to the editor Dr. Alan Haywood and anonymous reviewers for their
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Role of solar activity and Pacific decadal oscillation in the hydroclimatic patterns of eastern China over the past millennium
2022, Global and Planetary ChangeCitation Excerpt :For the neutral PDO phase (Fig. 11b), an anomalous weakening high ridge centered on NWC promoted the precipitation in parts of NC and middle and lower reaches of the Yellow River. Moreover, it was suggested that the eastward retreat of the weakening WPSH with an intensive India–Burma trough may induce higher moisture concentrations in parts of SC (Rong et al., 2020). For the warm PDO phase (Fig. 11c), a weakening WPSH combined with the intensified deep East Asian trough and the India–Burma trough would also promote high moisture concentrations in parts of SC and SEC.