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1. Introduction
According to the IPCC AR5, climate warming during the recent 30 years is the strongest since 1850 and the global mean air temperature has risen about 0.85°C during 1880-2012 (IPCC 2013). The Eurasian continent is one of the regions with the most significant warming (Hansen et al. 2010). As the largest continent, the land surface thermal anomalies of the Eurasian continent are very important to both regional and large-scale atmospheric general circulations. Recent studies suggested that significant summer land surface warming has been observed in the middle latitudes over East Asia, especially after the mid-1990s (Zhu et al. 2012; Chen and Lu 2014; Chen et al. 2016; Dong et al. 2016, 2017; Chen et al. 2017; Hong et al. 2017). Such warming has close relationships with both temperature and precipitation changes in China as well as with the East Asian summer monsoon circulation (Li et al. 2010; Xu et al. 2011; Zhu et al. 2012; Chen and Lu 2014; Chen et al. 2017). Therefore, it is necessary to investigate possible reasons for the changes in the anomalous thermal conditions over this region before understanding their impacts on weather and climate.
With regard to the reasons for recent land surface warming over the Eurasian continent, previous studies partly attributed the surface thermal anomalies over the Eurasian continent to the atmospheric circulation anomalies caused by the external forcing of the sea surface temperature and sea ice. Such large-scale atmospheric general circulation anomalies play important roles in the decadal variation of the surface temperature over Northeast Asia (Sun et al. 2008; Sutton and Dong 2012; Zhu et al. 2012; Gao et al. 2014; Dong et al. 2016, 2017; Hong et al. 2017). Based on numerical experiments, Dong et al. (2016) suggested that changes in sea surface temperature (SST) and sea ice can explain about 76% of the abnormal warming signal over Northeast Asia. Hong et al. (2017) found that the Atlantic multidecadal oscillation (AMO) can also modulate the activity of the atmospheric Silk Road teleconnection pattern and further amplify the land surface warming in Northeast Asia. In addition, more recent studies emphasized the impacts of greenhouse gases and anthropogenic aerosol emissions on the warming of the continent (Zhu et al. 2012; Dong et al. 2016, 2017)....