Decreasing Influence of Summer Snow Cover Over the Western Tibetan Plateau on East Asian Precipitation Under Global Warming

In recent years, some studies emphasized the influence of western Tibetan Plateau summer snow on the East Asian summer precipitation. With the temperature rise in the past decades, the snow cover over the western Tibetan Plateau in summer has significantly decreased. This raises the question whether the impact of the Tibetan Plateau snow has changed. The present study identifies a prominent change in the influence of the western Tibetan Plateau snow cover on the East Asian summer precipitation. Before the early 2000’s, positive precipitation anomalies extend from the southeastern Tibetan Plateau through the Yangtze River to Japan and Korea and negative anomalies cover southeast China corresponding to more Tibetan Plateau snow cover. After the early 2000’s, with the reduction of snow cover variability, below-normal and above-normal summer precipitation occurs over northern China-Mongolia and northeast Asia, respectively, corresponding to more Tibetan Plateau snow cover. The change in the influence of the Tibetan Plateau snow on the East Asian summer precipitation is associated with an obvious change in the atmospheric circulation anomaly pattern. Before the early 2000’s, the wind anomalies display a south-north contrast pattern with anomalous convergence along the Yangtze River. After the early 2000’s, an anomalous cyclone occupies Northeast China with anomalous southerlies and northerlies over northeast Asia and northern China, respectively. The Tibetan Plateau snow cover variation after the early 2000’s is associated with the northeast Indian summer precipitation. The model experiments confirm that the weakened influence of summer western Tibetan Plateau snow cover on the East Asian atmospheric circulation and precipitation with the reduced snow cover anomalies.

Impact of Eurasian Spring Snow Decrement on East Asian Summer Precipitation

In this study, the relationship between Eurasian spring snow decrement (SSD) and East Asian summer precipitation and related mechanisms were investigated using observational data and the Community Atmospheric Model, version 3.1 (CAM3.1). The results show that a west–east dipole pattern in Eurasian SSD anomalies, with a negative center located in the region between eastern Europe and the West Siberia Plain (EEWSP) and a positive center located around Baikal Lake (BL), is significantly associated with East Asian summer precipitation via triggering an anomalous midlatitude Eurasian wave train. Reduced SSD over EEWSP corresponds to anomalously dry local soil conditions from spring to the following summer, thereby increasing surface heat flux and near-surface temperatures. Similarly, the increase in SSD over BL is accompanied by anomalously low near-surface temperatures. The near-surface thermal anomalies cause an anomalous meridional temperature gradient, which intensifies the lower-level baroclinicity and causes an acceleration of the subtropical westerly jet stream, leading to an enhanced and maintained Eurasian wave train. Additionally, the atmospheric response to changed surface thermal conditions tends to simultaneously increase the local 1000–500-hPa thickness, which further enhances the Eurasian wave train. Consequently, significant wave activity flux anomalies spread from eastern Europe eastward to East Asia and significantly influence the summer precipitation over China, with more rainfall over northeastern China and the Yellow River valley and less rainfall over Inner Mongolia and southern China.