A climatological northern boundary index for the East Asian summer monsoon and its interannual variability

A long-term perspective on the spatial variation of the northern boundary of the East Asian summer monsoon(EASM) and the related physical mechanisms is important for understanding past climate change in Asia and for predicting future changes. However, most of the meteorological definitions of the EASM northern boundary do not correspond well to the actual geographical environment, which is problematic for paleoclimatic research. Here, we use monthly CMAP and GPCP precipitation data to define a new EASM northern boundary index by using the concept of the global monsoon, which is readily applicable to paleoclimatic research. The results show that the distribution of the 2 mm day^(-1) precipitation isoline(i.e., 300 mm precipitation)has a good relationship with the spatial distribution of modern land cover types, the transitional climate zone and the potential natural vegetation types, in China. The locations of the precipitation isolines also correspond well to the locations of major shifts in wind direction. These results suggest that the 2 mm day^(-1) isoline has a clear physical significance since the climatic, ecological,and geographical boundary can be used as the northern boundary index of the EASM(which we call the climatological northern boundary index). The index depicts the northeast-southwest orientation of the climatological(1981-2010) EASM northern boundary, along the eastern part of the Qilian Mountains-southern foothills of the Helan Mountains-Daqing Mountains-western margin of the Greater Khingan Range, from west to east across Northwest and Northeast China. The interannual change of the EASM northern boundary from 1980 to 2015 covers the central part of Gansu, the northern part of Ningxia, the eastern part of Inner Mongolia and the northeastern region in China. It can extend northward to the border between China and Mongolia and retreat southward to Shangdong-central Henan. There is a 200-700 km fluctuation range of the interannual EASM northern boundaries around the locations of the climatological northern boundary. In addition, the spatial variation of the interannual EASM northern boundaries gradually increases from west to east, whereas the trend of north-south fluctuations maintains a roughly consistent location in different regions.

“North-South”dipolar mode of precipitation changes in eastern China extends to the Last Deglaciation

Changes in the East Asian summer monsoon (EASM)are closely related to human wellbeing,and thus improving our knowledge of the evolution of the EASM is of major socioeconomic importance, as well as being of scientific interest.Previous studies have shown that during the Holocene (the present interglacial)a "North-South" dipolar mode of precipitation change occurred over eastern China (i.e.the so-called "northern drought-southern flood"scenario, or vice versa)on decadal [1,2],centennial [3]and millennial timescales ([4]and references therein).A recent high-resolution speleothem trace element-based precipitation record from Haozhu Cave in the Yangtze River Valley (YRV)[5],together with a pollenbased quantitative precipitation reconstruction from Lake Gonghai in North China [6],further suggests that this dipolar mode extended to the Last Deglaciation [5].This inference is based on the recorded anti-phased variation of precipitation between North China and the YRV during the Younger Dryas (YD),Bolling-Allerod (B-A),and part of Heinrich Event 1(H1)(Fig.1a,b).