Summer droughts in the northern Yellow River basin in association with recent Arctic ice loss

摘要

Drought is one of the most devastating natural disasters occurring in the Yellow River basin (YRB) in Central and North China, with strong impacts on human societies and natural environments. Based on datasets from ECMWF/ERA-Interim on atmospheric circulation and sea level pressure and on rainfall data from meteorological networks from 1979 to 2013, droughts in the YRB are investigated in the present work. Rainfall in summer in the northern YRB (semi-arid and semi-humid zones) has been low since 1990, which is associated with an anomaly of planetary waves around Lake Baikal. These waves are significantly affected by Arctic ice loss (particularly in the eastern Arctic) among other factors. Eastern Arctic ice loss results in a warmer surface and increases air temperatures over the Arctic and northern Siberia. These conditions are favourable for cyclonic anomalies in the lower troposphere, generating an ascending flow anomaly over the eastern Arctic and a positive anomaly of meridional wind in the low troposphere between high latitudes and the Arctic. This situation results in a weaker Polar cell and a stronger Ferrel cell in the three-cell latitudinal circulation, due to increasing the ascending flow at approximately 60 degrees N as well as the descending flow at approximately 35 degrees-53 degrees N. These conditions favour reinforcement as well as the width of subtropical high pressures, meridional shifts or northward elongation, the anomaly of planetary wave ridges around Lake Baikal, finally, more severe droughts in the northern YRB. Simulations performed with an atmospheric general circulation model confirm that Arctic sea ice loss can induce a northward shift and enhancement of subtropical high pressures, together with decreasing rainfall in the mid-latitudes of both North America and East Asia, especially in the northern YRB.