Interannual variations of spring drought-prone conditions over three subregions of East Asia and associated large-scale circulations

For proactive prevention of water shortage problems, it is important to understand the spatiotemporal variabilities of droughts and their development mechanisms. This study identified an interannual variation of spring drought-prone condition in each region of East Asia (EA)-northern, mid, and southern EA-for the period 1979-2015 by using the standardized precipitation evapotranspiration index (SPEI) and a hierarchical clustering analysis. According to our analyses, spring drought-prone condition in the northern EA region mainly depends on local surface heating and precipitation deficit due to adiabatic compression by a high-pressure anomaly over the region during the preceding winter to spring. The high-pressure anomaly was driven by an upper-level wave train propagating from the Europe related to a positive phase of the Arctic Oscillation. For drought-prone conditions in the mid and the southern EA regions, the local precipitation deficit caused by the negative moisture advection/convergence therein acts as a primary factor. During the preceding winter, the negative moisture advection/convergence anomalies both in mid and southern EA regions were induced by anomalous low-level cyclonic circulations over the western North Pacific related to the La Nina. In spring, however, the moisture condition in the southern EA region was affected by persistent La Nina while that in the mid-EA region was by the negative phase of North Pacific Oscillation. This gap makes the main location of the negative moisture advection/convergence anomalies just slightly different, but considerably alter the region exposed to the drought-prone condition.