资源环境科技发展态势分析平台

Persistent drought is a major meteorological disaster causing vast agricultural damage and long-term regional water crises. Over north China, this type of drought tends to begin in spring and to persist until summer with severe societal impacts. This paper aims to identify the large-scale dynamic drivers of the prolonged spring-summer drought (PSSD) over north China. This analysis has shown that the north China PSSD is associated with a persistent anomalous low-level cyclonic circulation spanning the midlatitude North Pacific from spring to summer with reduced moisture transport to north China, in combination with a tropospheric cooling along the East Asian upper-level westerly jet with dynamically forced anomalous descent above. Seven of the selected eight north China PSSD events occurred when La Nina transitioned to El Nino with a negative North Pacific Oscillation (NPO) phase in the preceding winter. The two key circulation anomalies in spring are set directly by a negative NPO phase generated in the preceding winter. The NPO, in turn, forces an El Nino onset in summer through the so-called seasonal footprinting mechanism. Consequently, sea surface temperature anomalies of El Nino in summer suppress Indian monsoon rainfall, triggering the tropospheric temperature cooling over East Asia through a circumglobal teleconnection along the East Asian upper-level westerly jet. Modeling analysis of the long preindustrial control simulation confirms the above findings. This study demonstrates that ENSO phase transition from La Nina to El Nino is one of the important precursors of north China PSSD.