The Characteristics of Flood‐Drought Abrupt Alternation in North China During Midsummer and Its Response to Atlantic Sea Surface Temperature

Abstract

ABSTRACT Based on the ERA5 reanalysis data, precipitation data provided by the China Meteorological Administration, and the Met Office Hadley Center Sea Ice and SST dataset, this study investigates the characteristics of midsummer flood‐drought abrupt alternation (FDAA) in North China and its correlation with Atlantic sea surface temperature (SST) by using the water vapour budget equation and the moist static energy (MSE) budget equation. The results indicate that FDAA, primarily driven by the anomalous vertical advection of moisture, occurs frequently in North China during midsummer, with dynamic processes playing a dominant role. In drought‐to‐flood years, the blocking high on the eastern side of the Ural Mountains guides cold air southward, while the western Pacific subtropical high (WPSH) is anomalously extended northward, leading to the convergence of cold and warm air over North China. Concurrently, North China is located on the northwest side of an anomalous anticyclone that extends from the Korean Peninsula to Japan over the northwest Pacific in the lower troposphere. This configuration induces southerly winds that transport high moist enthalpy air, especially warm air, into North China, favouring the transition from drought to flood. The opposite pattern is observed in flood‐to‐drought years. Quantitative analysis using the dynamic adjustment method reveals that the circulation component exhibits spatial correlations of 0.72 and 0.59 with the wind anomalies in DTFs and FTDs, respectively. Thus, the anomalous anticyclone (cyclone) in the northwest Pacific plays an important role in influencing FDAA in North China during midsummer. FDAA in North China is closely related to the tropical Atlantic SST anomaly (SSTA). In positive SSTA years, the tropical Atlantic SST warming in July can enhance convective activity in the Atlantic basin, which alters the Walker circulation and produces a sinking motion in the central Pacific. An anticyclonic circulation is generated over the northwest Pacific and centered around 20° N. Under the influence of prevailing northerly winds, precipitation decreases, resulting in a dry period in North China. The positive SSTA in the tropical Atlantic, which persists from July into August, promotes a northward shift of the anticyclonic circulation towards the southeast of North China in August. At this time, southerly winds prevail in North China, leading to anomalous upward motion and resulting in a transition from drought to flood. In negative SSTA years, the opposite pattern occurs.