Southeast China Extreme Drought Event in August 2019: Context of Coupling of Midlatitude and Tropical Systems

Abstract

An extreme drought occurred over Southeast China (SEC) in August 2019. We demonstrate synergistic effects of midlatitude and tropical circulation on this extreme event and highlight the impacts of the coupling and locking of two cyclones at different latitudes, which are otherwise ignored. We propose the relaying roles of the Tibetan Plateau (TP) and western North Pacific in connection with the tropical convection and SEC precipitation. The equivalent-barotropic anticyclone over the TP and lower-tropospheric cyclone over the western North Pacific both resulted from the positive Indian Ocean dipole and El Niño Modoki. The equivalent-barotropic cyclone over Northeast China originated from the dispersion of Rossby waves upstream along the subtropical waveguide associated with the North Atlantic tripole sea surface temperature anomaly pattern and the Rossby wave response to the TP precipitation deficiency. Further, they jointly contributed to this drought by inducing strong northerly wind anomalies in the entire troposphere over East China. These anomalous northerly winds led to decreased warm moisture from the south and substantial sinking motions, which inhibited the occurrence of the SEC local convection and precipitation. The SEC precipitation is closely related to convection over the Maritime Continent from a climate perspective. This relationship is verified by observations, linear baroclinic model experiments, and general circulation model sensitivity experiments with and without the TP, in which precipitation anomalies over the southern TP and Philippine Sea play important bridge roles. The results will advance the prediction of the SEC extreme drought events.