Eddy contributions at multiple timescales to the evolution of persistent anomalous East Asian trough

Abstract

Persistent strong and weak East Asian trough (EAT) cases during boreal winter are captured by projecting 12 UTC geostrophic vorticity onto that of a seasonal strong EAT. The persistent cases are investigated in terms of seasonal, intraseasonal, and synoptic temporal scales. The evolution of persistent strong and weak EAT cases displays characteristics of intraseasonal temporal variation. The onset and decay of strong (weak) cases is associated with the passage of an intraseasonal negative (positive) 500 hPa height anomaly from Northeast Asia to the midlatitude central Pacific, through the region of the EAT. The onset and decay stages of persistent strong (weak) EAT cases occur in conjunction with a seasonal stronger-than-normal (weaker-than-normal) EAT. In addition, it is noted that the difference between the number of strong and weak cases exhibits significant covariability with the strength of the seasonal EAT. Meanwhile the seasonal EAT also plays a role in the development and decay of persistent cases. The contributions of dynamic and thermodynamic processes during the evolution of persistent strong and weak EAT cases are investigated. Since the forcing of horizontal temperature advection and that of adiabatic and diabatic processes are likely to offset each other, dynamic processes make a more important contribution to the evolution of the EAT than do thermodynamic processes. Furthermore, horizontal absolute vorticity advection is dominant in the dynamic processes and is caused mainly by the advection of synoptic and intraseasonal vorticity by seasonal wind. The seasonal wind transports positive vorticity to (away from) the EAT, which results in a remarkable strengthening (weakening) of the EAT during the onset (decay) stage for persistent strong EAT cases.