Dynamical processes controlling the evolution of early-summer cut-off lows in Northeast Asia

Abstract

Cut-off lows are crucial extratropical circulation systems that can bring weather extremes over large areas, but the mechanism responsible for the life cycle of cut-off lows remains elusive. From a perspective of regional eddy-mean flow interaction, this study investigates the dynamical processes controlling the evolution of early-summer cut-off lows over Northeast Asia using the 6-hourly reanalysis data. Through the diagnostic of local wave activity (LWA) budget, we show that the cut-off low is initialized by a Rossby wave train originated from the subpolar North Atlantic, and then reinforced rapidly by zonal LWA flux convergence and local baroclinic eddy generation, and eventually decayed through energy dispersion by zonal wave activity advection. Furthermore, we show that the evolutions of the above dynamical processes are strongly modulated by the changes of background flow. In early summer, Northeast Asia is located at the eastern exit of the midlatitude jet to the north of the subtropical jet and exhibits a weak meridional gradient of potential vorticity, which favors frequent formation of cyclonic anomaly and energy accumulation. Prior to the onset of cut-off lows by several days, a Rossby wave train propagates along the Eurasian midlatitude jet, which initializes a cyclonic anomaly over Northeast Asia. With the aid of mean flow advection of anomalous zonal momentum, the zonal winds are then decelerated at the midlatitude jet exit and accelerated at the subtropical jet center. The former obstructs the wave packet proceeding downstream and the latter favors stronger baroclinic eddy generation below the subtropical jet. The two processes together maintain and strengthen the cyclonic anomaly in Northeast Asia rapidly.