北半球夏季のダブルジェットの力学変動

Abstract

This study investigates the dynamical variability of the double-jet structure in the Northern Hemisphere summer months of June and July. The summer northern annular mode (NAM), which is the leading mode of the empirical orthogonal function analysis in the extratropical troposphere for each month, is used for this analysis. The strong positive NAM is characterized as the double jets with the subtropical jet located at around 40°N and the polar front jet (the eddy drive jet) at 60°–70°N; the strong negative NAM is characterized as the single jet with the dominant subtropical jet at around 40°N. The vertical profile of momentum budget at 60°–85°N in the strong NAM months has peaks of the EP flux convergence and the poleward absolute vorticity advection at 300 hPa. The difference of zonal wind between the strong positive and negative NAM is estimated at around 5 m s⁻¹.