抄録
In the present study it is attempted to explain the mechanism of blocking formation in the Atlantic in connection with Rossby wave propagation from the upstream side. For this purpose, assuming a realistic monthly-mean non-blocking basic flow, the nondivergent vorticity equation on the sphere is numerically time-integrated in the linear and nonlinear models, with steady and non-steady (traveling) forcings located in the southern side of the North Atlantic jet.
Our numerical experiment can reproduce a blocking formation as follows. When the Rossby wave induced by traveling forcing propagates along a North Atlantic jet and the portion of the anticyclonic vorticity in the Rossby wavetrain reaches high latitudes (near 60°N), its amplitude is increased in the jet exit region. This process can be understood as a linear evolution, since it can be simulated by the linear model also. After this process, the meridional separation of induced anomalies takes place and the blocking flow is formed as a distinct dipole structure with positive anomaly (i.e., negative vorticity) northward and negative anomaly (i.e., positive vorticity) south of it, so that the blocking flow becomes quasi-stationary. This process cannot be simulated by the linear model. Therefore, nonlinear effects are considered to play a critical role in this process.
