Rossby Wave Propagation and Blocking Formation in Realistic Basic Flows

Abstract

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.