Numerical simulation of a life-cycle of atmospheric blocking and the analysis of potential vorticity using a simple barotropic model

Abstract

In this study, we conducted a series of numerical experiments to investigate atmospheric blocking, using a simple barotropic model that featured a wavemaker to excite synoptic disturbances. The model has a resolution equivalent to R20 and consists of only five physical processes: a wavemaker as baroclinic instability, topographic forcing, biharmonic diffusion, zonal surface stress, and Ekman pumping. Results of time integrations show that persistent dipole blockings appear one after another in the model, showing a reasonable life-cycle. In the model atmosphere, the synoptic disturbances are amplified exponentially by the wavemaker. The exponential growth soon saturates with nonlinear scattering of energy from synoptic to planetary waves associated with a Rossby wave breaking. The analysis of potential vorticity (PV) indicates that the onset of blocking is brought on by the Rossby wave breaking. The overturning of high and low PVs tends to occur at the topographic stationary ridge. Once a block is formed by the Rossby wave breaking, subsequent Rossby waves are blocked and undergo meridional stretch. The stretched wave then breaks down, depositing fresh low PV at the north and high PV at the south of the blocking system to maintain the block. The result is consistent with the so-called eddy straining mechanism. The result suggests that the exponential growth of synoptic disturbances is essential both for the onset and the maintenance of blocking.