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

In this study, we conducted a series of numerical experiments to investigat e atmospheric blocking, using a simple barotropic model that featured a wav emaker to excite synoptic disturbances. The model has a resolution equivale nt to R20 and consists of only five physical processes: a wavemaker as baro clinic instability, topographic forcing, biharmonic diffusion, zonal surfac e stress, and Ekman pumping. Results of time integrations show that persistent dipole blockings appear o ne after another in the model, showing a reasonable life-cycle. In the mode l 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 br eaking. The analysis of potential vorticity (PV) indicates that the onset o f blocking is brought on by the Rossby wave breaking. The overturning of hi gh 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 s o-called eddy straining mechanism. The result suggests that the exponential growth of synoptic disturbances is essential both for the onset and the ma intenance of blocking.