Blocking systems over an aqua planet

It is well established that forced planetary waves are critically important for blocking formation. To test whether blocking can be generated and maintained in the absence of topographic forcing, we carry out aqua-planet simulations using NCAR CAM2. Results show that blockings occur frequently under the aqua-planet conditions which have no forcing such as land-sea contrast, orography and stationary remote tropical forcing. Features of simulated blockings well resemble that in the real atmosphere, showing typical W-like patterns, persistence, and quasi-stationary behavior with occasionally westward shifting. It is found that the onset and maintenance of simulated blockings are due to interaction between quasi-stationary free Rossby waves and baroclinic eddies. For 10-year simulations, the blocking frequency is slightly higher than that in the Northern Hemisphere (NH) in the real atmosphere. These suggest that blocking is a natural consequence of rotating and baroclinic atmospheres, while locally topographic forcing and remote tropical forcing are not necessary conditions.