Dynamical Processes of Block Evolution

Abstract

The dynamical processes associated with block evolution are investigated by analyzing a GCM run, forced with perpetual January conditions. The core of the analysis lies on the temporal evolution of the blocks and on vorticity budget terms obtained from appropriate compositing procedures on a 350-mb model output. The results from the budget analysis are examined with barotropic model experiments, which allow the investigation of the influence of an individual dynamical process on block evolution. Results are presented for two composite blocks, one close to the Atlantic storm track and the other farther downstream. Although these two blocks are found to develop differently, they share the following characteristics. During the decay linear processes dominate, and the high- and low-frequency eddy fluxes contribute equally toward prolonging the lifetime of the blocks by 2 to 3 days. While the time average of the budget yields results that are consistent with previous diagnostic studies, it is shown that such an approach exaggerates the role played by high-frequency eddies. The barotropic model experiments show that the nonlinear self-interaction of the composite block anomaly plays a minimal role in the block evolution. It is the remaining part of the composite low-frequency eddy flux that contributes significantly toward the block evolution, indicating that case-to-case variability of the individual blocking events can be substantial, and that the nonlinearity of a slowly moving, nonsteady component of the flow plays an important role for the individual blocking events. The model experiments also demonstrate that the effect of divergence is crucial for correctly reproducing the structure of the blocking high. The implications of these results, as they apply to some of the prominent blocking theories, are also discussed.