On the maintenance of blocking anticyclones in a general circulation model
Blocking anticyclones that appear in perpetual January simulations of a general circulation model are examined. Blocks in three geographical regions are studied: the North Pacific, the North Atlantic and Western North America. Local time-averaged balances of vorticity and heat are evaluated for composite cases of blocking. The following common relationships emerged from these budgets.The time-mean divergence term is, in general, a first order term throughout the troposphere and its pattern over severe orography is closely related to the underlying topography. Above the surface layer, the horizontal advection of time-mean absolute vorticity by the mean wind mainly balances the divergence term with the net effect of the time-mean vorticity forcing being a tendency for the blocking pattern to propagate downstream. The transient eddy vorticity transports act in the sense as to shift the block upstream and hence they mainly offset the downstream tendency due to the time-mean flow; the magnitude of the eddy vorticity term is typically one-third to one-half that of the divergence or advection terms alone. Frictional dissipation is negligible everywhere except near the ground where it strongly offsets the divergence term.The horizontal advection of the time-mean field by the mean wind throughout the troposphere is a first order term and is mainly responsible for maintaining the block's thermal perturbations; it is predominately balanced by adiabatic heating in the free troposphere and by diabatic heating near the surface. Transient eddy heat transports act to dissipate the block's thermal perturbations at all levels, while diabatic heating does not exhibit a systematic relationship with the temperature field at any level.It appears that dynamical processes which strongly affect the vorticity balance (i.e., barotropic processes) may be more important to the maintenance of model blocks than processes which strongly affect the heat balance (i.e., baroclinic conversions). The mountains appear capable of influencing the shape of the model blocks through the divergence term, but preliminary results indicate that orographic forcing may not be absolutely essential for the blocking process to occur in the model.
- Atmospheric sciences