A Statistical Study of the Dynamics of Blocking

Abstract

An objective method is used to isolate instances of blocking ridges from instances of transient ridges for 10 winters of Northern Hemisphere data. The vorticity budgets and heat budgets of the blocking ridge cases are compared statistically with those of transient ridges for both the Pacific and Atlantic oceanic regions. It is demonstrated that there are statistically significant differences in the vorticity and heat budgets of blocking compared to transient ridges, which may be related primarily to changes in zonal advection. While the dominant scale of the vorticity pattern associated with blocking is represented by a zonal scale of about 60° of longitude, the local zonal wind reductions associated with blocks are contributed primarily by wavenumber 1 (?40%) with smaller contributions from the zonal mean (?20%). The most important mechanisms for sustaining blocking ridges are found to be different in the Atlantic and Pacific regions. In the Pacific the major difference between blocking and transient ridges is reduced eastward advection of vorticity, whereas in the Atlantic the blocking ridges are more baroclinic than the transient ridges.