Abstract
In this study, local energetics analysis is conducted for the blocking formation in the North Pacific, in order to investigate the condition of a transient ridge to become a blocking. Among the total number of 452 ridges, 88 are identified as blocking during 51 winters from 1950 to 2001. Kinetic energy budget is then performed in the framework of the vertical mean and sheared flows, and the energetics terms, including the barotropic-baroclinic interactions, C(Ks,Km), are analyzed. As a result, we find that a blocking becomes Ω type for large C(Ks,Km), and it becomes dipole type for small C(Ks,Km). It is also shown for the large CðKs;KmÞ, that a ridge develops to a blocking when the flux convergence of mechanical energy of the mean flow, B(Km + φm), is positive around the ridge. On the contrary, a ridge flows away downstream when B(Km + φm) is negative there. The positive B(Km + φm) around the blocking is associated with the enhanced negative BðKm þ fmÞ at the upstream jet, due to the intensified mechanical energy flux from the upstream jet. Therefore, it is found that the sign of the flux convergence of mechanical energy around the transient ridge is the condition for the ridge to become a blocking.
