Static Stability Associated with Blocking Onsets in the Southern Hemisphere

The variation of static stability prior to blocking onset is characterized through composite analysis of fourteen blocking events in the Southern Hemisphere. It is found that a local minimum of static stability in the upper troposphere and on the tropopause is achieved over the block-onset region when blocking onset takes place. From the perspective of isentropic potential vorticity, blocking onset is accompanied by extratropical tropopause elevation and a local low isentropic potential vorticity anomaly is formed right under the elevated tropopause. This low isentropic potential vorticity anomaly is coincident with a local minimum of static stability over the block-onset region. In addition, based on static stability budget analysis, it reveals that the decrease of static stability in the upper troposphere and on the tropopuase prior to blocking onset is attributable to horizontal advection of low static stability from subtropics to midlatitude as well as the stretching effect associated with upper-level convergence, with the former forcing being the primary contributor. On the other hand, the diabatic heating term is found to largely enhance the local static stability over the block-onset region. In addition, the vertical advection of static stability term is also found to oppose the decreasing static stability through advecting more stable air downward and, therefore, stabilize the local air over the block-onset region.