The Role of Deformation and Potential Vorticity in Southern Hemisphere Blocking Onsets

The relative importance of interactions between deformation and potential vorticity (PV) as a block-onset mechanism is examined in thirty cases of atmospheric blocking over the Southern Hemisphere (SH). The blocking cases are diagnosed with a quasigeostrophic model for the U-component of the geostrophic wind tendency. In this model, two mechanisms, the advection of the meridional gradient of PV, and interactions of deformation and PV, can force the weakening of westerly flow or increasing easterly flow associated with blocking. The first forcing mechanism, which does not directly include deformation, indicates that the advection of equatorward increasing cyclonic PV (or equatorward decreasing anticyclonic PV) could force a local weakening of geostrophic westerlies or increasing easterlies. The second forcing mechanism, which represents the net effect of interactions between deformation and PV, indicates that eastward increasing PV embedded in a cyclonically sheared flow, or equatorward increasing PV coincident with a stretching (diffluent) flow, could each force a weakening in the westerlies.

While deformation is a distinct signature of blocking, it may not always actively participate in the formation of blocking. Advection and interaction contributions generally opposed each other in both the diagnosed blocking and nonblocking cases. Weakening westerlies associated with block onset would occur when one effect (usually the advection effect) contributes more negatively to the wind tendency than the opposing, positive contribution from the other effect. When deformation is actively involving the formation of blocking, self interactions between synoptic-scale PV and deformation, and self interactions between planetary-scale PV and deformation contribute more importantly than synoptic-to-planetary-scale interactions between PV and deformation fields to the weakening of westerlies associated with block onsets.