Rossby wave breaking morphologies on the Southern Hemisphere dynamical tropopause

Abstract

Rossby waves are fundamental meteorological structures in the extratropics of both hemispheres. Several extremes and weather regimes have been linked to the amplification and breaking of Rossby waves propagating along the extratropical waveguide. The morphology and evolution of Rossby wave breaking (RWB) on the Southern Hemisphere dynamical tropopause is studied here through an objective classification algorithm. Although the well-known classifications of RWB morphologies (LC1, LC2, P1, and P2) work well, the objective algorithm identifies important distinctions between RWB events with higher and lower amplitude structures and more meridional and zonal orientations. The different morphologies reflect the differences in the structure of the Rossby wave packets, the degree of phase locking with the low levels, the strength of the diabatic processes, and the amplitude of the nonlinearities in the flow. Anticyclonic RWB morphologies are associated with the decay of a Rossby wave packet, often producing a cyclonic–anticyclonic potential vorticity (PV) dipole in its wake. The cyclonic PV cutoff in this PV debris field can be stirred back into the extratropical waveguide resulting in a cyclonically overturned PV contour. Unlike anticyclonic RWB morphologies, cyclonic RWB morphologies in the Southern Hemisphere upper troposphere are associated with Rossby wave packet generation and downstream development. SIGNIFICANCE STATEMENT : This work aims to determine how well the well-known and much-used two- and four-type classifications of Rossby wave breaking (RWB) represent the full range of morphologies and analyze the differences in their evolution. An objective clustering technique identifies important distinctions between high- and low-amplitude patterns and more meridional and zonal orientations that the two- (anticyclonic, cyclonic) and four- (P1, P2, LC1, and LC2) type classifications do not. Anticyclonic RWB leads to the decay of a Rossby wave packet, whereas cyclonic RWB generates another Rossby wave packet and downstream development. This fundamental difference in the evolution, together with differences in the physical processes shaping these morphologies, deepens our understanding of RWB in the Southern Hemisphere upper troposphere.