Southern Hemisphere summertime Rossby waves and weather in the Australian region

Abstract

Motivated by recent studies on the significant summertime weather patterns in the Australian region, the life cycle of summertime transient Rossby wave packets in the Southern Hemisphere is investigated. The strength of the interaction I between the wave packets and the jet is defined by the advection of potential vorticity by the divergent part of the wind on the 350 K isentropic surface. The magnitude of I is used as the basis for time‐lagged composites in distinct regions: southwest Chile, southwest Africa, over the Indian Ocean, southwest Australia, and over the West Pacific. The interaction is strongest on the stratospheric side of the jet core. Although in each case cyclogenesis from a pre‐existing disturbance in the upper troposphere precedes the generation of the Rossby wave packets, there are two types of interaction with the jet. In the souhwest Chile, southwest Africa and Indian Ocean cases, extratropical cyclogenesis and the associated convection lies on the poleward side of the jet, and hence I is positive. In the southwest Australia and West Pacific cases, extratropical cyclogenesis occurs 3 days earlier than the others, and I is negative as it describes the interaction of a downstream propagating Rossby wave packet with the subtropical jet. In all cases, as the cyclones develop, the wave activity propagates from low levels to the upper levels of the troposphere, and then downstream and equatorwards towards higher values of refractive index. The equatorward refraction of the southwest Australia and West Pacific wave packets is accompanied by enhanced tropical convection in the warm conveyor belt of the advancing front.