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.

The flow and moisture fluxes associated with ridging South Atlantic Ocean anticyclones during the subtropical southern African summer

“…The Rossby wave intensifies as it propagates north‐eastward towards the South African domain and the initiation of the ridging process coincides with positive MSLP anomalies as the latter propagates past the country and associated southward meridional excursions of air parcels aloft (red contours to the west of South Africa). The development and propagation of the Rossby wave trains are very similar to those described in O’Brien and Reeder (2017), even though the waves in that study exhibit a higher degree of wave reflectivity. Its eastward propagation is along the midlatitude waveguide (Ambrizzi et al 1995) that is provided by the midlatitude jet (O’Kane et al 2016).…”

The synoptic-dynamics of summertime heatwaves in the Sydney area (Australia)

“…Further afield, low-level wind anomalies were westerly in tropical north-west Australia, consistent with an active phase of the Australian monsoon, and were associated with the enhanced rainfall over the north of the continent. As shown by Parker et al (2013Parker et al ( , 2014 for heatwaves in Victoria, convection in the tropics may lead to enhancement of the upper level anticyclone either indirectly, through perturbation of the wave guide and subsequent wave amplification, or directly, through material advection of low-PV air from regions of moist convective outflow.…”

“…In this way, the diabatic processes contribute to the subsequent amplification of the upper-tropospheric anticyclonic anomaly. In addition, diabatically enhanced divergent outflow advects anomalously anticyclonic PV air poleward, further amplifying the anticyclone (Parker et al 2013). The upper-tropospheric anticyclone is accompanied by a surface anticyclone centred east of the continent over the Tasman Sea, and this anticyclone plays a major role in producing anomalously high temperatures at the surface in Victoria.…”

“…This results in different thermodynamic histories for these air parcels. In contrast to heatwaves over south-eastern Australia, which are associated with strong uppertropospheric anticyclonic circulations (Parker et al 2013; Quinting and Reeder 2017), strongly diabatic heatwaves in the Brisbane region are characterised by a midlatitude cyclonic PV anomaly to the south-east of the continent with a horseshoe of weaker anticyclonic PV on the eastern flank. Weakly diabatic heatwaves, on the other hand, show an upper-tropospheric anticyclonic PV anomaly close to the Brisbane region, with regions of cyclonic PV to the south and east (Quinting et al 2018).…”

On the Relationship Between the Madden‐Julian Oscillation and the Hadley and Walker Circulations

“…Such regions are not necessarily the source of the waves. O'Brien and Reeder (2017) have pointed out that I represents the deformation of the PV contours by the divergent part of the wind and can be interpreted as the local forcing of the barotropic Rossby wave equation in isentropic coordinates. The Rossby wave source S on the other hand represents the forcing of the barotropic vorticity equation in pressure coordinates.…”