Unveiling the dynamics of shallow fronts in Australia during Southerly Buster episodes (1994–2020)

Abstract

A Frontal Identification System (FIS), initially designed to track Galernas in the Bay of Biscay, has been adapted to monitor cold fronts across Australia using wind shifts derived from ERA5 hourly reanalysis data. This high-resolution system tracks shallow, cloud-free fronts during the warm season, which can trigger bushfires, dust storms, extreme heat, and coastal weather extremes like Southerly Busters (SB) on the east coast. SB episodes, marked by sudden, squally south winds, pose hazards for boating and aviation. Our analysis of 35 SB events from 1994 to 2020 indicates that SBs originate from frontogenesis in Bass Strait (40 %) or from prefrontal troughs crossing the strait (60 %). Preceding synoptic conditions involve a Southern Ocean cold front driving cool maritime winds into the Australian thermal low, creating shallow convergence fronts (∼1000 m deep) facing warm continental winds. Onshore acceleration into the thermal low sharpens these new fronts (Type 2 fronts of the southern coast) and weakens the trailing primary ocean front, which may disappear due to high-pressure wave propagation, cold advection, and subsidence over the sea. These fronts can penetrate deep inland (Central Australian fronts) and initiate SBs on the southeast coast after interacting with the Great Dividing Range. All 35 SB events show active shallow front frontogenesis/frontolysis affecting the southern coast and inland regions. Upper-level reversed pressure gradients between the thermal low over the continent and the ocean depression maintain a wind shear region over the shallow inland cold advection. Intense warm north-westerlies south of the surface front, with wind speeds of 35–50 m s⁻¹ between 700 and 550 hPa, contribute to mesofront formation preceding SB episodes. This jet also sustains strong cross-mountain winds over the Great Dividing Range, causing the lee trough at the coastal strip that precedes all SB episodes on the eastern coast. Understanding these dynamics can help predict and manage these events more effectively.