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

Abstract

Motivated by the record-breaking heatwaves of early 2017, the synoptic structure and evolution of summer (December–February) heatwaves in the Sydney area is investigated through composite and trajectory analyses. In the upper troposphere, the main features of the composite structure are an isolated upper-tropospheric anticyclonic potential vorticity (PV) anomaly to the south-east of Australia and cyclonic anomalies to the east and south. Back trajectories starting from within the upper-tropospheric anticyclonic PV anomaly on the first day of the heatwave fall into two groups: those that are diabatically cooled in the final 72 h and those that are diabatically heated. Those that are cooled come predominantly from the upstreammiddle troposphere over the Indian Ocean. The change in the potential temperature of these parcels is less than 3K, and so their motion is effectively adiabatic. In contrast, those parcels that are heated in the final 72 h are drawn predominantly from the lower half of the troposphere over the south-western part of the continent. As they ascended, their potential temperature increases by 10K in the mean due to latent heating. At low-levels, the main features of the composite are an anticyclone centred in the Tasman Sea, a broad low over the Southern Ocean and associated anomalous warm northwesterlies over the Sydney area. Five days prior to the heatwave, air parcels that become part of the near surface air mass are located predominantly offshore to the east and south of the continent. The anomalously high surface temperatures can be explained by adiabatic compression and surface sensible heating. For the next 48 h, the air parcels subside and their potential temperature changes little, whereas their temperature increases by around 15Kthrough adiabatic compression. In the final 72 h, as the parcels approach the surface and are entrained into the boundary layer, the potential temperature and temperature both increase by 5K, presumably through surface sensible heating. The record-breaking heatwaves of January and February 2017 are found to be very representative of previous heatwaves in the Sydney area, and in the mean they are synoptically very similar to heatwaves in Victoria, although dynamically there are differences.

Recurrent Rossby waves during Southeast Australian heatwaves and links to quasi-resonant amplification and atmospheric blocks

“…TAS, experience longer hot spells during periods when RRWPs occur. Interestingly, Northern Australia does not show such an association, which agrees with previous studies showing different dynamical pathways for Northern and Southern Australian heatwaves(Parker et al, 2019;Quinting et al, 2017). Other statistically significant areas over land include parts of South February 2004 heatwave (7-22 February) lasted for 16 days.…”

“…Broadly, heatwaves in SEA (Fig. 1), comprising the states of Victoria (VIC), New South Wales (NSW), South Australia (SA), and Tasmania (TAS), are associated with slow-moving transient anticyclonic upper-level potential vorticity (PV) anomalies over the Tasman Sea (e.g., Marshall et al, 2013;Parker et al, 2014a;Quinting et al, 2017;Parker et al, 2019).…”

Atmospheric and Land Surface Contributions to Heatwaves: An Australian Perspective

“…Research focused on identifying the role of atmospheric blocking for heatwave initiation and persistence has involved both composite analysis (e.g., Gibson et al, 2017; Loughran et al, 2018; Parker et al, 2019; Parker, Berry, Reeder, & Nicholls, 2014; Pfahl & Wernli, 2012; Risbey et al, 2017; Tozer et al, 2020) and Lagrangian trajectory analysis (e.g., Pfahl et al, 2015; Quinting et al, 2018; Quinting & Reeder, 2017; Schumacher et al, 2019). Composite analysis involves extracting relevant atmospheric fields (e.g., pressure and potential vorticity) for days classified as part of a heatwave event and aggregating them to identify key synoptic features associated with these events.…”

Exploring Potential Links Between Co-occurring Coastal Terrestrial and Marine Heatwaves in Australia

“…No equivalent local sensible heating of the air occurs during the THWs in Ningaloo and Hobart (Figures 8E,F). Previous studies using lagrangian tracking of air parcels have also suggested that adiabatic warming of descending air in the hours and days prior to a THW may play an important role in these events (Quinting and Reeder, 2017;Quinting et al, 2018;Parker et al, 2019). Although the composite analysis of vertical velocity hints at the possibility of these THWs being influenced by the vertical motion of air, other techniques would be needed to determine the relative importance adiabatic warming.…”