The synoptic-dynamics of summertime heatwaves in the Sydney area (Australia)
Tess Parker A D , Julian Quinting A B and Michael Reeder A CA School of Earth, Atmosphere and Environment, Monash University, Building 28, 9 Rainforest Walk, Clayton, Vic. 3800, Australia.
B Institute of Meteorology and Climate Research (IMK-TRO), Karlsruhe Institute of Technology, Karlsruhe, Germany.
C ARC Centre of Excellence for Climate Extremes, Monash University, Clayton, Vic., Australia.
D Corresponding author. Email: tess.parker@monash.edu
Journal of Southern Hemisphere Earth Systems Science 69(1) 116-130 https://doi.org/10.1071/ES19004
Submitted: 19 August 2018 Accepted: 5 April 2019 Published: 11 June 2020
Journal Compilation © BoM 2019 Open Access CC BY-NC-ND
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 upstream middle troposphere over the Indian Ocean. The change in the potential temperature of these parcels is less than 3 K, 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 10 K 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 north-westerlies 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 15 K through 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 5 K, 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.
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