Environmental interactions during the extreme rain event associated with ex-tropical cyclone Oswald (2013)

Abstract

Tropical cyclone (TC) Oswald made landfall over north-east Australia as a minimal or Category 1 TC on the Australian scale on 21 January 2013. As it moved southward, it intensified over land and produced extreme rainfall for nearly 7 days. Tornadoes were reported and confirmed. Tragically, seven people died and insurance estimates were ~$1 billion. It is demonstrated that the event was associated with an interaction between the ex-Oswald circulation and an amplifying Rossby wave, which propagated north-eastward from high latitudes. Diagnoses showed that as the wave amplified and broke, a potential vorticity (PV) anomaly (PVA) extended to mid-levels, moved equatorward, merged with or axisymmetrised the ex-Oswald circulation through mid-levels. Backward trajectories from locations regularly scattered within the mid-level circulation illustrated that the storm transitioned from an isolated vortex into a circulation which was strongly influenced by its environment for at least 5 days. During this interaction, PV was advected from the environment towards the storm through mid-levels. The heavy rain coincided with the commencement and maintenance of this PV injection. The PV injection is quantified and shown to be consistent with PV advection by the mean radial flow. In addition, eddy angular momentum convergence in the mid- to upper levels coincided with an intensification of the circulation through this region. This was first related to outward transport of anticyclonic momentum by the asymmetric outflow at upper levels, followed by inward transport of cyclonic momentum by the asymmetric inflow. It is shown that the environmental interaction had an impact on vortex structure changes, rainfall and tornado development. We propose that the environmental processes influenced the ascent within the storm (1) via differential vorticity advection and baroclinic forcing, as the mid- to upper level PVA approached the circulation and (2) by low- to mid-level warm air advection.

The meteorology of the 2019 North Queensland floods

“…Anticyclonic Rossby wave breaking also results in PV streamers that can extend into the Tropics and interact with or initiate tropical weather systems (Berry et al ., 2012; Hoang et al ., 2017; Leroux et al ., 2020). Indeed, a PV streamer and later cut‐off low is found near the monsoon low around January 26 (see Figure 7).…”

“…Although monsoon lows are tropical weather systems, they may be affected by disturbances from the midlatitudes (Berry et al ., 2012). For example, midlatitude Rossby wave breaking results in PV streamers that can either feed into and reintensify an existing tropical low (Leroux et al ., 2020) or initiate new ones (Hoang et al ., 2017). Similarly, monsoon bursts (sudden periods of heavy rain) are associated with an increased cyclonic circulation resulting primarily from absolute vorticity fluxes from the midlatitudes (Narsey et al ., 2017).…”

East coast lows and extratropical transition of tropical cyclones, structures producing severe events and their comparison with mature tropical cyclones

“…Severe lows over the period 2000-2020 are well represented in the lists of east coast floods (Callaghan and Power 2014), lists of severe ocean damage (Callaghan 2020) and Victorian ECLs (Callaghan 2019a) so studying this 20-year period should represent the period 1967-2020 well. The only events that did not form downstream of TUs were where the TU was already interacting with a TC advancing pole-wards such as ex-TC Oswald 2013 (Leroux et al 2019). The later part of this paper describes this general interaction in detail.…”

“…There are exceptions to the processes described above and one was related to the tragic circumstances associated with the Sydney Hobart Yacht Race of 1998. The other was the interaction of ex-TC Oswald with an upper trough (Leroux et al 2019). In both these cases there were more complex interactions between PV centres near a TU and the msl lowpressure system.…”

Weather systems and extreme rainfall generation in the 2019 north Queensland floods compared with historical north Queensland record floods

“…Falls of up to 1321 mm in 4 days in the Barron and Herbert districts 6.6 Extreme rainfall from extratropical transition -ex TC Oswald 2013 and ex TC Sid 1998 Both events occurred when the remnants of TCs interacted with deep layered trough systems in a similar way to extratropical transition events (Jones et al 2003) and the rainfall in both cases was characterised by winds which turned anticyclonic from 850 up to 500 hPa. From Leroux et al (2020) a record daily rainfall of 348.0 mm was recorded at Rockhampton on 25 January 2013 when ex TC Oswald moved down through eastern Queensland. A total of 6500 properties across 90 towns were either damaged or uninhabitable across Queensland, costing an estimated A$2.4 billion.…”