Article
BAMOS Dec 2024
17
Understanding the dynamics behind these events
The key to understanding these events lies in the upper troposphere . Although wet weather in the tropics is typically associated with the Australian monsoon , dry-season events are associated with transient Rossby waves propagating across the continent from the extratropics . When these large-scale atmospheric waves break , the flow becomes highly distorted , often leading to the formation of cut-off lows — low-pressure systems which become detached from the main flow . The upward motion on the eastern flank of these systems can lead to extensive and prolonged rainfall , as observed during August and September 2024 .
August rainfall event
Figure 2 highlights the synoptics of heavy rainfall and minor flooding over the Queensland and northern New South Wales coast in mid-August , where over 150 mm was recorded over several days along the coastline stretching between Coffs Harbour and MacKay . As a result of Rossby wave breaking , a high-amplitude trough was eventually cut off from the main westerly flow , producing a relatively slow-moving cut-off low , shown by the closed contour in the 500hPa geopotential height field in Figure 2a . According to quasi-geostrophic theory , upward vertical motion must occur on the eastern flank of the cut-off . Unsurprisingly , this is exactly what we observed here , with the upward motion shown by the red colours in Figure 2a . This upward motion led to significant precipitation over the region . The Rossby wave breaking event leading to the formation of this cut-off low can be observed in the upperlevel potential vorticity ( PV ) field as an overturned PV structure ( Figure 2b ). Overturning is visible as a northwest-southeast tilted large-magnitude PV structure ( pink colours ), stretching from the east coast of Australia towards New Zealand .
The Rossby wave breaking event also produced an upper-level ridge which can be observed as an anticyclonic PV anomaly over the Tasman Sea ( blue-white colors in Figure 2b ). The surface expression of this upper-level ridge is a surface anticyclone in the Tasman Sea ( Figure 2d ). This surface anticyclone plays an important role in this synoptic setup , enhancing easterly flow along the east coast and driving moisture from the Coral Sea onto the continent .
Figure 2 : Synoptic-dynamics of rainfall along the east coast of Australia between 10-15 August 2024 . Key components of the east coast rainfall event are highlighted with arrows . Data source : European Centre for Medium-Range Weather Forecasts ( ECMWF ) reanalysis ( ERA5 )