10
BAMOS Dec 2024
Article
Melbourne ’ s recent storm surge event : the future new normal ?
Dr Ben Hague , Bureau of Meteorology ( ben . hague @ bom . gov . au ) Dr Danielle Udy , Bureau of Meteorology ( danielle . udy @ bom . gov . au )
In late August and early September , Melbourne experienced a multi-day storm surge event leading to widespread flooding and road closures in various locations .
The event was linked to the passage of five strong cold fronts over Southern Australia and coincided with a meteotsunami — a tsunami-like wave generated by storms . During this event , sea levels reached 1.06 m above reference levels , marking the thirdhighest level since 1966 .
As sea levels continue to rise due to climate change , the high sea levels and impacts observed during this recent event could become the " new normal ", with chronic flooding occurring in the city and along the coast . This trend could have significant consequences for infrastructure , ecosystems , and communities .
Here , we explore the recent storm surge and meteotsunami event in Melbourne and discuss how it may offer a glimpse into a future new normal .
Storm surge and meteotsunami : two distinct phenomena that can combine
Storm surges are a temporary rise in sea level above the normal tidal levels , typically occurring during storms when strong winds push water toward the shore . They are also caused by the low atmospheric pressure associated with storms , which allows the water level to rise , a phenomenon known as the inverted barometer effect . Storm surges can cause severe coastal flooding , especially when combined with high tides . In tropical regions , they are often associated with tropical cyclones , while outside of the tropics , they are usually caused by extra-tropical cyclones and their associated strong cold fronts .
Although distinct phenomena , storm surges often co-occur with meteotsunamis . Unlike tsunamis triggered by seismic activity , meteotsunamis form as a storm moves over the ocean , creating a sudden drop or change in atmospheric pressure over a localized area . This rapid change in pressure generates a wave on the ocean surface which is small at first but soon amplifies as it reaches shallow waters through a process called resonance .
Melbourne storm surge and meteotsunami , August- September 2024
The broader synoptic-scale weather setup of this event consisted of multiple strong extratropical low-pressure systems located south of Australia and associated cold fronts extending north over southern Australia . The usual west-to-east movement of these extratropical cyclones was blocked by a large stationary high-pressure system to the east of New Zealand ( yellow-green color in Figure 1 ), leading to frequent cold fronts impacting southeast Australia ( purple-blue color in Figure 1 ). Between August 27 and September 2 , five cold fronts crossed southeast Australia with sustained westerly winds following each front ( Figure 1 ), leading to high sea levels .
Tide gauge observations show that the highest water levels were observed on the evening of August 28 and the afternoon of September 2 ( black lines in Figure 2 ). On August 28 , the maximum 3-minute storm surge — the highest surge level recorded within a three-minute interval — was 0.89 m , occurring at 1809 local time ( red lines in Figure 2 ). On September 2 , the maximum 3-minute storm surge reached 0.95m at 0400 and 1845 local time ( red lines in Figure 2 ). These times appear to coincide with high points in sea level oscillations likely linked to a meteotsunami ( blue lines in Figure 2 ).
This multi-day storm surge event resulted in flooding in numerous locations around inner Melbourne , especially in the suburb of Southbank , on the 28th and 2nd ( Figure 3 ). The
Figure 1 : Mean sea level pressure anomaly over the event ( 27th August 2024 to 2nd September ) and location and strength of anticyclonic blocking in the Southern Hemisphere . The blocking east of New Zealand is circled . Left image generated using Daily Climate Composites : NOAA Physical Sciences Laboratory . Right image is created from Climate Prediction Center - Observed Southern Hemisphere Atmospheric Blocking .