- Taswegian observes a meteotsunami
- High sea levels in the east next week
- Penguins a long way from home
Very warm sea temperatures continue in the southeast, in a mirror to the atmospheric heatwave over much of the continent. A strong high pressure ridge across Australia and the Tasman Sea has increased insolation for the last two weeks and exacerbated the developing heat wave.
In the meantime a cool anomaly continues on the mid-west coast of WA. It is enhanced by persistent southerly winds causing broadscale upwelling.
High sea levels
Large spring tides early next week are expected to combine with onshore winds to make sea levels clip Highest Astronomical Tide at many locations along the eastern seaboard.
From southern Tasmania to northern Queensland, red alerts on the BOM’s sea level viewer indicate HAT exceedence on Monday and Tuesday. Remote islands such as Noumea, Cocos Islands and Vanuatu are also expecting large tides.
An example location is Ulladulla. See the timeseries below which shows increasing tide heights in the week ahead. Check out the Sea Level Viewer for your location of interest.
During these ‘king tides’ there is an increased change of coastal inundation and nuisance flooding. Forecasters will be on alert.
How significant are these king tides? It depends on location, but the spring tides in January and February 2019 are expected to be close to the biggest in the year ahead.
Sub-Antarctic penguins in WA
At least eight Rockhopper Penguins have been spotted on WA beaches. Authorities and wildlife carers are surprised. They are a long way from home! The colony these penguins belong to is located on St Paul and Amsterdam Islands in the Sub-Antarctic Indian Ocean.
The penguins must be feeling very lost. This feeling would have been amplified when one penguin found itself at a New Years Eve party in the town of Dunsborough. Party-goers started taking Penguin Selfies before putting the bewildered penguin back in the water. However, the penguins have come ashore to moult and wildlife carers say that they need to be left alone.
There is speculation that the penguins travelled this way due to cooler than normal SSTs that have persisted in WA since winter (ABC radio).
That is a long way to swim for a cool anomaly.
A local resident of Stewarts Bay near Port Arthur contacted BOM on Wednesday to report on some strange tides:
“Around 8-9 pm local time on 15 Jan 2019 the tide was observed to fluctuate from high to low (and so forth) every 20 minutes, size on the order of 1 metre.”
The BOM Tide Unit checked the closest tide gauge, Hobart, and sure enough, there was a signature of a meteotsunami.
Meteotsunamis are generated by a moving atmospheric pressure disturbance. A change of 0.3 hPa per minute is sufficient to cause one. This can occur around squalls, thunderstorms, frontal passages and atmospheric gravity waves (Pattiaratchi and Wijeratne 2015).
Was there a pressure disturbance on Tuesday night?
Water vapour images from Himawari-8 overlaid with lightning strike locations (yellow and red dots) – BOM
Yes. There was a line of thunderstorms moving across the state from west to east. Coincidentally, there is also ground footage from Port Arthur.
Video of lightning over Slopen Main near Port Arthur (Source: ABC)
The high-frequency barometer at the Hobart tide gauge recorded the disturbance. Atmospheric pressure rose sharply then suddenly dropped 2.5 hPa in about 10 minutes before rebounding.
The evidence all points to a meteotsunami, but the effect of these events is highly dependent on location. The observer at Port Arthur saw regular and clear tidal signals of the order of 1 metre, while the measurements from Hobart are messier and much smaller at around 0.3 metres.
Pattiaratchi and Wijeratne (2015) write that small sea-level disturbances can be enhanced in particular locations due to wave shoaling and topographic resonance. A Google Earth comparison of the two locations show that the bays are orientated in similar directions, south-southeast to north-northwest. However the length and width of the bays is different which would affect both of these factors. Wave shoaling would occur in both bays, but Hobart would experience greater dissipation of waves travelling up the bay to the tide gauge location. The resonance would also be different due to a change in the characteristic frequency.
Direct observations of meteotsunamis are still uncommon and under-reported. Some argue that leads to the phenomenon being an under-rated hazard. What makes it so rare is that meteotsunamis need a very particular combination of location-dependent factors to amplify the wave amplitudes to hazardous heights. They are less likely to bring the destructive loss of life and property that can occur with seismic tsunamis.