A king tide inundated low lying parts of Woy Woy including outside the Woy Woy Hotel and the wharf. Pic Nick Moir 2 jan 2018 A king tide inundated low lying parts of Woy Woy including outside the Woy Woy Hotel and the wharf. Pic Nick Moir 2 jan 2018
Coastal regions around are experiencing unusually high tides this week, flooding some low-lying areas, thanks to an unusual confluence of astronomical and meteorological factors.
The so-called wolf moon – the first full moon of the year – also happens to be a “super moon”. At some 50,000 kilometres closer than its furthest extent – a phase of proximity that “makes men mad” according to Shakespeare – our nearest partner in space is exacting an extra pull on the world’s water.
The earth’s own orbital eccentricity with the sun also means that at this time of year, we are about 5 million kilometres closer to the sun – a point known as the perihelion – than at our most distant in July.
“With this super moon and perihelion, what we’re experiencing is the full might of the gravitational force” on the earth, said Alan Duffy, an astronomer and associate professor at Swinburne University of Technology in Melbourne. “You can expect a larger range in the tides both high and low.”
A king tide inundated low lying parts of Woy Woy including outside the Woy Woy Hotel and the wharf. Pic Nick Moir 2 jan 2018
The impacts on tides, though, will vary greatly depending on local conditions, including the weather.
For Sydney Harbour, the high tide will be about 2.07 metres at Fort Denison on Wednesday compared with the high tide of 1.72 metres in two weeks’ time with the new moon.
“If you’re close to a shoreline, 30 centimetres might not sound that much but it can be the difference between a beautiful sea view and being inundated,” Professor Duffy said.
The extra gravitational tug is already noticeable in areas with narrow channels, such as Ku-ring-gai Chase National Park or Woy Woy to Sydney’s north. Both regions had some flooding on Monday and Tuesday.
Impacts, though, are likely to be slight in other areas, such as Melbourne, where there is typically much less variation between high and low tides than in Sydney.
The high tide at Williamstown on Port Phillip Bay, for instance, will reach 0.88 metres on Thursday and recede merely to 0.77 metres at the new moon on January 17, according to the Bureau of Meteorology.
While the current high tides from the solar and lunar gravitational effects may be causing disruption for some places, the impacts will be made worse if they coincide with a low-pressure system bringing strong winds, Professor Duffy said: “That’s a far more impressive and damaging change in [tidal] heights.”
For Sydney and Melbourne, the weather is generally much more benign, at least as far as the risk of storm surges go.
A very high tide catches out unwary drivers at Apple Tree Bay in the Ku Ring Gai National Park. Pic Nick Moir 01 jan 2017
La Nina boost
Along with the heavenly influences on the tides, the current weak La Nina in the Pacific will also likely be playing a role.
In 2015, sea levels along the western Pacific dropped about 20 centimetres during the big El Nino, as the easterly equatorial trade winds weakened and reversed, piling up water along the eastern Pacific coasts.
La Ninas go the other way, with trade winds strengthening.
Over time, rising sea levels from global warming are also playing a role to exacerbate the effects of king tides. The warming planet means more melting from land-based ice sheets and glaciers, and a thermal expansion of the oceans as the sea warm.
According to the Permanent Service for Mean Sea Level, sea levels at Fort Denison have been rising at an annual rate of 1.68 millimetres since 1986, while those at Stony Point, south-east of Melbourne, have been increasing 2.3 millimetres a year.