[Editor’s note: This article is reprinted with permission from Afloat Magazine. The author writes a regular feature on topics related to marine weather and other items of interest to mariners. Visit http://www.afloat.com.au.]

In a letter to Afloat last month John Burch of Concord wrote about the strange tidal movements in Sydney Harbour on 28 February 2010 following the recent earthquake.
     “On 22 May 1960 there was a huge earthquake in Valdivia Chile which caused tidal waves up to 35 feet high as far as 6,000 miles away.
     “I don’t recall if it was the actual day of the quake, in fact I think it was a day or maybe two days later, but at Bobbin Head there were six high tides and six low tides in the space of five or six hours on that day.
     “Each tide that came in and went out was higher/lower and bigger and fiercer than the one before it and the pontoons were going up and down like yo-yo’s.
     “On the last ‘out tide’, in early afternoon I think, quiet Bobbin Head bay became more like rocky mountains rapids. The floating pontoons were damaged and some broken away and at least 40-plus boats and moorings were swept down the bay crashing into each other and either breaking or dragging the moorings.”
     When tsunami are generated, the Joint Australian Tsunami Warning Centre (JATWC) issue warnings for Australia and its territories. The timings of the arrival of the tsunami in these warnings are for the initial ‘wave’ albeit a trough (sea recedes) or a crest. However, when tsunami are generated, often the largest waves come some time after the initial wave. In the 1960 Chilean event the initial wave arrived at Hilo in Hawaii at 0015, the largest wave arrived at 0110 and waves of three to four metres continued for many hours after that.
     Tsunami travel directly across the oceans from their points of origin on great circle tracks. From the recent Chilean earthquake the travel time to NSW of a tsunami along the great circle route that goes south of New Zealand was approximately 15 hours. However, almost no energy was directed that way so miniscule tsunami arrived in the minimum time. From this quake the energy and resultant ‘westward’ tsunami were focussed, not along the great circle route south of New Zealand, but more into the central Pacific.
     The waves that affected Sydney actually arrived by a much longer journey north of New Zealand. This added thousands of kilometres to the distance the tsunami travelled and delayed the arrival of these larger waves by many hours from those on the shorter great circle route.
     The strange currents that you experienced were in fact these larger tsunami arriving in Sydney. There may have been a natural resonance in Sydney Harbour that enhanced the tsunami’s effect.
     On the graph of sea levels at Port Kembla times are in UTC, add 11 hours for AEDT. The purple line shows the estimated arrival time tsunami via the great circle 2130UTC/0830AEDT. However, tsunami started generating small sea level fluctuations at around 0030UTC/1130AEDT.
     The largest fluctuations in sea level arrived after 0430UTC/ 1530AEDT, and persisted for many hours during the afternoon and evening … when you were trying to berth your vessel.
     This is a good example of the sea level fluctuations caused by tsunami that can cause dangerous and unusual currents, even though their amplitude is quite small. It is these fluctuations in sea level and the associated dangerous currents that the JATCW is trying to capture when it issues Marine Threat Tsunami Warnings. (Also see http://www.bom.gov.au/tsunami/index.shtml)
     Another letter (from Angus Gordon) mentions seiche in Sydney Harbour and I agree with the majority of his comments. Pretty much all bays and semi enclosed waters have some type of seiche and are usually relatively small and fairly frequent (from 20 minutes to a few hours). If a force (weather, seismic) sends a wave into one of these bays and it is in sync with the natural seiche, the effects are enhanced; if it is out of sync then the effects are reduced.
     In this case the arrival of the tsunami at Port Kembla coincided with the period of strange fluctuations reported in Sydney Harbour and were more likely to be tsunami related.

*Malcolm Riley is the Public and Marine Officer for the Bureau of Meteorology in Hobart. He has worked in all States with the exception of Qld and is a Master V. He gives education courses on Marine Meteorology.