Tuesday, 14 May 2013

The Nodal Tidal Cycle - more evidence

You've probably never heard of Port Pirie. It's a mining town and port (really?) in South Australia, north of Adelaide, though Australians might call it a city. In common with many ports in Australia it has a tide gauge.

Nothing very spectacular to see there, just a pronounced dip caused by the intense 1997/8 El Niño, followed by a step rise and a relative stasis, and a low overall rate of rise. Apart from the lesser dips going back to 1969, all caused by previous El Niños, there's no obvious signs of any cycles in the data. That data provided by the BOM's National Tidal Centre, has a standard deviation included for the hourly data which is averaged to create the monthly averages plotted on the chart. The standard deviation reflects the variability of the hourly data through the tidal cycles, and that variability is increased or reduced by the effect of the 18.61-year period of the nodal tidal cycle (NTC). If the standard deviation is plotted something quite spectacular appears.

Proof if any is still needed, of the lack of any great influence of the NTC on mean sea-level. Using the standard deviation to show the effect of the NTC on tidal variability is a technique I'd like to be able to claim as my own invention, but I have to give all the credit to one David Pugh, author of several excellent books on things tidal. It's obvious when you think about it - I just didn't think about it - doh! In common with other regions worldwide, some Australian stations show the cycle clearly (Port Pirie is just the clearest example) and some don't. Fremantle in Western Australia is another Oz station which clearly shows the cycle; it has a much longer record than Port Pirie, and the cycle has an evident upward trend. First the gauge record -

..... and the standard deviation showing the NTC -

Note the obvious upward trend - this doesn't indicate any marked effect on mean sea-level, but shows that the tidal range, the difference between low and high water, is increasing somewhat. I don't know whether this is a feature of west-coast sites or not, but Sydney on the east coast shows a much steeper downward trend. What the cause of these trends might be, I don't now, and I can't find any literature which might throw some light on it. Perhaps there's something here waiting to be discovered.