Twice a year, at the Spring and Autumn equinoxes, the equatorial plane of the earth is aligned with the orbital plane around the sun and the biggest spring tides – known as the “Equinox Spring Tides” occur. Tides also follow an 18.6 year “nodal cycle” relating to the tilt of the moon and the elliptic nature of the moon’s orbit around the earth. This allows an extremely big spring tide to occur during this cycle.
This year we will experience a period of unusually high tidal ranges, which will reach a zenith in Galway around September 9th, October 8th and November 6th. In Dublin high tides are predicted around the 10th September, 9th October and November 6th. In Cobh high tides are predicted around September 9th, October 8th and November 5th to 7th (Information drawn from FES 2004 global tidal model). The differences in date are due to mitigating local factors concerning effects of the Irish Sea and the open Atlantic.
Accurate tidal prediction depends on having accurate tide height information. The Marine Institute and various partners are in the process of installing a permanent network of tide gauges with monitoring stations in Killybegs (DCMNR), Dublin Port (Dublin port Company), Rosslare (Irish Rail), Castletownbere (DCMNR), Galway Harbour (Galway Harbour Company) and the Kish Bank Lighthouse (Dublin City Council, Commissioners for Irish Lights) which provide regular accurate measurement of tidal height.
Some of these can be viewed online at www.irishtides.ie (this project is 3 years into an anticipated 10 year programme). The information they provide can be used to increase the level of accuracy of tidal prediction and, when used with weather information prepared by Met Eireann from the Irish National Weather Buoy Project and other sources can be used to predict storm surges that might cause flooding.
Storm surges occur when high tides coincide with weather patterns causing onshore winds and low pressure (storm) conditions. Onshore winds create waves that drive water ashore and low atmospheric pressure allows the tides to rise even higher against the reduced downward pressure of the air.
The Marine Institute Weather Buoys monitor air pressure, wave height and period and wind speed and direction which, together with numerical tidal computer models, will allow the forecasting agencies to predict and monitor storm surges and to issue warnings in the future. The Marine Institute provides national infrastructure in the form of weather buoys, tide gauges and coastal buoys which assist with forecasts but is not in itself a forecasting agency.
The Marine Institute is involved in the development of new tidal surge numerical models, managing the installation of hardware and routing data in support of such a scheme.
In Galway the tide gauge information is piped directly to the Harbourmaster at Galway as well as through the “Irish Tides” web site. While cycles of climatic variability and potential changes could be increasing the world sea level due to expansion of the oceans as temperatures rise, this effect is very small in relation to the effect of tide and wind on local sea level. Ireland resides in a complex ocean-atmosphere system.
Having a developing network of monitoring equipment in place through inland, shore-based and deep sea observing systems is allowing the true nature of the oceans to be understood, through high-powered computers being put to work to assimilate these data sets into formats than can be readily comprehended and put to work by engineers, commerce and the general public.