Tide data (real time)

Tides are the periodic rise and fall of sea level resulting from the gravitational interaction and motion of the Sun, Moon and Earth acting on ocean waters. Apart from astronomical factors, tidal patterns are modified by many other non-astronomical influences such as the shape and depth of oceans, and the weather. The combination of factors influencing local tides is complex and varies greatly from one site to another.

Tides are often distinguished by their cyclic patterns. One tidal cycle is the sequence from one high tide to the next high tide. Tides that go through one cycle during a single Earth rotation are described as diurnal tides. Those that go through two cycles are described as semidiurnal tides. A third common category is mixed tides. Mixed tides cycles are a combination of the other two and usually are characterised by two unequal high waters and two unequal low waters each day. In the northern part of the state (Carnarvon - north) the tidal cycle is semidiurnal. In the southern part of the state the tidal pattern is considered as mixed tides consisting of part semidiurnal, but predominantly diurnal tide cycles

The Western Australian coastline is renowned for its diverse range of tides. The difference between the height of the tide at high water and the height at low water is called the range of the tide. The tide range on the Western Australian coastline varies from approximately 12 metres at Collier Bay in the Kimberley to about 0.7 metres in Fremantle.

DoT tide stations measure sea levels at various location along the WA coast. The sea level can be simplified as comprising of tide prediction and residuals:

Sea level = Tide Prediction + Residuals or, Residuals = Sea level - Tide Prediction

Residuals are the difference between sea level and the tide predictions. They are due to non-tidal effects (weather) and they are a combination of short and long-term effects.

Short term water level changes can vary on a timescale from minutes to months. The main influence is weather or meteorological effects. For example, high atmospheric pressure reduces water level heights. Low atmospheric pressure raises water level heights. Onshore winds raise water levels while offshore winds depress water levels. Other short-term effects include seiches, tsunamis, Proudman resonance (meteo-tsunamis) and shelf waves.

Long term effects are of the order of years, decades, or even millennia. They comprise of long-term meteorological effects such as El Nino Southern Oscillation (ENSO), eustatic changes due glaciation and tectonic movements and sea level rise due to global warming. Depending on timescales under consideration, some of the components can be ignored.

Although tides are influenced by astronomical and non-astronomical factors, local tides can be predicted with a high degree of accuracy from the analysis of long-term tide records. The tide predictions provide the time and height of high and low water for a particular day at a particular place. Find out more about the tide predictions for locations around Western Australia.

Further information about tides and sea level measurements can be obtained from the following sites: