Australian Government, Bureau of Meteorology

Oceanographic Effects on Tides

Oceanographic conditions which differ from the average will cause corresponding differences between the predicted and the actual tides. Variations from predicted heights are caused mainly by large scale ocean currents, which can be predicted a few days in advance via the "Sea Level" links for each of the "Forecast Regions" on the Bureau's Sea Level Forecasts web pages.

Meteorological Effects on Tides

The following was published in the 2008 edition of the Australian National Tide Tables

Meteorological conditions which differ from the average will cause corresponding differences between the predicted and the actual tides. Variations from predicted heights are caused mainly by strong or prolonged winds, and by unusually high or low barometric pressure. Differences between predicted and actual times of high and low water are caused mainly by the wind

Barometric pressure. Tidal predictions are computed for average barometric pressure. A difference of 10 hectopascals (hPa) from the average can cause a difference in sea level of about 0.1m. This depression of the water surface under high atmospheric pressure, and its elevation under low atmospheric pressure, is often described as the inverted barometer effect. The water level does not adjust itself immediately to a change of pressure and it responds moreover to the average change in pressure over a considerable area. The average barometric pressure and information, in some instances, concerning changes in level which can be expected under different conditions for certain places, is given in Sailing Directions. Changes in sea level due to barometric pressure seldom exceed 0.3m but their effect can be important as they are usually associated with those caused by wind set-up since winds are driven by the pressure gradient.

The effect of wind. The effect of wind stress on sea level and hence on tidal heights and times is very variable and depends largely on the topography of the area. In general, it can be said that wind will raise sea level in the direction towards which it is blowing, this effect is often called wind setup. A strong wind blowing onshore will pile up the water and cause high waters to be higher than predicted, while winds blowing off the land will have the reverse effect. In addition, winds blowing along a coast tend to set up long waves which travel along the coast, raising the sea level at the crest and lowering it in the trough.

Storm surges. The combination of wind setup and the inverted barometer effect associated with storms can create a pronounced increase in the sea level. This is often called a storm surge. An additional process in the form of a long surface wave travelling with the storm depression can further exaggerate this sea level increase. A negative surge is the opposite effect, generally associated with high pressure systems and offshore winds, and can create unusually shallow water. This effect is of great importance to very large vessels which may be navigating with small under-keel clearances.

Seasonal Changes In Mean Level

The monthly variations in mean sea level do not necessarily repeat themselves exactly from year to year; hence the values for seasonal tidal constituents may be found to differ from observed values by as much as 0.1 m, even where the tabulated values are based on several years of observation. In practice, for some places, the figures may have been obtained by interpolation due to scarcity of data.

Variations in mean sea level over short periods may be considerably greater than the values given; mean sea level may remain as much as 0.3m above or below the average for as long as a month.

In addition, oceanographic effects such as Southern Ocean Oscillation (El Niņo) can produce large scale variations in mean sea level of up to 0.5m with corresponding changes in rate and direction of tidal streams.

© Australian Government 2010, Bureau of Meteorology