Ocean currents flowing through the Strait of Gibraltar cause the level of the Mediterranean Sea to rise and fall and ultimately affect the ecosystems of the region across a wide area.

Gibraltar's key location

New tide gauge location

Radar gauge (looking towards East)

These currents are both tidal and non-tidal in origin. The ocean tide is caused by the gravitational forces of the Moon and Sun, and result in large tidal amplitudes in the Atlantic and relatively small ones in the Mediterranean. If the Mediterranean was a closed 'lake', then its tides would be very small indeed. However, tidal flows in and out of the Strait result in quite measurable tides over much of the western Mediterranean Sea thanks to the input of tidal energy from the Atlantic. Non-tidal flows through the Strait are caused by the weather (both air pressure and the action of the wind across the sea surface) and by differences between the densities of the adjacent Atlantic and Mediterranean.

One way to monitor all these flows is to use a network of sea level stations, sometimes called 'tide gauges', and the station at Gibraltar provides a particularly interesting component of such a network. A number of countries are collaborating in the construction of Mediterranean monitoring systems and the Gibraltar gauge will contribute to those efforts and also to what is called the Global Sea Level Observing System.

Data from the Gibraltar tide gauge are also important for harbour operations as ships need to know how deep the water is underneath them. Data are also needed by coastal engineers in order to design structures near to the sea, and are of course required to give warning in the event of floods. Increasingly, data are also being used in a range of environmental monitoring activities.

New radar tide gauge
The measuring system consists of an Ott Kalesto radar gauge mounted on a quay edge stanchion. The quay edge system is powered by a 110 volt supply from a building nearby. The housing on the stanchion contains the power supply unit, a back-up battery and a radio transmitter.

The data is transmitted to the Port Authority offices 60m from the quay edge. The system housing in the port office contains a radio receiver, data logger, modem power supply unit, back-up battery and digital read-out. The Port Authority can collect data on a desk top PC. The data is collect by POL via the telephone.

Previous tide gauges
A tide gauge was operated at Gibraltar for many years (1961-2000) by the UK Hydrographic Office together with the Queen's Harbour Master. That gauge was what was called a 'float and stilling well system' and made a continuous trace of the level of the sea on a paper chart. All the historical data from that tide gauge are available from this web site.

However, as time went on it became apparent that a new tide gauge system was badly needed. In 2004 the Proudman Oceanographic Laboratory (POL) was commissioned by the Gibraltar Port Authority to provide a state-of-the-art radar gauge system at the North Mole. This provides continuous measurements of sea level in electronic form which are radioed to the port office and by telephone to a data centre at POL. The data are quality checked at POL and then made available via the world wide web to all interested users.