Spring tides. What are they and why do they occur?

MANUEL GONZÁLEZ. Researcher at AZTI’s Observation and Data Systems Area

September-October and February-March are times of high spring tides. This week, some of the extreme annual high and low tides will be recorded.

However, the phenomenon of these tides is not exclusive to these months. There are neap tides every month of the year and they occur when there is a new or full moon. When this occurs, the effect of the moon and the sun are adjusted and both are added together, giving rise to neap tides. However, in waxing or waning quarters the effect of the sun is opposed to that of the moon and then the tides are smaller, known as neap tides.

But why do the largest neap tides of the year occur in September-October and March? At these times the effect of the sun is maximised by the equinoxes, which occur twice a year (21 March and 21 September). The equinoxes are the times of the year when the length of the day (the hours of daylight) is equal to the length of the nights.

Attraction of the stars

The moon, the sun and the earth play a crucial role in tidal phenomena. Spring tides occur when these stars are more or less in a straight line. The moon is the main protagonist of the tidal cycle, while the sun plays a secondary role. The stars attract each other and the intensity of this force causes the liquid surface of the ocean to deform significantly and move across the oceans like a wave. In this way, the coasts of the Basque Country register almost five metres in spring tides.

Despite the fact that the Cantabrian coastline registers quite pronounced tidal differences, there are coastlines where this difference is even greater. For example, the Bay of Fundy in Canada has the greatest difference in tide level in the world at 20 metres. Close behind is Mont Saint Michel in France, where it can be as much as 15 metres.

The intensity of this attraction between the stars depends proportionally on the mass of the bodies and inversely on the square of the distance. As the moon is much closer to the earth than the sun, despite having a much smaller mass, the moon’s effect on the tides is approximately twice that of the sun. In other words, of the total tide, two thirds is due to the moon and one third is due to the sun.

On the Spanish Atlantic coast, the tide is semi-diurnal, i.e. there are two high tides and two low tides during each day, with an amplitude of between three and five metres approximately. In other places, tides can be diurnal, irregular diurnal and mixed.