Understanding tides
The sun and moon have a gravitational pull on the Earth’s oceans, causing water levels to rise and fall. The changes in water levels are the tides.
When the sun and the moon are aligned (at a full moon or a new moon), the sun and the moon pull together and the gravitational effect on the ocean is strongest. This is called a ‘spring’ tide, when there is a big difference in the height of the low tide and the height of the high tide.
When the sun and moon are in opposition (at right angles to each other), they cancel out some of the gravitational effect. This happens during the moon’s first and last quarters. At these times, there is less difference between ‘high’ and ‘low’ tides. These are called neap tides.
Due to the rotation of the Earth and orbit of the moon, there are two high tides a day, about 12 hours apart. The tides not only affect the depth of water over a dive site, they also affect the amount of water that floods in and ebbs out.
On those occasions when there is an exceptionally high ‘spring’ tide, there is also an exceptionally low ‘spring’ tide. A lot of water is shifted to and fro in the ocean in the same period of time, and the currents can be very strong. This can make diving either almost impossible or very spectacular.
Ocean currents
The land we live on stands on a continental shelf that stretches from the coasts far out beneath the oceans. When the level of the deep ocean rises with the tide, the water floods up over the shelf. When the level falls, the water ebbs back. The flow of this water is called a tidal flow or current.
Many famous diving destinations are affected by ocean currents. Out in the ocean itself, there may be only a few feet of difference in the depth of the water you dive in at different times, but the ocean currents can still be very strong. The currents are also driven by the dragging effect of winds on the ocean surface. Other causes of oceanic current are the jet stream (a band of very strong wind high above the Earth) and changes in seawater temperatures, such as those close to the Poles.
Local currents
The seabed is not flat; it has a bumpy surface caused by natural features, reefs and even wrecks. The sideways pressure of the water as it flows onto the continental shelf means it has to squeeze around, up and over, or even through some of these obstacles. Like the air passing over the wing of a plane, the water has to speed up to get past the obstacles, causing fast currents.
In other places, vortexes, or areas of low sideways pressure, are produced. These places are said to be in the ‘lee’ of a current.
The water squeezing up and over a reef or wreck can speed up dramatically. These areas of fast currents, or current points, often attract large marine animals.
Currents can change very quickly. You may start a dive when there is apparently little current at all, but while you are down there, the current is very likely to strengthen once again.
