Oceans Day: The sea in all its facets - a playground for sailors

Seen from space, the earth is the famous "blue planet", with around 70 per cent of its surface covered by water. Infinite expanses, infinite opportunities for sailors - no two areas are the same, each has its own special features, and anyone looking for a cruise to suit their particular taste is sure to find the opportunity somewhere.

Selected seas in comparison

The sports ground of superlatives

The Atlantic, Pacific, Indian, Arctic and Antarctic Oceans make up the largest part of the global water surface. In addition, there are the marginal and secondary seas, the term used to describe all sea areas that are only connected to the oceans by more or less narrow passages or lie on a continental shelf. The North Sea, the Baltic Sea and the Mediterranean, the main sailing areas for Germans, fall into this category. A large number of other seas around the world fall into this category, such as the Caribbean Sea, which is bordered by the Antilles Arc and Central America, as well as the South China Sea and the Red Sea.

Even if the total surface area of the oceans seems impressive enough, their true size usually remains hidden from the human eye. The highest mountains in the world can be found on the seabed; measured from the base, the Hawaiian Mauna Kea rises to a height of around 9,000 metres (Mount Everest, the highest mountain on land, measures only around 3,500 metres from the base). The longest mountain range on earth is also hidden under an average 2,000 metre thick layer of water: the Mid-Atlantic Ridge is around 15,000 kilometres long (its counterpart above water is the Andes with a length of around 7,500 kilometres).

However, the undersea mountains are only visible in the few places where their peaks emerge as islands. Iceland is one such elevation, while the Azores form the highest peak at 2,351 metres above the sea floor.

The drive of the seas

Current systems of enormous proportions are relevant for offshore sailing on the oceans. It is not only the sun and moon that set masses of water in motion; wind and gravity are the strongest driving forces for ocean currents. Both deep currents (from 1,000 metres) and surface currents are important for the global climate, but skippers and navigators are only interested in water movements on the surface. They accelerate or decelerate the ship when travelling over ground.

The large current systems such as the Gulf, Agulhas, Brazil and Canary currents or the equatorial current system are all mainly set in motion by two mechanisms: Wind and density differences.

The layers of water near the surface are pushed by the prevailing winds. When the air passes over the water, it exerts a force on its particles and sets them in motion. The resulting water movements are called drift currents. As the water particles are deflected by the rotation of the earth, they do not follow the exact direction of the wind, as would seem logical at first glance, but flow at an angle of around 45 degrees to the atmospheric current. With increasing depth, the flow velocity decreases and the deflection increases. The resulting flow profile is called an Ekman spiral.

Water in motion

However, the largest quantities of water are set in motion due to comparatively small differences in the water composition. Temperature and above all the salt content play the decisive role here.

The salt in seawater comes from the rocks in the earth's crust. It mainly enters the oceans via rivers. As it remains behind when seawater evaporates, it has accumulated over the course of the Earth's history to its current concentration of 3.5 per cent on average. Compared to the total amount of water, however, the salt input from rivers is so low that the salt content can be considered constant even over thousands of years.

However, there are large regional deviations from this average value. Over 200 rivers flow into the Baltic Sea, for example, and precipitation exceeds evaporation. As a result, the salinity decreases from around 3 per cent in the Kattegatt to 0.5 per cent in the Gulf of Bothnia. In the Mediterranean, on the other hand, more water evaporates than is discharged by precipitation and rivers, so the salinity here reaches up to 3.7 per cent, forming a mass of water that can still be detected thousands of kilometres away in the Atlantic.

The salt of the seas

Even if these differences appear very small at first glance, they have a major impact on currents in the sea. Salt and temperature determine the weight of a "water packet" - cold, salty water is heavier than warm fresh water. Wherever such masses of different densities meet, the heavier water sinks and the lighter water rises.

The temperature and salinity of seawater change through mixing in the ocean or through interaction with the atmosphere. The North Atlantic makes the most important contribution worldwide. In simplified terms, the following process takes place: In the Labrador and Greenland Seas, the surface water is cooled in winter by very cold Arctic winds. At the same time, the salt content increases due to ice formation. As salt is not trapped in the ice crystals, its concentration in the remaining seawater increases. The density of the water increases, it sinks, flows southwards at depth and reaches the other oceans, where it slowly warms up and returns to the surface.

Once back in the Atlantic, it flows northwards again and releases its heat energy into the atmosphere. This creates a global band of constant currents.

The most important for Europe is the Gulf Stream. Not only does it set off from the American east coast at speeds of up to 4 knots to the north-east and must therefore be closely monitored when navigating, it also transports the unimaginable amount of energy of one billion megawatts to the north. This is roughly equivalent to the output of one million nuclear power stations. This warm water heating of northern Europe increases the average winter temperature in Hamburg by around 10 degrees compared to southern Alaska, which is at the same latitude.

Fault in the system

Another example of the effects of heat transport in the ocean is the so-called El Niño in the tropical Pacific. The steady south-east trade wind in the tropical South Pacific pushes the cold water of the Peru Current (also known as the Humboldt Current) westwards with the South Equatorial Current. On its way across the Pacific, the water is strongly heated by the sun. This creates a temperature difference: in the east, off South America, the cold Peru Current and in the west, in the area of the Philippines, warm water.

The different water temperatures affect the air temperatures and therefore the air pressure. An area of high pressure forms over South America and a low over the Philippines. This pressure distribution supports the east-west circulation of the atmosphere and the system is stable. For reasons that are not yet fully understood, this circulation is disrupted every three to seven years: the south-east trade wind weakens to such an extent that the warm water sloshes back from the west to the east. The cold, nutrient-rich Peru Current is forced back.

Once the warm water has reached the South American coast after around two to three months, the atmospheric circulation reverses. The trade winds die out completely. Instead of the usual easterly winds, westerly winds now blow. The reversal of the wind system is accompanied by enormous climate changes.

The distribution of precipitation in the tropical Pacific is changing fundamentally. Tropical storms develop over the warm ocean, reaching as far as Mexico and California and causing major devastation there. Floods occur in South America, and Australia and Indonesia are hit by droughts that lead to crop failures and forest fires. The effects of El Niño even reach across the Indian Ocean to Africa.

Experience the sea

The specific sailing conditions in the individual sailing areas are the result of extremely complex processes and interrelationships. And apart from all the science, there is another essential factor: individual perception. Even if a cruising area remains the same every day, sailors can experience it anew time and again.