Viewed from space, the Earth presents itself as the famous "blue planet". Around 70 per cent of its surface is covered by water. These endless expanses offer pleasure boaters countless possibilities, as 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 one somewhere. To mark World Oceans Day on 8 June, the editorial team compares the characteristic aspects of the world's oceans and their most important marginal waters.
These oceans and seas exist
The oceans make up the largest part of the global water surface: the Atlantic, Pacific, Indian, Arctic and Antarctic Oceans. In addition, there are the marginal and secondary seas, 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, Baltic Sea and Mediterranean Sea, the main areas for German boaters, fall into this category.
Many other oceans around the world are included, 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. The true size of the oceans 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 some 9,000 metres into the air, while 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 stretches for around 15,000 kilometres.
What drives the oceans
Current systems of enormous proportions are relevant for offshore boating 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. However, skippers and navigators are only interested in the water movements on the surface, which accelerate or slow down the ship as it travels over the ground. The large current systems such as the Gulf, Agulhas, Brazil and Canary currents or the equatorial current system are mainly set in motion by two mechanisms: Wind and density differences. The water layers 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.
The salt content
However, the largest quantities of water are set in motion due to comparatively small differences in the water composition. Temperature and, above all, salinity play a decisive role in this. The salt in seawater originates from the rocks in the earth's crust and mainly reaches 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.
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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 Kattegat 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, which is why the salinity here reaches up to 3.7 per cent.
The North Atlantic
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 most important contribution worldwide is made by the North Atlantic. 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. 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.
The Gulf Stream: Europe's hot water heating system
The most important current for Europe is the Gulf Stream. Not only does it set off from the east coast of America 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.
El Niño: disturbance 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 southeast 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.
For reasons that are not yet fully understood, this cycle 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. The reversal of the wind system is accompanied by enormous climate changes that have an impact as far away as Africa.
In figures: plastic waste in the oceans
- Floating rubbish: 15% of the total waste
- Seabed: 70% of the waste sinks to the bottom
- Beaches: 15% of the rubbish is washed up
- Total quantity: Around 142 million tonnes of waste in the world's oceans
- Annual growth: 6-8 million tonnes
- Estimated duration of plastic degradation: About 500 years
