Tobias Aufmkolk
This text was originally published in the collection “Die Schweiz und die Ozeane” by the Swiss think tank Denknetz.
Everything flows
On its way from the Caribbean to Northern Europe, the Gulf Stream brings our continent a mild climate that is unique in these latitudes anywhere in the world.
Without the Gulf Stream, we would have to prepare ourselves every year for a winter as harsh and cold as in Siberia.
The oceans are constantly in motion. Some of these movements are barely visible to the naked eye, while others manifest themselves in raging currents and whirlpools. Even on a seemingly calm water surface, ships can be driven off their chosen route. The early seafarers of the 15th and 16th centuries were unable to explain this phenomenon.
Broadly speaking, there are three types of ocean movement: tidal movement, surface currents, and deep currents. While tidal movements cause the oceans to slosh back and forth only slightly in their basins, surface and deep-sea currents span the entire globe.
In view of these global current patterns, American oceanographer Wallace Broecker coined the term “ocean conveyor belt.”
The ocean conveyor belt
The most visible ocean movements are surface currents. They are caused by the force of the wind and friction. The wind sets the water masses of the oceans in motion. However, these do not flow in the direction of the wind as expected, but are deflected by the Coriolis force. The force described by the Frenchman Gaspard de Coriolis in 1835 states that objects moving on a rotating body are deflected. In this case, the Earth is the rotating body and the ocean current is the moving object.
Deep-sea currents, on the other hand, are caused by differences in water density. Temperature and salinity play a decisive role here. Cold water is heavier than warm water; salty water is heavier than low-salt water. Accordingly, cold, salty water is drawn down into the depths.
This phenomenon is most pronounced in the North Atlantic. On their way to the North Pole, the surface currents have become significantly saltier and colder due to evaporation. They also encounter cold water currents coming from the pole. The stratification of the water becomes unstable, and the water masses, which have become heavier, sink to deeper regions. There, at a depth of several thousand meters, they flow very slowly in the opposite direction through the ocean basins to the South Atlantic.
Here they encounter the circumpolar current, which flows around the entire globe in the south and mixes the water masses of the three oceans. The mixed water masses move back to the surface, where they are reheated on their way towards the equator. The cycle begins again.
Warmed by the sun
The Gulf Stream is therefore only a small part of the global conveyor belt, albeit a very significant one. It is one of the fastest, most powerful, and warmest surface currents in the oceans. It is fed by water from the North and South Equatorial Currents, which is warmed by the strong solar radiation near the equator. This water is then driven by trade winds from the coast of Africa to the Caribbean. At this point, the sea has warmed up to 30 degrees Celsius.
Due to the natural land barrier of the American continent, the water now has to find another way. It is forced through the Gulf of Mexico, which has only one outlet in the north: the Florida Strait. At a speed of two meters per second, the so-called Florida Current returns to the Atlantic Ocean. There it meets the Antilles Current coming from the south. It is only here that the current gets its name: the Gulf Stream.
The Gulf Stream flows along the American east coast for a good 1,000 kilometers before turning eastward at Cape Hatteras in North Carolina. The deflection of the Coriolis force and the prevailing westerly winds in this area force it to take this route. Now it heads straight northeast.
As the Gulf Stream has already lost speed, it does not move in a straight line, but in curved, wave-like movements. After about 1,500 kilometers, the Gulf Stream is joined by the ice-cold Labrador Current coming from the north. As a result, the water masses mix and the Gulf Stream loses power and heat.
The Gulf Stream splits
Scientists now refer to it as the North Atlantic Current, while the term Gulf Stream is still commonly used. Shortly after meeting the Labrador Current, the water masses divide into two large currents.
The Canary Current turns south, flows along the West African coast, and finally flows back into the North Equatorial Current. This warms the water up again and transports it back towards the American coast. The first circle is complete. The North Atlantic Current, on the other hand, moves towards the coast of Ireland, flows past northern Scotland and finally reaches the coast of Norway. It has lost a lot of heat along the way. In addition, the salinity of the current has risen sharply due to constant evaporation. The water becomes denser and denser, literally being pulled down into the depths. The North Atlantic Current near the surface dissipates, the water flows back into the Atlantic as a deep current, crosses the equator, ends up in the Antarctic Circumpolar Current, and soon reappears on the surface. The second circle is complete.
Europe's climate engine
The journey of the water masses is spectacular in itself, but the Gulf Stream's truly visible significance comes from the influence it has on Europe's climate. Western Norway lies at the same latitude as southern Greenland and eastern Canada.
While only very sparse vegetation grows on permanently frozen ground in Greenland and Canada, fruit trees, strawberries, and vegetables thrive on the coasts of Norway. Palm trees grow on Ireland's southwest coast, and lush rhododendrons grow in northern Scotland. None of these plants should really be found this far north. Only the Gulf Stream makes it possible. The warm water it carries also warms the air on the coasts it flows past. This creates such a mild climate that, for example, the ports along the entire west coast of Norway remain ice-free all year round.
Is Europe facing a new ice age?
But what would happen if the Gulf Stream were to dry up one day? Scientists have been studying this question for some time. Global warming is causing more rain and glaciers to melt faster. The salinity of the sea is decreasing, which means that water can no longer sink as easily. This would interrupt the global conveyor belt.
The most dire scenarios suggest that Europe would be several degrees colder on average in winter. In northern and western Europe, winter temperatures and snowfall would then be similar to those in Siberia. Scientists are still debating whether such a scenario could ever occur.
Most oceanographers see an urgent need for further research on this issue. Although our understanding of the Gulf Stream has improved in recent decades, many answers are still hidden in its waters. A study published in 2021 by the Potsdam Institute for Climate Impact Research5 shows that the Atlantic circulation, which includes the Gulf Stream, has very likely become less stable over the past 100 years. A collapse of the system could have serious consequences for the global climate. However, researchers still need to conduct much more research in order to arrive at more accurate findings.