Let’s immediately clarify that vortices and eddies are synonymous and are phenomena that can affect both air and water. Let’s start with the formation of vortices in lakes. To better understand how vortices form in lakes, imagine two currents of water traveling in opposite directions. When these meet and cannot find a way to continue freely, they begin to revolve around each other, creating a sort of “funnel” or suction. In fact, you might think about going for a swim in the lake to feel more at peace compared to the sea, but when you swim you might come across a vortex. Now let’s not be alarmist, because not all vortices are large enough to be dangerous, but it is important to learn to recognize them and stay away from them. In fact, sometimes they form very quickly and the force of the suction could be so strong that they drag you under water.
How eddies work in rivers and lakes
Whirlpools in lakes can form for various reasons, which most often influence each other: the strength of the wind, the shape of the seabed, the difference in water temperature and the currents generated by the entry of rivers or streams. To begin with, the force of the wind can obviously move the water in one direction, generating a current in the surface water of the lake. When the water moved by these surface currents encounters obstacles, such as banks or an uneven seabed, it can begin to rotate and give rise to a spiral movement. The obstacles create an effect similar to that of a river meeting a rock: the water is forced to move around this obstacle, generating turbulent movements which, in some cases, can actually become vortices. But that’s not all. As we were saying, temperature differences can also generate movements. Let’s say that on the surface I have a layer of warmer water, and below, deeper down, another colder one: if suddenly the air temperature drops and makes the water on the surface colder than the one at depth, this change it triggers a mixing of the water which can generate a rotary movement, i.e. a vortex. Why does this mixing happen? Because the water above, having become colder, has also become heavier, and therefore goes down, while the water below, lighter, will rise above. Finally, as I was telling you, even the currents generated by rivers or streams that flow into lakes can cause the formation of vortices. As? Basically, the river introduces a large mass of water, creating a current, which perhaps encounters an obstacle or an opposing current within the lake, generating a rotation and therefore the vortex.

How do large marine gyres form? What is a Maelstrom
We said it at the beginning, vortices are not only formed in lakes, but also in oceans. In this case they are called maelstrom And gyredepending on their size, and can reach hundreds of kilometers in width and even influence the climate on a global scale. The physical mechanisms are essentially the same as lake whirlpools, but here we have to think on a much larger scale.
Let’s start with the most dangerous of all, i maelstromNorwegian term meaning “crushing current”. We always talk about opposing currents colliding or seabeds influencing the movement of the water. But now what is at stake is not the rocks of the lakes but the canyons or large underwater reliefs, or the currents that cross straits or narrow passages between the islands or for example in the fjords.
Right in Norway there is the most powerful marine vortex on the planet, the Saltstraumen. With the change of the tide, that is, with the water level that rises and falls during the day, the flow of water rapidly reverses, creating powerful rotational currents: let’s talk about more 480,000 liters of water that they cross the Salten and Skjerstad fjords, at a speed of 40 km/h. This passage causes the formation of enormous vortices, which can reach even i 10 m wide and 5 m of depth.

There are also other impressive examples, such as the Naruto Vortex in Japan, which is not the cartoon, but takes its name from the strait of the same name in which it is generated; or the Corryvreckan, which is located in Scotland and is the third largest in the world. And then we also have them in Italy, for example in the Strait of Messina.

Then there are much larger, although less dangerous, oceanic vortices called gyres. Since in this case we are talking about widths that can exceed hundreds of km, another important factor also comes into play, namely theCoriolis effectthat is, an apparent force generated by the earth’s rotation. And it is precisely this effect that gives the typical spiral shape to these vortices. Let’s see how it works.
What the Coriolis Force is and how it works: the explanation
To explain the Coriolis effect, I want to give you an example: let’s imagine a balloon thrown by a person who is at the equator towards someone who is at the north pole. Obviously it’s an absurd example, but it helps to understand. Due to the earth’s rotation, the ball will not arrive straight at the recipient but will deviate slightly, as if it were subjected to a “lateral push”. This is the result of the Coriolis effect: when you move along the surface of a rotating object, such as the Earth, you experience a lateral deviation in motion. So basically instead of moving in a straight line, the object will trace a curve. This effect is not immediately visible, but it amplifies over large distances and long times – so I have given you the example of a launch from the equator to the north pole – becoming particularly evident in large masses of water and air.

Now, having understood the Coriolis effect, let’s go back to the formation of gyres. As with lakes or maelstroms, it is always the air and sea currents that generate the vortices, even if in this case we are talking about permanent winds such as the Trade Winds, or the Western Winds, which always blow in the same direction, and volumes of water like those of the Pacific or Atlantic. So, let’s take for example the trade winds, which blow from north-east to south-west and theoretically should move the water in this direction: due to the Coriolis force, however, this movement is deflected and therefore the mass of water, instead of moving in a straight line, it will instead move by tracing a curve that continues and continues until returning to the starting point, generating an enormous spiral movement, that is, this vortex we are talking about, the gyre. These vortices then, unlike eddies or maelstroms, are not momentary, they are always there, playing a fundamental role in the climatic balance of the areas in which they are found, such as the Indian Ocean Gyre, North Atlantic Gyre, North Pacific Gyre: the latter is very famous because huge “islands” of rubbish tend to be created here, the Great Pacific Garbage Patchwhich are an accumulation of plastic and waste trapped in the gyre.
I open a brief parenthesis without going into detail because we are talking about aquatic vortices, but the Coriolis effect, that is, this deviation of movement, is equally responsible for the rotary movement of air currents, which forms cyclones and hurricanes.
Does the Coriolis effect affect rotation in sink drains?
https://www.geopop.it/e-vero-che-lacqua-nel-lavandino-ruota-in-senso-opposto-a-seconda-dellemisfero/
Small gem, but is the movement of water in sink drains influenced by the Coriolis force? That is, is it true that in the southern hemisphere the water turns in one direction and in the northern hemisphere in another? No, guys, this is a hoax, for the reasons we have just seen. The Coriolis effect is only relevant on large scales, as we saw for gyres. On smaller bodies of water, such as in sinks but also in lakes, except for really large exceptions such as the American Great Lakes, the effect is too weak to have a significant influence.