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Before answering the question, keep in mind that I am a second year Biology student, with no experience in studying Physics and a very basic understanding of Mathematics. Thus, I would prefer a qualitative explanation if possible.


I do not understand why water drifts at an angle to the wind. I understand that,

Ekman transport occurs when ocean surface waters are influenced by the friction force acting on them via the wind. Source: Wikipedia.

I also understand that,

As the wind blows it casts a friction force on the ocean surface that drags the upper 10-100m of the water column with it. Source: Wikipedia.

However, I do not understand this point,

However, due to the influence of the Coriolis effect, the ocean water moves at a 90° angle from the direction of the surface wind. Source: Wikipedia.

I do not understand this point, because the wind itself is also veering off to the right when travelling from the equator to the poles and vice versa. What causes the differential in 'veering' to the right or the left, between the wind and the water travelling in a Northerly or Southerly direction, if both are affected by the Coriolis effect? In other words, simply stating Coriolis effect does not explain why water drifts at an angle to the wind since they are both affected by the Coriolis effect.

I know this phenomenon is explained by the Ekman spiral, which I partly understand. I understand that,

If the ocean is divided vertically into thin layers, the magnitude of the velocity (the speed) decreases from a maximum at the surface until it dissipates. Source: Wikipedia.

This makes sense to me, because pressure increases with water depth, meaning more wind energy is required to move 'heavier' water.

Albeit, I do not understand why,

The direction also shifts slightly across each subsequent layer (right in the northern hemisphere and left in the southern hemisphere). Source: Wikipedia.

My only idea is that it may have something to do with this,

The impact of the Coriolis effect is most significant with high speeds or long distances. Source.

Explained a little bit here,

Because it largely depends on how large the difference is between an object's velocity is and the ground's velocity, the effect is really only significant at high speeds (either type) or long distances (north/south especially). Source.

So the direction of water shifting slightly across each subsequent layer might have to do with the different velocities of the water layers compared to each other and the wind. Furthermore, I get why the Coriolis effect is greater over longer distances on Earth, because points on the equator are moving faster then points on the poles. However, what do these sources mean by high speeds exactly? Are they referring to the object's speed due to the Earth's rotation (pushing it in an Easterly direction) or due to the force pushing an object in a Northerly or Southerly direction for example? How do higher speeds make the Coriolis effect greater? The answers to these sub-questions will help me understand why water drifts at an angle to the wind.

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    $\begingroup$ You could start here: earthscience.stackexchange.com/questions/12030/… $\endgroup$
    – arkaia
    Mar 4, 2023 at 13:59
  • $\begingroup$ Thanks @arkaia the answer you provided was excellent, however, I must admit it is a bit mathematically technical for me (I know I need to learn some math) and I cannot find an answer to my question, within that answer, which may be my fault. If you could directly address my question here, that would be high appreciated, but you have no responsibility to do so if you do not want to. Thanks again. $\endgroup$ Mar 5, 2023 at 11:45

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I'm a student myself, but maybe this will help -

Consider that surface winds, while influenced by the Coriolis force, are not necessarily blowing in the same direction that the deep ocean is flowing. Both are influenced by the Coriolis force, yes, but that doesn't mean that they're always flowing in the same direction. This 'mismatch' between the wind direction + the direction of flow in the ocean interior is a conceptually simple way to understand the Ekman transport. The winds drag along the surface of the water, but because the lower ocean is in something called 'geostrophic balance' (meaning primarily that its flow is heavily influenced by the Coriolis force) the lower ocean will drag the water to the right of the velocity because of this Coriolis influence. This leads to a net water transport to the right of the wind. On the surface, this manifests as the water moving at ~20degrees to the right of the wind, as as you move down in depth the Ekman spiral describes the changes in velocities induced by the effects of the flow in the ocean interior - until you reach the bottom of the layer influenced by wind, and you end up in the deeper ocean flow.

Hope this helps!

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  • $\begingroup$ Thank you it would be nice to know why the water of the lower ocean is heavily influenced by the Coriolis force compared to water of the upper ocean or air for example. I will look into geostrophic balance if I can. FYI: I did not downvote your answer. Good to see a fellow student. $\endgroup$ Mar 7, 2023 at 13:46
  • $\begingroup$ Perhaps explaining 'geostrophic balance' in more detail would be instrumental in answering my question? $\endgroup$ Mar 7, 2023 at 13:50

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