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The Gulf Stream is said to be

  1. Largely driven by wind stress, and
  2. Up to 1000 m deep.

At the same time, I am reading that

  1. the effect of the wind usually does not reach deeper than 100 m
  2. in Ekman motion theory water at a depth of 100 m will flow in the direction opposite to the surface current.

How do these things fit together?

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    $\begingroup$ you need to include this en.wikipedia.org/wiki/Thermohaline_circulation into your question. $\endgroup$
    – trond hansen
    Sep 21, 2022 at 5:24
  • $\begingroup$ I see, thanks. I am reading here that the Gulf Stream is driven by both wind stress and thermohaline gradients: earthscience.stackexchange.com/a/8480/27611 So I guess that answers the first question (how can the Gulf Stream be 1000 meter deep, when the effect of the wind is said to only reach to a depth of 100 m). I think it doesn't answer the second one though: how can a flow be 1000 meter deep when Ekman's motion theory tells us that flows tend to reverse direction after about 100 m? $\endgroup$
    – shamiv
    Sep 21, 2022 at 11:26
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    $\begingroup$ The wind is not the main answer to the forcing. If it were just the wind, then the Canary Current off of West Africa should be as energetic as the Gulf Stream and it is not even close. The Gulf Stream is predominantly a thermohaline current as @trondhansen mentions. The main reason it is so strong is because of Earth's rotation (Coriolis effect): rwu.pressbooks.pub/webboceanography/chapter/… $\endgroup$
    – arkaia
    Sep 21, 2022 at 20:22
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    $\begingroup$ Ok thanks. I understand that the tendency of a flow to reverse directions at a depth of 100 m only applies to situations in which the "original" driving force (in this case wind stress at the surface) only directly affects the top layer. If another driving force - like a density gradient - acts on multiple layers at the same time, this tendency to reverse directions at a certain depth disappears. (As is for instance the case for the movement of air as a result of pressure gradient force in the atmosphere). Does that make sense? $\endgroup$
    – shamiv
    Sep 22, 2022 at 11:21
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    $\begingroup$ Just to add to that, I am reading on p. 56 of the IPCC AR6 compilation of FAQ (ipcc.ch/report/ar6/wg1/downloads/faqs/…) that the Gulfstream is indeed part of both the (density-driven) AMOC and the (wind-driven) subtropical gyre. $\endgroup$
    – shamiv
    Oct 5, 2022 at 21:14

1 Answer 1

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The winds over the north Atlantic are, basically, cyclonic: the southern winds blow westward and the northern winds blow eastward. Due to Eckman pumping, this creates a dome of water, and consequent higher pressure, in the Sargasso Sea. This, in turns, mandates a cyclonic flow of water around the higher-pressure dome with westward compression/intensification of the flow due to a latitude-dependent Coriolis effect.

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