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From some research, I have learned that atmospheric circulation - winds currents and ocean currents - depend on pressure and the Coriolis effect. But if the oxygen level is increased and the pressure is higher (as said in the question), how much would this effect be modified ?

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  • $\begingroup$ We enforce a "one question per post model". I have removed the second question, which would have made this too broad. $\endgroup$ – L.Dutch - Reinstate Monica Oct 21 '19 at 18:18
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Winds will be slower, but there are a lot of second-order factors

The oxygen content is irrelevant but the pressure has a significant effect.

Let's start by analyzing wind. In general, there are four forces:

  1. The pressure gradient force, which is defined as $\frac{1}{\rho} \frac{\Delta P}{\Delta x}$. Note here the density term: a higher density reduces this force. This makes intuitive sense: if the density is higher, there's more stuff to move. This is a large reason why winds are faster the higher up you go (ignoring a lot of other complexities).
  2. Coriolis force, which is defined as $2 \Omega v \sin \phi$. This is an apparent force thanks to the earth's rotation. While proportional to the speed of the wind, it isn't dependent on density.
  3. Friction. This slows down the wind, thereby reducing the coriolis force and changing the direction. Density isn't a big factor.
  4. Apparent centrifugal force. Again, density isn't a factor.

So, all else being equal, the higher atmospheric density will reduce wind speed to about 58% of what it was.

Another input, the pressure gradient ($\Delta P$) is also reduced. A major cause of pressure differentials is a temperature differential. For example, let's say we have an island. During the day, the land of the island will heat up more than the surrounding ocean, the air will rise, and thus there will be a surface low (and divergence aloft). At a high level, if a given air molecule (pedants, you know what I mean) has a higher temperature than the surrounding molecules, it will rise and get out of the way. What the ideal gas laws tell us is that the absolute number of molecules isn't relevant; it's the percentage of the molecules. From this angle, increased density means that you have to move a large number of air molecules to get the same change in pressure. This means the pressure gradient force is smaller than it would be on Earth. This is obviously just one example, without formal math behind it, but my suspicion would be that wind speeds are slower as a consequence here as well.

Note also that wind speed isn't quite the same as wind strength; it may be slower, but there's more air moving, so it will feel stronger for a given speed.

Nonetheless, winds being slower will in general cause ocean currents to slow as well, as the wind pulls the water along at the surface. This opens up a countervailing effect: with the winds and ocean currents slowing, temperature gradients will likely sharpen in some places, which will increase the pressure gradients and thus the winds. Ultimately, the weather system will settle into a new equilibrium which can't be analyzed without running a weather model under your conditions.

There may also be some interesting effects based on the heat absorbed by the added air mass that I haven't thought through.

One final note about extreme weather events:

  1. Thunderstorms won't be much affected, though some probability that they'll be less severe on average. Thunderstorms are caused by the rising of humid air. Severe thunderstorms are aided by wind shear (on the vertical axis), so they may be a little less likely to form if winds are slower.
  2. Tropical storms (ie hurricanes) will have slower winds for all the same reasons mentioned above, but will likely still do as much damage. More air moving at a slower speed will still have the same amount of momentum. Their energy source is warm sea surface temperatures, which is unaffected by the density.
  3. Tornados have about the same analysis as hurricanes, though they're driven by north-south temperature gradients. In general, you could probably make up whatever you want about their frequencies being set by the new equilibrium the climate settles into and I would believe it.
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  • $\begingroup$ Thank you so much for your answer ! You said that wind speed will be reduce to about 58%. Maybe I'm wrong, but it seem huge ; would sailing boat travels be affected ? Or the winds strenght would, as you said, bring balance ? (actually it would be really interesting if it slows down discovery of new territories, maybe it would be longer to colonize and history of exploration would be different, but maybe I am going too far ahah). $\endgroup$ – Emie Oct 22 '19 at 9:39
  • $\begingroup$ In theory, as long as the ships are lateen-rigged and don't have the wind at their backs, it won't make a difference. In practice, I would guess that it will still slow sailing. I'm no sailor though, so I don't know well enough to say for sure or how much. $\endgroup$ – Kofthefens Oct 22 '19 at 15:55
  • $\begingroup$ If I want to run a weather model, what elements would I need to reunite for that model to be settled ? (I already have the athmospheric characteristics of the planet, a map of plate tectonics, montain range and oceanic and winds currents, and the heat absortion formula - even if I don't know how to calculate it), what would I need more ? $\endgroup$ – Emie Nov 7 '19 at 10:00
  • $\begingroup$ I should preface this by saying that running a proper weather model with your custom world is going to be hard. As in, it may well take months, or longer if you're not good at compiling weird systems. Your best bet is probably the WRF model, simply because it's got a good community. Their forums are here: forum.mmm.ucar.edu/phpBB3/… $\endgroup$ – Kofthefens Nov 7 '19 at 17:17
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    $\begingroup$ Imagine that the air is molasses sloshing around in a jar. It flows slower, and it's harder to get it moving in the first place. This is the dominant factor. Now imagine that the person holding the jar tilts it back the other way when they see the fluid moving too much to one side. They're not perfect, so there's still a bit of sloshing, but less than there would have been. If it was water in the jar, they'd see that it's moving almost instantly. But with molasses, it can build momentum. This is secondary -- it still sloshes less than water -- but it does counteract the first force $\endgroup$ – Kofthefens Nov 8 '19 at 5:30
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The Coriolis effect is generated by the rotation of the planet, thus content of oxygen doesn't affect it.

Also, different content of oxygen has negligible effects on the thermal properties of the atmosphere, since it is a mixture of gases anyway.

The increased pressure would make winds stronger, as there would be more mass of air withing the same flowing volume.

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  • $\begingroup$ Thanks for your answer ! When you say "stronger", do you mean that storms or hurricanes could be eventually more devastating ? $\endgroup$ – Emie Oct 21 '19 at 18:53

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