If an earth-sized planet were tidally locked to a star, and was in the life zone of its star with an earth-like atmosphere, what would its wind patterns be like?

The planet would have one hot hemisphere and one cold hemisphere, so intuitively it seems like air currents would drive from the hot side to the cold side, but this seems like it should be counterbalanced by some stream of air from the cold side to the hot side. Would Coriolis effect be strong enough to direct these air currents? What would these wind patterns look like?

up vote 20 down vote accepted

There is many studies (e. g. Yang 2013, Hu & Yang 2013, indirectly Joshi 1997 or Joshi 2003) that investigate this in context of tidally-locked planets of red dwarf stars. The reason is that such planets have to be very close to their parent star in order to maintain liquid water. Their orbital periods are short (15 - 40 days) and tidal-locking occurs quickly and naturally. Because of the quick rotation period, which is the same as the orbital period, the Coriolis force is crucial.

The most important feature, already pointed out by PipperChip and Vincent, is that the hot air is rising at the substellar point, which causes lot of rains, and the upper winds move towards the dark side. The winds at the level of ground move towards the substellar point, replacing the missing air. However, the Coriolis force turns the winds in the direction of rotation, which causes strong westward winds and maybe even superrotation as on Venus.

Winds according to model of Hu & Yang

Image from Yongyun Hu and Jun Yang, PNAS 111 629–634 (2013), doi: 10.1073/pnas.1315215111

You can notice on this Figure from meteorological model od Hu & Yang that the substellar point is not the hottest place, thanks to the wind. The hottest places are two points just above and below it.

However, if Earth got tidally locked, the Coriolis force would be almost non-existent and the most dominant feature would be air rising at substellar point and near-ground winds moving towards it. (There some videos exploring this scenario 1, 2, 3, although not the wind pattern itself.) This situation was described in Vincent's and PipperChips answers.

Do not miss: The transition from wind pattern of slow rotating planets without Coriolis force and quickly rotating planets of red dwarfs is nicely illustrated in Oceaniis' post at the Forum of Speculative Evolution. Interesting source is also this post at the SpaceEngine forum. Interesting source for deeper study is also thesis of R. A. Edson.

Note: While this isn't a complete answer, it turns out that your intuition is mostly right.

The Wind

Wind currents would, generally, go from the day side to the night side. This depends, however, on a lot of factors. Such factors can be temperature of the star, landmasses, density and composition of the atmosphere, oceans, and others. Cornell University did a neat study on tidally locked planets, if you would like to take a look. They even address Coriolis Effects! As a simplification and summary, the faster your planet goes around your star, the more your winds get mixed up.

We know that, eventually, the air from the cold side will "want" to go to the warm side, due to pressure, gravity, and diffusion. This may form the equivalent of Hadley, Ferrel, and Polar Air cells, except it involves air going from the day side to the night side instead of equator to pole. It turns out modern simulations find that such circulation would actually keep the night side quite warmer than previously thought. See the wikipedia article about the Habitability of Red Dwarf Systems.

The Air Cells

It would be generally assumed that the air cells on our tidally-locked planet would look like Hadley/Ferrel/Polar Air cells going from the day side to the night side. Some people think there would be areas of permanent rain where hot/cold air meet, and that there would at least be a ring of habitability somewhere around there.

Climate Models Are Complicated

You should therefore recognize that this answer has some assumptions built in. The Atmosphere needs to be neither too thick or too thin. It appears that oceans ought to exist to make it habitable. Many studies don't actually look at wind, but rather heat transfer or other effects. Your question is closely tied to exoplanet study, and it is a huge field.

  • Nice answer. Do you have some reference that Hadley and other cells are expected? I read few studies and they never mention it. The cells arise in normal planets, which are heated around whole equator. But for tidally locked planets, the problem has very different symmetry - the planet is heated only at the substellar point. So a lot of things will be different and I would not expect Hadley cell structure unless some meteorological model shows it.. – Irigi Dec 1 '14 at 23:13
  • @Irigi This is mostly a me doing "spherical cow" approximations. Full Climate Models are not simple things; there are a lot of factors. As a generality, I wouldn't expect air to move only in one direction, and I would expect hot air to rise and then move, but then cooler air to take its place. This can be the basis for cell formation. – PipperChip Dec 2 '14 at 0:46

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