There's a tidally locked planet with native life or not, but with an atmosphere that could allow a decent degree of heat transfer between hot and cold side and a rather broad, hospitable region near the terminator.

Even without rotation, we could expect vortices around the poles - looking at the dayside: Cool air streams, near to the ground, from the night side towards the sunpole, to be heated, rise, and stream back above the incoming cool air. If cool flow is disturbed in any way (say a mountain range roughly along the terminator), this would could introduce a small angular momentum to the incoming cool air which would be greatly accelerated as we get closer to the pole. This alone could in theory lead to a strong polar vortex.

ETA: Coriolis forces are not necessary to create a vortex, as can be observed when emptying a bathtub. A short explanation is found here, indicating that the effect of the coriolis force is rather weak, as shown by an experiment where all other disturbances where eleminated and it took 15 minutes for a vortex to develop due to coriolis effects.

Now my question is: Would such a circumpolar vortex trap hot (or cold) atmosphere, the way the polar vortices on earth do? Or would it rather be part of a sort of atmospheric conveyor belt, transporting hot and cold air masses around the globe?

For the purpose of this questions, I'm only interested in atmospheric vortices, I think asking about marine phenomena would complicate things too much. The worldbuilding aspect is that a tidally locked planet could have those vortices and relatively benign conditions near the terminator - but the vortices break down every few years or decades, sending masses of scorching or freezing air to the terminator. The inhabitants would study chaos theory with religious devotion.

Note: A previous version of this question considered a rotation around the sunpole-nightpole axis to aid in vortex formation, as a learned discussion in the comment section here and under this (wrong) answer make clear this is impossible.

  • $\begingroup$ It is not possible for the planet to be "rotating around it's daypole - nightpole axis". A solid body can rotate around only one axis (because geometry, and we cannot argue with geometry), and if the planet is tidally locked then it is already rotating around an axis perpedincular to its orbital plane. $\endgroup$ – AlexP Sep 17 '20 at 12:04
  • $\begingroup$ I thought I read somewhere that this was possible but what you say is entirely plausible - I'll remove that part, thanks! $\endgroup$ – mart Sep 17 '20 at 12:06
  • $\begingroup$ ‘Sunpole’ is a very cool term. You might want to look into Saturn’s Hexagon for some interesting reading on polar vortices. $\endgroup$ – Joe Bloggs Sep 17 '20 at 12:07
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    $\begingroup$ @JoeBloggs: That accepted answer to the "this one" question is manifestly wrong. JDługosz's comments on that answer are right. Planets are by definition almost spherically symmetrical; there are indeed small asymmetries, which will induce precession of the axis of rotation, but the precession will be very very much slower than the rotation. (For example, Earth rotates once in 23 hours 57 minutes; its axis of rotation precesses once in about 26,000 years.) $\endgroup$ – AlexP Sep 17 '20 at 12:28
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    $\begingroup$ General note, since it seems to be coming up: Tidally locked planets with very short orbital periods do have a Coriolis effect, because the planet is rotating to keep one face toward the sun. This has been modelled. worldbuilding.stackexchange.com/questions/4850/… $\endgroup$ – rek Sep 17 '20 at 15:46

Models of some tidally locked Earthlike planets (TLE) have supported middle atmospheric polar-adjacent vortices, but the upper and lower atmosphere continue to mix or form vortices elsewhere:

Cold TLE (CTLE):

Model of Cold TLE horizontal windspeeds at 24, 36, and 60 km

Warm TLE (WTLE):

Model of Warm TLE horizontal windspeeds at 24, 36, and 60 km

Snapshots of the last model step of the CTLE and WTLE horizontal wind at stratospheric and mesospheric altitudes are shown in [Cold TLE and Warm TLE, above].... The CTLE and WTLE stratospheric horizontal wind at 36 km is shown [above]. In both simulations, a westward global zonal jet stream with an accompanying vortex located at polar latitudes is present. The vortex can be seen on the left hand side of the Southern and Northern Hemispheres [above]. The blue coloured regions on the right hand side of the Southern and Northern hemispheres, on the other hand, are regions of low HW speeds. The WTLE horizontal wind is slightly weaker and has a wider jet stream compared to the CTLE.

The situation is different at mesospheric altitudes. At 60 km altitude, the zonal jet stream is replaced by large-scale vortices in both the CTLE and the WTLE, located at different geographical locations.

— (Proedrou, E., Hocke, K. & Wurz, P.; The middle atmospheric circulation of a tidally locked Earth-like planet and the role of the sea surface temperature.)

In modelling TLEs of different radii and short orbital periods, this study found that in TLEs of R>1.75 and orbital periods less than 12 days, the upper atmosphere can undergo superrotation, effectively forming polar hemispheric vortices with wind speeds around 300km/h and trapping heat:

For orbital periods shorter than 12 days, towards the very inner edge of the habitable zone, the situation becomes more complex. We found that tidally locked planets on very tight orbits can assume first two different climate states (Climate I and II) and for orbital periods shorter than 5 days even three different climate states (Climate I, II and III).

Climate I is associated with so called equatorial superrotation: This is a very fast and strong eastward wind jet along the equator in the upper atmosphere that can reach in our simulation wind speeds of 300 km/h and more. This climate state has already been observed for tidally locked gas planets. For rocky planets, superrotation apparently stops rising heated air over the day side from being transported towards the night side. The heat is thus stuck on the day side and the surface temperatures there can reach the boiling point of water – in particular for a two Earth-radii planet. Climate II, on the other hand, assumes instead two weaker eastward wind jets that circle the planet at higher latitudes, well away from the equator.

—(Ludmila Carone, Rony Keppens, Leen Decin; Connecting the dots II: Phase changes in the climate dynamics of tidally locked terrestrial exoplanets)

Climate models for TLEs of varying radius

However, as the surface figures show, no polar vortex is apparent.

So current models of tidally locked planetary atmospheres do not seem to support the idea of fixed polar vortices that reach the surface. However it's interesting to note hurricanes are supported on planets near the inner edge of the habitable zone, and seem capable of forming anywhere:

Atmospheric models of a tidally locked aquaplanet

We find that hurricanes can form on the planets but not on all of them. For planets near the inner edge of the habitable zone of late M dwarfs, there are more and stronger hurricanes on both day and night sides. For planets in the middle and outer ranges of the habitable zone, the possibility of hurricane formation is low or even close to zero, as suggested in the study of Bin et al.(2017).

— (Mingyu Yan, Jun Yang; Hurricanes on Tidally Locked Terrestrial Planets: Fixed SST Experiments)


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