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Possible repeat of this, but the northern ocean wasn't really addressed.

Picture a mars-like planet, with a rocky, mountainous southern hemisphere, and a flatter, low-elevation northern hemisphere. Enough water for 10-30% surface coverage. It's also tidally locked to a red dwarf.

The terminator is habitable. The sun side is too hot for human survival, but presumably below 100 Celsius.

enter image description here

My uneducated guess for weather patterns: the night side of the ocean would freeze, at least on the surface. The day side would dry up due to heat, except closer to the pole, with less direct light. The terminators would accumulate more lakes and small seas, possibly? And if winds converged to the center of the day side, the rains would send water flowing north, towards low elevation?

enter image description here

Is this close?

If it's not too broad, please correct my errors or let me know any other unusual outcomes of the basic geography.

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It's a lot more complex than you think

But we can simplify it to get to your goal. Imagine, if you will... a billiard ball. This hypothetical billiard ball has an atmosphere and its star is simulated by a radiant heater. The ball sits placidly before the heater and never moves (which is basically what a tidally locked planet is doing).

What's happening?

  1. The atmosphere on the sun-side gets hot and stays hot. That means a low pressure zone more-or-less at the center of the sunward hemisphere. Hot air wants to move into the darkside.

  2. The atmosphere on the darkside is cold and stays cold. That means a high pressure zone more-or-less at the center of the darkside hemisphere. Cold air wants to move into the sunside.

  3. You don't just have storms at the terminus, you have storms invading both the sunside (but not to the center point) and the darkside (but not to the center point). The more water you add to this mix, the stronger and deeper the storms become. You also get some honking nasty wind at the terminus (which is hardly a garden) due to the constant battle between cold, humid and hot, dry air.

The center point of the sunside is a desert. Depending on how close to the star you are, it could be a wailing hot and dry desert. However, most deserts have some life because water isn't just in the atmosphere, it's also in aquifers, and given the nature of the general climate, it's reasonable to believe that you could have sunside aquifers that are recharged through the terminus from the darkside. You have a lot of variability here because there isn't a definitive answer without an unreasonably detailed geological map of your world. The consequence is that you can have some evaporative water on the sunside... which just makes all the storms worse. But it increases the quality and quantity of life.

Generally speaking, thermal zones cycle between each other

And this is the point I'm making. hot-to-cold-to-hot. Humid-to-dry-to-humid. That billiard ball is still sitting there with a heater in front of it, but the atmosphere isn't static at all. In fact, without a stronger rotation (i.e., not tidally locked), you lose the benefit of things like prevailing westerlies that mix things up pretty thoroughly and help moderate planetary average temperatures.

And just to make everything really messy, I've not discussed what happens between the equator and the poles.... Frankly, the most habitable locations on the planet just might be the two darkside locations just off the poles. The hot air will want to force its way though those moderate areas rather than battle the stronger temperature gradients at the two equatorial terminus locations, which means living in a perpetual twilight but with moderate temperatures and lower wind (still have wind, though).

That's my I'm-not-a-climatologist-by-any-stretch-of-the-imagination take on it.

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    $\begingroup$ an alternative view though for something a bit further away from the sun than Earth is (like Mars?) the old "all of the water and maybe even all of the atmosphere will precipitates out and freeze in a giant ice cap on the dark side" model may still work? $\endgroup$
    – Pelinore
    Commented May 10 at 12:00
  • $\begingroup$ @Pelinore That link makes a good point. A lot of this depends on just how perfectly the planet rests in its goldilocks position - but I think the article understates the darkside component. If the planet rests with at least a nearly perfectly circular orbit at just the right distance from the sun, I think it's arguable that the dayside centerpoint would be tropical forest fading to a colder perhaps alpine style biome at the terminus. But the icecap would definitely be on the darkside. I just find that level of perfect a bit unbelievable. $\endgroup$
    – JBH
    Commented May 12 at 19:10

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