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If the polar cap of a tidally locked planet was all ocean, what weather patterns would emerge? The ocean couldn’t be half frozen/half boiling, right? I need a weather pattern so the planet isn’t just a sun blasted desert on the bright side and a frozen wasteland on the dark side. If one (or both) of the poles is an ocean that covers the light and dark sides, could the temperature differentials of the water create wind and other weather patterns?

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    $\begingroup$ What's your single specific question with definite parameters which has an identifiable best answer? $\endgroup$ – A Rogue Ant. Sep 16 '19 at 22:58
  • $\begingroup$ Ok, I’m tampering with the idea of a tidally locked planet. I need a weather pattern to cool the sun-facing side so the planet isn’t just blasted desert on one side and frozen wasteland on the other. If the pole is an ocean that covers portions of the light and dark sides, could those temperature differentials create winds and varying weather patterns? $\endgroup$ – Csraves Sep 16 '19 at 23:18
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    $\begingroup$ Tidally locked planets don't have polar ice caps. The poles are indistinguishable from any other point around the planet's terminator in terms of insolation/climate. It will likely have an antistellar ice cap however. $\endgroup$ – Arkenstein XII Sep 16 '19 at 23:27
  • $\begingroup$ @ArkensteinXII is an antistellar ice cap located on the “equator” on the cold side? $\endgroup$ – Csraves Sep 17 '19 at 0:15
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    $\begingroup$ @ArkensteinXII I’m postulating an assumption that a large body of water shared between the light and dark sides will be able to moderate slightly, allow some vegetation and normal weather patterns. I’m just asking if the presence of that ocean allows me to normalize the climate slightly while maintaining scientific accuracy of the situation $\endgroup$ – Csraves Sep 17 '19 at 1:05
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The easiest way I can think of to explain a tidally-locked planet without extreme differences in temperature between the day and night sides would be through the structure and density of its atmosphere. Let me explain:

The atmosphere of a planet can have many different effects on a planet's temperature: highly dense atmospheres can have a runaway greenhouse effect that warms the planet considerably; the atmosphere also has the benefit of creating winds that can circulate temperatures between the hotter and colder halves of the planet.

But there is one bottom line: The side facing the sun will be hot, and the side facing away from the sun will be cold. You can't really avoid this fact, because one side is being constantly heated and the other side is getting barely any heat at all. You could have a dense atmosphere, but that will simply make it so that the near side is scorching and unlivable and the far side is simply "less cold."

"Strong constant heating of a planet on one side can change or even control how much weathering occurs on the planet, which can lead to significant and even unstable climate changes. These dramatic climate effects could make a planet that otherwise has the potential for life to instead be uninhabitable."

If you want to have a livable tidally-locked planet, you could:
- 1) have one with a thin Earth-like atmosphere, where there is a habitable zone on and near the border of the light and dark side. However, this would mean the sun is always very low in the sky and near the horizon in those locations, like a constant state of sunset. To these people, completely blue skies would be bizarre and foreign.
- 2) have people living underground on the hotter side or create complicated man-made systems that isolate people from extreme temperature (for example, a giant structured dome)
- 3) give the planet a certain degree of libration. This means that although it is always facing the sun, it would "wobble" back and forth, giving a greater range of variation in temperature and potentially increasing the areas of livability.

You can also read this paper for more helpful information on a tidally-locked planet.

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Tidally locked planets can be habitable

In actuality, having a tidally locked planet isn't such a big problem for habitability w.r.t. climate. Take a look at what PlaSim generates for a tidally locked earth with solar constant of $1100\text{Wm}^{-2}$ (real earth value: $1360\text{Wm}^{-2}$). Notice how the climate on the day side ends up quite similar: the region around the pacific ocean is habitable.

The atlantic ocean ends up much like the north pole in winter: a few metres of ice covering liquid water below, and temperatures around $-30^{\circ}\text{C}$. Landmasses on the cold side can get quite cold, the model seems to indicate $-110^{\circ}\text{C}$ as the minimum average temperature. One interesting observation: a tidally locked earth can have liquid surface water down to $950\text{Wm}^{-2}$, while the real earth is a snowball below $1200\text{Wm}^{-2}$ and would need more CO2 to be habitable.

So what happens here? Two effects actually make tidal locking less of a problem than it seems. First, the weather patterns. On a tidally locked world, the atmosphere and oceans turn into efficient heat transport mechanisms from the light to the dark side. You can see it in the model results: Comparing the heat transport:

  • Earth: $ H \approx 1.7 \text{Wm}^{-2}\text{K}^{-1}$
  • Locked Earth: $H \approx 13.0 \text{Wm}^{-2}\text{K}^{-1}$

The temperature difference between the two sides is inversely proportional to the heat transport coefficient, and proportional to the diffrence in solar illumination, or: $\Delta T \propto H^{-1}\Delta G$. What this tells us about the climate on a tidal locked planet is some combination of:

  • Average wind speeds are (much) higher.
  • For tidal locked planets near the inner edge of the HZ: Hurricanes can be very strong and more common (even without the coriolis effect), as the 'pole of heat' on the day side forms a large area where storm systems can form. Similarly, dry, cold storms can form on the night side.
  • Rain will be more common, especially around the transition zone.
  • Weather tends to be more predictable.

The second effect is that of oceans. Oceans are great absorbers of heat, and take many years to heat up even from the sunlight on them. It takes one calorie to heat 1g of water up one degree (at standard pressure and temperature). Or:

$C_{\text{aq}} = 4.186\text{Jg}^{-1}$

Given an ocean column of $d = 5\text{km}$ deep, and surface area $A=1\text{m}^2$, how long would it take the midday sun to heat it up a single degree?

$$t =\frac{ V\rho C}{G} \approx \frac{10^6dC}{G} \approx 1.53 \cdot 10^7s \approx 0.61 \textbf{ years} $$

Which means even with the slow ocean currents the deep ocean would have a uniform temperature. You might think that if you lived on a tropical island near a deep trench, the seawater down in the abyssal depths would be warm just like the seawater on the surface. In reality, the ocean has a steep thermocline. The water deep down is surprisingly cold.

So nearby oceans will have a large moderating effect on local temperatures. Have a look at this model of a tidally locked ocean planet. This model tells us that we don't need to have a deathly cold night side.

Increasing the ease at which oceans can transport heat by not having north-south continental shelves near the transition zone, or having more ocean and less land can make your exoplanet have a stabler climate with less variation.

You can see this on earth too. The most extreme climates (e.g. Siberia) are inland, with the dominant wind direction away from the sea. Milder climates (such as say southern Tasmania, or equatorial islands) are often maritime.

Notice how in this model also there are cold areas on the 'light' side (near the poles) and warmer areas on the dark side (near the equator).

The former are caused by the same mechanism as on earth (yes, there is constant sunlight, but it's of low intensity, equivalent to the light the North pole gets in April around noon).

The latter are caused by heat transport and a tidally locked planet's version of hadley cells. This video provides a nice overview of some of the basics.

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