Precipitation on a world where the entire ocean is covered by plants?

Question

My very fancy planet, has a green blotch approximately the size of Africa on its surface, which looks like a forest from space but is actually the planet's ocean. A sea, which, over millions of years, has been colonized by a free-floating species of halophyte. This plant was so prolific that nearly 100% of the planet's ocean is covered by it (with some holes here and there). This plant mat is about 3 to 5 inches thick and floats thanks to pockets of air. If you think about it, it's basically a continent-sized lawn bubble.

Other than that the planet is completely earth-like. Size, rotation, distance from the sun, atmosphere and so on. It's the kind of planet that humans would look for but it's also meant to be a desert planet. Or just generally speaking a very arid planet.

With conditions like these, what would the weather look like? Do the plants make a difference?

On hindsight I should have asked about this before making the flora and fauna, but I guess I'll have to make adjustments as I go. What I care about first and foremost is the amount and distribution of rain.

Answer 1

It would look startlingly like Arrakis

If I've understood your question correctly, your entire planet the size of Earth has a single ocean the size of Africa and that ocean is covered in plant life.

So, rather than the surface being 71% water, it's only 6% water.

• 90%-98% of your planet (at a rough guess) is raging sandstorms and desert. An ocean that small can't supply enough water to allow a planet the size of Earth to flourish. In fact, if I took the time to do some ugly research, I bet an ocean that small on a world this big tips the planet's entire ecology into "dead in no time from a geological standpoint" territory. There's almost no water on your planet.
• Some water is locked away at the poles similar to Mars. The poles are cold so any water that does get around in the atmosphere condenses to ice at the poles, just making everything worse.
• Your ocean is the only place going where life can be abundant, so it's little wonder your world evolved a plant that would protect the ocean from evaporating, finally killing the planet.

Consequence

1. Those sandstorms seriously threaten your ocean. Not a little, a lot. Let's ignore that.
2. Water must recycle or it stagnates, becoming over saturated with stuff (usually the stuff of decay) that locks up free oxygen and promotes deadly bacteria. Let's ignore that.
3. You don't actually say where on your world (what latitude) your ocean is. That actually matters in this case. I suspect a tropical location (as in, "the Tropic of Capricorn") would promote more rain than the equator, which would cause the plants to lock everything down since heat expansion in the atmosphere would draw water too far away from the ocean to return. But let's ignore that.

Everywhere on the planet other than near the ocean (within 100 miles of the ocean at most) is devastatingly dry. Terrible sandstorms, terrible heat... what can I say? Terrible.

Near the ocean it will be somewhat muggy, condensing into small water sheds that drain back into the ocean. Almost no rain, just condensation. By definition your ocean must be (for all practical purposes) the lowest point on the planet or the ocean would be diversified into groups of small lakes planet-wide. So everything drains to this point. Death valley with fauna-locked water.

The garden spot of Ceti Alpha VI.

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Thoughts I've had over time...

1. On Earth water is constantly evaporating. There's exposed water under sunlight 24/7. On your world, your ocean is exposed to daylight only 50% of the day. That's a serious hit on evaporation as the ocean must reheat from nighttime cooling every day. On Earth with sunlight constantly striking the oceans what develops are currents that carry the heat around, raising the average temperature to facilitate maximum evaporation. You won't have that effect — and with so much surrounding desert cooling so quickly at night, despite the "hot" nature of the desert during the day, I expect your ocean will be on average substantially cooler than Earth's even if it's located at the equator. To give you an idea of this impact: Your ocean is 8.5% the size of Earth's oceans with no better than half the direct sunlight time (4.25%) and the need to reheat every morning, which is a difficult to calculate effect. Let's be generous and say your planet will enjoy only 3% the evaporation of Earth. At best that's 3% the precipitation of Earth, some of which is lost at the poles, and anything that gets out of the ocean's watershed is forever lost for the value of rain (sinks into the sand or bound into fauna/flora and is proverbially never heard from again). Earth's average rainfall is 39 inches, so your planet annually will see 1.17 inches of rain—pretty much all of which must fall within the watershed of the ocean for the planet to survive. And that's not taking the insulative effect of the plant life into account. 3–5 inches of plant life and you might not see a drop of rain at all anywhere on the planet. (Which believably solves the problem of the ocean drying up from evaporative loss. There's still those sandstorms, though.)

Answer 2

That depends a lot of specific properties of the plants.

On one hand, transpiration through the extended surface area of leaves can enhance evaporation. On the other hand, plants on average have higher albedos than open ocean, which means the plant cover will have a cooling effect which decreases evaporation. How the evaporation balances out comes down to specific details of the plants' structure.

Less heat means less intense storms. Less evaporation along with less heat means less water to produce rainfall downwind. More evaporation with less heat means more water to produce heavier rainfall whenever those air masses get to a place where they can form a storm.

The salinity and thus conductivity of ocean water makes lightning strikes over ocean less common but more individually powerful than over land. The plant cover might tip the balance to make oceanic lightning storms a little more terrestrial-like, but with only a few inches of insulating cover, the effect probably won't be very large.

Plant cover will also suppress waves, which has more significant effects. Since winds won't be able to dump energy into waves as efficiently, wind across the ocean will tend to get faster and stronger than it does over Earth's oceans. Lack of waves will also suppress sea spray at the shores, which is another fairly minor effect but would slightly suppress lightning strikes near shorelines compared to Earth.

Answer 3

First, about the plants: evaporation could plausibly be in a range more, less than, or just the same as it would be in open water. Many real-world plants increase evaporation by exposing so much leaf area. In other words, whatever you want it to be.

The ocean should have some sort of top-to-bottom circulation or else whatever marine animal life there is will consume all the oxygen in deep waters and the lower levels of the ocean will become completely anoxic.

Next, assuming there's some sort of equilibrium in the water cycle, at least as much water has to fall as rain, somewhere, as evaporates from the ocean. That means that humidity levels have to reach 100% at least some of the time, over part of the planet. If the air is sucked dry of moisture by mountain ranges, etc. before it reaches the remoter regions, it's possible that it never rains on much of the planet.

If rain does fall everywhere, it's implausible that water in distant areas can find a path to drain all the way back to the ocean. The alternative is that there are inland salt seas, along the lines of Great Salt Lake in Utah, here and there. Your "ocean" might well be considered just the largest of these.

The Earth's weather is more or less divided into latitude bands--certain latitudes where it rains a lot, others where deserts occur. This is driven by long-term continental-scale convection patterns where rising air creates rain and falling air doesn't. Your planet might have the same thing, in which case all the clouds and rain only happen in certain horizontal bands around the planet.

Or maybe your planet's water cycle isn't driven by rain after all--isn't it kind of a cop-out to assume the planet is like Earth in every other way? There could also be a geological water cycle in which water circulates through the planetary mantle and crust by plate tectonics, trapped in rock layers, and your ocean is the only area where it reaches a liquid form. Mars is suspected of having enormous amounts of water trapped beneath the surface as ice. You could also plausibly create a system without a steady equilibrium if the planet is geologically young and its ocean is fated to last for "only" a few tens of millions of years.

Answer 4

Marine plants provide 70% of the Earth's oxygen, so this green patch should be more than enough to provide enough oxygen for the planet. Transpiration would increase precipitation in regions close to the ocean, making them damper than regions on a similar planet with a blue ocean.