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tl;dr : the earth has been gradually flooded, over the course of a few hundred years. Also, there are now large lumps or ice raining out of the sky (not very often), causing massive tsunamis.

I have recently watched a sort-of documentary, about what would happen if a roughly moon-sized icy asteroid collided with the moon, causing icy shrapnel of various sizes to rain down, smallest first. The long-term results are that the entire planet gets flooded, even mount Everest, due to the smaller chunks of space-ice depositing huge amounts of water-vapour in the atmosphere, resulting in "biblical flooding". After the rain stops, the larger chunks start falling down, causing massive tsunamis. Humanity has moved to floating cities, which can survive these waves, by riding over them. Probably not comfortable, but survivable.

My question is what weather would look like during and after this period of mega-tsunamis. My question is not about what we would do, but what our problems might be.

A theory I had was that there would be constant (as in Stormlight archive Highstorms. Not Star Wars Kamino) hurricanes, since there would be no land to stop them. Also, currents should be interesting.

Note: I have tagged this question with planetary-rings, since that is where all the water and tsunami-causing space-rocks come from.

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  • $\begingroup$ @KorvinStarmast the asteroid collides with the moon, forming some icy rings, causing ~300 years of rain (I missed the beginning, so I don't know if the rain is non-stop everywhere, or just about 3/4 times more). This question is set about the years 250-350 after the collision. Not editing the question, since this isn't really relevant to the meteorology, which is what the question is actually about. $\endgroup$ – Mark Gardner Sep 29 '18 at 12:51
  • $\begingroup$ Ok, that's clearer (the time horizon) so I'll remove that comment. As I understand the premise, the wave action / tsunami is like "ripples in a pond from throwing in a rock" but obviously orders of magnitude greater. Have you already thought through the food source problem? The majority of fishing is from the two hundred fathom curve, and in, in the littoral areas. That's due to the transition zone being favorable to both unersea plants and fish. (likely a separate question) $\endgroup$ – KorvinStarmast Sep 29 '18 at 12:56
  • $\begingroup$ @KorvinStarmast that is correct. $\endgroup$ – Mark Gardner Sep 29 '18 at 12:59
  • $\begingroup$ Oh, and by the way, you need to consider that all this water will be fresh. The oceans will become significantly less salty (like, 1/5 as much), and a lot of fish species will die as a result. Plus, of course, there will be virtually no equivalent to areas like the Grand Banks, so no groundfish. Water higher than Everest means almost nowhere with depths less than about 10,000 feet. $\endgroup$ – WhatRoughBeast Sep 30 '18 at 21:25
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I can argue for constant violent storms and a snowball earth event, it depends on the frequency with which the big chunks keep coming and how much, if any, sulfur they contain.

If the barrage is still fairly constant, a lump or two every couple of years, then high altitude water vapour deposited by their transit through the atmosphere has the potential to block a great deal of solar radiation from reaching the surface. If the cometary debris contains sulfur compounds then sulfur aerosols will exacerbate such an effect. High atmosphere blocking/absorption would mean that the Earth was Ice-age cold, or worse, if bad enough there could be ice bergs right up into the tropics.

If on the other hand the bombardment is lighter, one or two pieces every couple of decades, then the main weather driver is the fact that water absorbs all the solar energy that hits it. This means that the oceans in the tropics will continuously be in the high 30s to mid 40s Celsius spawning a steady stream of Hypercanes/Super Typhoons. The exact balance that settles out of this scenario is a bit hard to judge since cloud reflectance is high variable and somewhat poorly understood but current models suggest the world will be quite a bit warmer than today (possibly to the extent of runaway global warming, so called "Sauna Earth").

A light bombardment that is high in sulfur will cause a seesaw effect, the weather will be warm between pieces of debris but temperatures will drop markedly for relatively short periods, 1-5 years, every time another lump deposits a new round of sulfur aerosols. This will be similar to the year without a summer but occurring repeatedly over an extended period.

Couple of notes, a world ocean is probably poorly mixed below, at most, 200m (the maximum depth of sunlight penetration), meaning that the water at depth, and the seabed everywhere, will be de-oxygenated and dead. This means that the ecosystems are going to be very surface, and so are the weather systems; no deep water currents moving heat around the globe and mixing oxygen into the sea as a result. In any scenario with a world ocean that doesn't have any global conveyor temperature differences between the poles and the equator are going to be more marked, this will enhance winds that blow north/south.

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