Weather on a Flat, Infinite Sea

Question

Imagine a vast sea, stretching out to encompass the horizon much like a real sea. But unlike such a sea, this sea sits upon a flat plain, and stretches out over 100 million miles in each direction(except for the shore of course). A day begins with a glow spreading from one horizon to eventually encompass the whole sky with an even light. This diminishes after 12 hours, followed by 12 hours of starlit, moonless night. Some things to note for clarity;

• This flat plain that the ocean stretches on has a maximum depth of 10,000 feet, and the atmosphere over this sea extends above the water only 10,000 feet.
• We will say there is a hard barrier at the top of the atmosphere, keeping the air in. Why is beyond the scope of this question.
• Gravity is equal to Earth's gravity at all points under the ocean without variation.
• The daylight radiation is equal to a tropical summer day on earth.
• The light begins on the east horizon, moving across the sea toward the west. Any given point on the ocean will experience a total of 12 hours of day and 12 hours of night.
• There are no landmasses in this sea, be they islands, continents, or subsurface mountain ranges. Only the flat bottom of the ocean.

My question is this; What weather(waves, winds, storms) would form in such an environment.

Answer 1

Without permanent temperature differentials across the water plain, the whole place gets the same daily total insolation, you wouldn't have any permanent ocean currents. Not related to the weather but this means that the water will be anoxic at depth and life as we know it will have serious issues surviving.

What you will see is weak, transient, fluid circulation events on a daily cycle, the subsolar point will be slightly hotter than the areas around it at any given point creating a relatively high level of evaporation and atmospheric convection above the point of maximum radiation absorption. This is going to produce an upwelling in both the atmosphere and the underlying ocean but not particularly strong ones. The evaporated moisture is going to form clouds as it rises but without Coriolis these are going to simply rise to their dew point and then dump their load. Rather than hurricanes etc... there will simply be relatively shortlived ranks of clouds behind the subsolar point that bleed out their water over the course of the afternoon and the night that follows, dawn should find the sky clear. I would expect overnight fog to be a given since the ocean is always going to be warmer than the air after the sun goes down and surface evaporation should continue.

Waves are going to be minimal except where the winds feeding into the subsolar low pressure cell are active, the waves that are "chasing the sun" (approaching the subsolar point in the same direction as the subsolar point is moving across the surface) have the ability to build on themselves somewhat and will be larger than those coming the other way.

Oops, I hadn't done the math, the solar radiation sweep is moving so fast that tropical intensity radiation is insufficient to create a meaningful energy budget for almost any weather phenomena to form. You'd need to be looking at something like being 1AU from Sirius A to get any major weather. You would still get waves if the radiation source is a star though, gravity induced like a tide but going millions of miles an hour.

Answer 2

Water has a really high heat capacity, which is why a lot of coastal regions have milder climates than continental interiors. In this case, you clearly have a lot of water, so it's likely that most temperature variation would get sort of just evened out. Depending somewhat on the overall/average temperature of the ocean, you might see differing levels of humidity at various points in the day where you'd otherwise see temperature variation... but I'm not 100% sure on that. It'd probably have to be on the warmer side.

Increased humidity is more particles in the air, so even without much temperature variation there is a world where you could have small pressure differentials, with higher pressure where you have more energy entering the system. In any case, the dominant characteristic of this place no matter what is likely to be a mild and largely unchanging climate.