I’ve just learned that different parts of the ocean can have different levels. How does this work? If I drop water into a cup, the entire level of water in the cup rises. How is this not true for the ocean? Are their just parts of the ocean that are uneven? Where do they meet?
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I can finally flex my knowledge on a niche topic! It has to do with the moon!
Basically, the earth is a rock, a spherical rock with its own gravity. This gravity holds liquid water to its surface. So imagine if you will, that the earth is perfectly smooth, and that the whole world was covered in a thin layer of water. With no interference, the water will cover the earth evenly.
Now add the moon with its own gravity. It starts pulling the earth towards it, and vice versa. Since the earth is a hard rock, it doesn’t change shape, but water can change shape, so it bulges towards the moon a little. So now, the side with the moon is a little deeper than the side without the moon.
TLDR: The moon pulls water towards itself. That bulge will be deeper and follow the moon around.
When thinking about sea level or tides and currents, a few things drive the sea as a whole. Wind, water temperature, gravity.
Gravity’s influence on tidal level according to Degrasse Tyson can be thought of the earth( solid part) kind of getting pulled, by the moon, back and forth within the body of water. It’s not that the sea actually rises or falls. The earth is pulled in the water. There is a video on YouTube where he talks about it.
Wind and temperature just compound all of this in various ways.
If you’re asking if the polar ice melting creates deeper water in that region? I guess it could very slightly and it evens out over time.
Oceans have different levels primarily due to the effects of gravity, but also due to varying levels of ocean temperatures. For gravity, it’s pretty simple: the side facing the moon feels gravity’s effect more, so it gets pulled toward the moon slightly. Not enough to take off, but enough to vary a small amount. On the opposite side of the earth, the *earth* is being pulled toward the moon more than the water, so there the water level is higher as well. On the sides not facing the moon, the water level is lower, since more of the water is moved towards the sides where the water is being pulled away from the earth or the earth pulled away from the water.
For temperatures, areas of the ocean that are warmer will have slightly higher water levels, as water expands very slightly with temperature.
> If I drop water into a cup, the entire level of water in the cup rises. How is that not true for the ocean?
It does – eventually, anyway, as the drop needs time to spread out, but if you put water in, it raises the level of the ocean as a whole.
> Where do they meet?
Everywhere. Think about it this way: the ocean is like a very, very, very gently sloped hill. There’s no part where there’s a sudden elevation change, but wander a quarter of the way around the earth and maybe you change elevation by a few meters.
The biggest changes in levels are due to local differences in the earth’s gravity. Some places just happen to have concentrations of denser-than-average rocks near the surface and that makes gravity a little stronger there. Where this happens in the ocean, the sea level is higher because the higher gravity is attracting slightly more water.
The coordinate system used by GPS (WGS84) is based on a perfectly smooth, mathematical ellipsoid. Differences in gravity mean that actual sea level is different from GPS sea level by up to roughly 100 metres, depending where you are on the earth (see [the map](https://www.researchgate.net/figure/Difference-in-meters-between-the-WGS84-spheroid-and-the-approximate-mean-sea-level-as_fig1_224239584)). This is a much larger difference than that caused by tides, air pressure and winds, which all change daily.
Local temperature differences don’t cause a change in local levels because the water is free to flow. Globally, a change in the average ocean temperature would cause a global change in levels but this is a very slow process.
> If I drop water into a cup, the entire level of water in the cup rises.
The same thing happens in the ocean. If you could add enough water in one place in the world then, within hours or days, average water levels would rise globally. It’s just that the sea level would start out and finish uneven.
Where a river empties into the sea it rises levels locally in part from the sheer amount of water, in part because it’s not salty and therefore lighter and can float on top of the seawater. Where strong currents meet the land they can pile water up against it raising the height
A sheet of ice weighs billions of tons. When one melts off land, that weight is removed from one area of the crust and so that part of the crust rebounds, actually rising up, like when you lift your finger off a balloon.
This means that other areas are going to face even more sea rise.
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