eli5: how are oceans different heights?

Question

i know the moon pulls the water around and the gravity hole near india, but like, would their still be a height difference without lunar tides? also isnt it one big whole body of water? how is one side taller than the other

In: 2

Answer 1

> would their still be a height difference without lunar tides?

Yes. Winds also shape the surface of the seawater somewhat and consistent wind patterns effectively create ‘hills’ of seawater over many kilometres laterally, though they are only metres high at the most.

We can average out both the effects of winds and tides with satellite altimetry though (given enough passes over the whole Earth from satellites in different orbits). Even then, there is a difference between different parts of the world because (1) the Earth is an oblate spheroid so water bunches up nearer the equator, (2) the strength of the gravity field does not just follow a simple shape based on (1) because different rocks have different densities and there are also lumps/bumps in the seafloor (not to mention continents surrounding ocean basins). These all contribute a bit more or less to the gravity at any one point, so the [resulting reference shape of sea level](https://en.wikipedia.org/wiki/Geoid) is lumpy, but only by a 200 metres when comparing absolute lowest and highest bits. So it’s still incredibly smooth on the scale of the whole Earth, but it’s far from perfectly smooth. (3) Different ocean basins are underlain by different tectonic plates, with different spreading (or subduction) rates. The newer a portion of oceanic crust is, the hotter and more buoyant it is, so that it sits higher in the underlying mantle. This creates a higher sea level when measuring as distance from centre of the Earth.

There’s a nice answer on r/askscience which I think is actually very Eli5 friendly and goes through the different reasons why [here](https://reddit.com/r/askscience/comments/143xu8a/is_0_elevation_sea_level_the_same_everywhere/jne8dnb) Important for these sorts of questions is what you want to use as a reference point to start measuring from. In most cases, the geoid is the best reference for zero sea level.