MadSci Network: Earth Sciences

Re: Is 'sea level' different in Atlantic and Pacific? How?

Date: Mon Sep 14 13:19:34 1998
Posted By: Karen Culver-Rymsza, Biological Oceanographer
Area of science: Earth Sciences
ID: 904871333.Es


You were taught correctly. The Pacific and Atlantic Oceans have different heights with the highest sea surface in the Western Pacific. But don't worry about all that Pacific water rushing into the Atlantic and flooding the Texas coast.

As far as equalizing levels, this has occurred but introduces an idea that many people find hard to imagine. The difference in heights is a result of a "dynamic equilibrium". Dynamic means changing, but equilibrium means balance, which we usually think of as no change. How can this be? Think of an unchanging balance as one like you can find on a scale where you place the same weight on both sides and it will balance -- always and forever. There is no new weight added or removed, no energy added or removed. This is Static equilibrium, an unchanging balance. A dynamic equilibrium involves the balance of changes. Imagine on your scale that you have cups instead of weights. Each cup the same size and is being dripped into and overflows but it will still balance if the fluid dripping in and out balances on both sides. Here there is a dynamic equilibrium with changes in mass or weight. If we want to imagine an equilibrium that also includes energy, let's have one side of the scale have a smaller cup than the other. Water drips in and overflows, but the two sides can never balance because the two cups can never hold the same amount of water. This gets a little complicated, so hang in there. The cups can't balance each other with weight alone. The lighter cup will go up and the heavy cup will go down. You can make a dynamic equilibrium by pushing down on the lighter cup. Now you have two cups with different amounts of water that balance. Hmmm? This can only happen with the addition of energy.

This is similar to how the sea surface heights can be different -- there is a dynamic equilibrium. It is indeed affected by the rotation of the Earth which is where some of the energy used to maintain the balance comes from. More comes from the Sun and its heat. The energy acts through winds on the sea surface and currents which tend to cause water to "pile up" where currents are swift. Imagine the surface of your morning juice in its cup. If you swirl that juice, the surface is no longer 'level' but slanted (and the slant rotates with the swirl). This slant is maintained as long as you continue to swirl the cup. You have an equilibrium that requires energy. The moment you stop adding energy by swirling, the surface of the juice levels out. It is now a static system with a static equilibrium that is simple to understand.

There is a huge science project (yes, scientists do science projects too) called TOPEX/Poseidon that looks at the sea surface (and other ocean information) from orbiting satellites. And there are oceanographers who specialize in studying the sea surface and what causes it to be different from flat. Some of this information is available on the Internet including really cool pictures made from satellite information. You can start looking at these locations: research/

TOPEX site:

Dynamic height image:
topo with current speed arrows:
changes in sea surface height:

Hopefully helpful,
Karen Culver-Rymsza, PhD
Graduate School of Oceanography
Rhode Island

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