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Isostasy
alcaray — (archived)

Here's a cool write-up about historical efforts to map the earth's geoid. It doesn't get to the point you are discussing here till the very end, where it discusses the huge contribution that GRACE made. It is a good read, throughout, though.

From March through about July the geoid in the Amazon is somewhat higher than average but this elevation transitions into lower heights from September through December. Such variations suggest what is known as isostatic adjustments. Isostasy refers to the tendency for the Earth's outer sphere to experience forces that cause its surface to rise or fall as loads are added or removed. The response is not instantaneoous but occurs over time. For instance, if much of a continent is covered with ice (glaciation), then the crust beneath will sink under this load; when the ice is removed that crust will rise (rebound). Part of the isostatic mechanism involves plastic flow adjustments in the upper mantle. In the South American case pictured above, the added load, leading to the blue stage, is the accumulation of water in the Amazon Basin during the rainy season. GRACE is sensitive enough to monitor the resulting elevation changes.

It should be noted that all of the change over time that GRACE detects, is caused by moving around things that have more gravity than air. When water (frozen or liquid), or magma accumulates somewhere, it brings its mass with it. Differences in air pressure will not appear in GRACE's dataset because it has a microscopic and undetectable effect.

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