Global Mean Sea Level is an averaged measure. The averaging has to take account of local temporary factors such as the tidal flows, windspeed, sea currents, etc. etc.. It also allows for the permanent effects of the Earth's rotation - whereby the poles are 'flattened'. And factors-in corrections for the curvature of the Earth.
But, in addition, there are various locations on the planet which have permanent differences to the mean sea level, caused by local variations in the Earth's gravitational pull. Where the gravitational pull is lower, the downward-forces on the sea are lower, so it 'thins out'. These gravity variations are mostly unexplained.
Sea Slopes are inclined gradients between 'normal' sea level and these anomalies - which can be surprisingly large.
As an example, the Indian Ocean Geoid Low (IOGL) is an area covering approx. three million square kilometers, and has comparatively low gravity. The mean sea levels near the centre of the the IOGL are permanently around 100 metres lower than the global average. Although the sea level difference is substantial, the sea slopes are very slight due to the extremely large area of the anomaly.
In other areas, the sea is 'piled-up' by higher gravity, and can reach 50 meters or so above average.
The variations were originally discovered in the 1940s by ship-based gravity surveys, and have now been extensively confirmed by radar readings from satellites.
Some of the gravity variations are easily explained - e.g. the sea levels around the coats of Greenland are slightly higher than average due to the gravitational pull of the vast land-based ice-sheets. But in other areas, such as the IOGL, the reasons for the low gravity can only be surmised. A recent theory suggests that it may be due to variations in the flows of molten rock under the Earth's crust. See Geophysical Research Letters Volume 50, Issue 9.
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