What is Sea Level Fingerprint?

When people talk about sea level, the above figure is probably what we had in mind. The recent mass increase over the ocean is contributed by ice sheets melting (Antarctica and Greenland), land water mass changes (including hydrological cycle, glacier and ice cap melting), and atmospheric water content variations. The figure shows how each component contributes to the “global” mean sea level changes [Hsu and Velicogna, 2017]. Try clicking off some of the components in the figure to see more detail variations on the rest.

Global mean sea level, however, cannot represent local sea level changes. Local sea level changes are affected by many different regional physical processes that vary spatially. Sea level fingerprint is one of the processes that change the regional sea level due to water mass redistribution in the entire Earth system.

While the explanation above makes a lot of sense to me, I understand it still lacks a detailed description. For instance,

1. ” What is water mass redistribution in the entire Earth system? “
2. ” Why it causes regional sea level changes? “

Let us focus on the first question. What is water mass redistribution in the entire Earth system? There is a total of three reservoirs that can store water in the Earth system. They are the atmosphere, the ocean, and the continent. In each reservoir, water is stored in the form of water vapor, liquid water, or solid ice. These different forms of water (water mass) can move around (redistribute) between reservoirs based on some physical processes like river runoff (continent to the ocean), precipitation (atmosphere to ocean or continent), evaporation (continent or ocean to atmosphere), ice discharge (continent to the ocean), etc.

Base on the above concept, it seems quite straightforward that the sea level would change if there is any water mass move from the atmosphere or continent into the ocean. In another word, the global sea level rises when water mass is added into the ocean and the global sea level falls when water mass is removed from the ocean. You may notice that I emphasize the word “global” in the above sentence since we do not have enough information to deduce how the water mass added/remove from the ocean would affect the sea level regionally.

Most people or even some scientists would just assume that this water redistribution between ocean and atmosphere (or between ocean and continent) causes the sea level to change uniformly. This assumption is not true because that the tides and ocean currents can move the water around. However, even without the influence of tides and ocean currents, sea level will not change uniformly over the entire global ocean when you take away the water mass from the continent (or the atmosphere) and add that into the ocean. The regional sea level rising signal would look like this based on the recent decade of ice melting on Greenland.

The redistribution of water mass causes two instantaneous responses within the Earth system.

1. Changes of gravity field due to mass redistribution.
2. Changes of loading on the crust due to remove/add water mass at a certain location.

Take the above figure as our example, when the Greenland ice sheet on land started to melt away, the gravity attraction decreased due to the missing ice mass on land. This causes the nearby ocean water to move away from the region where the ice melted. Besides the changes of the gravity field, the surface of the Earth (crust) moved up due to less weight on top. This is called the elastic response which is similar to the spring bounding back up once the force on top is removed. This upward movement of the crust resulted in the uplift of the seafloor near the region of ice melt. The combined effect is the decreasing height between the sea surface and seafloor (sea level drop) in the region close to the melting ice and increasing height (sea level rise) in the region far away from the melting ice. This unique spatial pattern of sea level is also called sea level fingerprint or self-attraction and loading effect.