Water And Oceanography

Understanding what an Ocean Bulge is



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There is so much water on Earth compared to land (71 percent of the surface) that many prefer to call it the "water planet," and some are even of the opinion that "earth" is a misnomer. The presence of so much water, all in a single body called the ocean, along with the proximity of a massive satellite, the moon, gives rise to a phenomenon that coastal people experience twice daily, the tides. In places, the shore water levels rise several meters from their neap-tide low to their spring-tide high in a space of six hours and 12 minutes, then recede to their low again in a further six and twelve. These tides are merely result of a huge bulge in the ocean caused by the gravitational pull of the moon.

Astronomers speaking of tidal forces is a common occurrence. After the primary gravitational effects, such tidal forces constitute "secondary" effects. The primary gravitational force determines the overall trajectory of the astronomical body. Tidal forces are minuscule in comparison, causing minute topographical changes in the body, and over long periods of time determine how the body rotates on its axis. Compared to what takes place generally, the ocean tides on Earth constitute a spectacular display hardly to be found anywhere else in the solar system.

The most superficial explanation of the ocean bulge is that the gravitational force of the moon pulls the entire ocean toward it. But why does it pull only the ocean, and not the rest of the planet too? This is where the subtlety comes in. The rest of the planet is also pulled, but water, being liquid, relents more easily. The advantage would have been unimaginably small if not for the fact that there is so much water. It is as if the moon is dealing with a rubber ball here.

This is still not enough. Here is where the proximity of the moon comes into play. The moon is so close to the Earth (in astronomical terms) that the nearside of the latter experiences a greater force than the far side. Of course, this is always the case with tidal forces, but usually the difference between the near and far sides is negligibly small (only with things falling into a black hole do you get things as exciting as the earth tides). Even with the moon it is only a fraction of a percent, but this is enough. The waters on the far side of the ocean are not pulled so strongly, but something else quite extraordinary happens. The solid earth as a whole cannot help but be pulled toward the moon, while the rubbery water is left behind! So there is a bulge on the far side of the ocean, not because anything is pulling at it, but simply because the ocean floor moves away.

Now, you have to keep in mind that all these things are small, but they are adding up. And there is another thing that contributes, too. The circumference of the Earth at right angles to the near and far points is not pulled at all, where in normal circumstances it would be, with the moon in any part of the sky other than the horizon. So the waters in these parts can depress, at the same time as they are rising everywhere else. The overall effect is the unparalleled phenomenon known as the ocean bulge. In the end, at the point nearest to the moon, the solid Earth's surface moves one meter toward the moon, while the surface of the ocean adds one more meter to that. A similar bulge appears on the other side, the point furthest from the moon.

Only one meter! This is depressing. Remember, this is only on the exact spots nearest and furthest from the moon. With 71 percent of Earth’s surface covered by water, these two points are likely to fall almost always in lonely ocean expanses. Everywhere else the rise will be much less. But the fact is that there are places on Earth that experience tidal increases of up to 12 meters. How does this come about?

The key here is momentum. One meter does not sound like much, but the ocean is so big that there is a lot of water moving the moon's way, and this represents a mighty momentum. If the earth was only water, the water level increases would have gone entirely unnoticed. But due to the 29 percent landmass scattered here and there, the momentum of the ocean waters comes before impediments. It is much like carrying a bucket full of water, and the momentum imparted to the water, after hitting the bucket walls, causes splashing, and might give rise to standing waves that cause the water to overflow. As a result, some places will witness spectacular tides. By the same token, other bases get them belatedly, like in the Mediterranean coasts, cut off from the oceans by the narrow Gibraltar Straits.

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ARTICLE SOURCES AND CITATIONS
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