What does the professor mean by this statement:

Questions 1 through 4. Listen to part of a discussion in an oceanography class.
W1: We know a fair amount about the circulation of water in the shallow ocean, but much less about the deep ocean. The deep ocean is vast—about 90 percent of the total volume of the ocean—and we know very little about it. We do know there’s a close relationship between the ocean and the atmosphere. A lot happens at the surface of the ocean, where water meets air, including some of the forces that drive ocean currents, such as ... what? What’s one thing that happens at the surface?
W2: The wind blows at the surface. The wind has a big effect.
W1: Yes, that’s right, it does. The wind has a major impact on surface water currents. In fact, water circulation at the surface is driven mainly by the wind. But what about the water in the middle and lower layers of the ocean? What drives the circulation of deep water?
M: Well, we know there are deep currents in the lower layers and near the bottom. Our book says that water movement in the deep ocean is slow, compared to how it is at the surface. That’s because the deep water is so much colder and so much heavier than the water at the surface.
W1: That’s right. This colder, heavier water moves much more slowly. The movement of the deep water is driven by gravity and caused primarily by changes in the density of seawater.
W2: So, it’s gravity that drives the water circulation in the deep ocean.
W1: That’s right. OK. Another thing we know is, what happens at the surface has an impact on what happens in the deep ocean. For example, one change at the surface is an increase in the water’s salinity—or saltiness—which can happen when the water evaporates or sea ice forms. And what happens when water gets saltier?
M: It gets heavier and denser. If it gets dense enough, it will sink downward until it reaches a level of equal density, or until it reaches the sea bottom.
W1: Yes, that’s right. The deeper you go, the denser it gets. The deepest water has the greatest density. And all of these properties—temperature, density, salinity—all play a crucial role in the ocean’s flow.
1. What does the professor mean by this statement:
"We know a fair amount about the circulation of water in the shallow ocean, but much less about the deep ocean."
2. According to the discussion, what factors influence the movement of water in the ocean?
3. How is the ocean’s deep water different from the water at the surface?
4. Why does the professor say this:
"The deeper you go, the denser it gets."