The Sun dumps a lot of thermal energy by sunlight into the atmosphere, and especially into the oceans. Basically, currents arise as warm equatorial waters move to the NORTH carrying heat, and as part of this flow, the cold polar waters go south and there get heated. Then, the SUN is like an engine which drives ocean currents.

The sun affects ocean currents in a couple of ways. When the sun heats water at the surface of the ocean, some of the water will evaporate and increase the concentration of salt in the water at the surface. Increased salt concentration means the water is more "dense." But if we think about it, things that are more dense tend to sit below things that are less dense. For example, think about putting a rock in a tank of water. The rock will sink to the bottom of the tank, because it is more dense than the water. For the same reason, the denser water with higher salt that formed because of evaporation will start to sink, and less dense water will rise and take its place. This process, combined with fresh water flowing into the ocean from river and changing the salt concentration of the water, contributes to ocean currents. In certain parts of the oceans, the dense water will sink very deeply and form deep water masses. These will flow very slowly in the deep ocean currents.

The sun also heats up the air in the atmosphere at the equator. This air will move toward the poles and cool over time. In the process, wind that occurs due to air currents will induce currents at the surface of the ocean. The currents at the surface are sort of "propagated" down deeper because of differences in the forces on different parts of the water.

When we talk about ocean currents we normally talk about the wind that drives those currents. But what creates that wind? The sun! The heat of the sun controls what we refer to as the model of thermohaline circulation, or more simply, the “Global Conveyor Belt” Cold water at the poles is more dense than warm water and so it sinks, it heads south, and then it reemerges farther south. At the same time, the solar energy of the sun heats up the surface water at the equator. As the cold water from the poles is forced back to the surface, the heated water is pushed farther north. As it moves north, it too gets less energy from the sun, which causes the water to cool, increase in density, until it finally sinks again and head back down to the equator. This process takes around 1,000 years to complete!

The sun adds energy to the Earth, and that energy tries to escape back into space for the same reason that energy in the air tries to escape into space. Ocean currents help this happen.