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Because That Message In A Bottle Wont Float Itself To Africa

Currents are essentially just the movement of water. They can be caused by tides (tidal currents), gravity, or just the Coriolis effect (open ocean currents).
     Tidal currents occur when the tidal bulge moves across the earth. This bulge draws water with it and as that water flows into a bay or past a reference point, the current can be noted. For example, when a high tide is coming into San Francisco Bay, the current under the Golden Gate Bridge can be very strong leading into the bay.
     Tidal currents in most areas are described as Reversing because the flow of the tide comes in, stops, then flows out. When the tide is coming in, or transitioning from a low to a high tide, it is said to be Flooding. When the current is going out, or transitioning from a high to a low tide, it is said to be Ebbing. The strength of the current is highest halfway between the high and low tides, and it 0 or close to 0 exactly at the time of high or low tide. The strength of the current depends largely on the height of the tide. Unusually high tides cause unusually strong currents because a larger volume of water must flow into an area in a set amount of time.
    Rotary Currents are also tide driven, but rather than being reversing as described above where the flow of water stops and reverses, water is always flowing past, but it is constantly changing direction.

Time of Slack/Max Current

Speed at Any Time

Time at Any Speed

Duration of Slack

     Open Ocean Currents are caused by the differing densities of ocean waters. Water at the equator is hot and water at the Poles is cold. As a result, the warm water moves toward the colder denser water, forcing the cold water to move as well. Because the Earth is spinning, the Coriolis Effect deflects the water movements, causing currents that skirt the edge of most ocean basins. Click the link below to see a diagram of Open Ocean Currents.

Ocean Current Patterns