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(a) Deep currents

Explanation:

Streamlike movements of ocean water  located far below the surface are called  deep currents. Deep currents are not  controlled by wind.  Deep currents form in parts of the ocean  where water density increases. The  density of the ocean is affected by  temperature and salinity.

When the water molecules of the ocean become heated, they expand. Extra space is created by this expansion into which salt and other molecules can fit. Since warmer water thus can hold more salt and other molecules than cold water; it can have a higher salinity. To relate this to ocean currents, the higher the salinity of ocean water, the more dense it becomes. When the salinity is high enough, the water will sink, starting a convection current. This means that cold water can sit on top of warm water if the warm water has a high enough salinity, and that the natural flow of a current actually can be reversed based on the related density, salinity and temperature of the ocean water.