How do deep currents form ???

The sinking of large masses of cool water gives rise to the (aka thermohaline circulation), which is derived by density gradients due to variations in the temperature and the salinity. The earth's rotation on its axis also influences deep ocean currents (the high and low tides).

Explanation:

Salinity is the saltiness or amount of salt dissolved in a body of water, called saline water- Dictionary term

Density is the degree of compactness of a substance.- Dictionary term

Hope that helps