Surface currents are powered by the wind. The flow of the ocean's surface currents is powered by the steady, prevailing winds through a series of processes. Surface currents are contolled by wind direction and speed as well as the location of the continents. Wind speed and direction vary on a different time scale than does continental locations. A very specific pattern of surface current flow is present in the modern ocean.
Trade winds drive the north and south equatorial currents.
Water density is principally controlled by its temperature and salinity Decreasing water temperature increases water density. Increasing water salinity increases water density. Temperature is commonly the more important of these two properities. Circulation in the deep ocean is driven by differences in water density. Water at the surface of the ocean will become more dense as its temperature decreases with cooling or its salinity increases with sea ice formation or evaporation.