The temperature of the sea is dependent on the amount of solar radiation falling on the surface. In the tropics, with the sun nearly overhead, the temperature of the surface layers can rise to over while near the poles the temperature in balance with the sea ice is about . Cold water is denser than warm water and tends to sink. Cold water is denser than warm water and tends to sink. There is a continuous circulation of water in the oceans. Warm surface currents cool as they move away from the tropics, the water becomes denser and sinks. The cold water moves back towards the equator as a deep sea current, driven by changes in the temperature and density of the water, eventually welling up again towards the surface. Deep sea water has a temperature between and in all parts of the globe.