Wind blowing on the ocean stirs the upper layers leading to a thin mixed layer at the sea surface having constant temperature and salinity from the surface down to a depth where the values differ from those at the surface. The magnitude of the difference is arbitrary, but typically the temperature at the bottom of the layer must be no more than 0.02-0.1°C colder than at the surface. Note that the both temperature and salinity must be constant in the mixed layer. We will see later that mean velocity is not constant. The mixed layer is roughly 10-200 m thick over most of the tropical and mid-latitude belts.