the surface part of the ocean, also called the "mixed layer", is not much colder even when we go deeper down. below this surface zone is a layer of sudden temperature difference, called a "thermocline". this is a middle layer hat is from the surface zone down to about 2,600 feet (800 m). thermoclines may happen only at seasons or permanently, and may change depending on where and how deep it is. as evaporation happens, it begins cooling, and if the water evaporates very quickly, the water becomes saltier. the salty, cold water is denser, so it sinks. this is why warm and cold waters do not easily mix. most animals and plants live in the warm upper layer. below the thermocline, the temperature in the deep zone is so cold it is just above freezing - between .