The coldest surface waters occur near the poles where the Sun is low on the horizon. Cold ocean water of the poles sinks below warmer waters and fills the deep ocean basins creating cold bottom waters throughout the world's oceans. In mid-latitudes during the summer and always in the Tropics, solar heat absorbed by surface water is mixed into a surface layer to depths of a few hundred meters by waves and wind action. In mid-latitudes in winter, the thermocline may be very gentle or even missing. The thermocline is always present in the Tropics and generally absent in the polar regions.
Currents move warm water to cooler regions and cold water into warm latitudes.
Currents will therefor be an important factor in characterizing the overall temperature distribution of the world's oceans. D. Water Masses and Water Types 1. A water mass is a water body with a characteristic range of T and S. A water type has a specific T and S value. Therefore, the masses can be defined by a range of sT and a water mass by a single value of sT. 3. Major water masses are usually named for their place of origin.