Deep Water Layer 

The deep water layer is the bottom layer of water, which in the middle latitudes exists below 1,200 meters. This layer is characterized by fairly constant cold temperatures, generally less than 4C.

To better understand the basic vertical temperature distribution, look once again at figure 1-2-1. At high latitudes in winter, the water is cold from top to bottom. The vertical temperature profile is essentially isothermal (no change in temperature with depth). In middle latitudes, the structure is like that illustrated in figure 1-2-2. In low latitudes, the mixed layer extends to a depth of about 300 feet. Here, the main thermocline is encountered and the temperature drops about 8C more than it does in the mid-latitudes. This sharper drop is due to the higher surface temperature in the lower latitudes. The thermocline extends to 2,100 feet, where the deep layer is encountered. 

Learning Objective: Define water mass and water type and identify the properties used in their classification; recognize the oceans basic vertical structure with regard to their latitudinal distribution; and recognize their source regions and how they are formed.

The concept of visualizing water masses as we do air masses is possible because both are based on the physical properties that go into their makeup. The properties of temperature and salinity are used to classify both water types and water masses.

A water type has a single value of salinity and a single value of temperature associated with it, while a water mass takes into account a range of temperatures and salinities. For example, Red Sea water is a water type characterized by a temperature of 9C and a salinity of 35.5 . On the other hand, North Atlantic Central Water (a water mass) is characterized by a range of temperatures (4C to 17C) and salinity (35.1 to 36.2 ). A water mass may be considered to be made up of a combination of two or more water types.

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