• Salinity in surface waters is controlled by currents and temperature and averages approximately 35 parts per thousand.
• Salinity values are relatively uniform in well-mixed surface waters of the open oceans but are more extreme in restricted waters of coastal seas.
• Salinity and temperature change with depth with the most rapid change occurring in a depth zone labeled the halocline.
• Ocean temperatures of 27^oC are typical of tropical surface waters and temperatures of 2^oC are typical for deep ocean waters.
• Cold water is more dense than warm water, but ice (frozen solid water) is less dense than liquid water.

Salinity and Latitude
eawater contains dissolved salts. The concentration of salt in seawater is salinity. Salinity varies around the world's oceans depending on temperature and the  mixing action of ocean currents.  Salinity is measured in parts per thousand (ppt; 10 ppt = 1%) of salt in water. The salinity of the warm, well-mixed surface waters over much of the world's major ocean basins ranges from 33 to 37 parts per thousand (Fig. 6).

Figure 6. Map of salinity at the ocean surface. Numbers represent salinity values in parts per thousand. Salinity in the open ocean is greatest in tropical regions and decreases in the isolated Arctic Ocean. Map generated at University of Tokyo website.

Higher- and lower-salinity values are observed in smaller, restricted ocean basins and seas (Fig. 7). For example, salinity values of 20 to 30 ppt are recorded for the high-latitude Arctic Ocean and values of over 40 ppt occur in the narrow tropical Red Sea basin between north Africa and the Arabian peninsula. Salinity is higher at low latitudes because high temperatures at these locations promote evaporation which removes water but leaves the salt it contains behind. Salinity values are lower at high latitudes because of the lack of evaporation, high precipitation, and the influx of freshwater from melting ice sheets. The isolated Baltic Sea between Sweden and Finland has salinity values that approach freshwater along its northern shore.

	Figure 7. High and low salinity in restricted seas in low (Red Sea) and high (Baltic Sea) latitudes. Click on image to view larger version. Maps generated at University of Tokyo website.

Salinity and Depth
Salinity values are variable in the shallow (e.g., 0-200 m) ocean but are much more uniform in deeper waters below 2,000 meters (6,600 feet; Fig. 8). Salinity may decrease with depth in the tropics but increases with depth at high latitudes (+60^oN/S). The salinity in the Arctic Ocean (north of 70^oN latitude) increases with depth from 30 to 35 ppt. Salinity in this relatively isolated ocean basin remains uniform below a depth of approximately 300 meters (1,000 feet).

The change of salinity occurs over a depth zone known as the halocline. The depth range for the halocline is from approximately 200 to 1,000 meters (660-3,300 feet) but will show some variation with location. Salinity is uniform with a value of 34 to 35 ppt below the halocline.

Figure 8. North-south profile through the Pacific Ocean along the 155.5 meridian illustrating the range of salinity with depth and latitude. Numbers represent salinity values in parts per thousand. Cross section generated at University of Tokyo website.

Temperature and Latitude
Solar radiation strikes Earth more directly at the equator and tropics than in polar regions (Fig. 9). Radiation strikes Earth at a lower angle near the poles and the Sun’s rays must therefore penetrate a greater thickness of atmosphere. Some of the solar radiation is scattered in the atmosphere and more heat energy is lost near the poles as a result of scattering. Earth's surface at the equator receives 2.5 times more insolation, incoming solar radiation, than the atmosphere above the poles. The highest average annual ocean temperatures (~27^oC) are present along the equator and temperatures decrease symmetrically to the north and south approaching 0^oC at high latitudes (Fig. 10). 

Figure 9. Solar radiation is distributed over a wider area and must penetrate a greater thickness of atmosphere at the poles, reducing the amount of solar energy reaching Earth's surface. Consequently, ocean temperatures are greater near the equator.

Figure 10. Map of world's oceans illustrating the average annual range of temperature with  latitude. Numbers represent temperature in degrees Celsius. Map generated at University of Tokyo website.

Water has two relatively unusual thermal properties that make the oceans a great storage reservoir for heat energy and contribute to global oceanic circulation patterns. First, the heat capacity of a material is measured as the amount of heat required (in calories) to raise the temperature of 1 gram of the substance by 1^oC. Materials with high heat capacity, such as water, can absorb substantial quantities of heat without any significant change in temperature. The ability of the oceans to store heat plays a crucial role in controlling global climate patterns.

Second, cold water can be both less dense and more dense than warm water. Water density increases as water temperature decreases down to approximately 4^oC. Below that temperature water density decreases, especially when water changes state from a liquid to solid (ice) form. Consequently, dense cold water can sink below less dense warm water but ice will float on the ocean's surface.

Temperature and Depth
The major oceans can be divided into layers of relatively warm waters at shallow depths and cold waters at greater depths (Fig. 11). Surface waters are warmed by solar radiation and currents cause thermal mixing that results in relatively uniform temperature distributions by latitude. Sunlight doesn't penetrate more than a few hundred meters below the ocean surface and the impact of current activity diminishes with depth.

Temperatures exceed 20^oC over much of the tropical ocean's surface but decline to a chilly 2^oC below 2,000 meters (6,600 feet)depth. The depth zone in which temperature decreases rapidly is known as a thermocline. The base of the thermocline is at a depth of approximately 1,000 meters (3,300 feet).

Figure 11. North-south profile through the Pacific Ocean along the 155.5 meridian illustrating the range of temperature with depth and latitude. Cross section generated at University of Tokyo website.