1. Salinity and Density Differences VERTICAL STRUCTURE, THERMOHALINE CIRCULATION & WATER MASSES

2. Three Effects of Salinity on Water

3. 1. Freezing Point Pure water freezes at 0 ºC. Salt lowers its freezing point: Salt lowers its freezing point: Seawater with a salinity of 35 º / ºº freezes at -1.91 ºC -1.91 ºC

4. 2. Density Salt molecules have a greater atomic mass than H 2 O molecules, so the density of water increases with salinity. As a result fresh water will “float” on salt water.

5. 3. Vapour Pressure As salinity increases, vapour pressure drops. This means that freshwater evaporates at a faster rate than seawater

6. Density of H 2 O The density of water depends mainly on salinity and temperature Cold salty water is more dense then less salty warm water Density of seawater is 1.020 to 1.030 g/cm 3

7. Seawater’s density increases when: 1. salinity increases 1. salinity increases 2. pressure increases 2. pressure increases 3. temperature decreases 3. temperature decreases

8. Density Structure of the Ocean Much of the ocean is divided in to three density zones 1. surface (mixed) zone 2. pycnocline 3. deep zone

10. 1. Surface (mixed) Zone Consists of water in contact with the atmosphere and exposed to sunlight Least dense zone Surface to approx. 1000m

11. 2. Pycnocline The zone where density increases with depth This zone isolates surface waters from the denser zone below 18% of ocean waters

12. 3. Deep Zone Lies below the pycnocline Density does not change much with increasing depth in this zone 80% of oceans water

13. More on Density Differences Density varies within the world’s oceans depending on the location. Factors that cause differences in density: - High rates of evaporation - High inputs of freshwater (river run-off) - High inputs of freshwater (ice melting) - Temperature differences

14. Thermocline – graph representing change in temperature Halocline – graph representing change in salinity Pycnocline – graph representing change in density

15. Done!

16. A WATER TYPE is a body of water which has uniform temperature and salinity The deep and bottom waters vary little in temperature and salinity, and so are represented almost completely by water types. They are not influence by wind and local climate changes and therefore reveal much about their life history. They are not influence by wind and local climate changes and therefore reveal much about their life history.

17. SEASONAL THERMOCLINE In the subtropical gyres, at latitudes where seasonal variations in wind strength and solar heating occur, a SEASONAL THERMOCLINE develops in addition to the permanent or main thermocline. In the subtropical gyres, at latitudes where seasonal variations in wind strength and solar heating occur, a SEASONAL THERMOCLINE develops in addition to the permanent or main thermocline. During winter, strong winds typically mix the upper ocean layers down to 100 m or more, but then in spring when winds moderate and heating increases a thin warm layer develops at the surface with a sharp seasonal thermocline beneath. During winter, strong winds typically mix the upper ocean layers down to 100 m or more, but then in spring when winds moderate and heating increases a thin warm layer develops at the surface with a sharp seasonal thermocline beneath. Later periods of storms may mix the seasonal thermocline down but not usually as deep as in winter. Subsequent periods of heating and mixing may form step like structures in temperature, forming a seasonal thermocline continuous with the permanent one. Later periods of storms may mix the seasonal thermocline down but not usually as deep as in winter. Subsequent periods of heating and mixing may form step like structures in temperature, forming a seasonal thermocline continuous with the permanent one. Come winter, vigorous mixing distributes the heat added at the surface over the upper few hundred meters. During the course of the winter, this heat is released slowly to the atmosphere warming the local climate

20. DEEP SALINITY LAYERING The bulk of the deep ocean is relatively uniform, especially in the Pacific. However the subtropical surface layers have a higher salinity because of excess evaporation, compared to the northern polar areas which are diluted (lower salinity) because of the effects of massive precipitation and runoff However on looking more closely, it is seen that there is significant salinity variation in the deep Atlantic layers. First, there is a clear tongue of high salinity stretching south through the equatorial regions. Second, there is a core of high salinity water at 1000 m depth evident especially in the eastern sector of the North Atlantic. However on looking more closely, it is seen that there is significant salinity variation in the deep Atlantic layers. First, there is a clear tongue of high salinity stretching south through the equatorial regions. Second, there is a core of high salinity water at 1000 m depth evident especially in the eastern sector of the North Atlantic. These features represent water masses, which are distinct bodies of water formed in a restricted location, spreading out through the ocean by being carried with the current (advection) and by mixing with surrounding waters (diffusion). These features represent water masses, which are distinct bodies of water formed in a restricted location, spreading out through the ocean by being carried with the current (advection) and by mixing with surrounding waters (diffusion). The two in question here are the North Atlantic Deep Water and Mediterranean Intermediate Water, which has escaped out through the Strait of Gibraltar. In the Pacific, there is no clear contrast of deep water masses but two tongues of lower salinity water are seen to sink and extend towards the equator from the subpolar regions. These are known as the Arctic and Antarctic Intermediate water masses. The two in question here are the North Atlantic Deep Water and Mediterranean Intermediate Water, which has escaped out through the Strait of Gibraltar. In the Pacific, there is no clear contrast of deep water masses but two tongues of lower salinity water are seen to sink and extend towards the equator from the subpolar regions. These are known as the Arctic and Antarctic Intermediate water masses. The Antarctic Intermediate Water is also visible in the Atlantic section. The spreading of these water masses is evidence of the THERMOHALINE CIRCULATION. Which means that subsurface flow patterns are produced by density differences not by wind

21. WATER MASSES At this point we consider what is meant by the term WATER MASS. There are 4 main water masses: CENTRAL, INTERMEDIATE, DEEP & BOTTOM CENTRAL, INTERMEDIATE, DEEP & BOTTOM A water mass is a kind of fingerprint, which allows the origin of particular water parcels to be identified even when they are far removed from their source. A water mass is a kind of fingerprint, which allows the origin of particular water parcels to be identified even when they are far removed from their source. The predominant regions where the temperature and salt content of sea water are altered are at the surface. Once away from the sea surface the only mechanisms which alter temperature and salinity are mixing processes. So in general water masses tend to maintain their characteristics and can be identified even if they have traveled far away from their original location.

22. CHARACTERIZING GLOBAL WATER A WATER MASS is an extensive body of water which has a limited range of temperature and salinity A WATER MASS is an extensive body of water which has a limited range of temperature and salinity The water masses of the world's oceans can be summarized by a few points: The warm, less dense surface waters make up only 20% of the total volume, but they encompass the layers of greatest variation. The warm, less dense surface waters make up only 20% of the total volume, but they encompass the layers of greatest variation. The upper layers vary significantly from one ocean to another because they are strongly affected by heat and water fluxes through the surface locally. The upper layers vary significantly from one ocean to another because they are strongly affected by heat and water fluxes through the surface locally.

23. Although it would be possible that both temperature and salinity could have values independent of each other, in fact, it is observed that they vary throughout the oceans in a very related manner. Although it would be possible that both temperature and salinity could have values independent of each other, in fact, it is observed that they vary throughout the oceans in a very related manner. Since density must increase with depth to maintain stability within the water column, it is known that density increases with depth. Since water with a higher salinity is more dense it to will increase with depth to maintain a stable water column. Therefore as the depth increases so do both the temperature and the salinity. So the Halocline and the Pycnocline will follow the same patterns.