Water mass

This ppt will help to understand the concept of water mass in the world ocean. It gives an overview of it's origin, formation, distribution etc. in the world. It is also helpfull for Fisheries Science students.

1. Water mass Author: ANUJ SHARMA Kerala University of Fisheries and Ocean Studies, Kerala

2. Definition An oceanographic   water mass is an identifiable body, in terms of density, of water with a common formation history which has physical properties distinct from surrounding water. Water mass are stable, large water bodies and for their formation, water has to sink down/ strong downwelling. It is defined on the basis of uniform density, not in terms of temperature or salinity. 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 eg. .                                                                                                        cont...

3. Cont..... Red Sea water ( lying between Africa and Asia)  is a water type characterized by a temperature of 9°C and a salinity of 35.5 ‰. On the other hand, North Atlantic Central Water (a water mass) is characterized by a range of temperatures (4°C to 17°C) 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. 

4. Formation majority of water masses are formed in the polar latitudes, and are not formed in tropical waters and temperate waters. Cold, highly dense surface water sinks, spreads out horizontally, until it reaches a level having the same constant density.  The manner in which it spreads out depends on its density in relation to the density of the surrounding water. Downwelling is more required for water mass formation and more pronounced under: a) brine extraction process, b) in absences of Pycnocline (not well developed in Polar region)

5. Common water masses in the world ocean Antarctic bottom water ( AABW),   North Atlantic Deep Water(NADW),  Circumpolar Deep Water(CDW),   Antarctic Intermediate Water(AAIW), Subantarctic Mode Water (SAMW),  Arctic Intermediate Water (AIW), North Pacific Intermediate Water (NPIW), the central waters of various oceanic basins, and various ocean surface waters. Different regions with different temperatures form water masses. Some water masses are more dense than others depending on salinity. Freshwater will float because it is less dense than salt water. 

6. There are 6 water masses sources region in world ocean: North Pacific ( near Aleutian Island) South Pacific(ANTARTICA) Norweigian Sea East Greenland(Arctic region)  Weddell Sea( Antartica) Lazarev Sea( Part of Antartic Basin)

7. Source: internet

8. Distribution In low and middle latitudes the vertical arrangement of water is such that we can distinguish a surface layer, upper water (central and equatorial), intermediate water, deep water, and in some localities, bottom water. SURFACE LAYER:  properties vary widely from one area to another, depending on current variations, evaporation, precipitation, and various seasonal changes, especially in the middle latitudes, so, not classified as a water mass or water type. The surface layer is separated from deeper water by a transition layer (the main thermocline) CENTRAL WATER MASSES: found in relatively low latitudes although its source region is in the region of the subtropical convergence.  Central water is not usually discernible at the surface and is generally relatively shallow. Its greatest thickness is observed along its western boundaries.  Cont.....

9.     EQUATORIAL WATER MASSES: found in the Pacific and in the Indian Ocean. In the Pacific it is thought to originate on the southern side of the equator.  Equatorial water is found in the northern part of the Indian Ocean is having  higher salinities,  probably due to its mixing with the waters of the Red Sea. Cont.... Fig:  Global distribution of upper waters (0-500 m).  (source- Emery et al.,2001)

10. INTERMEDIATE WATER Intermediate water is found below central water in all oceans. Intermediate water includes Antarctic intermediate water, Arctic intermediate water, Mediterranean water, and Red Sea water.    One of the characteristics of Antarctic intermediate water is its low salinity (34.1 ‰ to 34.6 ‰). In comparison to the water around it, it displays the lowest salinity values. Fig:     Global distribution of intermediate water (1000-3000 m )     ( source:  Emery et al.,2001  )

11. DEEP AND BOTTOM WATER MASSES.  In the deep ocean basins below intermediate water, high density deep and bottom water exists. These water masses form in both hemispheres.    In the Southern Hemisphere, Antarctic bottom water forms near the Antarctic continent, while in the Northern Hemisphere, Arctic deep and bottom water forms in northwestern Labrador Basin    These water masses form at the surface, sink, and spread out to fill the deep-ocean basins. FIg :  Global distribution of deep and abyssal waters (3000....)bottom. (Source:  Emery et al.,2001)

12. TS-time diagrams In oceanography,  temperature-salinity diagrams , sometimes called  T-S diagrams , are used to identify water masses.  The water mass can be identified by a relation of the properties pertaining to temperature and salinity. Even though, density depends on the temperature and salinity, it cannot be used for identification, bcz two water mass may have same density but temp. And salinity may different.  In a T-S diagram,  potential temperature (on the vertical axis) is plotted versus salinity (on the horizontal axis).  Fig:  Water masses of the deep ocean as seen in a TS-diagram and associated source water types. (source: Tomczak et al., 2000)

13. T-S Diagram, Example Source: Coursera

14. THE THREE-LAYERED OCEAN    Using bathythermograph information (temperature versus depth profiles), the oceans display a basic three-layered structure: the mixed layer, main thermocline, and deep water layer.  The mixed layer is the upper layer, constant warm temperatures which, in middle latitudes, extends from the surface to a maximum depth of about 450 meters, or 1,500 feet. Mixing processes are done by mechanical and convective process. Fig: layers in ocean (source: internet)

15. The main thermocline is the central layer of the ocean. It is found at the base of the mixed layer and is marked by a rapid decrease of water temperature with depth.  The seasonal thermocline comes about from the gradual warming of the surface waters. The warming takes place in the upper few hundred feet of the surface, and results in the seasonal thermocline becoming super-imposed on the main thermocline.  The mid-latitude summer thermocline is more pronounced than the thermocline of spring or autumn. Fig: Seasonal thermocline.          (Source: internet)

16. 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 4°C. Below 3,300 feet to a depth of about 13,100 feet, water temperature remains constant. At depths below 13,100 feet, the temperature ranges from near freezing to just above the freezing point of water as depth increases. Fig: reprentation of layers with depth ( source: NOAA)

17. THE GLOBAL CONVEYOR BELT Source: National  Geographic

18.   It is a system of ocean currents that transport water around the world. While wind primarily propels surface currents, deep currents are driven by differences in water densities in a process called thermohaline circulation.   1) At the poles, cold ocean water becomes saltier and denser from evaporation and ice formation. The cold, salty, dense water sinks and slowly spreads. 2)  Because there are no continents to block the water, the current can flow freely around Antarctica. Some water moves into the Indian Ocean while another current travels to the Pacific Ocean.  3)  Deep water surfaces in a process called upwelling. 4) The warm surface current flows north toward Greenland, completing the cycle.

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