Physical Properties of Seawater and Basic Circulation Flashcards
Why are the oceans a key element in the climate system?
They transport heat and freshwater and exchange these with the atmosphere.
What is WOCE?
The World Ocean Circulation Experiement was a part of World Climate Research Programme which used resources from about 30 countries to make satelitte obervations of the global ocean between 1990 and 1998 to observe physical processes.
What are the main oceanic components in terms of salinity?
Na^+, Mg^2+, Cl-, SO4^2-
What does salinity variation depend on?
The balance between evaporation and precipitation and upon mixing between surface and the deep oceans/
What does excess evaporation and excess precipitation coincide with in terms of salinity?
Higher precipitation adds more freshwater and lowers salinity; higher evaporation increases the salinity
What are Isohalines??
They are lines joining points of equal salinity
What is the general N-S trend of oceanic salinity
Surface maximum at 20N and 20S where E>P and decreases towards equator and poles
What is the Halocline?
Area of strong below surface gradient in salinity
Where is the most vertical salinity variation?
Surface waters
What does sea surface temperature depend upon?
Incoming radiation depends on the latitude and the season
What are Isotherms?
Lines joining points of equal temperature
What is the thermocline?
The rapid change in temperature with depth
What is the general N-S trend of sea surface temperature?
Highest at equator, (warmest equator area in Pacific and Indian Oceans N of Australia. Temperature decreases with latitude increase toward poles.
What is the temperature difference in Low, Mid and High latitude profiles?
Low lats have the largest surface temperature gradient, High lats have smallest and mid lats are more intermediate.
Explain potential temperature of water.
The CTD measures the ‘in situ’ temperature of water because sea water is compressable any vertical movement of the water will cool or warm the water as it expands or contracts. The ‘potential temperature’ is essentially therefore the temperature a parcel of water would be if it were brought tot he surface.
What is Density of seawater a function of?
Temperature, salinity and pressure.
What is potential density?
The density of water should it be brought to a reference level such as the surface
What is the pycnocline?
A great change/ gradient in density.
Which is more dense, cold or warm water and salty or fresh water?
Colder water is denser, as is saltier water. Dense water will sink.
How can water masses be defined?
Water masses can be defined by T and S and hence by density. They maintain characteristics over great distances and time.
Name the 10 main oceanic water masses.
AABW (Antarctic Bottom water); CPW (Circumpolar water); NADW, (North Atlantic Deep Water); PDW (Pacific Deep Water); AAIW (Antarctic Intermediate Water), AIW: (Arctic Intermediate Water); MedW (Mediterranean Water); RedSW (Red Sea Water); Central Water; Surface Water
What are T-S diagrams
Diagrams that show non-linear relationship between temperature and salinity and can be used to assess density and stability.
What is meant by a stable water column?
Some common parameters analyzed in the water column are pH, turbidity, temperature, salinity, total dissolved solids, and other chemicals. A ‘stable’ water column is one where these ‘layers’ do not mix.
What do T-S diagrams show?
Show layering of water masses, a full line to show the situation prior to mixing and a dahsed line to show the water column after mixing.
Where is Antarctic bottom water formed?
Mainly in the Weddel and Ross Seas by deep convection and sills all ocean basins below 4000m depth. In Pacific and Indian ocean mixes with Atlantic deep water becoming Circumpolar water
How is North Atlantic deep water formed?
Product of deep convection in the Arctic ocean, Greenland Sea and Labrador Sea.
How is Central Water formed?
The water of the permanent thermocline is formed by subduction at the tropics.
How are Mediterranean and Red sea water formed?
Intrusion of high temperature, high salinity waters from two inland seas. Dense water is a product of cooling and evaporation
What are oceanic currents
Large scale water movements that occur in all oceans
Where does oceanic movement come from?
Energy from the sun - winds, T S and density. Rotation of the earth imparts a velocity of fluids on the surface and decreases towards the poles.
What is Eulerian measurement of currents?
Instruments are fixed and current flows past giving temporal changes
What is Lagrangian measurement of currents?
Tracking of floats and objects - this is cheap but hard to interpret.
What are the four methods of current measurement?
Eulerian; Lagrangian; Chemical tracing and satellites
How are wind and current directions related to compass directions?
Currents are TO the compass direction and Winds are FROM the compass direction. So therefore a northerly wind will produce a southerly current
Explain boundary currents
As circulations swing poleward around the western margins of the oceanic sub-tropical high pressure cells, there is a tendacny for water to pile up against the continents, affecting sea level. Due to high temperatures and vertical stability it continues poleward driven by dominant surface airflow - through this movement the current gains anticyclonic vorticity leading to narrow current of high velocity such as the Gulf stream. These are known as western boundary currents. Eastern boundary currents are weaker and more diffuse.
What are the main differences between eastern and western boundary currents?
Eastern, eg Canaries are broad, shallow and slow with diffuse upwelling. Western boundary currents are narrow, deep and swift and sharp with not upwelling
How can current be variable?
Eddies and meanders; zones separating one area of water from another, (oceanic front), from change in temperature, salinity and different biological characteristics.
Explain Coriolis force
Because the earth rotates, the shallow layer of surface water set in motion by the wind is deflected to the right of the wind direction in the N.Hemisphere and to the left in the S.Hemisphere. Frictional coupling propagates wind induced movement downward through successive ocean layers, each layer deflecting respectively and forming the Ekman spiral effect.
Where is Coriolis effect zero?
The equator
Explain upwelling
If surface water moves away from a coast it is replaced from below.
What is the global rate of upwelling
~4m/year
What is THC
Thermohaline Circulation
Explain THC
The thermohaline circulation is driven by density differences. Density depends on temperature and salinity. Temperature and salinity differecnes arise from heating and cooling at the sea surface and from freshwater fluxes.
Contrast THC to wind driven currents
THC is not confined to surface waters, but can be regarded as a big overturning of the world ocean from top to bottom.
What processes does THC consist of?
Deep water fomation (sinking of water masses); Spreading of deep waters; Upwelling of deep waters; Near surface currents.
What drives THC?
Pressure gradients at depth, resulting from the density gradients of the overlying waters.
Which are more variable; surface waters or deep waters?
Surface waters are more variable than deep waters
