Ocean Circulation Flashcards
(L1)
What is meridional ocean (thermohaline) circulation?
A global ocean circulation system which connects the worlds oceans.
What is meridional ocean circulation driven by?
Wind and variations in water density.
What does the term Thermohaline not encompass?
The importance of wind in driving the circulation system.
Why is meridional ocean circulation important?
- It caries heat from tropical Atlantic to higher latitudes
- Ventilates the deep ocean
- Redistributes nutrients, oxygen, carbon, pH
What is sea density a function of?
Temperature and salinity
Which part of the globe has the highest sea-surface density?
The poles
What are isopycnals?
Contours of equal density (sigma)
Why do we use potential temperature (theta)?
It is in situ temperature corrected for the effects of compression at depth
What density is seawater?
1.025-1028g cm⁻³
What are the conditions of Antartic bottom water?
Very cold, high salinity
Where does AABW form?
On the shelves of the Weddell Sea, Prydz Bay, Adelie Land and Ross Sea Polynyas
How does deep water form from coastal/latent polynyas?
Coastal polynyas occur where the wind drives newly formed ice off shore creating areas of open ocean. The continual freezing cools the surrounding waters and increases it’s salinity. The sinking water circulates over the continental shelf and then forms deep waters/
How does deep water form from open ocean/sensible polynynas?
Heat is lost from the water by convection/conduction (sensible heat loss) and the cooled water sinks.
How are open ocean polynyas kept open?
By the upwelling of warm waters
Where does deep water go after it is formed?
Most remains trapped in the deep basins around the Antarctic but some escapes and contributes towards the world oceans.
Where does Antarctic bottom mix with less dense water?
Antarctic Circumpolar Current to form Antarctic Circumpolar Current bottom water.
What causes AABW formation reduction?
Freshening of surface Antarctic Waters due to increased glacial melt and possibly increased precipitation.
Where is North Atlantic Deep Water (NADW) formed?
In the Labrador sea (upper) and the seas between Iceland, Greenland and Norway (lower).
Where is NADW formed in the Labrador sea?
Cold, polar surface water flows through the Fram straight in the East Greenland Current. This mixes with warmer water Atlantic water, rounds the southern tip of Greenland and flows into the Labrador sea as the west Greenland Current.
How is NADW formed in the Nordic Seas?
Fresh polar water in the Nordic sea and the warm salty Atlantic water mix. Heat from the Atlantic is released to the Air above, the mixing and cooling increases the density of the surface water so it becomes dense enough to sink.
What are the three routes taken by NADW to cross across the shallow ridge between Greenland and Scotland?
a) The Denmark Strait
b) The Shetland Faroe ridge
c) Scotland Faroe channel
What doe the Atlantic Intermediate water mic with to become NADW?
Nordic overflow
What two methods are used to track NADW?
RAPID and OSNA
What do the moorings monitors measure in order to analyse Atlantic meridional overturning circulation?
Conductivity(salinity), temperature, depth, current direction and intensity.
What has been the decadal change in north flowing NADW current?
Significant decrease in volume.
What has been the decadal change in the south flowing current?
Significant increase in volume.
What is the reason for this anomaly?
Reflects the decadal variability in the AMOC.
At what rate doe the thermohaline current move?
1cm/s
Why is there currently no deep water formation in the North Pacific
The surface water is too fresh for deep water formation.
What is the fingerprint for AAIW, AABW and NADW?
Temp-4 Salinity - 34.4, Temp - 0 Salinity - 34.7, Temp-2 Salinity- 34.9.
(L2)
What does the Coriolis Affect?
Items moving across the ground that are not frictionally bound to it.
What direction does the Coriolis force senf objects from the Equator?
To the right in the north and to the left in the south.
What direction do the main ocean gyres rotate?
Clockwise in the northern hemisphere and anticlockwise in the southern hemisphere (caused by the Coriolis effect).
Why is the sea surface higher in the centre of the gyre than at the edges?
Ekman transport (using coriolis) forms a dome which sinks compressing the layers beneath and forcing the layers to spread.
When do we reach geostrophic balance?
When the coriolis force equals the pressure gradient causing flow perpendicular.
Where is the Coriolis effect at its greatest?
At the poles
What impact does this phenomenon (known as western intensification) have on water water flowing in the gyre at the equator?
It is deflected much less than water flowing at higher latitudes.
What type of currents does westward intensification create?
Fast deep narrow western boundary currents (i.e. gulf stream) and shallow slow wide eastern boundary currents (i.e. Canary current)
What are currents that travel in different directions to the wind called?
Counter currents
Where are the divergences on a world map?
At the equator, beside Antarctica and above Norway and west of Canada.
What happens to NADW as it moves north?
It gains heat from the atmosphere and is freshened by excess precipiatation -> now called AASW
What water body does Antarctic surface water converge with?
The sub antarctic water (which is much warmer).
What then happens to the antartic surface water?
It sinks to form the Antarctic intermediate water
(L3)
What does ocean mixing reflect?
Molecular diffusion and turbulence diffusion (chaotic flow with irregular fluctuations in speed and direction)
Which is quicker-horrizontal or vertical turbulent diffusion?
Horizontal by about 10^5
What can the spreading of fluorescent dyes be used to estimate?
Turbulent diffusivity
What inhibits vertical ocean mixing?
Density
Which mix more-unstratified or stratified waters?
Unstratified
What drives vertical changes in seawater?
Temperature
What does sea surface temperature reflect?
Balance of heat gain (solar) and heat loss (albedo, conduction, evaporation)
What drives vertical changes in seawater density?
Seawater salinity
What are isotherms?
Lines joining points of equal temperature are called isotherms.
Where do salinity and temperature become more constant?
Further from the surface.
What do we call the surface section where temperature shows little variation with depth?
Mixed surface layer.
What is the name given to the section of the profile where temperature changes very rapidly?
Thermocline
Most solar energy is absorbed within a few meters of the surface of the ocean. How can heat be transmitted deeper?
By turbulent mixing, conduction and downwelling.
At which time of year would we expect to see less efficient mixing of the surface warming with the waters at depth and stratified layers?
Summer months.
What is the name given to the section of the profile where salinity changes very rapidly?
Halocline.
What can we use to work out density?
Temperature and Salinity.
What are the lines joining points of equal density called?
Isopycnals
What is the name of the section of the profile where density changes very rapidly?
Pynocline
Which water would have the greatest pynocline (the greatest variation in density) - equator, tropics or high latitude?
Equator, it has a very low density at the surface (warm water)
Why does the tropical Atlantic not show a boom in nutrients over summer?
It has a permanent Thermocline so nutrients at depth are not recycled to the surface.
Why does the northern Atlantic show a boom in nutrients over summer?
It has a seasonal Thermocline which breaks down over the winter allowing nutrients in deeper waters to be recycled to the surface.
What is the impact of climate change on ocean stratification and primary production?
Surface warming-increased water stratification, decreased vertical mixing/overturning of surface waters and decreased primary productivity.
Which is faster- the diffusion of heat or salt?
Heat (100x)
What happens if warm salty water overlies cold fresh water?
The water column will overturn/mix
What is salt fingering?
When sinking cells of salty water alternate with rising cells of fresher water.
Where does salt fingering occur?
When warm, saline waters overlie cool, fresh waters.
Does Salt fingering mic water faster than local turbulence?
Yes- forms a Thermohaline staircase.