Week 7: Ocean circulation Flashcards
What is going on?
Two currents meet at Cape Hatteras (Northwest Atlantic Shelf) creating rough and treacherous water
Why should we care?
Fastest warming regions in global ocean
Ecologically and economically valuable species
What happened in summer 2008?
Gulf stream migrated closer to tail of the grand banks (TGB) in a shift toward higher sea surface temp
- impinges flow of cold, O2 rich water
Ocean currents
Continuous directed movement of water from one place to another
Wind driven- surface currents
Horizontal movements in upper 10% of water
Density-driven (thermohaline)- deep currents
Vertical movement, mixing; 90% of all ocean water
- slow moving 10-12km/year
Where do currents go?
Direct or indirect observation and measurement
Diff methods for surface and deep currents
(ex. current buoys, flow meter, drift carts)
Tracking currents- flotsam
Flotsam= floating wreckage of a ship or cargo
How much water is being transported?
Measured in sverdrups (SV) (massive amounts of water)
One sverdup= 1,000,000m3 per sec
Surface currents
Occur above and within pycnocline
Primary force behind movement– wind
Which way does the northern hemisphere create flow?
To the right of wind direction
Ekman spiral
Ocean circulation model
Surface layer in contact w wind- 45 degrees to the right of the wind direction in northern hemisphere due to coriolis effect
Ekman spiral- what is happening?
Visualize many water layers
Moves at an angle to the right of the overlying layer
Energy passed from layer to layer
Slows down due to friction
Ekman transport
Net motion of water down to 100 m
Why- eventually water flows in direction opp of surface current
Theoretically 90 degrees to right northern hemisphere and 90 degrees to left southern hemisphere
Coastal upwelling
Upwards movement of cold, nutrient rich water, high biological productivity and essential for coastal fisheries
caused by…
Ekman transport= movement away from shore, deeper water moves in to replace missing surface water
Coastal downwelling
Downward movement of surface water to deeper parts of ocean
- Low productivity, nutrient poor
- Ekman transport= water stacks up against shore, moves downwards
Coastal upwelling and weather
Strong control on regional weather
- Cooler temps, fog and precipitation
ex. coast of Peru
Open ocean- coastal upwelling
Divergence and convergence- ekman transport
Surface currents and gyres
Ekman transport moves surface water to centre of subtropical gyre
Gyre
Large scale circular patterns of surface circulation
Surface currents- geostrophic currents
In northern hemisphere ekman transport results in movement towards centre of gyre and water piles up in centre but gravity pulls water downhill creating a pressure gradient (hi to lo pressure)
- water deflected by Coriolis effect resulting in clockwise current
Subtropical gyres
Collection of currents w diff properties
Centered around 30 degrees north or south
Direction of subtropical gyres in northern hemisphere
Clockwise
Direction of subtropical gyre in southern hemisphere
Counter clockwise
How many major current circuits in the world?
6
5 are subtropical gyres
1 is the eastward-flowing Antarctica Cicumpolar current driven by westerlies
5 subtropical gyres
- North Pacific
- South Pacific
- North Atlantic
- South Atlantic
- Indian Ocean
Western boundary currents - Gulf stream (North Atlantic)
Fast moving- average 6.4 km/hr to a depth of 450m
Narrow and deep (<100km)
Transports warm water northwards (swift speed in large amounts- 50 Sv)
Nutrient poor
Sharp boundaries
Little or no upwelling
Eddies
Cold and warm core rings
Creation of meanders from eddies
Over time warm water eddy rotates and rotates and eventually breaks off creating a patch of warm water within cold water
Western boundary currents - Kuroshio current
Temp diff support distinct populations
Warm, nutrient poor
Eddies aren’t persistent features
Eastern boundary currents
Shallow and broad (1000km across, <500 m deep)
Moves cold water towards equator (slow)
Poorly defined boundaries, rare eddy formation
Low volume of water transported (10-15 Sv)
Nutrient rich (coastal upwelling common)
Eastern boundary currents- Coastal desert in Namibia
Caused by Benguela current
Low water vapour content, air cooled by current
Types of eastern boundary currents
- Canary current (North Atlantic)
- Benguela current (South Atlantic)
Westward intensification
Currents on western side of a gyre are faster, deeper and narrower than currents on Eastern side bc Coriolis effect isn’t equal everywhere on earth
Diff in speed at high latitudes
Greater difference
(earlier deflection to right)
ex. Gulf stream
Diff in speed at lower latitudes
Lesser differences
(later deflection to right)
ex. Canary current
Transverse currents
Currents that flow east to west= equatorial currents
Currents that flow west to east= northern/southern boundary currents
Thermohaline circulation (global ocean conveyer belt)
Driven by diff in water density
Vertical movement, horizontal flow
Links major surface and deep water currents in the Atlantic, Indian, pacific and southern oceans
Transports heat, delivers O2 to deep water
Surface water
To depth of 200 m
Less dense, warmer, less saline
Central water
Bottom of main thermocline- latitude dependent
Intermediate water
to 1500m
Deep water
Water not in contact bottom (4000m)
Bottom water
In contact w sea floor
Denser, colder, more saline
Temp of intermediate water masses
-1.5- 14 degrees celcius
Temp of deep and bottom water masses
-10.5- 4 degrees celcius
Conservative water masses
Consistent temp, salinity; no sinks or sources in ocean interior
Non-conservative water masses
Changed in course of time by chemical, physical or biological processes
Deep water masses
North Atlantic Deep water (NADW)
Antarctic bottom water (AABW)
Caballing
Mixing of two water masses
ex. AABW
AABW
Salinity= 3.47%
Temp= -0.5
Density= highest of all water masses
Most oxygenated deep water
How is AABW created?
Strong wind blowing off Antarctica creates ice-free areas of water- polynyas
Water is now exposed to cold wind- sea ice formation and brine expulsion
Salt is forced out and concentrates in remaining water
Cold, saline water sinks
North Atlantic deep water (NADW)
Layer of high salinity, high O2 and low nutrients btwn 1500-3500 m depth
Forms in Labrador and Greenland seas
Lower density than AABW
How is NADW created?
As warm water flows north salinity increases
- Wind blows over the water and cools it
- Evaporation (causes increased salinity) and downwelling occur
Upper vs Lower NADW
Upper
- warmer, less dense
- convection in winter
- production depends on NAO(north atlantic oscillation)
Lower
- cooler
- overflow water- Greenland- Iceland - Scotland ridge
Formation of Lower NADW
Wind blows across water, causes freezing and brine expulsion
Sea ice forms and it sinks
Henry Stommel
Westward intensification
Global circulation (surface water sinks in polar regions)
Feeds deep boundary currents on Western sides
What will happen to water masses in about 1500 years?
NADW- most nutrients depleted, Co2 depleted and well O2 water
Pacific ocean water masses- most nutrient rich, CO2 rich and oxygen depleted water
AMOC- Atlantic Meridional Overturning circulation
Southern to north Atlantic ocean
Major tipping point in climate change
Currently observing slowing down
AMOC
Series of surface and deep-water currents
- Northward flowing warm water in upper layer of Atlantic (GULF stream)
- Deep convection in nordic and labrador seas
- Southward flowing of colder water at depth (NADW)
Why is AMOC slowing down?
Climate change
-Warm water= less prodcution of deep, cold, saline water
Freshening
- Fresh water in sea water= less salinity
Thermohaline circulation and climate change
- Gulf stream is slowing; cooler NW europe and more water piles up on east coast of north america(sea level rise)
- Southward shift in rain belt
