Topic 9: Ocean Circulation

Gulf stream migration

In summer 2008, migrated to a position closer to the Tail of the Grand Banks (TGB)

Consequences of Gulf Stream Migration/Collapse

• Impinges flow of cold, O2-rich water

• Among the fastest warming regions in the global ocean

• Home to many ecologically and economically valuable marine species

Ocean currents

Continuous, directed movement of water from one place to another

2 types of ocean currents

1. Wind-driven: surface currents

2. Density-driven (thermohaline): deep currents

Surface currents

Wind-driven; horizontal movement in upper 10% of water, occur above and within the pycnocline

Deep currents

Density-driven (thermohaline); vertical movement, mixing, move 90% of all ocean water

• slow-moving – 10 to 12 km/year

Flotsam definition

Floating wreckage of a ship or its cargo

Use of flotsam

Movement of flotsam can be used to track currents

sverdrups (Sv)

Units to measure massive amounts of water; one sverdrup = 1,000,000 m3 per second (1 Sv = 1 x 106 m3/s)

Primary force behind movement of surface currents

Wind!

• 2% wind’s energy is transferred

• Energy transfer from air to water via friction

Direction of surface current flow in N. Hemisphere

Currents flow to the right of wind direction (major wind belts)

2 reasons why surface waters move in a direction different than the wind

1. Ekman spiral

2. Ekman transport

Ekman Spiral

Ocean circulation model:

Surface layer in contact with the wind moves 45 ̊ to the right (left in S. Hemisphere) of the wind direction in northern hemisphere (due to the Coriolis Effect)

Ekman spiral moves downwards: visualize many water layers moving at an angle to the right of the overlying layer (CREATES A SPIRAL)

Ekman spiral slows down due to friction as energy is passed from layer to layer

Ekman Transport

The net motion of water (down to about 100m) due to the blanace between:

1. The Coriolis effect

2. Turbulent drag forces within surface waters and geographic features (shoreline and seabed)

Theoretical direction of Ekman transport

• 90 ̊ to right - Northern Hemisphere

• 90 ̊ to left – Southern Hemisphere

Causes of upwelling and downwelling

• Water movement

• Wind-driven

• Ekman transport plays an important role

Upwelling

Upward movement of deep, cold, nutrient-rich water to surface waters

Downwelling

Downward movement of surface water to deeper parts of ocean

How Ekman transport influences coastal upwelling

• Ekman transport moves water to the RIGHT of wind direction

• Wind moves parallel to shore so movement occurs away from shore

• Deeper water moves up (upwelling) to replace the ‘missing’ surface water

3 advantages of coastal upwelling

1. Brings cold nutrient-rich water to the surface

2. High biological productivity (biodiversity)

3. Essential for coastal fisheries (economy)

Influence of coastal upwelling on climate

Strong control on regional weather, causes:

• cooler temperatures

• fog and precipitation

Effect of coastal downwelling on deeper waters

Supplies deeper ocean with heat, dissolved materials and water rich in dissolved oxygen (water is low productivity, nutrient-poor)

How Ekman transport influences coastal downwelling

• Wind blows (from the S) parallel to shore

• Ekman transport moves water to the RIGHT of wind direction

• This cases water to stack up against shore and move downwards

Convergence of water causes…

Downwelling

Divergence of water causes…

Upwelling

Gyre

Large-scale circular patterns of surface circulation

How gyres form

• In the Northern Hemisphere, currents flow to the right of wind direction (major wind belts)

• Ekman transport moves surface water to centre of a subtropical gyre

Geostrophic Currents

In idealized N Hemisphere, Ekman transport results in movement towards centre of the gyre where water piles up in the centre

However, gravity pulls water downhill –’flow downhill’ (water flow from region of higher pressure to lower pressure)

The movement of water ’downslope’ deflected by the Coriolis effect results in a clockwise current— GEOSTROPHIC CURRENT

Major currents flow…

… around the periphery of ocean basins

6 current circuits in the world

1. 5 subtropical gyres

2. 1 is the eastward-flowing Antarctica Circumpolar Current (driven by westerlies)

Boundary currents + 2 types

Ocean currents with dynamics determined by the presence of a coastline

1. Western boundary currents

2. Eastern boundary currents

9 Characteristics of Western boundary currents

1. Warm

2. Narrow (< 100 km)

3. Deep

4. Swift (hundreds of km a day)

5. Large (usually 50 Sv or greater)

6. Sharp boundaries

7. Little or no coastal upwelling

8. Nutrient-poor

9. Form meanders and eddies

Westward intensification

Currents on the western side of a gyre are faster, deeper, and narrower than currents on the eastern side

Why does westward intensification occur

The Coriolis effect gets stronger with higher latitudes, it is stronger in the latitudes of the westerlies than in the latitudes of the trade winds

Example of Eastern boundary current

Canary Current (North Atlantic)

Example of Western boundary current

Gulf Stream (North Atlantic)

Purpose of Western boundary current

Transports warm water northwards

Where are subtropical gyres found

Centered around 30 degrees N or S of the equator

Direction that subtropical gyres circulate in

• Northern Hemisphere = clockwise

• Southern Hemisphere = counterclockwise

9 Characteristics of Eastern boundary currents

1. Rarely forms meanders and eddies

2. Cold

3. Broad (~ 1000 km)

4. Shallow (< 500 m)

5. Slow (tens of km per day)

6. Small (typically 10-15 Sv)

7. Diffuse (poorly defined) boundaries

8. Coastal upwelling common

9. Nutrient rich

Purpose of Eastern boundary currents

Moves cold water towards the Equator

Transverse currents

Currents that are connected by eastern- and western boundary currents within a certain gyre; driven by trade winds + westerlies

Currents that flow from east —> west

Equatorial currents

Currents that flow from west —> east

Northern/southern boundary currents

Where thermohaline circulation occurs

Links major surface and deep water currents in the Atlantic, Indian, Pacific, and Southern Oceans

Direction of thermohaline circulation movement

Vertical movement, but also horizontal flow

Cause of thermohaline circulation

Driven by differences in water density

Purpose of thermohaline circulation

Transports heat and delivers O2 to deep water

5 kinds of water masses

1. Surface water

2. Central water

3. Intermediate water

4. Deep water

5. Bottom water

Surface water

From the surface to depth of 200 m; least saline, warmest, least dense

Central water

From end of surface water to bottom of main thermocline (latitude dependent)

Intermediate water

From bottom of main thermocline until depth of about 1500m

Deep water

Water not in contact with the bottom, to a depth of ~4000 m

Bottom water

Water in contact with the seafloor; densest, coldest, and most saline

2 named water masses

(water masses are named according to where they originate from)

1. North Atlantic Deep Water (NADW)

2. Antarctic Bottom Water (ABW)

Temperatures where intermediate water masses form

T = -1.5 to 14.0 degrees C

Temperatures where deep/bottom water masses form

T = -10.5 to 4.0 degrees C

Non-conservative characteristics of water masses

Changed in the course of time by chemical, physical or biological processes (dissolved O2, Δ14C)

Conservative characteristics of water masses

A water mass characteristic which has no sinks or sources in the ocean interior (temperature, salinity)

Where do water masses form

At the surface, can have subsurface mixing

• surface conditions reflect in the characteristics of a water mass

Isopycnals

Lines of the same density across different pressures and temperatures (shape of line due to position of water masses)

Caballing

When 2 distinct water masses (a,b) combine to produce a new water mass (c) of greater density

5 Characteristics of Antarctic Bottom Water (AABW)

1. High salinity (3.47%)

2. Low temperature (-0.5ºC)

3. Highest density of all water masses (1.0279)

4. Most oxygenated deep water

5. 20 to 50 million m3 of brine/sec

4 Steps to formation of Antarctic Bottom Water (AABW)

1. Wind blowing off Antarctica creates ice-free areas of water (polynyas)

2. The water is now exposed to cold wind, causing sea ice formation

3. Brine expulsion as it is added to sea ice

4. Salt concentrates in the remaining water

5. Water “flows downslope” and sinks (now AABW)

Cause of water masses having different characteristics

Surface conditions are reflected in the characteristics of a water mass

How AABW production is tracked

Use of seals with attached instrumentation – able to dive down underwater canyons

6 Characteristics of North Atlantic Deep Water (NADW)

1. High salinity

2. High O2

3. Low nutrients

4. Between ~ 1500 and 3500 m depth

5. Found throughout the length of the Atlantic (may extend as far as 10 degrees S)

6. Density less than AABW density

Where does North Atlantic Deep Water (NADW) form?

Forms in the Labrador and Greenland seas (Upper and Lower NADW)

Steps to formation of NADW

1. As warm water flows North, salinity increases

2. Wind blowing over the water - cools it and causes an increase in evaporation

3. Downwelling occurs and sea ice forms

4. Water mass sinks (now NADW)

4 Characteristics of upper NADW

1. Slightly warmer

2. Less dense

3. Convection in winter

4. Production – dependent on NAO (North Atlantic Oscillation)

3 Characteristics of lower NADW

1. Overflow water from Greenland-Iceland-Scotland Ridge

2. Colder

3. Denser

Global conveyor belt

System of thermohaline circulation driven by temperature and salinity (takes 1500 years to fully circulate, SLOW moving)

Purpose of global conveyor belt

Transfers heat and nutrients and brings O2 to deep water

NADW vs Pacific Ocean water masses

• NADW= most nutrient-depleted, CO2 depleted and well oxygenated water

• Pacific Ocean water masses= most nutrient rich, CO2-rich and oxygen-depleted water

AMOC (Atlantic Meridional Overturning Circulation)

• Series of surface and deep-water currents

• Northward flowing warm water in the upper layer of the Atlantic Ocean (Gulf Stream)

• Deep convection in the Nordic and Labrador seas

• Southward flowing colder water at depth (NADW)

Danger faced by AMOC

Slowing down— major tipping point in climate change

4 Consequences of AMOC slowdown

1. Southward shift in rain belt

2. Weakening of African and Asian monsoons

3. Drying in Europe

4. Impacts food production