Brodie Pearson

Assistant Professor

College of Earth, Ocean, & Atmospheric Sciences

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Chapter Scientist & Contributing Author for IPCC AR6

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Thermohaline Circulation & Ice Sheets

OSU School of Ice – August 2022

Definition and Importance

Thermo = Temperature

Haline = Salinity

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• The Thermohaline Circulation (THC) is a critical component of the large-scale ocean circulation that is driven by global density variations

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Set up Lab: Melting Ice

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• The thermohaline circulation moves substantial heat from the equator to the poles.

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• If this heat didn't get moved, the Equator would get hotter, and the poles would get colder [i.e. some past climates].

Components of the Thermohaline Circulation

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Imagine this cross-section as a north-south “slice” through the north Atlantic Ocean

Equator

Greenland &

North Pole

1) Deep water formation in polar regions

North Atlantic: Labrador and Greenland Seas

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Southern Ocean: Weddell and Ross Seas

Components of the Thermohaline Circulation

What happens to dense water once it sinks?��Demo Lab: Michelle’s Density Tank

Components of the Thermohaline Circulation

2) Spreading of deep waters throughout the ocean basins; inter-basin exchange via the Southern Ocean

Components of the Thermohaline Circulation

3) Slow upwelling & mixing of deep water

Occurs throughout the ocean, but thought to be most prevalent in the North Pacific and Indian Oceans (although this is of some debate)

Components of the Thermohaline Circulation

4) Slow return flow in the near-surface

Water takes 500-1000 years to travel along this path!

Driving the Thermohaline Circulation

What forms dense water?

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• Heat loss to atmosphere
• Thermal energy flow from warm to cold

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• Brine rejection as ice forms
• Ocean loses freshwater to ice, leaving saltier water behind

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• Net evaporation (evap minus precip)
• Makes surface water saltier and more dense

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Arctic sinking

Flow rates are immense and measured in Sverdrups

1 Sverdrup is about 10 million bathtubs flowing through every second!

Arctic sinking

Flow rates are immense and measured in Sverdrups

1 Sverdrup is about 10 million bathtubs flowing through every second!

�Most Arctic deep water is formed by surface heat loss in ice-free times & locations�

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Magenta line shows typical ice edge

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Antarctic Sinking

Most AABW is formed by brine rejection as sea ice forms�

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• Orange line shows typical ice edge

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Antarctic Polynyas (“ice factories”) create saltiest, densest water

Stratification/layering of water in the Atlantic Ocean

NADW = North Atlantic Deep Water

AABW = Antarctic Bottom Water

How might stratification affect the thermohaline circulation?

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Revisit Lab: Melting Ice

Stratification

• Weak stratification is necessary for convection and deep-water formation

A Historical Example: Great Salinity Anomaly

Oceanus, Woods Hole Oceanographic Institution

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In the late 60’s, a mass of freshwater discharged into the North Atlantic from Greenland

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It freshened the surface and increased stratification of the adjacent Labrador and Nordic Seas.

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This fresh salinity anomaly was later observed to have inhibited deep water formation

Summary

• The thermohaline circulation is critical for maintaining Earth’s climate

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• Ice can drive, or inhibit, this circulation
• More Arctic sea ice could slow it down (blocks atmosphere)
• More Antarctic sea ice could speed it up (brine rejection)
• Melting land ice could shut it off (stratification too strong)

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Effect on Convection�

Oceanus, Woods Hole Oceanographic Institution

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Four Key features of the Global Thermohaline Circulation

1. At poles, dense water sinks to the bottom of the ocean

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• These deep waters spread across global ocean

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• Slow upwelling of deep waters at low- & mid-latitudes

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• Poleward return flow in near-surface currents, which completes the loop

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Flow rates are immense and measured in Sverdrups

1 Sverdrup is about 10 million bathtubs flowing through every second!

Water takes 500-1000 years to travel along this path!