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MARI201 > The Southern Ocean (L18-23) > Flashcards

The Southern Ocean (L18-23) Flashcards

1
Q

Why is the Southern Ocean important for the global climate?

A

The global ocean absorbs ~25% of anthropogenic CO2 emissions annually, 40% of that happens in the Southern Ocean
The global ocean has absorbed more than 90% of excess heat due to global warming

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2
Q

What is a front?

A

A boundary between waters with different properties and motion

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3
Q

What affects the circulation of the Southern Ocean?

A

Surface wind (lows circulating Antarctica) and sea level pressure (changes in low and high pressure systems causing movement due to PGF)

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4
Q

What are the three types of winds that flow around the Southern Ocean?

A

Persistent circumpolar westerlies (unimpeded by anything)
Coastal easterlies
Offshore katabatic

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5
Q

Why is wind stress important for circulation?

A

Ekman transport pushes water towards lower latitudes, therefore upwelling is needed to replace the water, moving water from the poles equatorward
Wind stress is stronger over the Southern Ocean due to being unimpeded by any physical features

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6
Q

Why is sea surface height lower in the Southern Ocean?

A

Due to Ekman transport

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7
Q

How does Ekman transport along with westerly winds form a geostrophic current?

A

The PGF is higher closer to the coastline due to the sea surface height, Ekman transport is occurring along the coastline as upwelling is occurring, and the Coriolis force is moving in the opposite direction. This causes a geostrophic current underneath the westerly wind

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8
Q

What gyres do the Antarctic Circumpolar Current form?

A

The Ross Sea Gyre (Pacific side) and the Weddell Gyre (Atlantic side)

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9
Q

Why does the Antarctic Circumpolar Current only flow within certain boundaries?

A

It follows the seafloor bathymetry due to topographic steering

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10
Q

How much water does the Antarctic Circumpolar Current move and how quickly?

A

173.3 Sv, at 0.11m/s eastward

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11
Q

What is potential density?

A

The density of the water if you remove the effect of pressure

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12
Q

What are the different water stability conditions?

A

Statically stable - positive gradient (would rise)
Neutral stability - zero gradient (would stay where you put it)
Statically unstable - negative gradient (would sink causing stratification)

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13
Q

Why does stability change?

A

If the gravity force is larger than the pressure gradient force, the resulting motion moves the parcel back to its original position

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14
Q

What is baroclinity and how does it occur?

A

The mismatch between isobars (lines of equal pressure) and isopycnals (lines of equal density)
This occurs due to upwelling raising the isopycnals as dense water is moving close to the surface

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15
Q

Why is baroclinity important?

A

It is a powerful mechanism for mixing

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16
Q

How does upwelling cause mass transport?

A

It brings the North Atlantic Deep Water (NADW) toward the surface through the Antarctic Circumpolar Current, where it is transformed into the Southern Ocean water masses that return to the global ocean

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17
Q

What properties are most effected by fronts?

A

Sea surface height (decreases), potential temperature, and salinity

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18
Q

What moves water horizontally?

A

Geostrophic currents
Fast, deep westerly jets
Bathymetric steering
Barotropic (poleward) and baroclinic (equatorward) conditions

19
Q

What fronts are found within the Antarctic Circumpolar Current?

A

SACCF - Southern Antarctic Circumpolar Current Front (surface water 0°C)
PF - Polar Front (Antarctic Surface Water (AASW), Circumpolar Deep Water (CDW))
SAF - Subantarctic Front (Subantarctic Mode Water (SAMW), Antarctic Intermediate Water (AAIW))

20
Q

What is different about the upwelling pathway?

A

There is a lot of mixing, meaning similar and uniform platforms are found throughout

21
Q

How does bathymetry steer ocean currents?

A

Due to changes in potential vorticity, as the water column height (H) is changed, the latitude of the water column (f) also has to change
PV must be conserved
Increases in H requires increases in f, requires increases in latitude, turns poleward
Decreases in H requires decreases in f, requires decreases in latitude, turns equatorward

22
Q

How does flow change around fronts?

A

Within fronts - fast flow and vertical motion
Between fronts - property mixing

23
Q

How does the mixed layer change seasonally?

A

Winter - deeper and colder
Summer - shallower and warmer

24
Q

What are the surface heat flux processes?

A

Radiative heat - short and long
Sensible heat - conduction due to dT/dz
Latent heat - evaporation phase change and melting

25
Q

What are the surface freshwater flux processes?

A

Precipitation and evaporation
Sea ice formation and melt
Terrestrial freshwater (rivers, melting glaciers)
Entrainment and Ekman transport

26
Q

How do surface fluxes change?

A

They vary seasonally, giving us strong changes in water properties

27
Q

How do heat and salinity fluxes change buoyancy?

A

Due to the changes in the surrounding water
Thermal expansions causes volume to increase and density to decrease
Haline contraction causes volume to decrease and density to increase

28
Q

What are the units of buoyancy flux?

A

N/s or force/time
Changing the force balance on the water, more or less likely to flow

29
Q

What has more of an effect on the state of seawater (temperature or salinity)?

A

At higher temperatures, temperature has a larger effect on density
At lower temperatures, salinity has a larger effect on density

30
Q

How do seasonal changes in the mixed layer affect stratification and stability?

A

Seasonal changes in temperature and salinity lead to seasonal change in stratification and stability

31
Q

What occurs south of the Polar Front?

A

Cooling occurs as summer ends
Mixed later cooling by conduction, wind, and evaporation (sensible and latent heat cool the surface layer)
Mixing by storm activity

32
Q

What does cooling south of the Polar Front do to the density?

A

It causes the water to be statically unstable (negative buoyancy flux), where dense water sinks and the mixed layer is lower than -2°C
Once this temperature is reached, the water starts to freeze (dependent on salinity)

33
Q

What does surface freezing do?

A

Sea ice formation removes freshwater (brine rejection), and the cold higher salinity water sinks, removing salt from the mixed later, and the sea ice stores the fresh water at the surface

34
Q

What is the annual sea ice cycle?

A

Autumn - slow expansions
Spring - rapid retreat

35
Q

What is a polynya?

A

Where wind moves sea ice northward, opening up the water below the ice, and the open water allows for more sea ice growth

36
Q

What is the cold high salinity water resulting from sea ice formation and where does it sit?

A

Dense Shelf Water (DSW) or High Salinity Shelf Water (HSSW) and it sits on the continental shelf

37
Q

How does sea ice cause a freshwater flux?

A

Cooling, freezing, and evaporation causes an increase of freshwater, this gets moved by Ekman transport towards the north, where warming, melting, and freshening occurs (due to rain), leading to fresher water towards the north

38
Q

What are the different water masses?

A

Subantarctic Mode Water (SAMW) - winter cooling and mixing in the SAF zone

Circumpolar Deep Water (CDW) - upwelling and mixing through the ACC
Upper CDW experiences mixing with AASW
Modified CDW is near/over the continental shelf

Dense shelf waters (DSW and HSSW) - cold, higher salinity shelf waters sink south of the SACCF

Antarctic Bottom Water (AABW) - sinking shelf waters mix with the lower CDW

SAMW - cool, fresh Subantarctic Mode Water water mixes north into subtropical gyres

AAIW - cool, fresh Antarctic Intermediate Water subducts northward

AABW - cold, dense, Antarctic Bottom Water fills the abyss

39
Q

What is meridional overturning?

A

Where the Southern Ocean water masses ventilate the deep ocean, affecting global heat and carbon budgets

40
Q

Why is change detection in the Southern Ocean difficult?

A

There is large natural variability and limited observations (especially earlier in the record), so we need to be careful that the differences observed/measured are actually changes

41
Q

How is the Southern Ocean changing?

A

Temperature - warming centred around 45°S (0 to ~1500m), subsurface cooling between 30° and 36°S (250 to 2000m)

Salinity - surface freshening south of 45°S (into the interior and northward), salinification north of 45°S (top 500m)

Attribution - surface heat and freshwater fluxes due to greenhouse gas emissions and stratospheric ozone depletion (ozone absorbs radiation)

42
Q

How does the Walker Circulation change pressure around Antarctica?

A

On the western side of pacific ocean, warm water accumulates and makes a big pool of warm water, which heats up the atmosphere above and causes evaporation, air at the surface gets lighter and rises, condenses and forms moisture and gets cooler and drier, and sinks, which sets up a regular pattern of relative highs and lows in the atmospheric pressure called the Rossby wave train

43
Q

Why are low elevation locations around Antarctica more susceptible to warming?

A

The places where ice is below sea level it is very easy for water and glacial ice to interact and cause more melting

CDW intrusions onto the continental shelf is 2°C, and is quite warm compared to ice, causing a lot of melting (increase in intrusions over the summer)

MARI201 (5 decks)

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