Antarctic Oceanography

Objectives
Antarctic Oceanography
• What defines the Antarctic Region and Southern Ocean
• What are the main surface currents in the region
• What are the primary water masses
– What are their T, S characteristics
– Where/how are they formed
• Why is North Atlantic Deep Water important for the
region
• What are the major frontal systems and how are they
connected to the various water masses
Ocean Fronts
• Sharp boundaries between water masses
• Large gradient between T & S or both across
front
• Can be caused by many processes:
– FW runoff from rivers
– Boundaries of ocean currents
– Regions of subsurface convergences
• Important for biological impacts, acoustics,
eddy formation
Cabbeling
• When two water masses mix, because of the nonlinear dependence of
density on temperature and salinity, the mixture is denser than either
source.
• Observed in Antarctica when cold shelf water (formed during winter
ice freezing) mixes with circumpolar deep water, becomes denser than
either, and descends down the shelf slope to form AABW
Increasing Temperature
Surfaces of
constant density
ISW
Increasing
Density
CDW
AABW
CDW
ISW
AABW
Increasing Salinity
Temperature (left) and Salinity (right) sections
showing Cabbeling at 132 East in Antarctica
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Antarctic Basins
Antarctic Basins
• Continent surrounded by ocean
• Southern Ocean has free communication with all
of world’s oceans – except the Arctic
– Ocean basins 4000-6500 m deep
– sills separating basins are about 3000 m deep
– basins are essentially extensions of Atlantic,
Pacific, and Indian Ocean Basins
– no geographic impediment to water transport
• Narrow and shallow shelf areas, with exception of
Weddell and Ross Seas (400 m deep, 400 km wide
shelves)
The Southern Ocean
The Southern Ocean
• Southern Ocean is defined as area south of
subtropical convergence (~40ºS), comprises ~23%
of world’s ocean surface area
• Area south of Antarctic Polar Front defines the
polar water masses (~50-60ºS), comprises ~11%
of world’s ocean surface area
– volume is 11% of world total
• Major mixing region for world’s water masses
• Major source of deep water in global ocean
• Driver of global thermohaline circulation
2

Antarctic
Polar
Front
Antarctic Surface Currents
Sub-Antarctic
Front
Area in between
is Antarctic
Polar Frontal
Zone (APFZ)
Antarctic Surface Currents
Mean SLP in Southern Ocean
South Polar High over the
continent
Symmetric low over
Southern Ocean
Statistical lows over
Weddell and Ross Seas
Surface currents are winddriven
(though the gyres
have a thermohaline
component)
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Antarctic
Surface
Currents
• Antarctic Circumpolar
Current
• Antarctic Coastal
Current
Antarctic Surface Currents
• Antarctic Circumpolar Current (ACC) (West Wind Drift)
– Flows eastward, driven by westerly winds
– Largest current on Earth, ~130 Sv (with max up to 290 Sv!),
compared to 85 Sv for Gulf Stream, and ~7 Sv for Leeuwin
Current
– Northern limit of current defines the Southern Ocean (near
subtropical convergence)
– Fairly slow current (4-15 cm/s)
– Extends very deep
• Most currents extend only a few hundred meters
• In Drake Passage, at 2500 m, current is 10-30% of surface
current
• Why
Antarctic Circumpolar Current
• Wind drives surface current eastward
– Coriolis causes motion away from continent: sea surface slopes
away from continent
• Little stratification in Antarctic
– Vertical salinity and temperature gradients are small Pressure
force due to horizontal density differences is distributed
throughout water column (whereas in tropics much of it happens
in the pycnocline)
– Weak stratification allows transfer of momentum to lower water
much more easily than for strong stratification
– Therefore current is transmitted to great depths
• Deep Current unimpeded all the way around continent – therefore
water from Atlantic and Pacific and Indian Ocean is mixed well –
Antarctica is earth’s main mixing region
Antarctic Surface Currents
• Antarctic Coastal Current (not ACC!)
(East Wind Drift)
– Between ACC and continent - narrow
– Opposite flow from ACC (polar easterlies)
– Current speed ~0.1 m s-1, transport ~8 Sv
– Current interrupted by Antarctic Peninsula
and Ross Sea – forms gyres with ACC
pushed Shackleton’s party away from coast
4

Major Antarctic Water Masses
Antarctic Region
Major Antarctic Water Masses
• Three Main layers (South of APFZ):
– Antarctic Surface Water (AASW)
– Circumpolar Deep Water (aka Warm Deep Water,
aka Antarctic Circumpolar Water) (CDW or WDW or
ACW)
– Antarctic Bottom Water (AABW)
• Other Water Masses
– Antarctic Intermediate Water (AAIW) (North of APFZ)
– Ice Shelf Water (ISF)
Red lines are isotherms
Antarctic Water Masses
AASW
Antarctic Surface Water (AASW): -1.9 to +4ºC, S < 34.5
Ant. Circumpolar Water (ACW): 1.5-2.5ºC (top), 0-0.5ºC (bot.); S~34.7
Ant. Intermediate Water (AAIW): 2-4ºC, S=34.2
Ant. Bottom Water (AABW): -2.0ºC, S=34.75, very dense
1. Antarctic Surface Water (AASW)
– Between continent and Antarctic Polar Front (APF)
– Cold (-1.9° to 4°C), relatively low salinity (S < 34.5
ppt) in upper 50-200 m north of continental slope
– Low salinity from fresh water input
• Where does the fresh water come
– Forms from deep water that has upwelled and
modified by cooling and ice formation at the surface
• North Atlantic Deep Water upwelling
• Brings nutrients to surface – important for marine life
– Influenced by ice melt and surface freezing
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Antarctic Water Masses
2. Circumpolar Deep Water (CDW)
– 1.5° to 2.5°C near surface, 0° to 0.5°C near bottom;
salinity ~34.7 ppt
– All around continent from just below surface to 4000
m deep
– Fairly uniform T-S properties around continent
– Source is North Atlantic Deep Water (NADW)
– Also called Warm Deep Water (esp. in Weddell
Sea) and Antarctic Circumpolar Water
Antarctic Water Masses
3. Antarctic Bottom Water (AABW)
– Formed in Weddell and Ross Seas
– Mixture of shelf and CDW
– Flows down slope into south Atlantic as
bottom water below 4000 m (also into
Pacific and Indian)
– Traced to 40°N in Atlantic, 50°N in Pacific
Antarctic Water Masses
4. Antarctic Intermediate Water (AAIW)
– 2-4°C, 34.2 ppt
– Formed by AASW sinking at APF
– Flows northward across equator at 800-
1000 m deep (traced by high oxygen and
salinity minimum)
Spread of
AAIW
Blue numbers indicate
depth of layer (m)
Black numbers indicate
salinity (ppt)
6

Antarctic Water Masses
Fronts and Zones
5. Ice Shelf Water (ISW)
– Formed when water on continental shelf comes in
contact with Ice Shelves, melt them slightly and
cools
– -1.9 - -2.4°C, ~35 ppt
– Coldest water on earth
– Unique to Antarctica
– Source water is either brine rejected in coastal
polynyas or CDW that has shoaled up onto the
continental slope and circulates under the ice shelf
North
Sub-Antarctic
Zone
Convergence
Subtropical SubAntarctic Polar
Convergence Front Front
Antarctic
Polar
Frontal
Zone
Antarctic
Slope Front
Antarctic
Zone
Divergence
Continental
Zone
Continent
South
Antarctic Fronts
Antarctic
Polar Front
zone
Shaded region
between
Antarctic (Polar)
Front and Sub-
Antarctic Front is
APFZ
Both Fronts are
characterized by
strong jets
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Antarctic Fronts
1. Continental Zone (CZ)
– Shelf region between continent and Antarctic Zone
– Southern boundary of Antarctic Zone called Antarctic
Slope Front (ASF)
– Separates upwelling NADW from downwelling cold water
off shelf
2. Antarctic Zone (AZ)
– ACC and Coast current flow in opposite directions
– divergence (Antarctic Divergence at ~65°S)
– Upwelling brings water from great depths (2500 m),
compared to coastal upwelling
– Important because it brings up NADW and nutrients for
biological productivity
Subtropical Subantarctic
Conv. Front
Subant. Zone APFZ
x
x
40°S
Vertical
Cross-Section
(looking east)
AAIW
NADW
Ant. Polar
Front
50°S 60°S
downwelling
AZ CZ
x • •
AABW
Antarctic
Slope Front
upwelling
70°S
downwelling
Surface
Current
Antarctica
N
x-direction:
x = into image
(eastward)
• = out of image
(westward)
y
z
Antarctic Fronts
4. Subantarctic Zone
– Northern edge of APFZ is the Subantarctic Front
– North of front, lies the Subantarctic Zone
– Water still moves with ACC
– Edge of downwelling region
– Northern limit is the Subtropical Convergence (~ 40° S), a
region of very large temperature contrasts
Antarctic Fronts
3. Antarctic Polar Frontal Zone (APFZ)
– Northern boundary of Antarctic Zone is called Polar Front
or Antarctic Front
– Northern boundary of APFZ is Sub-Antarctic Front
– Region of surface convergence, thus downwelling
– Where AAIW is formed
– Incorporates main branch of ACC
– First front found, originally called “Antarctic Convergence”
– Position varies regionally and seasonally
– Its position controls the northern limit of sea ice (upwelling
brings warm water to surface which melts ice)
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Subtropical Subantarctic
Conv. Front
Subant. Zone APFZ
x
x
40°S
Vertical
Cross-Section
(looking east)
AAIW
NADW
downwelling
Ant. Polar
Front
50°S 60°S
AZ CZ
x • •
AABW
Antarctic
Slope Front
upwelling
70°S
downwelling
Surface
Current
Antarctica
N
x-direction:
x = into image
(eastward)
• = out of image
(westward)
y
z
Subtropical
Front
Subant. Zone
Subantarctic
Front
APFZ
Ant. Polar
Front
ACC ACC ACC
40°S 50°S 60°S
x – downwelling
(convergence)
• - upwelling
(divergence)
AZ
AABW
formation
AAIW NADW
formation upwelling
x • x
Antarctic
Slope Front
Volume
CZ transport
Coastal
70°S
Antarctica
N
E
Overhead
View
Role of the Southern Ocean in Global
Circulation
• Major source of world’s bottom water
– Bottom water circulates around Antarctic and
penetrates into Atlantic, Pacific and Indian Oceans
– Drives global thermohaline circulation
• Bottom water formed primarily on Antarctic Shelves
through density driven convection
• Two processes for bottom water formation in Antarctica
– Coastal polynya
– Cabelling
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