Antarctic Oceanography
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Transcript of Antarctic Oceanography
Antarctic Oceanography
Objectives
• 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
Temperature (left) and Salinity (right) sections showing Cabbeling at 132 East in Antarctica
Incr
easi
ng T
empe
ratu
re
Increasing Salinity
Surfaces of constant density
CDW
ISW
AABW
Increasing Density
ISW
CDW
AABW
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)
Antarctic Basins
Waddell Sea
Ross Sea
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
The Southern Ocean
Antarctic Polar Front
Sub-Antarctic Front
Area in between is Antarctic Polar Frontal Zone (APFZ)
Antarctic Surface Currents
Antarctic Surface Currents
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 – 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, transport ~8 Sv– Current interrupted by Antarctic Peninsula
and Ross Sea – forms gyres with ACC pushed Shackleton’s party away from coast
Red lines are isotherms
Antarctic Region
Major Antarctic Water Masses
Major Antarctic Water Masses
• Three Main layers (South of APFZ):– Antarctic Surface Water (AASW)– Antarctic Circumpolar Water (ACW)– Antarctic Bottom Water (AABW)
• Other Water Masses– Antarctic Intermediate Water (AAIW) (North of APFZ)– Ice Shelf Water (ISW)
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
AASW
Ant. Bottom Water (AABW): -2.0ºC, S=34.75, very dense
Antarctic Water Masses
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
2. Antarctic Circumpolar Water (ACW)– 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)
Antarctic Water Masses
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)
Antarctic Water Masses
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
Fronts and Zones
North
South
Continent
Sub-AntarcticZone
AntarcticPolar
FrontalZone
AntarcticZone
ContinentalZone
Subtropical Convergence
SubAntarcticFront
PolarFront
AntarcticSlope Front
Divergence
Convergence
Antarctic Fronts
Antarctic Polar Front
zone
Shaded region betweenAntarctic (Polar) Front and Sub-Antarctic Front is APFZ
Both Fronts are characterized by strong jets
Antarctic Fronts1. 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
NADW
AABW
AAIW
x • • x x
y
z
x-direction:x = into image (eastward)• = out of image (westward)
Surface
Current
Antarctica
N
CZAZAPFZSubant. Zone
AntarcticSlope Front Ant. Polar
FrontSubantarctic
FrontSubtropical
Conv.
downwelling upwelling
downw
elling
70°S50°S 60°S40°S
VerticalCross-Section(looking east)
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)
Antarctic Fronts
NADW
AABW
AAIW
x • • x x
y
z
x-direction:x = into image (eastward)• = out of image (westward)
Surface
Current
Antarctica
N
CZAZAPFZSubant. Zone
AntarcticSlope Front Ant. Polar
FrontSubantarctic
FrontSubtropical
Conv.
downwelling upwelling
downw
elling
70°S50°S 60°S40°S
VerticalCross-Section(looking east)
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