antarctic oceanography

33
Antarctic Oceanography

Upload: zek-man-top

Post on 10-Apr-2015

192 views

Category:

Documents


0 download

TRANSCRIPT

Page 1: Antarctic Oceanography

Antarctic Oceanography

Page 2: 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?

Page 3: Antarctic Oceanography

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

Page 4: Antarctic Oceanography

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

Page 5: Antarctic Oceanography

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)

Page 6: Antarctic Oceanography

Antarctic Basins

Waddell Sea

Ross Sea

Page 7: Antarctic Oceanography

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

Page 8: Antarctic Oceanography

The Southern Ocean

Page 9: Antarctic Oceanography

Antarctic Polar Front

Sub-Antarctic Front

Area in between is Antarctic Polar Frontal Zone (APFZ)

Page 10: Antarctic Oceanography

Antarctic Surface Currents

Page 11: Antarctic Oceanography

Antarctic Surface Currents

Page 12: Antarctic Oceanography

Antarctic Surface Currents

• Antarctic Circumpolar Current

• Antarctic Coastal Current

Page 13: Antarctic Oceanography

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?

Page 14: Antarctic Oceanography

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

Page 15: Antarctic Oceanography

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

Page 16: Antarctic Oceanography

Red lines are isotherms

Antarctic Region

Major Antarctic Water Masses

Page 17: Antarctic Oceanography

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)

Page 18: Antarctic Oceanography

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

Page 19: Antarctic Oceanography

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

Page 20: Antarctic Oceanography

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

Page 21: Antarctic Oceanography

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

Page 22: Antarctic Oceanography

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)

Page 23: Antarctic Oceanography

Spread of AAIW

Blue numbers indicate depth of layer (m)

Black numbers indicate salinity (ppt)

Page 24: Antarctic Oceanography

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

Page 25: Antarctic Oceanography

Fronts and Zones

North

South

Continent

Sub-AntarcticZone

AntarcticPolar

FrontalZone

AntarcticZone

ContinentalZone

Subtropical Convergence

SubAntarcticFront

PolarFront

AntarcticSlope Front

Divergence

Convergence

Page 26: Antarctic Oceanography

Antarctic Fronts

Page 27: Antarctic Oceanography

Antarctic Polar Front

zone

Shaded region betweenAntarctic (Polar) Front and Sub-Antarctic Front is APFZ

Both Fronts are characterized by strong jets

Page 28: Antarctic Oceanography

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

Page 29: Antarctic Oceanography

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)

Page 30: Antarctic Oceanography

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

Page 31: Antarctic Oceanography

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

Page 32: Antarctic Oceanography

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)

Page 33: Antarctic Oceanography

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