Physical Oceanography Seminar

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Physical Oceanography Seminar

• Role of Polar Oceans on Global Climate

Motivation Why is Sea Ice Important for Climate?
by Kenneth J. Boda
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Global Thermohaline Circulation
Solar Heating Max at Equator, Min at Poles
Resulting in

• The Great Ocean Conveyor Belt

Atmospheric Circulation
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Properties of Seawater

• Density ? the prime mover of seawater
• is a function of
• Salinity S (S?, ?? S?, ??)
• Temperature T (T?, ?? T?, ??)
• Pressure P (P?, ?? P?, ??)
• howevercold water is more compressible than
  warm water, know as the Thermobaric Effect

Another property of seawater is d18Ohow much
heavy oxygen is in the water. This shows if
the water has glacial water origin.
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Major Water Masses

• A water mass is a body of water with similar
  properties.
• There are 3 major water masses created in polar
  regions
• Antarctic Bottom Water (AABW)
• North Atlantic Deep Water (NADW)
• Antarctic Intermediate Water (AAIW)
• These are the densest water masses in the world
  ocean.

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Major Ocean Water Masses
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Definitions

• Isopycnals are surfaces of equal density. When
  isopycnals are not horizontal, gravity will
  attempt to neutralize this by moving denser water
  down.
• Fronts are places where two water masses of
  different T/S properties meet.
• Polynyas are areas of open water in ice covered
  seas.

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Water Mass Modification

• Mixing along fronts
• blends the two water masses, properties of each
  (T,S) become averaged.
• Modification by the Atmosphere
• Evaporation (S?), Precipitation (S?)
• Solar Heating (T ?), Cold-Wind Cooling (T?)
• Freezing (S?,T?), Melting (S? T?),
• Modification by the Land
• River input, Glacial Meltwater (S?)

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Antarctic Bottom Water Formation
Ross Sea, Antarctica Sea Ice and Mt. Erebus
(Largest Active Volcano in the World)
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AABW Formation Water Masses

• Offshore Water Masses
• Circumpolar Deep Water (CDW) a.k.a. Warm Deep
  Water (WDW)
• warm (1ºC) salty (34.7)
• Antarctic Surface Water (AASW)
• cold (-1ºC) and very fresh (34.2)
• Antarctic Bottom Water (AABW)
• moderately cold (0ºC) and salty (34.7)

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AABW Formation Water Masses

• Continental Shelf Water Masses
• High Salinity Shelf Water (HSSW)
• Very cold (-1.8ºC) very salty (34.8)
• Low Salinity Shelf Water (LSSW)
• Very cold (-1.6ºC) moderately salty (34.5)
• Ice Shelf Water
• Wicked cold (-2.0ºC) moderately fresh (34.4)

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Latent Heat Polynya Dynamics
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(No Transcript)
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AABW Production
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North Atlantic Deep Water Formation
Labrador Sea, between Canada and Greenland
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North Atlantic Deep Water Formation
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Sensible Heat Polynya Dynamics
Violent-mixing the period when the surface
buoyancy forcing associated with the seasonal,
synoptic meteorology, having exceeded some
critical level, initiates convection at the
preconditioned site
Sinking and spreading the long-term,
post- forcing restratification of the water
column
Preconditioning preconditioned when there exists
a laterally extended deep region of relatively
weak vertical density stratification capped by
a locally shallow Thermocline, also known as
doming of isopycnals
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A time series of Deep Convection
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Great Salinity Anomaly

• A decadal event where an anomalously low-salinity
  water mass (.1 psu) in the upper layers (surface
  500m) of the ocean was observed to travel
  cyclonically around the subpolar gyre of the
  North Atlantic Ocean for 14 years.

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Great Salinity Anomaly
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Hypothesized Causes of the GSA

• Advective Hypothesis A 250 nm eastward shift
  of the upper-ocean T-S structure, a physical
  movement of water masses and fronts (Pollard and
  Pu, 1985)
• Arctic Oscillation Hypothesis an anomalous high
  pressure cell forms over Greenland which
  accelerated transport ice south through the Fram
  Straight (Dickson, et al. 1988)
• Runoff Hypothesis Anomalously high annual
  river runoff from North America into the Beaufort
  Sea moved to Fram Straight by the Transpolar
  Drift Stream (Mysak, et al. 1990)

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Great Salinity Anomaly Chart
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Impact of GSA on Climate

• GSA appeared to play a crucial role in the
  formation of large sea-ice anomalies in the
  Greenland and Labrador Seas, a 16 month lag of
  ice max follows observed salinity minima.
• GSA caused the suppression of deep convection,
  and hence deep water formation in different parts
  of the northwest Atlantic.

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Deep Water formation and Thermohaline Circulation

• NADW formation has been hypothesized to be a
  pump that drives global thermohaline
  circulation.
• If NADW formation is shut down (by an event such
  as the Great Salinity Anomaly) the result could
  affect the entire global climate.

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Great Salinity Anomaly Discussion