CLIM 752 Ocean General Circulation

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CLIM 752 Ocean General Circulation Barry Klinger,
George Mason University

• You have already seen
• Description of major ocean gyres
• Linear theory of wind-driven barotropic
  circulation
• (Sverdrup, Stommel, Munk)
• Description of deep meridional overturning and
  water masses
• Stommel-Arons theory of horizontal flow patterns
  in deep ocean
• Some questions this class will address
• How does the ocean circulation affect climate?
• What does the 3D wind-driven circulation look
  like?
• How do we deduce this 3D flow from first
  principles?
• How are subduction regions and equatorial
  upwelling linked?
• What determines the strength of the deep
  meridional overturning?
• Can the meridional overturning undergo
  catastrophic change?

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• Brief Course Outline
• Heat Transport and Streamfunctions kinematics,
  Definitions, Observations
• Review of wind-driven gyres observation and
  theory
• Three-dimensional barotropic flow
• Subduction and Luyten-Pedlosky-Stommel theory
• The Equatorial Undercurrent
• Shallow Overturning Tropical and Subtropical
  Cells
• Review of "thermohaline circulation" observation
  and theory
• Theories of thermohaline overturning strength
• Mixing, buoyancy, wind, geography, and the deep
  meridional overturning
• Multiple states and time variability of the deep
  meridional overturning

Reference Material Pedlosky, 1996 Ocean
Circulation Theory, Springer-Verlag Siedler,
Church, and Gould, eds., 2001 Ocean Circulation
and Climate, Academic Press Tomczak and Godfrey,
1994 Regional Oceanography, Pergamon Press
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• Some ways ocean circulation affects climate
• biological productivity ? gas exchange
• direct transport of gases (e.g., the carbon
  cycle)
• ice ? albedo
• heat exchange with atmosphere

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Ocean Circulation, Heat Transport, and Kinematics
Barry A. Klinger
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Oceanic Heat Transport

• p T cp
• 0 -2 3989
• 0 31 4002
• -2 3844
• 5000 2 3854

See for instance Gill (1982)
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• Several possibilities including
• Q 0 (thermally passive ocean)
• D H small, Q ?Et dV (ocean is thermal
  flywheel)
• D ?Et dV small, Q H (ocean moves heat
  around)
• 1 ? good approx global and annual average
• 2 ? good approx some places for timescales
  seasonal
• 3 ? good approx annual average

What determines Q(x,y)? Need to consider both
atmosphere dynamics and global oceanic velocity
and temperature field.
Similar expressions exist for transport of salt
and other tracers.
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From COADS, W/m2
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Net Annual-Average Heat Flux into Ocean

• General pattern
• absorbs heat near equator, loses heat at
  high latitudes
• (as expected)
• Typical values O(50 W/m2)
• Some interesting details
• heat gain largely in east, heat loss in
  west
• Pacific heat gain especially big
• Atlantic heat loss especially big

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When calculating H, we often choose domain so
that most of the lateral boundaries are walls,
except for a 1 or 2 zonal sections (as shown).
u?n 0 on bottom, and if we can neglect u?n
through the top, H is simply meridional heat
transport.
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Ocean and Atmospheric Meridional Heat
Transport Ocean clearly important in
tropics Latest observations ? smaller ocean role
at high latitudes (still significant
uncertainties)
total
ocean
1 PW 1015 W
atmosphere
Trenbirth and Caron, 2001 J Clim, 3433-3443
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Coupled models indicate significant atmospheric
response to oceanic circulation change
Difference Between System with 2 Different Ocean
Circulations
Manabe and Stouffer, 1988 J Clim, 841-866
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See for instance Batchelor (1969), Kundu (1990)
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Can Find Streamfunction in Other Dimensions

• No divergence ? no flow from eastern or western
  boundaries of basin.
• Could also define Zonal Overturning
  Streamfunction
• but hasnt been useful in oceanography.
• To calculate streamfunction
• Set arbitrary value at some boundary
• Calculate either ? -? V dz or ? ?W dy

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Example Near-global numerical model (HYCOM),
climatological forcing
Klinger et al (2005), unpublished
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Meridional Overturning Streamfunctions (same
model)
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Not necessarily clean relationship between
streamfunction and heat transport
Temperature field is similar
same model as previous pages