What drives the oceanic circulation

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What drives the oceanic circulation ?

• Thermohaline driven
• Wind driven

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some of the observed main global surface current
systems.
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• Ocean waters respond to the wind stress because
  of their low resistance to shear (low viscosity,
  even after viscosity magnification by turbulence)
  and because of the relative consistency with
  which winds blow over the ocean.
• Good examples are the trade winds in the tropics
  they are so steady that, shortly after
  Christopher Columbus and until the advent of
  steam, ships chartered their courses across the
  Atlantic according to those winds hence their
  name.
• Further away from the tropics are winds blowing
  in the opposite direction. While trade winds blow
  from the east and slightly toward the equator,
  midlatitude winds blow from west to east and are
  called westerlies.

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The water column can be broadly divided into four
segments

• At the top lies the mixed layer that is stirred
  by the surface wind stress. With a depth on the
  order of 10 m, this layer includes Ekman dynamics
  and is characterized by d rho/dz ? 0.
• Below lies a layer called the seasonal
  thermocline, a layer in which the vertical
  stratification is erased every winter by
  convection. Its depth is on the order of 100 m.
• Below the maximum depth of winter convection is
  the main thermocline, which is permanently
  stratified. Ist thickness is on the order of 500
  to 1000 m.
• The rest of the water column, which comprises
  most of the ocean water, is the abyssal layer. It
  is very cold, and its movement is very slow.

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History

• The discipline began with the seminal works of
  Harald Sverdrup, who formulated the equations of
  large-scale ocean dynamics (Sverdrup, 1947)
• and Henry Stommel beginning with the first
  correct theory for the Gulf Strean (Stommel,
  1948).

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Basic equations
Geostrophy
Hydrostatic balance
continuity equation (mass conservation for an
incompressible fluid)
conservation of heat and salt (density)
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Definitions

• u, v and w are the velocity components in the
  eastward, northward and upward directions,
• rho0 is the reference density (a constant),
• rho is the density anomaly, the difference
  between the actual density and rho0,
• p is the hydrostatic pressure induced by the
  density anomaly
• This set of five equations for five unknowns
  (u, v, w, p and rho) is sometimes referred to as
  Sverdrup dynamics.

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Sverdrup Relation
Pressure eliminated
Conservation of mass
vertical stretching (), or squeezing (-)
demands a change in meridional velocity
Streching -gt shrink laterally -gt (zetaf)/h
requires vorticity to increase The parcel has no
choice but to migrate meridionally in search for
a better f df/dt ß v
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Sverdrup Balance
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Ekman

• the vertical flow from the surface Ekman layer
  into the geostrophic interior is

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Sverdrup Balance
relates the integral meridional flow throughout
the vertical extent of the treated layer to the
local windstress curl.
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Sverdrup Balance
we can introduce a Sverdrup streamfunction
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Being that the curl is negative throughout the
subtropics, it follows that the meridional flux
must be everywhere equatorward. But such a
situation, if sustained, will progressively empty
the midlatitude oceans, while piling-up more and
more water along the Equator a clear physical
impossibility! There must be somewhere a return
poleward flow that drains' the Equatorial region
while replenishing the midlatitude missing
volume.
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Boundary Current

• The vorticity generation by the interactions of
  boundary currents northward-flowing boundary
  current,
• The sense of the generated vorticity is shown for
  northern hemisphere flows.