Upwelling and Downwelling

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Upwelling and Downwelling

• Winds can drive vertical motions
• When the winds blow away from an area, we get
  divergence, which causes water to upwell from
  below
• Upwelling is important because because it brings
  cold water, rich in nutrients up from below the
  pycnocline
• This is very important for the biology, making
  most upwelling regions rich in biological life

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Distribution of production in the ocean
CZCS ocean chl-a composite
Oceanic deserts (subtropical gyres)
Productive areas (upwelling, shelfs, etc.)
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Upwelling and Downwelling

• Winds blowing towards an area lead to the
  convergence of ocean currents
• This causes water to be forced down, produced
  downwelling (sometimes called subduction)
• This means that there are thicker layers of warm
  water above the pycnocline
• Usually this will mean few nutrients available in
  the upper water column and thus these regions are
  poor for biological life

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Coastal Up/Downwelling

• When the wind is aligned with the coast such that
  the transport is offshore, surface water is
  transported offshore and replaced by deeper water
  that moves inshore and is upwelled
• The reverse occurs when the wind is oriented in
  the opposite direction

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Boundary Currents and Upwelling

• Western Boundary Currents
• Often have a very deep pycnocline, with lots of
  warm water
• Thus, even if upwelling occurs, it doesnt bring
  up cold, nutrient rich waters
• Thus, these regions are not good for much
  biological life (note coastal regions inside the
  boundary current are often very productive)
• Eastern Boundary Currents
• Are shallow, such that even weak offshore Ekman
  transport will lead to upwelling of cold nutrient
  rich waters

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Atlantic WOCE A16 Section Salinity Oxygen
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Ocean Vertical Structure

• Temperatures high in the tropics due to solar
  heating
• Salinity high in lower latitudes due to high
  evaporation (with lower salinity at the Equator
  because of the ITCZ)
• Although the equatorial surface waters have high
  salinity, their high temperatures keep the
  density low and prevent those waters from sinking

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Ocean Vertical Structure

• When the pycnocline is sharp, there is little
  motion across it, as large amounts of energy are
  needed to move water across strong density
  gradients
• Thus water masses tend to flow mainly horizontal
• Deep waters are effectively isolated from the
  surface

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Ocean Vertical Structure

• At high latitudes, there is generally no
  pycnocline because of strong heat loss to the
  cold atmosphere above the ocean
• This leads to the surface waters having a high
  density, causing them to sink and form
  intermediate and deep waters
• Since other than this sinking, motion is mainly
  horizontal, we can determine where water
  originates from in the ocean by tracing contours
  of salinity and temperature back to where that
  water last was in contact with the atmosphere

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Ocean Vertical Structure

• When water sinks, the water above it has lower
  density and the water below it has higher density
• This keeps each layer in the ocean separate,
  defining each as a separate water mass
• Temperature, salinity and density only vary
  slightly in a water mass as it moves away from
  the source
• The main changes to T and S are by vertical mixing

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Density Layering
Segar, 2007
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Upper Ocean Intermediate Waters (Mode Waters)
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STMW in the North Atlantic Ocean
Generic winter location of STMW formation
18.2C Isothem
18C Isothem
Geostrophic recirculation pathways of STMW
17.8C Isothem
Transect Data R/V Oceanus, June 2000
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Northern Hemisphere Deep Water Formation Locations
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Convection

• Actual deep water formation process often called
  convection
• Small scale process that occur on scales of kms
  to tens of kms
• Normally three factors determine the locations of
  the convection sites
• Strong wintertime heat loss
• Strong winds that drive deep mixing of the mixed
  layer
• Cyclonic ocean circulation, leading to doming of
  the isopycnals and pre-conditioning

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Deep Convection
Stage 1 Preconditioning
Stage 3 Sinking and Spreading
Stage 2 Violent Mixing
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Deep Water Formation

• Indian Ocean
• does not reach high latitudes ? no deep water
  formation
• North Pacific
• Isolated from polar regions by land, island
  chains and shallow water
• High precipitation means low salinity
• ? No Pacific Deep Water formation

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Deep Water Formation

• North Atlantic Intense cooling occurs in the
  Labrador, Greenland and Norwegian Seas
• Forms North Atlantic Deep Water (NADW)
• Then sinks and flows south filling the worlds
  oceans (most voluminous water mass in the world)
• Higher salinity and temperature than more dense
  AABW