How Does Salinity Vary

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How Does Salinity Vary

• Why is the ocean layered?
• Surface waters
• Deep waters
• What governs the density of seawater
• Variations in salinity and temperature give rise
  to ocean rivers. They drive thermohaline
  circulation and the overturn of the oceans.
• Also govern life in oceans, due to effects on
  nutrients

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Surface ocean variation of salinity

• Polar regions salinity lower, lots of rain/snow
  and runoff
• Mid-latitudes salinity higher, high rate of
  evaporation
• Equator salinity lower, lots of rain
• Thus, salinity at surface varies primarily with
  latitude

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Surface ocean variation of salinity
Fig. 5.20
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Surface salinity variation

• High latitudes have low surface salinity
• High precipitation and runoff
• Low evaporation
• Tropics have high surface salinity
• High evaporation
• Low precipitation
• Equator has a dip in surface salinity
• High precipitation partially offsets high
  evaporation

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Surface salinity variation

• Pattern of surface salinity
• Lowest in high latitudes
• Highest in the tropics
• Dips at the Equator
• Surface processes help explain pattern

Figure 5-20
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Deep ocean variation of salinity

• Surface ocean salinity is variable
• Deeper ocean salinity is nearly the same (polar
  source regions for deeper ocean water)
• Halocline, rapid change of salinity with depth

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Salinity variation with depth

• Curves for high and low latitudes begin at
  different surface salinities
• Halocline layer of rapidly changing salinity
• At depth, salinity is uniform

Figure 5-22
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Seawater density

• Factors affecting seawater density
• Temperature ?, Density ? (inverse relationship)
• Salinity ?, Density ?
• Pressure ?, Density ?
• Temperature has the greatest influence on surface
  seawater density

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Density of seawater
Fig. 5.22
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Density of seawater

• 1.022 to 1.030 g/cm3 surface seawater
• Ocean layered according to density
• Density seawater controlled by temperature,
  salinity, and pressure
• Most important influence is temperature
• Density increases with decreasing temperature

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Density of seawater

• Salinity greatest influence on density in polar
  oceans
• Polar ocean is isothermal

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Density versus depth

• Density differences cause a layered ocean
• Pycnocline, abrupt change of density with depth
• Thermocline, abrupt change of temperature with
  depth

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Fig. 5.23
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Pycnocline and thermocline

• Pycnocline layer of rapidly changing density
• Thermocline layer of rapidly changing
  temperature
• Present only in low latitude regions
• Barrier to vertical mixing of water and migration
  of marine life

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Ocean layering based on density

• Mixed surface layer (surface to 300 meters)
• Low density well mixed by waves, currents, tides
• Upper water (300 to 1000 meters)
• Intermediate density water containing
  thermocline, pycnocline, and halocline (if
  present)
• Deep water (below 1000 meters)
• Cold, high density water involved in deep current
  movement

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Layered ocean

• Mixed surface water
• Pycnocline and thermocline
• Deep water
• High latitude oceans
• Isothermal
• Isopycnal

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