The vertical structure of the open ocean surface mixed layer

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The vertical structure of the open ocean surface
mixed layer
11628320 Dynamics of Marine Ecosystems

• Phytoplankton need light and nutrients for growth
  and reproduction
• Light comes from above, nutrients come from
  below
• In a layer near the surface the euphotic zone
  there is enough light for photosynthesis
• The process of supplying nutrients is dominated
  by ocean physics
• This lecture the physical processes that affect
  the vertical structure of light, heat and
  nutrients required for phytoplankton primary
  production

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1025 Density
1027.5 km m-3
µmole/liter
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• If there were no ocean physics to mix things
• Surface nutrients would be low (consumed)
• Deep nutrients would be high (re-mineralization)
• Molecular diffusion would slowly flux nutrients
  upward
• The ocean is stirred and mixed by turbulent
  processes acting on a variety of time and length
  scales associated with
• Winds
• Waves
• Currents
• Buoyancy (density differences)

Stirred not shaken
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• If there were no ocean physics to mix things
• Surface nutrients would be low (consumed)
• Deep nutrients would be high (re-mineralization)
• Molecular diffusion would slowly flux nutrients
  upward
• The ocean is stirred and mixed by turbulent
  processes acting on a variety of time and length
  scales associated with
• Winds
• Waves
• Currents
• Buoyancy (density differences)

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approximately 1-D vertical processes
Characteristic time scales for processes of
vertical exchange between the euphotic zone and
the ocean interior
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Physical processes on time scales of hours to
days that stir and mix the upper ocean
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Typical vertical structure in the open ocean

• Warmer, lighter upper mixed layer
• Cooler, heavier lower stratified layer
• Separated by region of rapid change
• thermocline, and also
• pycnocline
• nutricline
• The maximum chlorophyll (phytoplankton abundance)
  and primary productivity (phytoplankton growth
  rate) do not necessarily coincide, and may not
  occur at the sea surface because of
  interactions between physics and biology

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• Heat that warms upper ocean and sunlight for
  photosynthesis come from the sun
• Only a portion of the solar radiation at the top
  of the atmosphere reaches the sea surface due to
  several factors

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Some radiation physics

• Incoming radiation from the sun is in the
  shortwave band(wavelengths of 280 nm to 2800 nm)
• Wavelength of emitted radiation depends on the
  black-body temperature (Wiens Law) Lmax c/Tk
  where c 2.9 x106 nm K
• Our Sun has surface temperature Tk of about 5800
  K
• Ultraviolet 300 nm to far infrared 2400 nm
• Averaged over the Earth we receive about 340 W/m2
  at the top of the atmosphere

Wilhelm Carl Werner Otto Fritz Franz Wien
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• Visible radiation
• violet 360 nm
• red 750 nm
• Were most interested in the (visible)
  photosynthetically active radiation (PAR)
• Shortwave radiation is absorbed by water
• intensity decreases exponentially with depth in
  the ocean

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Incoming radiation from the sun is in the
shortwave band (wavelengths of 280 nm to 2800 nm)
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Spectra of downward radiation at different water
depths Sea surface, 1 cm, and 1, 10, and 100
meters depth
violet
red
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• Vertical profiles of radiation for selected
  wavelengths of light
• infrared
• red and blue visible, and
• typical total shortwave

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Atmosphere-ocean heat exchange

• Shortwave radiation warms the ocean
• Ocean temperature is 17oC or 290 K
• Ocean emits radiation too, which cools it
• Ocean radiates in the long-wave (infrared)
  wavelengths why?
• Long-wave is emitted only from the very surface
  of the ocean why?
• Downward long-wave arrives at the sea surface
  because of emission from water vapor in the
  atmosphere

(because of Wiens Law)
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Atmosphere-ocean heat exchange

• Sensible heat
• Conduction
• Depends on difference of air and sea
  temperature(can be warming, or cooling)
• Exchange rate affected by wind speed
• Latent heat
• Evaporation (cools)
• Depends on air relative humidity and saturation
  vapor pressure of moist air
• Exchange rate affected by wind speed

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Calculating temperature increase in the mixed
layer

• Average summer day in North Atlantic at 40oN
• Heat gain 500 W m-2 x 12 hours 21,600 kJ m-2
• If the mixed layer is 5 m deep, about 76 is
  absorbed above 5 m depth 17,100 kJ m-2
• Loss over same period 10,400 kJ m-2
• Net energy gain during the day is Q 6700 kJ
  m-2
• Temperature change is T Q/(mass x specific
  heat) mass is density x volume 1000 kg m-3 x 5
  m3 specific heat is 4.2 kJ kg-1 oC-1 T
  6700/(5000 x 4.2) 0.3oC increase in 1 day

Box 3.01 in Mann and Lazier
View live met data at http//mvcodata.whoi.edu/cgi
-bin/mvco/mvco.cgi
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• Solar heating is exponentially distributed with
  depth
• Temperature profile is not exponential because
  turbulence stirs and mixes the water column
• Mixing that entrains cool water from below the
  thermocline cools the mixed layer (dilutes with
  cold)
• Zero net air-sea heat flux plus mixing gives
  net cooling

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• Mixing works against the gravitational stability
  of the pycnocline
• Displace a dense parcel of water up, it is heavy
  and falls down
• Displace a light parcel down, it is buoyant and
  bounces up
• Density interface will undergo oscillations of a
  fixed frequency
  Brunt-Vaisala frequencyg 9.8 ms-2,
  density difference 1 kg m-3 over 1 m depth,
• Get N 0.1 s-1 or a period of 2p/N 60 s (a bit
  high for reality)
• Real values are more like 10 minutes

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• Mixing works against the gravitational stability
  of the pycnocline
• Displace a dense parcel of water up, it is heavy
  and falls down
• Displace a light parcel down, it is buoyant and
  bounces up
• Density interface will undergo oscillations with
  frequency
  Brunt-Vaisala frequencyg 9.8 ms-2,
  density difference 1 kg m-3 over 1 m depth,
• Get N 0.1 s-1 or a period of 2p/N 63 s
  (this frequency is a bit high
  for reality)
• Real values are more like 10 minutes period

From Box 3.03 in Mann and Lazier
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Mixing, stability and stratification

• Mixing and stirring displaces water and down and
  averages their density
• This work uses up the stirring kinetic energy
• by increasing the potential energy of the water
• The stronger d?/dz the more work there must be
  done against gravity
• The pycnocline acts as a barrier that inhibits
  mixing and limits the depth of the mixed layer

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Cooling and convection

• Night time cooling (long-wave and sensible heat
  loss) decreases the ocean temperature only very
  close to the sea surface
• Cool water above warmer water is unstable, and it
  convects
• Convection ceases when the water column becomes
  stably stratified

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Growth and decay of a diurnal pycnocline
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Growth and decay of a diurnal pycnocline
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Heating-cooling-mixing balance through the seasons

• In Winter, cooling dominates causing max MLD to
  steadily deepen through March
• After solstice, increase in solar energy allows
  daily formation of ML
• Gets steadily shallower as heating increases
• After equinox, maximum MLD decreases because heat
  gained during day is not lost overnight, and
  winds are weaker (less stirring)
• Through spring and early summer the ML becomes
  more stable. The change in depth min/max
  decreases because density change is larger the
  same stirring effort (work) against gravity mixes
  a smaller depth of water
• Fall cooling takes over and erodes the mixed
  layer (convection)

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Vertical temperature profiles month by month
Depth of certain isotherms as a function of month
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Temperature at a given depth as function of month
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Model simulation of the diurnal variation of the
mixed layer over the year. Note the diurnal
movement of the thermocline from January to
October.(WS winter solstice SS summer
solstice)
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Nutrient fluxes across the base of the
thermocline

• Turbulent mixing that entrains water across the
  pycnocline
• entrains higher nutrient water and fertilizes
  the mixed layer
• which is circulated throughout the mixed layer
  by continuous stirring

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• Rate of nutrient flux depends on
• Physics
• Entrainment rate due to mixed layer turbulence
  (wind strength)
• Limited by strength of pycnocline density
  gradient
• Aided by convection
• Stratification depends on air-sea heat flux
• Available nutrient concentration below the
  nutricline (Liz)
• If there is enough light, get photosynthesis
  (Heidi)

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• References and reading
• Mann and Lazier, chapter 3
• Knauss, J., An Introduction to Physical
  Oceanography, 2nd ed., Prentice-Hall, 1997
• Lalli and Parson, Biological Oceanography, The
  Open University.