The vertical structure of the open ocean surface mixed layer

1
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

2
1025 Density
1027.5 km m-3
µmole/liter
3

• 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
4
approximately 1-D vertical processes
Characteristic time scales for processes of
vertical exchange between the euphotic zone and
the ocean interior
5
Physical processes on time scales of hours to
days that stir and mix the upper ocean
6
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

7
(No Transcript)
8

• 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

9
(No Transcript)
10
(No Transcript)
11
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
12

• 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

13
Incoming radiation from the sun is in the
shortwave band (wavelengths of 280 nm to 2800 nm)
14
Spectra of downward radiation at different water
depths Sea surface, 1 cm, and 1, 10, and 100
meters depth
violet
red
15

• Vertical profiles of radiation for selected
  wavelengths of light
• infrared
• red and blue visible, and
• typical total shortwave

16
(No Transcript)
17
(No Transcript)
18
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)
19
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

20
(No Transcript)
21
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
22
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
23

• 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

24

• 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

25
(No Transcript)
26

• 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
27
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 dr/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

28
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

29
Growth and decay of a diurnal pycnocline
30
Growth and decay of a diurnal pycnocline
31
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)
32
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)

33
Vertical temperature profiles month by month
Depth of certain isotherms as a function of month
34
Temperature at a given depth as function of month
35
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

36
(No Transcript)
37

• 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
  (Oscar)

38

• 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.