The vertical structure of the open ocean surface mixed layer

1. 11:628:320 Dynamics of Marine Ecosystems The vertical structure of the open ocean surface mixed layer

2. The vertical structure of the open ocean surface mixed layer Research Vessel “Flip”

3. 11:628:320 Dynamics of Marine Ecosystems The vertical structure of the open ocean surface mixed layer • 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

4. 1025Density 1027.5 kg m-3 µmole/liter

5. 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

6. approximately 1-D vertical processes Characteristic time scales for processes of vertical exchange between the euphotic zone and the ocean interior

7. Physical processes on time scales of hours to days that stir and mix the upper ocean

8. 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

10. 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

12. 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 (Wien’s Law) Lmax= c/Tkwhere 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

13. Visible radiation • violet 360 nm • red 750 nm • We’re most interested in the (visible) photosynthetically active radiation (PAR) • Shortwave radiation is absorbed by water • intensity decreases exponentially with depth in the ocean

14. Incoming radiation from the sun is in the shortwave band (wavelengths of 280 nm to 2800 nm) 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

17. 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 Wien’s Law)

18. 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. Calculating heating of the mixed layer • Average summer day in North Atlantic at 40oN • Heat gain 200 W m-2 x 24 hours = 17,000 kJ m-2 • If the mixed layer is 5 m deep, about 75% is absorbed above 5 m depth = 13,000 kJ m-2 • Loss over same period ~ 8,000 kJ m-2 • Net energy gain during the day: Q = 5000 kJ m-2 • Temperature change is ΔT = Q/(mass x specific heat) mass is density x volume = 1000 kg m-3 x 5 m3 specific heat of water is 4.2 kJ kg-1oC-1 ΔT = 5000/(5 x 1000 x 4.2) = 0.24oC 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

21. 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

22. 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 up z

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31. 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 frequency • g = 9.81 ms-2, density difference ~ 0.1 kg m-3 over 10 m • Get N = 0.01 s-1 or a period of 2π/N = 630 s (about 10 minutes period) From Box 3.03 in Mann and Lazier

32. 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 dρ/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 Mixing a stratified ocean uses up the wind energy

33. Which arrangement of blocks is more stable? Unstable: mass distribution causes vertical motion Stable: mass resists vertical motion m1 m3 m2 m2 m1 m3

34. If all the boxes have the same volume, then mass per unit volume is density Unstable: density distribution causes vertical motion Neutral: density does not influence on vertical motion Stable: density resists vertical motion ρ1 ρ3 ρ2 ρ2 ρ2 ρ1 ρ3

35. The static stability of the water column is controlled by the vertical distribution of density. Unstable: density distribution causes vertical motion Neutral: density has no influence on vertical motion Stable: density resists vertical motion ρ1 ρ3 ρ2 ρ2 ρ2 ρ3 ρ1

36. Where is the center of mass of these two columns of water? Before mixing After mixing ρ1 ρ2 ρ2 ρ3

37. 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

38. Vertical temperature profiles month by month Depth of certain isotherms as a function of month

39. Temperature at a given depth as function of month Depth of certain isotherms as a function of month

40. 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 mixed layer • Gets steadily shallower as heating increases • 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)

41. 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

43. 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)