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” https://www.youtube.com/watch?v=azZIcoPI_CU

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 (Sorry James)

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

9. 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

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 (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

12. 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

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

14. Vertical profiles of radiation for selected wavelengths of light • infrared • red and blue visible, and • typical total shortwave Depth of penetration where only 1% of surface light remains Blue (440 nm) Red (675 nm) Infrared (1000 nm)

16. 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)

17. 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

19. 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-2 200 x 86400 = 17.28 x 106 J m-2 • If the mixed layer is 5 m deep, and 75% is absorbed above 5 m depth = 13,000 kJ m-2 200 x 86400 x 0.75 = 12.96 x 106 J m-2 • Loss over same period ~ 8,000 kJ m-2 (from long-wave radiation and latent heat loss) • Net energy gain during the day: Qnet = 13 x 106 minus 8 x 106 = 5 x 106 J m-2 • i.e. every 1m x 1m column of the ocean is absorbing 5 x 106 J each day • Temperature change is ΔT = (Q x area)/(mass x specific heat) mass is density x volume = 1000 kg m-3 x 5 m3 = 5000 kg specific heat of water is 4200 J kg-1oC-1 ΔT = 5 x 106 /(5000 x 4200) = 0.24oC increase in 1 day Specific heat: Cp = the heat (Joules) required to raise the temperature of 1 kg water by 1o C. Box 3.01 in Mann and Lazier View live met data at http://mvcodata.whoi.edu/mvco/tsplots/MetindexE.html

20. 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

21. 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

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

23. Mass per unit volume is 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 ρ1 ρ3

24. 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

25. Where is the center of mass of these two columns of water? Before mixing After mixing Mixing raises the center of mass of the water column. So some of the Kinetic Energy of the mixing is converted to Potential Energy of the water column (by lifting mass against gravity). ρ1 ρ2 ρ2 ρ3

26. Cooling and convection • Night time cooling (long-wave and sensible heat loss) decreases the ocean temperature 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

27. Vertical temperature profiles month by month

28. Vertical temperature profiles month by month Temperature at a given depth as function of month

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

30. 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)

31. 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

33. 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)