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

The vertical structure of the open ocean surface mixed layer Research Vessel “Flip” https://www.youtube.com/watch?v=azZIcoPI_CU

The vertical structure of the open ocean surface mixed layer 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 Plants need water, light, nutrients, CO2 (no shortage of water or CO2 in the ocean, but nutrients and light are limiting) Where do the nutrients come from? The ocean is like a compost bin … sinking organic matter is decomposed back into inorganic nutrients at depth, away from light

µmole/liter 1025 Density 1027.5 kg m-3 The open ocean is stratified in most properties. Warm at the top – cold at the bottom – dominates the density stratification (light at top, heavy at bottom) Ocean mixing and biogeochemical processes act to stratify the vertical pattern of nutrients (and many other constituents) 1025 Density 1027.5 kg m-3 µmole/liter

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) Ocean is stirred, not shaken Mixing is not strong enough to make the entire ocean the same. Stirred – not shaken (Sorry James)

approximately 1-D vertical processes At short time scales the depth over which processes act are typically small. It takes rather long time scales to influence the full depth of the water column. Characteristic time scales for processes of vertical exchange between the euphotic zone and the ocean interior

Physical processes on time scales of hours to days that stir and mix the upper ocean This lecture will concentrate today on processes on short time scales – several days – and how they vary seasonally

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

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 Now we take some time to consider the physical processes that control the heating of the ocean surface (physics) and the light available for photosynthesis (biology)

100% at top of atmosphere is about 340 W/m^2 on average (The red balloons in this slide track how I like to follow the sums on where the radiation goes)

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/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

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 The sky is blue because of scattering Blue is more strongly scattered because of its wavelength favors Rayleigh scattering, so it dominates when looking skyward away from the sun violet red

Vertical profiles of radiation for selected wavelengths of light infrared red and blue visible, and typical total shortwave Anyone SCUBA dive? Ever take pictures? Your eye corrects for the light and helps you see what it really looks like, but what color dominates the photos? (The figures that follow show several different ways of looking at the light field dependency)

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 Anyone SCUBA dive? Ever take pictures? Your eye corrects for the light and helps you see what it really looks like, but what color dominates the photos? (The figures that follow show several different ways of looking at the light field dependency) Blue (440 nm) Red (675 nm) Infrared (1000 nm)

1 % light level is a commonly charted quantity by coastal oceanographers. 1 % can be enough to sustain photosynthesis If enough light reaches the seafloor what might you expect about the ecosystem (benthic algal production)

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) Next, go through the processes that contribute to net heat flux. The ocean doesn’t emit shortwave (because of Wien’s Law), so any shortwave coming up from the ocean surface must be directly reflected incoming solar.

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

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 = 5 x 106 J m-2 Every 1m x 1m column of the ocean is absorbing 5 x 106 J 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-1 oC-1 ΔT = 5 x 106 /(5000 x 4200) = 0.24oC increase in 1 day Box 3.01 in Mann and Lazier 500 W/m^2 is 500 J/s/m^2 so multiply by 12*60*60 seconds to get 21600 x10^3 J/m^2 It takes 4200 Joules to raise the temperature of 1 kg of water (1 liter) by 1 degree C This is the “specific heat” of water 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

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

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 Consider a stack of blocks of different weights but the same size (i.e. different density) A steel block, a wooden block, a foam block – what arrangement is more stable? Think about this in terms of gravitational potential energy. Mixing due to e.g. winds or tides does WORK on a stratified water column and actually raises the potential energy of the water column by raising the center of mass Mixing a stratified ocean uses up the wind energy

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

If all the boxes have the same volume, then 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 ρ3 ρ1

Unstable: density distribution causes vertical motion 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

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

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

Vertical temperature profiles month by month

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

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

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)

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

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)