This lecture will concentrate today on processes on short time scales – several days – and how they vary seasonally

The radiation received from the sun is not actually of a single wavelength but spread over a spectrum, with peak at around 500-600 nm (nanometers). The radiation received at the sea surface has distinct local mimima because of absorption by molecules in the atmosphere.

Shortwave radiation is absorbed by water 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

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)

Straight lines on a log scale emphasize that the shape of the vertical profile is exponential

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 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 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 Box 3.01 in Mann and Lazier View live met data at http://mvcodata.whoi.edu/cgi-bin/mvco/mvco.cgi

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

Net Surface Radiation Why not maximum at the qauator? Why is local minimum off-set from the equator?

Net surface thermal (long wave) radiation Why largest (most negative) over continents? Why spatial variability over continents?

Net surface heat exchange--- note that there is a loss of heat throughout the Entire North Atlantic! But not in North Pacific.

Svante Arrhenius. The large picture shows him in Spitsbergen, Norway, seeing off an Arctic balloon expedition in 1896. In 1896, when he published his greenhouse calculation, Arrhenius was Professor of Physics and Rector at the Stockholm Högskola. He was already famous for showing how dissolved salts separate into charged particles ("ions"). In 1903 he was awarded the Nobel Prize in Chemistry for "the extraordinary services he has rendered to the advancement of chemistry by his electrolytic theory of dissociation.

New York Times 1890

100% at top of atmosphere is about 340 W/m^2 on average About half shortwave reaches the sea surface (the rest is reflected or absorbed in the atmosphere) What reaches the ocean surface is re-emitted as long wave radiation, sensible, and latent heat flux This warms the atmosphere, which emits long-wave back to the sea surface (this is the natural greenhouse effect that keeps the planet habitable)

Increase in total radiative forcing To earth’s ~ 2 W/m2

Figure 5.14 Changes in solar constant (total solar irradiance) and global mean temperature of Earth’s surface over the past 400 years. Except for a period of enhanced volcanic activity in the early 19th century, surface temperature is well correlated with solar variability. From Lean, personal communication.

Note that heat transport in Atlantic extends further north than Pacific.

Figure 5.14 Changes in solar constant (total solar irradiance) and global mean temperature of Earth’s surface over the past 400 years. Except for a period of enhanced volcanic activity in the early 19th century, surface temperature is well correlated with solar variability. From Lean, personal communication.

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 is 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