Ocean's Radiative Balance Fundamentals of PHysical Oceanography

The electromagnetic Spectrum

Figure: Illustration of the different wavelengths of the electromagnetic spectrum.

What is light? Some say it's made of waves, others say it's made of particles, and still others say that it's both, just depends on how you look at it. Let's take the classical wave view, light is a part of a wide spectrum of electromagnetic radiation ranging from wavelength (L) on the order of nucleons to the size of skyscrapers. The speed of any electromagnetic wave in vacuum is c = 299,792,458 m/s. Amazingly, it is constant, no matter how fast you move relative to it. For each wavelength, there is a corresponding frequency (f = c / L) and a temperature.

Figure: The effect of atmosphere on electromagnetic radiation transmission.

Mechanisms that drive the transfer of energy on Earth surface

Shortwave Radiation (Q_SW):

Electromagnetic radiation in the UV (10 - 400 nm), visible (400 - 700 nm), and near-infrared (700 - 2500 nm) range. Majority of sun's radiation reaching the surface of Earth is shortwave radiation. To calculate the amount of radiation reaching the surface of Earth from the Sun, we apply the Stefan-Boltzmann Law: 𝜎T^4 = j = energy flux, where 𝜎 = 5.670367×10^−8 W⋅m^−2⋅K^−4 and T is the black body temperature of Sun's surface. Assuming that the surface of the sun is 5,780 Kelvin, what is the energy flux at the top of Earth's atmosphere?

Figure: Shortwave radiation Q_SW reaching the surface of Earth. Positive numbers indicate energy gain.

Long wave radiation (Q_LW):

This is infrared radiation with wavelength of 4 to 100 microns (10^-6 m). This is what we normally consider as 'heat'. If we calculate the wavelength of black body radiation from Earth, it would fall into the long wave radiation range. Green house gases in Earth atmosphere partly blocks long wave radiation from both entering and leaving Earth. That is why cloudy days can feel warmer than clear days because the long wave radiation back to the surface from the cloud.

Figure: Average long wave radiation (Q_LW) at the surface of Earth. Negative numbers indicate heat loss.

Latent Heat (Q_L):

This is the heat associated with phase change of materials, mainly water in the geophysical context.

Condensation: Phase change from gas to liquid as energy is removed from the gaseous phase of the material (water vapor).
Evaporation: Phase change from liquid to gas as energy is added to the liquid phase of the material (water).
Figure: Illustration of how heat energy can be moved from warm water to cold atmosphere via latent heat transfer..
Figure: Latent heat flux at Earth's surface (Q_L). Positive number means heat loss. Top panel is for southern hemisphere winter, and the lower panel is for southern hemisphere winter.

Sensible Heat (Q_H):

This is heat exchange via conduction. In the geophysical context, it is typically between air and ocean. The rate of exchange is directly proportional to the temperature difference between the two media. Stronger wind also lead to stronger rate of exchange as the wind can help maintain the temperature gradient.

Figure: Sensible heat exchange in the ocean. Positive numbers means heat loss. Notice that Q_H is not significant for most parts of the ocean except along the western boundary currents. Why?

Net mean surface heat flux (Q_net):

Q_net = Q_SW + Q_LW + Q_L + Q_H

Be aware of the signs for each above term. The equation assumes that negative is heat loss from the ocean and positive is heat gain by the ocean. On the global scale over the course of a year, what do you think Q_net is?

Figure: Net heat flux at Earth's surface. Notice that majority of the heat gain occurs at tropical oceans and most of the heat loss occurs in the temperate and polar oceans.
Figure: Latitudinal dependence of heat gain and loss is shown in figure above. Notice the boundary currents play a crucial of heat transfer.
Figure: Average heat fluxes through the various components of Earth's surface and the atmosphere.
Created By
Donglai Gong
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Created with images by NASA Goddard Photo and Video - "Ponds on the Ocean" • gr33n3gg - "Sun"

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