Subsurface Energy Flux

Since the subsurface heat flux has fundamentally different mechanisms on land and on a water body, it will be discussed separately. On land ground, heat flux is normally one order of magnitude smaller than other fluxes and it plays a limited role in surface energy balance. The main reason for this is the small thermal diffusivity of the material composing the ground in comparison with the eddy diffusivity of the near-surface atmosphere and ocean.

The thermal diffusivity of common soils and rocks is 10^-4 - 10^-6 [1], when usually the observed values for the eddy diffusivity in the first 10 m of the atmosphere are taken as unity. Nevertheless, this heat flux determines the thermal conditions of the near-surface layers of the pedo- and lithosphere. Ground heat flux turns downward in spring and reaches the year’s maximum heat flow in early summer, usually 1 month ahead of the summer solstice.

The flux declines throughout the summer and turns upward in autumn until the following spring. At the year’s maximum, it attains about 10% of the net radiation. The phase and the magnitude of the ground heat flux can be profoundly affected by the snow cover, the thickness, the timing of its onset, and the melt. The annual mean ground heat flux observed for more than several years is very close to zero or becomes the same as the geothermal heat flux, which is the order of 0.1 Wm^-2.

A unique exception is the significantly larger downward heat flow in the lower accumulation areas of the cold glaciers in the polar regions. The melt water in summer penetrates through the snow cover and refreezes in the lower layers, where latent heat of fusion is released upon refreezing. On these glaciers it is common to observe several degrees Kelvin warmer temperature at 10 m below the surface than at the surface. In this case, the subsurface heat is carried not by thermal conduction, but through the mass percolation that carries latent heat.

Another exception is the subsurface flux on the snow and ice. Snow and ice are semitransparent for solar radiation. Consequently, the main portion of the energy flow into the subsurface layer happens by absorption of solar radiation rather than slow process of heat conduction. This is the reason why the title of the present section is the subsurface energy flux, and not the ground heat conduction. In spring and summer, the energy flow through the absorption of solar radiation is often more than 10 times larger in comparison with the heat conduction.

This is the main reason how the glacial katabatic wind continues flowing in the polar summer after the net radiation turns positive. A substantial portion of the net radiation is the absorbed solar radiation below the surface, which does not contribute to heating the surface atmosphere, allowing the atmospheric cooling to continue to accelerate the katabatic wind on the Antarctic and Greenland ice sheets.

The subsurface energy flux on the ocean surface is the ocean surface heat flux or net downward heat flux in oceanography (Figure 13). This component is substantial in diurnal and annual surface energy balance as well as in climate changes. The importance of the ocean in the climate system is due to its large heat-storing and transporting capacity.

Figure 13. The annual mean subsurface heat flux by ECHAM4T106

This function is not only due to the large specific heat of water, but to a great extent to the transparency of ocean water for solar radiation, and to the oceanic turbulence and convection that promote heat diffusion. The annual amplitude of the ocean surface heat flux reaches up to 200 W m^-2 on the midlatitude ocean.

In the course of a year, the ocean heat flux flips between a heat sink (energy gain for ocean) in summer and a source (energy loss for ocean) in winter on the hemispheric scale. Regionally viewed, there are three belts of heat gains for the ocean (energy sinks for the surface and atmosphere), one around the equator and the other two in the eastern flanks of the major oceans at the higher midlatitudes around 40° - 50° N and S where a substantial amount of heat flows downward.

Of these three, the region under the equator is a permanent energy source. In the equatorial ocean, the large shortwave incoming radiation, supported by relatively weak longwave cooling (due to large cloud amount) and smaller turbulent heat losses (due to higher humidity and lower SST), forms the basis of the permanent energy source for the ocean (sink for the surface).

The other two belts in the higher midlatitudes are heat sources on an annual basis, but are in a delicate balance between the gain in summer and the loss in winter. Unlike ground heat flux, the annual mean ocean surface heat flux can assume large absolute values. The eastern half of the higher midlatitude ocean surfaces where the temperature is lower is especially important as a source, as witnessed in the regions of the Californian Current, Azores Current in the Northern Hemisphere, and Peru Current and Benguela Current in the Southern Hemisphere.

There are several regions with strong heat loss (gain for the surface and the atmosphere). They are the surfaces of the western boundary current; Gulf Stream and Kuroshio in the Northern Hemisphere; and Eastern Australian Current, Brazil Current, and Agulhas Current in the Southern Hemisphere. Further in higher latitudes, in the Arctic Ocean (Central Polar Ocean including marginal oceans, such as Norwegian, Barents, Kara, Laptev, East Siberian, and Beaufort Sea) and the Antarctic seas near the continent, the ocean delivers heat to the surface and a moderate amount of heat to the atmosphere, keeping the polar and subpolar regions relatively mild.