Mixed Layers, Thermoclines, and Hot Towers

The Transfer Laws of the air-sea interface link various fluxes to low-level properties of the air and to surface properties of the water. For, say, latent heat transfer to occur at all, the low level air has to be drier than the saturated air in contact with the sea surface. The resulting transfer of vapor moistens the air, however, and reduces or even eliminates the transfer. A steady supply of drier air to low levels is necessary to counteract the moistening and maintain vapor transfer.

Upper level air is much drier than the air near the interface, and if it extended downward to low levels, it would certainly maintain high vapor flux across the interface. The upper layers of the atmosphere are, however, insulated from direct contact with the interface by the Trade Inversion in the Tropics, and similar “thermoclines” poleward of the trades. Atmospheric thermoclines are layers in which the “virtual” potential temperature, θv, a measure of buoyancy taking into account humidity, sharply increases with height (θv = θ + 0.61qT, where θ = T + gz/cp is potential temperature). Meteorologists refer to such layers as strong “inversions,” because the absolute temperature T increases in them with height, instead of decreasing as it does everywhere else. The temperature gradient across an inversion would force strong downward Reynolds flux of heat, were not the associated buoyancy flux a strong sink for turbulent kinetic energy, TKE, as discussed in Chapter 1, in connection with the Transfer Laws.