Vertical and horizontal heat transfer within an oceanic surface mixed layer

Abstract

A well-mixed warm layer forms at the sea surface in cases of light to moderate winds and strong solar insolation. In one such case the heat content changes and irradiative absorption were measured throughout the layer enabling the determination of the local heat balance within the layer, which is between the outward flow of heat per unit volume; f; the absorbed radiation, i; and the heat content change rate, ∂h/∂t.

The quantity f represents both vertical and horizontal flows of heat. The horizontal components are due to a large-scale advection and local variability. The latter are removed from the reduced data by use of a reference temperature from below the layer, while the former are very small compared to the other components of the heat balance; thus f ultimately represents a vertical heat transport.

The data indicate that with weak solar insolation, heat was transported upward in the top meter and downward below. With strong insolation the heat transfer was downward at all depths. In the uppermost meter of the water, f almost exactly balanced i, while below the top meter f balanced ∂h/∂t. Because a stable density interface formed at the bottom of the layer due to turbulent mixing and radiative heating in the layer, it should inhibit non-conductive heat fluxes from the bottom of the layer. These measurements verify this and show f to be very small below the interface.