Abstract

The gas flux of weakly soluble gases through the air–sea interface is controlled by the transport mechanism in the aqueous diffusive boundary layer. The combination of molecular and turbulent transport and of secondary motions near the interface determines the exchange rate. This layer is difficult to access experimentally, so a combination of observation and physical interpretation is necessary.

Typical modes of fluid motion at the interface and their potential to further gas exchange are reviewed: organized motions, like cell and helicoidal rolls or Langmuir circulation on one hand, waves and wind induced shear flow on the other. Special attention is given to wave dissipation in the form of wave breaking. The secondary flow and irregular motions of breaking waves, as well as possible rolling motions of smaller waves on the slopes of larger waves, are seen as enhancing gas transfer through surface renewal.

Observations of surface streaming obtained by dying the sea surface are discussed in terms of the above-mentioned models of surface renewal. A set of observations by Gemmrich is used to assess the effectiveness of secondary motions, as found at a given wind speed at sea, to enhance gas transfer. It is found for the natural mix of wind speeds (e.g. for the North Atlantic Ocean) that enhancement of gas transfer, compared with undisturbed boundary-layer flow, occurs in about 20–25% of cases.