Interactions between the ocean and atmosphere occur at the air-sea interface. The ocean surface is a material interface that is a barrier to the exchange of heat, moisture, momentum and trace constituents. Away from the surface both fluids are usually in turbulent motion, but near the interface turbulence is supprssed and transport is controlled primarily by molecular processes. To quantify the exchanges at the interface it is necessary to understand how the turbulent layers of the ocean and the atmosphere are connected via the molecular sublayers either side of the sea surface. In turn we need to understand how the turbulent layers transport the properties of the interface into the interior of these fluids, the extent to which changes in the interior feed back to the interface, and how processes at the surface affect the structure of the deep ocean and free atmosphere. The fundamental processes that connect the atmosphere and the ocean are the energy input to the ocean by the wind, the net freshwater flux, expressed primarily as precipitation minus evaporation, and the net surface heat flux.
As Charnock [1951] pointed out the energy transmitted by the wind to the ocean is a tiny fraction of the radiation received at the surface, yet wind-driven currents largely determine the regions where the ocean energy is fed back into the atmosphere that sets the pattern of cloudiness, which in turn determines the radiation input. The ocean-atmosphere system is intrinsically coupled, although feedbacks across the air-sea interface are often masked by temporal and spatial differences. As in the example above, we may understand the processes that connect the ocean and the atmosphere, but we do not understand well enough the distribution of energy within the system. Many of the problems that vexed early attempts to measure the structure of the upper ocean and lower atmosphere remain paramount; for example, flow distortion around measurement platforms, the difficulty of obtaining measurements in the open ocean, and the relationship between point measurements and large-scale fields. The latter is of particular importance when trying to apply the results of process studies to large scale monitoring of the earth by satellites [ Liu, 1993].
To the list of basic physical processes mentioned above we must
also add gas exchange across the air-sea interface. The most
significant are carbon dioxide (CO
), a greenhouse gas,
dimethyl sulfide (DMS), which may be the main source of
non-sea-salt sulfate aerosols, and nitrous oxide (N
O), which
is both a greenhouse gas and may play an important role in the
destruction of ozone [ Watson et al., 1990, 1992].
Geernaert [1990] has outlined much of the experimental basis for air-sea interaction research tracing its history from the 1950s where most studies of wind stresses were conducted over small bodies of water, through the late 1950s, where open ocean measurements of the fluxes became more prominent, to the more contemporary large scale field programs,---Barbados Oceanographic and Meteorology Experiment (BOMEX), Atlantic Trade Winds Experiment (ATEX), Global Atmospheric Research Program Atmospheric Tropical Experiment (GATE), Joint Air-Sea Interaction Experiment (JASIN), Marine Remote Sensing Experiment (MARSEN), Storm Transfer and Response Experiment (STREX), Humidity Exchange Over the Sea (HEXOS), Marginal Ice Zone Experiment (MIZEX), Frontal Air-Sea Interaction Experiment (FASINEX). The reader is referred to Geernaert [1990] for a more detailed overview of these studies and their results. More recent field programs include TOGA COARE [ Webster and Lukas, 1992] that was completed at the end of February 1993, the Surface of the Ocean, Fluxes and Interactions and Atlantic Stratocumulus Transition SOFIA/ ASTEX experiment devoted to air-sea interactions and cloud development in the Azores region of the Atlantic Ocean that took place in June 1992 [e.g., Dupuis et al., 1993; Albrecht et al., 1994], and the ongoing WOCE [e.g., Taylor, 1989].
Understanding of the transfer of heat, moisture, momentum and
mass across the air-sea interface is fraught with difficulties.
One of the most complex problems is understanding the effect of
wind waves on the momentum flux [ Brown, 1990;
Geernaert, 1990]. Also understanding what happens at the
air-sea interface requires knowledge of how energy is transferred
across the stable layers connecting the interiors of the
atmosphere and ocean with their respective boundary layers.
These processes are frequently intermittent andinextricably
linked to the larger scale circulations of these
[4]
fluids.