The large-scale dynamics of the ocean and the atmosphere are closely related. Energy is transferred from the atmosphere to the ocean surface mixed layer driving the circulation of the upper ocean. In turn, energy from the ocean is fed back to the atmosphere affecting the atmospheric circulation, the weather and the climate. The concept is deceptively simple, but as we explore the coupled earth system, we are frequently limited by our lack of understanding of the interchanges between the atmosphere and ocean. As Brown [1990] pointed out, there are two gaps in our knowledge of the air-sea interface: the theory is inadequate and the data are sparse. More specifically, Kraus and Businger [1994] highlighted several areas that continue to require more attention, the interaction of the wind and surface waves, for example, the parameterization of subgrid scale processes in large-scale circulation models, and the transfer of gases across the air-sea interface, among others.
It has been convenient to divide air-sea interaction studies into two categories: small- and large- scale ocean-atmosphere interactions. However, this often belies the fundamental precept that the basis of the interaction of the atmosphere and the ocean is the exchange of matter and energy across a material interface---the sea surface. An exchange that, for the most part, occurs on molecular scales, involving both turbulent and laminar processes modified by wave breaking, surface tension, the structure of the planetary boundary layer and the ocean mixed layer and other effects. A satisfactory understanding of these processes remains elusive, but is essential if we are to address adequately the larger scale ocean-atmosphere problems.
In the past many proponents of large-scale studies, such as global climate and ocean circulation relied heavily on the veracity of the parameterization of small-scale air-sea exchange processes, often overlooking the uncertainties in the basic measurements and their interpretation. More recently, we have recognized the importance of connecting small-scale process studies, investigating the exchange of heat, moisture, momentum and trace constituents across the air-sea interface, with the large-scale problems of global climate change and ocean circulation that rely heavily on numerical models and highly averaged fields. Studies, such as the Tropical Ocean Global Atmosphere Coupled Ocean Atmosphere Response Experiment (TOGA COARE) and the World Ocean Circulation Experiment (WOCE), are leading the way by highlighting the importance of process studies for a satisfactory understanding of global climate and ocean general circulation problems [e.g., Webster and Lukas, 1992; Taylor, 1990].
The small-scale exchange processes are generally related to the global-scale problems via parameterizations of the fluxes that use readily obtained mean quantities [ Liu et al., 1979] measured from various platforms such as buoys, ships and satellites. These parameterizations are also used extensively in operational meteorological models as well as many research general circulation models of the coupled ocean-atmosphere system [e.g., Meehl, 1993]. Large uncertainties exist in the derivations of the bulk parameterizations due to the difficulty of measuring surface fluxes directly, and the difficulty of applying these measurements to scales greater than a few hundred kilometers and several hours. The problems are particularly acute in the tropics where low wind speeds and very high sea surface temperatures result in primarily buoyancy-driven fluxes that are not well parameterized by most prevailing methods [ Bradley et al., 1991; Miller et al., 1992; Webster and Lukas, 1992], and coastal regions where fetch, topography and water depth vary considerably [ Geernaert, 1990; Rogers, 1995].
While there is resistance to the establishment of canonical parameterizations of the fluxes, the TOGA COARE research community recognized that such an approach has the advantage of focusing the attention of a large group of researchers to exchange and compare data and rapidly transfer this information to a broader community whose interests require surface fluxes but whose focus is elsewhere. Despite these efforts, it is unlikely that in this decade of large scale climate and ocean circulation studies we will know the surface fluxes as well as we would like. A continued effort will be required to improve our knowledge of heat, moisture, momentum and trace constituent fluxes, to increase our understanding of the uncertainties in the fluxes we measure, and to improve our knowledge of the relationship between boundary layer processes on each side of the interface to the surface exchange mechanisms.
Air-sea interaction is a broad field and there has been considerable progress in many areas; most notably related to El Ni¤o phenomena and the circulation of the oceans. In the following sections we will review current progress in our understanding of the surface processes, and the application of this research to the global issues of ocean circulation and climate. Some of the comments have universal application, others are primarily addressed to specific processes. A vital and important step forward is the connection between advances in our fundamental understanding of surface processes and the application of this knowledge to the largest scales. With this in mind, we discuss the importance of satellite remote sensing and numerical modeling as bridges between the small-scale process studies and large-scale circulation of the ocean-atmosphere system.