Processes at the air-sea interface are directly affected by the boundary layers of the upper ocean and the lower atmosphere, which in turn are modulated by the exchanges of heat, moisture, momentum and trace constituents across the air-sea interface. The boundary layers are also affected directly by processes within the interior of the fluid that affect the transfer of energy across the inversion and the pycnocline. The organization of distinct, coherent structures, such as convection waves and rolls in the atmosphere and upper ocean eddies, fronts and filaments in the ocean, contribute to the intermittency and the variability of the exchange processes at the surface [e.g., Mourad and Brown, 1990; Smith, 1992; Etling and Brown, 1993; Weller, 1991; Flament et al., 1994; Friehe et al., 1991].
In the atmosphere, clouds connect the boundary layer with the free troposphere modulating the ventilation of the boundary layer. These structures are well-defined coherent features that impose spatial and temporal scales on the boundary layer and the intermittency of the surface exchange processes. For example, clouds develop in the boundary layer in response to the moisture input to the atmosphere from the ocean. These clouds may be associated with a specific spatial scale imposed by the boundary layer, but they will also adjust the depth of boundary layer and hence their own scales. Precipitation events will transfer these scales back to the ocean producing a sea surface temperature and density structure that is a direct consequence of the structure of the marine atmosphere and the initial structure of ocean boundary layer. Mourad and Brown [1990], who summarized many of the observations of atmospheric rolls, defined two classes of longitudinal perturbations. The first has a single dominant horizontal length scale, which is related to rolls of approximately circular cross-section. The second is made of several perturbations and involves different scales. Thus different features of the marine boundary layer may be identified with different spatial scales. For example, near the surface there may be a strong signal associated with the surface-driven buoyancy flux, whereas at the top of the boundary layer cloud-streets may be associated with wave-wave interactions and mechanisms that couple processes within the inversion layer nonlinearly with energetically favored roll vortices [ Kraus and Businger, 1994].
An ocean eddy may have a distinct surface signal that produces a large discontinuity in the horizontal temperature field. Air flowing across this boundary is rapidly modified, changing the stability of th surface and the momentum transferred to the ocean [ Friehe et al., 1991]. In FASINEX a decrease in the wind stress by a factor of 2 as air flowed over a 2 C surface temperature discontinuity was observed. This resulted in the development of a stable internal boundary layer that effectively decoupled the surface layer from the upper part of the boundary layer restricting the vertical transport of heat and moisture [ Koracin and Rogers, 1990]. Similar features are observed in the coastal ocean where the surface stress may be modulated by gravity waves propagating through a stable internal boundary layer ( Rogers et al. [1995]; for a more detailed discussion of coastal meteorological phenomena see Rogers [1995]). Large coherent structures in the ocean and their atmospheric response have been observed in the tropical Pacific also, where, with light winds, internal waves contribute to structure of the surface temperature field and hence the surface fluxes [ Walsh et al., 1994b; Hagan and Rogers, 1994].
Studies of coastal upwelling phenomena in the California current have described a system of very energetic near surface eddies and jets embedded within a mean southward surface flow. The scales of these features are typically less than 100 km and in some cases near the core of the jets temperature fronts of 1 C in less than 1 km have been observed (see Flament et al., [1995] for a more detailed discussion). From surface wind and simultaneous sonar measurements Smith [1992] has shown the development of Langmuir circulations in the upper ocean in response to a rapid increase in wind speed. The cells grow as the mixed layer deepens in response to wind stirring and associated enhancement of surface cooling.