Near-Boundary Processes and Their Parameterization

We think of the ocean as being driven by a variety of forces: tides, wind, surface buoyancy fluxes associated with heating, cooling, evaporation, precipitation and ice formation, and geothermal heating. Apart from tidal forces, which act throughout the ocean, geothermal plumes that rise far into the ocean interior, and locations of deep convection, these driving forces communicate their influence to the ocean via boundary layers that are thin compared to the ocean depth. In a sense, therefore, the behavior of the ocean is controlled by what happens in these boundary layers. The response of the boundary layers to forcing sets boundary conditions of fluid injection or removal that force a response in the rest of the ocean. We are familiar, for example, with the role of “Ekman suction” as the means by which the wind drives ocean circulation. Quite apart from these dynamical considerations, we also recognize that gas exchange with the atmosphere can only influence the bulk of the ocean after it has passed through the surface boundary layer. Similarly, sedimentary and biochemical processes at the sea floor involve the bottom boundary layer of the ocean, and only affect the ocean interior through exchange between it and the boundary layer. Quantifying these exchanges clearly requires a thorough understanding of the nature and behavior of the boundary layer, or, to be more general and avoiding for now a definition of what is meant by “boundary layer”, we could refer to the “nearboundary” region. Some of the influence of the driving forces is more subtle in that it involves the radiation into the ocean interior of internal waves that carry with them both momentum and the energy that can lead to turbulence and mixing elsewhere. Quantifying these effects again requires a detailed understanding of the near-boundary region. For example, a very simple model of the energy flux associated with wind-driven internal waves shows that it depends on the thickness of a surface homogeneous layer. The Thirteenth ’Aha Huliko’a Hawaiian Winter Workshop was convened from 21 to 24 January 2003 to review the general theme of “Near-Boundary Processes and Their Parameterization” with the support of the Office of Naval Research and the participation of 25 invited speakers. The emphasis was on the physics of the regions of the ocean near the surface, the bottom, and the sloping sides, with full recognition that predictive models for ocean behavior require that the effects of small-scale, near-boundary processes be “parameterized”, i.e., represented by formulae in terms of model variables rather than just expressed as numbers that might be correct only for present conditions. Large-scale numerical modelers are, of course, trying their best to include more appropriate parameterizations into their models but they face substantial barriers. Their models might not resolve the variables that the parameterizations require. Numerical errors might overwhelm the physical effects. Indeed, well known effects like numerical diffusion, numerical entrainment, and spurious diapycnal mixing become aggravated near boundaries when topographic and isopycnal slopes become large and when boundary layers and overflows become thinner than the typical vertical grid spacing. Considerable work is underway to design numerical algorithms that keep these numerical effects in check and allow physics to rule, but these efforts were only reviewed in passing at the meeting. The meeting also did not deal directly with important issues of sediment transport, gas transfer, or biogeochemical processes, but we hope that the report helps to establish the physical framework within which these issues are considered. In fact, we did not even focus extensively on the details of the boundary layers themselves. Instead, emphasis was placed on interaction with the ocean interior, recognizing that this is a two-way process: near-boundary regions influence the ocean interior but are also themselves affected by the properties of the interior. The report that follows only gives a broad summary of the meeting and a few references. More details and references can be found in the workshop proceedings which are available from http://www.soest.hawaii.edu/ PubServices/AhaWebPage.html.