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A winning proposal for the Innovative Research Program, 2008: Windrows in Global Models: How Much Do Langmuir Circulations Matter for Climate?Investigators: Baylor Fox-Kemper (Cooperative Institute for Research in the Environmental Sciences, University of Colorado at Boulder, bfk@colorado.edu), with: Keith Julien (Department of Applied Mathematics, University of Colorado at Boulder), Gregory P. Chini (Department of Mechanical Engineering, University of New Hampshire), and Edgar Knobloch (Department of Physics, University of California at Berkeley) Abstract: The research proposed here is an estimation of the effects of the Langmuir Cells-small wind and wave-driven overturning cells in the near-surface ocean–in the context of global climate models. The goals of the research are implementation of a rough parameterization of Langmuir Cells into the ocean component of the NCAR Community Climate System Model and documentation of the climate impact versus a control run. Including this effect will quantify a novel climate sensitivity, foster interdisciplinary cooperation, and provide a framework for bringing new mathematics into practical use in climate models. Background: Langmuir cells are one of the most recognizable near-surface mixing processes in the ocean. The long, parallel windrows of surfactants and bubbles that form along the direction of the wind at the convergences of the Langmuir cells (Fig. 1) are well-known to beach-goers. Since the early description of this phenomenon (Langmuir, 1938), and its mathematical underpinnings (Craik, 1977; Leibovich, 1977), much progress in modeling (e.g., McWilliams et al., 1997) and theory (e.g., Chini, 2008; Chini et al., 2008) has been made. It is obvious from the windrows themselves that the transport of tracers and water due to Langmuir cells can dominate other near-surface processes at times, and this transport has been documented to overturn the entire mixed layer in a matter of hours (e.g., Smith, 1998).Langmuir cells are an intriguing phenomenon in themselves, but it is their impact on climate that is the issue to be raised here, a largely unexplored topic. Recently, other small-scale, near-surface phenomena have been shown to have substantial climate impact (e.g., submesoscale eddies: Fox-Kemper and Ferrari, 2008; Fox-Kemper et al., 2008). The climate impact derives from the important role of the near-surface ocean (often called the mixed layer) acting as a alter for the air-sea exchange of heat, momentum, and gases. Furthermore, these near surface phenomena should have a large effect on biology and the carbon cycle, as the euphotic zone of the ocean—where there is enough light for photosynthesis–exists mostly within the mixed layer (G. McKinley, M. Follows, pers. comm.). Near-surface phenomena often act in concert with treatments of fine-scale boundary layer turbulence that mixes the mixed layer (e.g., Large et al., 1994), yet they are sensitive to quite different parameters than traditional boundary layer turbulence. For example, submesoscale eddies are sensitive to horizontal density gradients and mixed layer depth, and so they are selectively active throughout the world (Fox-Kemper and Ferrari, 2008; Fox-Kemper et al., 2008). Langmuir cells are sensitive to both wind stress and wave strength. Boundary layer turbulence parameterizations do depend on wind stress, but in a different way than Langmuir cells, and the effects of surface wave magnitude are neglected altogether in climate models. Regions with both significcant wave activity and strong winds are often where air-sea contact is important, e.g., Mode/Deep water formation sites, the Southern Ocean, and tropical upwelling regions. 
Figure 1: Images of Langmuir circulation windrows: (a) a photograph of Rodeo Lagoon in CA (from Szeri, 1996), (b) an infrared image of the surface of Tampa Bay (courtesy of G. Marmorino, NRL, D.C.), and (c) the evolution of surface tracers in a LES of Langmuir turbulence (McWilliams et al., 1997). Reproduced from Chini et al. (2008). Proposed Research: The goal here is to implement the simplest form of Langmuir circulation into a global ocean model and document the changes that result. Promise for more accurate parameterizations of the Langmuir circulation derive from recent mathematical progress made by the co-investigators (Chini, 2008; Chini et al., 2008). However, before spending time on a complex model it is important to determine the likely magnitude of climate impact. The research plan is:
It should be possible to complete this project within a short time frame, with preliminary results by the end of summer, 2008. Summary: An estimation of the effects of Langmuir circulation on global climate is proposed. If sensitivity is found in regions of climatic importance, it will support continuation of a highly interdisciplinary collaboration (four specialities are represented by the investigators), and bring developments at the cutting edge of applied mathematics into practical use in the world of global climate modeling. A novel climate sensitivity with potentially substantial impact will be documented, and the groundwork for implementation of a full parameterization will be laid. References: |
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