Generation of unbalanced motion at horizontal boundaries in the atmosphere and the oceans

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The dynamics of the atmosphere and oceans at mid-latitudes is well described by focusing on the slow, large-scale motion, in nearly hydrostatic and geostrophic balance. However, fast small-scale motion in the form of gravity waves and associated instabilities plays a crucial role in a number of processes (such as momentum transport, turbulence, vertical mixing, and dissipation), which impact on large-scale circulations, middle-atmospheric circulation and ocean thermohaline circulation in particular. The mechanisms of generation of the fast, unbalanced motion have therefore received a great deal of attention in recent years. However, the so-called spontaneous-generation mechanisms, whereby the evolving hydrostatically and geostrophically balanced motion directly excites unbalanced motion, remain poorly understood, in spite of their importance. One difficutly is that these mechanisms are only efficient when the balanced motion evolves sufficiently rapidly, and that such a rapid evolution is rather atypical in the bulk of the atmosphere and oceans. Near horizontal boundaries, however, a rapid evolution is typical. By horizontal boundaries, we refer not only to the ocean surface and solid earth, but also to the tropopause, which plays a similar role. Not coincidentally, these are precisely the regions where strong unbalanced activity is observed. These also are regions where unbalanced motion has crucial consequences, the effect of turbulence on airplanes being the most obvious, if not the most important. The proposed research will examine how the generation of unbalanced motion at horizontal boundaries takes place. This will be done by analysing simple models which isolate the key feature of the boundary dynamics that is reponsible for unbalanced-motion generation, namely the formation of very active structures in the surface temperature, with both small spatial scales and short temporal scales. This feature has been well demonstrated in the context of the (balanced) surface quasi-geostrophic model, but its impact on unbalanced motion remains to be described. The proposed research will carry out this task for the first time. It will consider not only situations of frontal collapse but also the more complex cascade of small-scale instabilities which characterise surface evolution and leads to a form of turbulence. The aim is to relate the characteristics of the unbalanced motion to those of the surface fields with, as ultimate goal, the parameterisation of its impact on mixing and dissipation.