Paul Williams (1), Thomas Haine (2) and Peter Read (3)
(1) Department of Meteorology, University of Reading, UK (p.d.williams@reading.ac.uk) (2) Department of Earth and Planetary Sciences, Johns Hopkins University, USA (3) Atmospheric, Oceanic and Planetary Physics, Oxford University, UK
In a laboratory study using a rotating two-layer annulus, we find that all evolving large-scale flows emit inertia-gravity waves spontaneously, as proposed by Ford, McIntyre & Norton (2000). The appearance of the waves is well-predicted by the radiation term derived by Ford (1994), following Lighthill (1952).
Two important issues arise from this study. First, the amplitude of the laboratory inertia-gravity waves displays a roughly linear variation with Rossby number (Ro) over nearly one decade in Ro. This finding is in disagreement with some dynamical theories and needs to be better understood. For example, exponential asymptotic theories predict that the amplitude should decay exponentially with decreasing Ro. A companion paper by Ring, Haine & Eyink further explores possible explanations for this linear scaling in Ro.
Second, we estimate that the balanced large-scale waves in the laboratory lose roughly one per cent of their energy per rotation period into the inertia-gravity waves. Extrapolation of this result suggests that the spontaneous-emission mechanism might make a significant contribution to the energy budgets of the ocean and atmosphere. For example, we crudely estimate that O(1TW) is being lost from balanced mesoscale ocean eddies into the internal wave field, suggesting that this mechanism might be a significant player in maintaining the deep ocean stratification.