Internal wave observations in the South China sea: The role of rotation and non-linearity

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Observations of internal waves travelling across the deep basin of the South China Sea provide an opportunity for exploring the effects of rotation and non-linearity on their evolution. Time series measurements using inverted echo-sounders at three locations illustrate the progressive steepening of the internal tide generated in Luzon Strait and the subsequent development of short non-linear internal wave trains. Potential mechanisms for internal tide generation are discussed in terms of tidal beam interaction with near-surface stratification and mode 1 response to flow over a ridge. For transformation of an internal tide under the influence of non-linearity and rotation, we apply Boyd's (2005) criterion for wave stability in a rotating flow to separate waves dominated by non-linearity, which can be expected to steepen and break, from waves that are inhibited from breaking due to rotational dispersion of energy into internal inertial gravity waves. Wave breaking in this context refers to the point at which the wave becomes steep enough for non-hydrostatic effects to come into play, with the subsequent generation of a short period non-linear internal wave train. For a monotonic M2 internal tide generated over the eastern ridge, breaking is predicted close to the location at which remotely sensed images first indicate the presence of short non-linear internal waves; steepening of a K1 internal tide, on the other hand, is predicted to be dispersed by rotational effects before steepening. Time series observations acquired just west of the eastern ridge provide initial conditions for applying the stability criterion to mixed internal tides over a spring-neap cycle and for comparison with time series measurements at two additional sites further west. When the stability criterion is applied to wave slopes, initial conditions predicted to be unstable generally result in the formation of high frequency non-linear internal wave trains. In the case of initial conditions for which rapid steepening is not predicted we see evidence of the interaction between rotation and non-linearity, specifically the formation of 'corner waves' with the characteristic parabolic shape first predicted by Ostrovsky (1978).

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Atmosphere - Ocean