‘Contamination’ of the Pelagic Linear Barotropic Tides by Nonlinear Barotropic Tides

More accurate representations of the gravitationally-forced pelagic tides are being demanded by both ocean and earth scientists. Improved accuracy will require, among other things, a better understanding of the complexities of the interactions between the pelagic and coastal tides. It has only recently been demonstrated (e.g., Ray, 2007, Geophys. Res. Lett.) that nonlinear interactions among the tide constituents in the shallow seas and on continental shelves (which are all principally forced by the pelagic linear barotropic tides) can result in a flow of nonlinear barotropic tide energy back into the open ocean. The impact of this energy flux is potentially significant not only for modeling the diurnal and semi-diurnal tides themselves, but for the energetics of phenomena dependent on the tides, such as abyssal mixing. Consequently, many avenues for research are suggested. 

The question discussed here is “how much do the coastal nonlinear barotropic tides ‘contaminate’ what is generally considered to be the linear barotropic diurnal and semi-diurnal tides in the pelagic ocean?” The answer provides an explanation for the origin of the ‘tidal jitter’ phenomenon first described in Honolulu, Hawaii, tidal admittance functions by Munk and Cartwright (1966, Phil. Trans. Roy. Soc., London). Using the burgeoning set of bottom pressure data from the North Pacific, evidence will be presented showing that admittance function extrapolations beyond the eight major tidal constituents produce inaccurate estimates of the observed lesser constituents due to the presence of additional deterministic signals that are probably nonlinear barotropic tides. The spatial structures of the amplitudes and phases of the nonlinear components in several tide constituents will be presented where the bottom pressure data is sufficiently dense. The nonlinear components can be as large as the linear components. The probable coastal sources of the nonlinear tides will be discussed.