Nonlinear internal waves forced by tides near the critical latitude

Abstract

Generation of semi-diurnal internal tides at high latitudes is investigated theoretically and on the basis of in situ data, collected in the Barents Sea. The experimental data had revealed evidence of semi-diurnal internal tides in records obtained at the critical latitude and above it, whilst the existing theories of baroclinic tides (linear or nonlinear) predict a suppression of tidal activity in the vicinity of the critical latitudes. The analytical solution for linear internal tides excited over the oceanic ridges predicts a substantial reduction of the efficiency of their generation. A similar result was obtained also in the frame of a fully nonlinear, nonhydrostatic model, developed and applied for typical conditions of the Barents Sea: weakly nonlinear baroclinic tides at high latitudes are suppressed by the Earth's rotation and have characteristics inherent to linear tides (small amplitude, large wavelength and small group speed). However, when applied to conditions of the southern slope of the Spitsbergen Bank, where the nonlinearity of the generated waves was estimated to be sufficiently strong, the mathematical model demonstrated generation of topography-scale short internal waves with characteristics close to those of unsteady lee waves. Propagating away from their source these waves produce wave patterns which may be considered as nonlinear internal tides of relatively short wavelength, although their genesis and features significantly differ from common internal tides. In particular, under conditions of strong nonlinearity and rotation the energy transfer from the barotropic tidal flux to the baroclinic modes leads to an effective generation of multiple tidal harmonics.