  Observational evidence is presented for nonlinear interaction between
mesoscale internal Kelvin waves at the tidal -- $\omega_t$ or the inertial --
$\omega_i$ frequency and oscillations of synoptic -- $\Omega $ frequency of the
background coastal current of Japan/East Sea. Enhanced coastal currents at the
sum -- $\omega_+ $ and dif -- $\omega_-$ frequencies: $\omega_\pm
=\omega_{t,i}\pm \Omega$ have properties of propagating Kelvin waves suggesting
permanent energy exchange from the synoptic band to the mesoscale $\omega_\pm $
band. The interaction may be responsible for the greater than predicted
intensification, steepen and break of boundary trapped and equatorially trapped
Kelvin waves, which can affect El Ni\~{n}o. The problem on the parametric
interaction of the nonlinear Kelvin wave at the frequency $\omega $ and the
low-frequency narrow-band nose with representative frequency $\Omega\ll\omega $
is investigated with the theory of nonlinear week dispersion waves.
