A Didactic Approach to Linear Waves in the Ocean
  The general equations of motion for ocean dynamics are presented and the waves supported by the (inviscid, unforced) linearized system with respect to a state of rest are derived. The linearized dynamics sustains one zero frequency mode (called buoyancy mode) in which salinity and temperature rearrange in such a way that seawater density does not change. Five nonzero frequency modes (two acoustic modes, two inertia--gravity or Poincar\'{e} modes, and one planetary or Rossby mode) are also sustained by the linearized dynamics, which satisfy an asymptotic general dispersion relation. The most usual approximations made in physical oceanography (namely incompressibility, Boussinesq, hydrostatic, and quasigeostrophic) are also consider, and their implications in the reduction of degrees of freedom (number of independent dynamical fields or prognostic equations) of, and compatible waves with, the linearized governing equations are particularly discussed and emphasized. 
F. J. Beron-Vera