The vertical propagation of inertial waves in the ocean

Abstract

A set of vertical profiles of horizontal ocean currents, obtained by electromagnetic profilers in the Atlantic Ocean southwest of Bermuda in the spring of 1973, has been analyzed in order to study the vertical structure and temporal behavior of inte'rnal waves, particularly those with periods near the local inertial period. An important feature of the observed structure is the pol ari zati on of horizontal velocity components in the vertical. This polarization, along with temporal changes of the vertical wave structure seen in a time series of profiles made at one location, has been related to the di-recti on of verti ca 1 energy fl ux due to the observed waves. Whereas the observed vertical phase propagation can be affected by horizontal advecti on of waves past the point of observation, the use of, wave polarization to infer the direction of vertical energy propagation has the advantage that it is not influenced by horizontal advection. The result shows that at a location where profiles were obtained over smooth topography, the net energy fl ux was downward, indi cati ng that the energy sources for these waves were located at or near the sea surface. An estimate of the net, downward energy flux (-.2-.3 erg/cm2/sec) has been obtained. Calculations have been made which show that a frictional bottom boundary 1 ayer can be an important energy sink for near-inerti al waves. A rough estimate suggests that the observed, net, downward energy fl ux coul d be accounted for by energy losses in this frictional boundary layer. A reflection coefficient for the observed waves as they reflect off the bottom has been estimated. In contrast, some profiles made over a region of rough topography indicate that the rough bottom may also be acting to generate near-inertial waves which propagate energy upward. 'Ca1culations of vertical flux of horizontal kinetic energy, using an empirical form for the energy spectrum of internal waves, show that thi s vert i ca 1 fl ux reaches a maximum for frequenci es 10%-20% greater than the local inertial frequency. Comparison with pro-filer velocity data and frequency spectra supports the conclusion that the dominant waves had frequencies 10%-20% greater than the inertial frequency. The fact that the waves were propagating energy in the verti ca 1 is proposed as the reason for the observed frequency sh ift.