Normal-Mode Decomposition of Small-Scale Oceanic Motions

Abstract

Abstract Small-scale oceanic motions consist of vortical motion and internal waves. In a linear or weakly nonlinear system these two types of motions can be unambiguously separated using normal-mode decomposition in which the vortical mode carries the linear perturbation potential vorticity, whereas the gravity mode does not. Normal-mode decomposition can be easily achieved using the fields of horizontal divergence, relative vorticity, and vortex stretching. An attempt to estimate these three fields is made using the Internal Wave Experiment (IWEX) measurements. Estimates of horizontal divergence and relative vorticity using the three-point array are attenuated at horizontal scales smaller than the size of the array and mutually contaminated at the horizontal separation scale of the sensors. Estimates of vortex stretching using vertically separated vertical displacement measurements are also attenuated at small vertical scales. The observed frequency spectra represent oceanic wavenumber frequency spectra ...