Importance of Background Vorticity Effect and Doppler Shift in Defining Near-Inertial Internal Waves

Abstract

Near-inertial waves contain a significant fraction of the ocean's internal wave energy and can propagate long distances from their source before dissipating. However, a varying background flow velocity can alter the wave propagation in two ways. The background vorticity modifies the lower bound of the wave frequency bandwidth while Doppler shifts alter the wave intrinsic frequency. Both effects complicate the identification of the waves and the quantification of their energy content. This study analyses the output of a realistic simulation of the North Pacific using adaptive frequency filters to isolate the effects of vorticity and Doppler shift on the apparent wave energy. Spectral filters neglecting background vorticity effects results in apparent near-inertial energy being underestimated by 40% in anticyclonic structures and overestimated by 100% in cyclonic structures. The asymmetry in energy bias is reinforced when both background vorticity and Doppler shift effects are omitted from the frequency filters.