Impacts of ocean model vertical resolution on acoustic propagation predictions in a stratified ocean
Eddies and fronts drive the vertical structure and variability of the upper ocean at meso-and submesoscales. Associated vertical distributions of sound speed in the mixed layer and pycnocline modulate acoustic energy propagation through the stratified ocean. While modern ocean models, including HYCOM and NCOM, have increasingly high horizontal resolution and fine vertical resolution within the upper hundred meters, insufficient vertical resolution of subsurface features within the pycnocline can result in unrealistic predictions of acoustic propagation. This study examined acoustic implications of ocean model limitations in reproducing the vertical structure of the South Cyprus Eddy, a semi-permanent mesoscale feature in the eastern Mediterranean. In situ observations were used to characterize the eddy vertical structure during several winter-spring transitions. Observed mixed layer properties and resulting acoustic structures were compared to HYCOM and three configurations of NCOM with varying horizontal and vertical resolutions. Our results suggest limited model ability to resolve the observed double-core structure of subsurface anticyclonic eddies and mixed layer depth in surface-intensified mesoscale features. These deficiencies resulted in misrepresentation of secondary sound channels and underestimation of sonic layer depth. The study has demonstrated the importance of resolving subsurface mesoscale structure for reliable prediction of acoustic propagation through the stratified ocean.