Viability of the mixed layer duct with observed dynamics

Abstract

While the upper ocean of the north Pacific is largely mixed due to air-sea interaction, a 1000 km sea-soar transect shows the existence of stratification in the mixed layer that persists over many tens of kilometers. The effects of this stratification on mixed layer acoustic propagation depends on the sound speed differences across the different density layers, and observations show the sound speed varies rapidly both between and along the isopycnals. Acoustic models are used to separately quantify the importance of the mixed layer stratification and the density compensated sound speed variability on propagation in the mixed layer duct. The observed sound speed field is modeled as the superposition of three fields: a smooth background, the stratification field with slow isopycnal sound speed variability, and the remnant unstratified sound speed variation. The sound speed variation in the stratified mixed layer is attributed to internal waves and eddy filaments and in the unstratified field to ocean spice. Both types of observed sound speed variation significantly change, or completely block, mixed layer propagation. Examples are presented of blocking features in the tilt or spice fields separately, and where they exist only in the superposition of the two fields.