North Atlantic upper ocean sound channel variations

Historical profile observations characterize sound channel distributions across the North Atlantic Ocean and their relationships to water masses as well as support assessment of numerical ocean model performance. The focus of this study is sound channels above 500 m depth, below the sonic layer, and having a cutoff frequency of less than 200 Hz. Sound channels occur frequently around the Rockall Trough, the Reykjanes Ridge, the Labrador Current off the Grand Banks, and the Gulf Stream. Seasonal effects result in higher occurrences in spring and summer when the sonic layer is shallower. Water mass properties at sound channel axes exhibit lower salinity than at channel tops and bottoms on average, which implies cold, low salinity surface waters have been subducted into the water column. As ocean numerical model resolution is increased vertically and horizontally, model results are more consistent with observed sound channel occurrence rates as well as water mass properties. In addition to low salinity waters laterally interleaving with warmer, more saline waters of equal density, several ocean processes can move relatively cold low salinity surface waters to the subsurface including subduction from surface fronts along isopycnals and winter convection. Model deficiencies are considered relative to water mass distributions and possible physical processes.