Subtropical Mode Water as an acoustic waveguide at the Gulf Stream

In this work, we describe the distribution and variation of subsurface acoustic ducts formed by North Atlantic Subtropical Mode Water in the region where the Gulf Stream encounters the New England Seamount chain. High-resolution observations were obtained during two cruises (November 2022 and May/June 2023) from ships and teams of up to seven gliders as part of the Predictions of AcousticS with Smart Experimental Networks of GlidERS (PASSENGERS) project. Low stratification mode water layers of varying thickness were widely observed from gliders, shipboard CTD, and towed underway CTD casts. The cruises bracket the wintertime formation of the mode waters and are located near the region of maximum production. Variation in the mode water properties on the scale of the local baroclinic Rossby radius was observed resulting from an eddy field associated with the southern flank of the Gulf Stream. A strong streamwise/spanwise asymmetry in the ducts exists in the region. The seasonal production and erosion of mode water south of the Gulf Stream extension imprints on the subsurface duct, with thicker ducts and lower sound speed minima in the spring relative to the fall.