The Deep Western Boundary Current of the Southwest Pacific Basin: Insights From Deep Argo

The deep western boundary current (DWBC) of the Southwest Pacific Basin (SWPB) is the main pathway through which the deep and bottom waters formed around Antarctica are transported northward and distributed throughout the Pacific Ocean. However, historical observations of this current are sparse. Here, we used an unprecedented number of deep-ocean observations collected by Deep Argo floats since 2016 to examine temperature, salinity, and velocity in the DWBC of the SWPB. Deep Argo trajectory velocities were fastest along the western side of the Kermadec Trench, with an average velocity of $0.057\pm 0.012$ m s⁻¹. Trajectories confirmed the existence of a tight recirculation on the eastern side of the Kermadec Trench ($-0.021\pm 0.008$ m s⁻¹). This recirculation was likewise seen in an independent eddy-resolving ocean reanalysis. For the DWBC within the northern Kermadec Trench (26–30°S), Deep Argo profiles and the ocean reanalysis demonstrated seasonal isopycnal heaving of the deep-ocean that was likely driven by local Ekman pumping and may influence seasonal DWBC transport. At the northern end of the Kermadec Trench, the deep-ocean salinity maximum was eroded as the DWBC exited the trench to the north through the Louisville Seamount Chain collision zone, thus revealing a previously unidentified region of enhanced deep-ocean mixing. Although Deep Argo observations accurately estimated vertical turbulent diffusivity in the Samoan Passage (6.1 $\times$ 10⁻³ to 1.57 $\times$ 10⁻² m² s⁻¹), mixing within the Louisville Seamount Chain collision zone was not due solely to vertical turbulent diffusivity. A global Deep Argo array could reveal wind-driven seasonal heaving and unexplored deep-ocean mixing hotspots in other DWBCs.