Gunnar Voet (University of Washington Applied Physics Laboratory)
"Pathways, Volume Transport and Mixing of Abyssal Water in the Samoan Passage"
The flow of dense water through the Samoan Passage accounts for the majority of the bottom water renewal in the North Pacific, thereby making it an important element of the Meridional Overturning Circulation. Spatially highly resolved measurements, obtained using a combined setup of shipboard CTD and LADCP and autonomous untethered microstructure measurements and accompanied by moorings deployed for the duration of the ship survey, show the complex pathways and high variability on various time scales of the abyssal flow through the Passage. Volume transport estimates for the dense northward current at several sections across the Passage, calculated using direct velocity measurements from a Lowered Acoustic Doppler Current Profiler, range from 3.9 to 6.0 Sv. To close the heat budget for the coldest waters along the Passage, strong mixing with vertical diffusivites of O(10^-2) m^2/s is inferred. Results from an advection-diffusion model support these high levels of vertical diffusivity. Average vertical diffusivities derived from finestructure measurements via Thorpe scales, validated against microstructure measurements from a Vertical Microstructure Profiler, are smaller by a factor of about two to five in these bottom layers.