Flow and Mixing around a Small Seamount on Kaena Ridge, Hawaii

Abstract

Microstructure observations over a small seamount on the Kaena Ridge, Hawaii, showed asymmetry in the along- and across-ridge directions. The ∼400-m-high seamount is on the southern edge of the ridge (centered at 21°43′49″N, 158°38′48″W), 42 km northwest of Oahu. A 1-km-resolution numerical simulation shows that the flow within the depth range of the seamount tends to be accelerated around the seamount rather than going up and over it. The flow patterns, however, are more complicated than for an isolated seamount because of the influence of the ∼3000-m-high Kaena Ridge. Comparison with the numerical simulations indicates that the across-ridge asymmetry, in which dissipation on the north-northeastern side of the seamount was higher and more concentrated toward the bed than on the south-southwestern side, is consistent with an M2 tidal beam generated at the northern edge of the ridge. The along-ridge asymmetry, with higher dissipation on the east-southeastern flank than on the west-northwestern flank, is in qualitative agreement with M2 shear variance from the model simulation. The average observed dissipation rate over the seamount was ɛ = 6.2 × 10−8 W kg−1, and diapycnal diffusivity was Kρ = 1.3 × 10−3 m2 s−1. Dissipation measurements following the 1000-m isobath south-southwest of the seamount suggest along-ridge internal tide generation caused by topographic steering that creates an along-ridge current over critical topography northwest of the seamount.