Observed energy and momentum budget of a hydraulically controlled dense overflow in the Samoan Passage

The flow of dense bottom waters of Antarctic and North Atlantic origin through the Samoan Passage is strongly modified by turbulent mixing as it passes over several topographic constrictions within the Passage. Here we present recent highly-resolved towed and moored measurements of the flow over one of its major sills. The sill is tall enough that the topographic Froude Number $N h_m/U_0\approx1$, with bouyancy frequency $N$, sill height $h_m$ and upstream velocity $U_0$, such that a hydraulic jump downstream is expected and indeed observed. Turbulent dissipation downstream of the sill and within the hydraulic jump reaches $10^{-5}$~W/kg. The sill exerts considerable form drag on the flow as expressed by a pressure drop in the flow over the sill. The form drag per unit length of the sill is about $10^5$~N/m and power loss due to form drag is above 1~W/m$^2$. Moored time series reveal tidal modulation of the hydraulically controlled flow and associated turbulent mixing. Current work is investigating the generation of lee waves by interaction of the mean flow with the sill.