Cold bottom water events observed in the Hawaii Ocean Time-series: implications for vertical mixing

During 1991–1999 six near-bottom, cold water anomaly events occurred at the Hawaii Ocean Time-series deep-ocean hydrographic station (22°45′N, 158°W), situated on the northern flank of a seafloor basin known as the Kauai Deep. Cold anomalies of at least 0.01°C, and as much as 0.024°C, were observed in near-bottom CTD profiles during each event. The anomalies relaxed towards more typical conditions over a period of several months. On the basis of water mass properties, we infer that the cold anomalies in the Kauai Deep originated when colder, saltier water from the adjacent Maui Deep spilled over the sill that separates these abyssal basins. The cause of these cold overflow events is not yet known, but inverted echo sounder observations and TOPEX/Poseidon altimeter data suggest that they might be associated with eddies entering the region of the Maui Deep. The transport across the sill and the vertical structure of the eddy diffusivity parameter were estimated with a one-dimensional advection–diffusion model, considering the evolving shape of the anomalous potential temperature profile during the third event onset and the apparent overflow duration. The best subjective fit to the observations yields a transport across the sill of 0.18 Sv, and a constant vertical eddy diffusivity of 7×10−4 m2 s−1. The observations during the recovery of the anomalous temperatures for four of the events were used to calculate the terms of a one-dimensional diffusion model, yielding the vertical and temporal variability of the turbulent diffusivity. The results show a consistent picture of the eddy diffusivity reaching median values of 40×10−4 m2 s−1 near the region of the sill depth and decreasing above and below it. This maximum increase seems to be due to enhanced levels of turbulence near the depth of the controlling sill. The diffusivities also peak at the beginning of the relaxation period, decreasing by one order of magnitude by the end of each event. The fifth eventʹs overflow was apparently captured in progress with two full-depth CTD profiles obtained 46 h apart, during which the near-bottom temperature decreased 0.035°C, the salinity increased 0.003, and the dissolved oxygen increased 7 μmol kg−1. A seiching motion or deep baroclinic basin mode resulting from the event could explain this and other large differences in near-bottom temperatures observed between deep casts taken a few days apart.