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Abstract 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 x 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 x 10-4 m2 s-1 near the region
of the sill depth and decreasing above and below it. This 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 at the end of each event.
The fifth event's overflow was apparently captured in progress with two
full-depth CTD profiles obtained 46 hours apart, during which the near-bottom
temperature decreased 0.035 °C, the salinity increased 0.003, and the
dissolved oxygen increased 7 umol 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.
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