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 degrees 45'N, 158 degreesW), situated on the northern flank of a seafloor basin known as the Kauai Deep. Cold anomalies of at least 0.01 degreesC, and as much as 0 .024 degreesC, were observed in near-bottom CTD profiles during each event. The anomalies relaxed towards more typical conditions over a period of sev eral 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 Maul Deep spilled over the sill that separates these abyssal basin s. The cause of these cold overflow events is not yet known, but inverted e cho sounder observations and TOPEX/Poseidon altimeter data suggest that the y might be associated with eddies entering the region of the Maui Deep. The transport across the sill and the vertical structure of the eddy diffusivi ty parameter were estimated with a one-dimensional advection-diffusion mode l, considering the evolving shape of the anomalous potential temperature pr ofile 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) m(2) 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 di ffusion model, yielding the vertical and temporal variability of the turbul ent diffusivity. The results show a consistent picture of the eddy diffusiv ity reaching median values of 40 x 10(-4) m(2) 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 controllin g sill. The diffusivities also peak at the beginning of the relaxation peri od, decreasing by one order of magnitude by the end of each event. The fift h event's 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 degreesC, the salinity increased 0.003, and the dissolved o xygen increased 7 mu mol kg(-1) A seiching motion or deep baroclinic basin mode resulting from the event could explain this and other large difference s in near-bottom temperatures observed between deep casts taken a few days apart. (C) 2001 Elsevier Science Ltd. All rights reserved.