R. Lukas et al., Cold bottom water events observed in the Hawaii Ocean Time-series: implications for vertical mixing, DEEP-SEA I, 48(4), 2001, pp. 995-1021
Citations number
30
Categorie Soggetti
Aquatic Sciences","Earth Sciences
Journal title
DEEP-SEA RESEARCH PART I-OCEANOGRAPHIC RESEARCH PAPERS
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.