HOT-75 COMMEMORATIVE SCIENCE SYMPOSIUM

Cold Bottom Water Events Observed in the Hawaii Ocean Time-Series: Modelling and Implications for Vertical Mixing

Fernando Santiago-Mandujano and Roger Lukas

School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, HI 96822

During early 1991, late 1993 and 1995, three bottom water events occurred at the site of the Hawaii Ocean Time-series (HOT; 22 45'N, 158 W); cold anomalies of at least 0.024 C were observed in near-bottom CTD profiles following each event, taking several months to develop. The anomalies relaxed towards normal over a period of several months. The cause of these ventilations of the Kauai Deep is not known.

It is presumed that the cold anomalies in the Kauai Deep originated from the overflow of colder water from the adjacent Maui Deep, through the controlling sill that separates them. A one-dimensional advection-diffusion model was used to estimate the upwelling velocity and the vertical structure of the eddy diffusivity parameter. The best subjective fit to the observations (the one that reproduced best the magnitude and shape of the observed profiles) during the onset of the third event was with a constant eddy difftisivity of 5 cm²/s , and a transport across the sill of 0.15 Sv, yielding a mean upwelling of 1. 6 x 10^-4 m/s in the basin.

The relaxation of the temperature field after the second event was modeled with a one- dimensional diffusion model. The shape of the distorted potential temperature profile, along with the relaxation time to the "typical" mean profile, was used to estimate the vertical structure of the eddy diffusivity parameter. The best subjective fit to the observations was with a constant 6 cm²/s below 4700 dbar and 12 cm²/s above. This increase away from the bottom seems to be due to enhanced levels of turbulence near the depth of the controlling sill in the Oahu Seamounts separating the Kauai Deep from the Maui Deep.