Low-Frequency Modulation of Turbulent Diapycnal Mixing by Anticyclonic Eddies Inferred from the HOT Time Series

Profiles of potential density obtained from CTD measurements during the Hawaii Ocean Time series (HOT) program in the vicinity of the island of Oahu, Hawaii, are used to evaluate low-frequency variability of turbulent kinetic dissipation rates based on a finescale parameterization method. A distinct seasonal cycle, as well as an increasing trend of dissipation rates, is found in the upper 300–600 m. The trend is mainly due to the much weaker diapycnal mixing in the first four years of the record, that is, 1988–92. In the upper 300–600 m, enhanced diapycnal mixing is found under anticyclonic eddies with the mean dissipation rate about 53% larger than that under eddy-free conditions. The modulation of dissipation rates by anticyclonic eddies becomes more evident with increasing eddy strength. The role of cyclonic eddies in modulating diapycnal mixing is almost negligible compared with that of anticyclonic eddies. The mean dissipation rate under cyclonic eddies is comparable to that under eddy-free conditions with a difference of less than 10%. Seasonality of the dissipation rates is partly modulated by the seasonal variation of anticyclonic eddies.