Moored observations of episodic abyssal flow and mixing at station ALOHA

Abstract

Moored measurements of abyssal velocity and temperature are presented with a focus on episodic cold overflow events first observed by Lukas et al. (2001) in the Hawaii Ocean Time‐series (HOT), a 23‐year‐long time series of ≈ monthly CTD profiles at station ALOHA (22.75°N, 158°W). Three major cold events were observed in our 2.5‐year record, of which we present one in detail. The event appeared in two pulses spaced by about two weeks, wherein potential temperature anomaly was <−0.015°C over the bottom 600 m. Flow was about 10 cms−1 to the southwest, confirming earlier interpretations of the events as overflows from the Maui deep to the east. Between the two pulses, flow veered to the northwest, possibly associated with seiching. Speed decreased rapidly below the sill depth (≈4625) m, suggesting sheltering by the basin walls. The associated shear, even smoothed over 200 m and not including internal waves, was nearly unstable to Kelvin‐Helmholtz instability. During this period, a large mixed region was observed wherein the lower 240 m was homogenized, remaining so for 14 hours (1.2 buoyancy periods). From Thorpe scale analysis, the implied diffusivity of the event was (0.5–4.5) × 10−1m2s−1. No other overturning events greater than 50 m high were observed in the record, suggesting that abyssal mixing is strongly intermittent. We suggest that such intermittency in abyssal mixing and flow is likely the rule rather than the exception, calling for more highly temporally resolved observations.