Spatial Analysis of Abyssal Temperature Variations Observed From the ALOHA Cabled Observatory and WHOTS Moorings

The ALOHA Cabled Observatory (ACO) has measured temperature variations at 4726 m (1.8 mab) in the Kauai Deep since June 2011. Starting in June 2012, temperatures 37 m above bottom were also measured from the Woods Hole-Hawaii Ocean Time-series Site (WHOTS)-9 (-10) moorings approximately 11 km to the east (10 km to the southeast) of the ACO, at depths of 4631 (4720) m. RMS potential temperature (θ) differences between ACO and WHOTS were 4.2 mK over 2 years, significantly greater than the accuracy and precision of the Sea-Bird instruments that were used. From mid-2013 onward, three distinct cold events were observed at each of the sites. Two events in 2014 saw drops in temperature >20 mK over only a few days, followed by large oscillations (10-60 days) of about 10 mK. ACO-WHOTS Δθs were greater than 5 mK on numerous occasions during these events, lasting days to weeks. During the 3-day overlap of the WHOTS-9 and -10 moorings (separated by 12 km meridionally and 89 m vertically), dual temperature sensors on each mooring showed near-bottom θ colder than the ACO by 5-8 mK and 3 mK respectively. ACO/WHOTS-9(-10) Δθs were largest during these events suggesting that relatively small spatial scales are important in the dynamics of the cold events in the Kauai Deep. The correlation of WHOTS-9(-10) abyssal θ with the ACO record was 0.69 (0.66). Cross-spectra of Δθ are distinctly different for the two mooring periods, with squared coherence > 0.8 for periods longer than 114 days during the cold events observed by WHOTS-10. A relative coherence peak near 20 days is found in both cases. Temperature - velocity spectra and cross-spectra also suggest energetic non-tidal dynamics.