Challenges of Measuring Abyssal Temperature and Salinity at the Kuroshio Extension Observatory

Nathan D Anderson,Dongxiao Zhang,Meghan F Cronin,Kathleen A Donohue,Makio C Honda

doi:10.1175/jtech-d-19-0153.1

Abstract

AbstractThe deep ocean is severely undersampled. Whereas shipboard measurements provide irregular spatial and temporal records, moored records establish deep ocean high-resolution time series, but only at limited locations. Here, highlights and challenges of measuring abyssal temperature and salinity on the Kuroshio Extension Observatory (KEO) mooring (32.3°N, 144.6°E) from 2013 to 2019 are described. Using alternating SeaBird 37-SMP instruments on annual deployments, an apparent fresh drift of 0.03–0.06 psu was observed, with each newly deployed sensor returning to historical norms near 34.685 psu. Recurrent salinity discontinuities were pronounced between the termination of each deployment and the initiation of the next, yet consistent pre- and postdeployment calibrations suggested the freshening was “real.” Because abyssal salinities do not vary by 0.03–0.06 psu between deployment locations, the contradictory salinities during mooring overlap pointed toward a sensor issue that self-corrects prior to postcalibration. A persistent nepheloid layer, unique to KEO and characterized by murky, sediment-filled water, is likely responsible for sediment accretion in the conductivity cell. As sediment (or biofouling) increasingly clogs the instrument, salinity drifts toward a fresh bias. During ascent, the cell is flushed, clearing the clogged instrument. In contrast to salinity, deep ocean temperatures appear to increase from 2013 to 2017 by 0.0059°C, whereas a comparison with historical deep temperature measurements does not support a secular temperature increase in the region. It is suggested that decadal or interannual variability associated with the Kuroshio Extension may have an imprint on deep temperatures. Recommendations are discussed for future abyssal temperature and salinity measurements.

Highlights

Even as scientific curiosity about the oceans and observing technology expands, the deep ocean remains undersampled compared to the upper ocean
Mean u6000 from the three Kuroshio Extension Observatory (KEO)-proximity Kuroshio Extension System Study (KESS) CTDs is 0.01208C warmer than predicted from the linear fit to the KEO data (0.01468, 0.01048, and 0.01098C warmer for April 2004, May 2004, and June 2005 casts, respectively, corresponding to 4.1, 2.9, and 3.1 times the standard deviation of the detrended u6000 time series), further suggesting that extrapolation of the linear regression back to the time frame of KESS is unrepresentative of temperatures at that time
Warming trends are beginning to emerge in the deep ocean (Purkey and Johnson 2010) and in Western boundary currents (WBCs) regions, where the deep ocean can be very active with strong currents and benthic storms (Zhang et al 2018)

Summary

Introduction

Even as scientific curiosity about the oceans and observing technology expands, the deep ocean remains undersampled compared to the upper ocean. Compared to the temperature trend and salinity drift anomaly presented variations in mooring depths (see pressure in Fig. 6d) are insignificant in the context of vanishingly small vertical gradients.

Results

Conclusion