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Hiroshi Uchida, Takeshi Kawano, and Masao Fukasawa

Abstract

To monitor changes in heat content and geostrophic volume transport of abyssal water accurately, 50 moored conductivity–temperature–depth (CTD) recorders used for density measurements were calibrated in situ by simultaneous observations with accurate shipboard CTDs. Comparisons of the data from the moored and shipboard CTDs showed pressure sensitivities of 0–3 mK at 6000 dbar for the temperature sensors of the moored CTDs. From the in situ calibrations, the uncertainties of the moored CTD data for the deep ocean (≥3000 dbar) were estimated to be 0.6 dbar, 0.6 mK, and 0.0026 for pressure, temperature, and salinity, respectively, relative to the shipboard CTD reference. Time drifts of the moored CTD data, estimated from the in situ calibrations before and after 17- or 14-month mooring deployments in the deep ocean, were considerably smaller than typical stabilities as specified by the manufacturer. However, time drifts of the pressure sensors tended to be negative and the result suggests that pressure data from most present Argo floats, which use the same type of pressure sensor, may have a systematic negative bias. Time series salinity data calculated from the in situ–calibrated CTDs were slightly biased (mean of +0.0014) with respect to the shipboard CTD salinity data, based on potential temperature–salinity relationships, possibly due to a disequilibrium of the moored CTD conductivity sensors during the in situ calibrations.

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Hiroshi Uchida, Koji Shimada, and Takeshi Kawano

Abstract

A data processing method to obtain high-quality data from an expendable conductivity–temperature–depth (XCTD) profiler is proposed. By adjusting the mismatch of the response time of the temperature and conductivity sensors, systematic error (on the order of −0.05) in XCTD salinity data can be eliminated from regions having a strong vertical temperature gradient (>0.2°C m−1), such as the main thermocline of the nearshore side of the Kuroshio axis and the seasonal thermocline of the subarctic North Pacific. The systematic errors in XCTD depth and temperature data from two cruises were evaluated by comparing the CTD and XCTD data taken simultaneously during each cruise. The XCTD depths were in good agreement with the CTD depths from one cruise, but depth-dependent depth errors from the other cruise were found. The cause of the depth error is unknown but may have occurred because the terminal velocity for the XCTD probes was much less (−0.0428 m s−1) than that provided by the manufacturer for the later cruise. The results suggest that XCTD and expendable bathythermograph (XBT) observations may have a similar depth error because XBT and XCTD do not have pressure sensors, and therefore depth is inferred from the fall rate of the probe. Systematic positive biases (0.018°C on average) were found in XCTD temperature data. The viscous heating effect may contribute to the thermal bias because flow past the XCTD temperature probe is relatively fast (>3 m s−1). Evaluation of XBT/XCTD data by using simultaneous CTD observations data is valuable for validation of statistical corrections of the global XBT/XCTD dataset.

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Hiroshi Uchida, Takeshi Kawano, Ikuo Kaneko, and Masao Fukasawa

Abstract

Eleven optode-based oxygen sensors were used for shipboard hydrographic casts in the North Pacific. Oxygen data from the optode sensors were compared with high-quality oxygen data obtained with discrete water samples, and the performance of the sensors was evaluated. The response of the sensing foil of the optode decreases with increasing ambient pressure, and this pressure effect was found to decrease the response by 3.2% (1000 dbar)−1. A new calibration equation for the optode sensors was proposed. On the basis of oxygen data from water samples, the optode sensors were calibrated so that the reproducibility was less than 1%. High-quality oxygen profiles from the optode were obtained for fast-profiling conductivity–temperature–depth (CTD) observations, by compensating for the temperature-dependent delay in the optode data due to the slow response time of the optode.

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Hiroshi Uchida, Toshiya Nakano, Jun Tamba, Januarius V. Widiatmo, Kazuaki Yamazawa, Satoshi Ozawa, and Takeshi Kawano

Abstract

The uncertainty of deep ocean temperature (~1°C) measurement was evaluated. The time drifts of six deep ocean standards thermometers were examined based on laboratory calibrations as performed by the manufacturer in triple point of water (TPW) cells and gallium-melting-point (GaMP) cells. The time drifts ranged from −0.11 to 0.14 mK yr−1. Three of the six thermometers were evaluated at the National Metrology Institute of Japan in five TPW cells and a GaMP cell, and the temperature readings agreed with the realized temperature of the national standard cells of Japan within ±0.14 and ±0.41 mK for TPW and GaMP, respectively. The pressure sensitivities of the deep ocean standards thermometers were estimated by comparison with conductivity–temperature–depth (CTD) thermometers in the deep ocean, and no notable difference was detected. Pressure sensitivities of the two CTD thermometers were examined by laboratory tests, and the results suggest that the deep ocean standards thermometers have no pressure sensitivity, at least up to 65 MPa. The position and attitude motion of the CTD system can affect temperature and salinity data quality. The overall expanded uncertainty of the deep ocean temperature measurement (up to 65 MPa) by the CTD thermometer calibrated in reference to the deep ocean standards thermometer is estimated to be 0.7 mK.

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Hiroshi Uchida, Takeshi Kawano, Toshiya Nakano, Masahide Wakita, Tatsuya Tanaka, and Sonoka Tanihara

Abstract

We expanded the batch-to-batch offsets of The International Association for the Physical Sciences of the Oceans (IAPSO) Standard Seawater (SSW) batches P145–P163 by intercomparison measurements using salinometers. On the basis of our results, we recommend using the correction factors instead of the offsets to correct the batch-to-batch differences, especially for salinity data outside the range of 30–40 g kg−1. We evaluated the expanded batch-to-batch correction factors by applying them to time series salinity data collected in the northwestern North Pacific Ocean and found that they are effective for detecting recent freshening (−0.6 ± 0.1 × 10−3 g kg−1 decade−1) in the deep North Pacific, which might be related to a reduction of the formation rate of Antarctic Bottom Water. We also evaluated the SSW linearity pack by applying the batch-to-batch correction factors. Linearity errors of the salinometers estimated from decade resistance substituters were consistent with the results of the linearity pack measurements. To correct the linearity errors of a salinometer, it might be suitable to use the more detailed distribution of those estimated from the decade resistance substituter than the linearity pack measurements. Since the cause of large batch-to-batch differences is still unclear, a reference seawater that is more robust and stable than SSW might be necessary to establish a high-level of international comparability of salinity measurements; the Multiparametric Standard Seawater (MSSW) currently under development might be a candidate for such reference seawater, because MSSW is expected to be more stable than SSW not only in practical salinity but also in absolute salinity.

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