Abstract
The use of the International Association of Physical Sciences of the Ocean (IAPSO) Standard Seawater (SSW) is essential for salinity measurements to ensure intercomparability. However, batch-to-batch comparisons indicate that the certified Practical Salinity may be imprecise by about ±0.002, and that imprecision may be too large for reliable detection of slight salinity changes in the deep ocean. Although offsets in certified values can be estimated from batch-to-batch comparisons, the estimation errors can accumulate and cause critical errors in the estimated salinity changes. We compared a robust and stable reference seawater, the Multiparametric Standard Seawater (MSSW), with SSW, and found a possible increase in the Practical Salinity (0.1 × 10−3 y−1) of SSW over time. We took into consideration this increasing trend to reevaluate the batch offsets of SSW batches P29–P167. We evaluated the batch correction by applying it to time-series salinity data in the North Pacific from stations K2 (47°N, 160°E) and ALOHA (22°45′N, 158°W). We found apparent salinity increases on isotherms (0.29 ± 0.09 × 10−3 g kg−1 decade−1 at a potential temperature of 1.09 °C) associated with pure warming (3.3 ± 0.1 × 10−3 °C decade−1) at the bottom of station K2 and warming (2.1 ± 0.3 × 10−3 °C decade−1) and freshening (−0.25 ± 0.08 × 10−3 g kg−1 decade−1) on pressure surfaces due to deepening of isotherms at station ALOHA (pressure ≥ 3500 dbar), probably due to slowing of the northward abyssal flow.
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