“Freshwater” in the Ocean is Not a Useful Parameter in Climate Research

Abstract

Ocean water is freshwater with salt. The distribution of salt concentration in the ocean changes by addition and removal of freshwater in the form of precipitation, continental runoff, and evaporation, and by a flow of saline ocean water that gives rise to a salt flux divergence. Often, changes in salinity are described in terms of "freshwater content" changes and oceanic "freshwater transports", defined as fractions of freshwater. But these freshwater fractions are arbitrary, because they are defined by a non-unique reference salinity. Also all temporal and spatial comparisons and anomalies of such freshwater fractions in the ocean depend on the choice of reference salinity in a nonlinear way, because in the definition of the fraction it appears in the denominator. Consequently, any conclusion based on the comparison of freshwater fractions is ambiguous. Since there is no definite physical constraint for a unique reference salinity, freshwater fractions are declared not useful for the assessment of the state of ocean regions and the associated changes. In the light of ongoing changes in the water cycle and the global nature of climate science, scientific results need to be expressed in a way so that they can be easily compared and integrated in a global perspective. To this end, we recommend to avoid freshwater fraction as a parameter describing the ocean state. Instead, one should use the terms of the salt budget to obtain unique results for quantifying and comparing salinity.

Dominant Terms in the Freshwater and Heat Budgets of the Subpolar North Atlantic Ocean and Nordic Seas From 1992 to 2015

“…Recent studies have pointed to problems associated with the use of reference salinities (Bacon et al, 2015; Schauer & Losch, 2019; Tsubouchi et al, 2012) or reference temperatures (Forget & Ferreira, 2019). For example, Bacon et al (2015) pointed out the ambiguity of choice of reference salinity and reasons that the only appropriate reference value is a boundary‐mean salinity calculated for an assumed closed‐volume freshwater supply.…”

Understanding Regional and Seasonal Variability Is Key to Gaining a Pan-Arctic Perspective on Arctic Ocean Freshening

“…For example, budgets are most often calculated as an anomaly based on a reference salinity, typically relative to a salinity of 34.8, but regional constraints in the use of a reference salinity exist (cf. Carmack et al, 2008;Bacon et al, 2015;Schauer and Losch, 2019). Still, even with the crude resolution of budget integrations, the importance of regionality of FW storage is strikingly clear (see Figure 4 of Aagaard and Carmack, 1989).…”

Response of Arctic Freshwater to the Arctic Oscillation in Coupled Climate Models

“…For reference, Serreze et al (2006) estimate the pre-2000 climatological mean liquid FWC to be 74,000 ± 7,400 km 3 , based on hydrographic observations. Intermodel comparisons of absolute values of freshwater as commonly defined in the literature are ambiguous (Schauer and Losch 2019). To provide parity, we report our linear response functions in terms of the fractional change in each model's Arctic FWC reservoir.…”