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''Freshwater'' in the Ocean Is Not a Useful Parameter in Climate

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''Freshwater'' in the Ocean Is Not a Useful Parameter in Climate SEPTEMBER 2019 S C HAUER AND LOSCH 2309 ‘‘Freshwater’’ in the Ocean is Not a Useful Parameter in Climate Research URSULA SCHAUER AND MARTIN LOSCH Alfred-Wegener-Institut Helmholtz-Zentrum fur€ Polar- und Meeresforschung, Bremerhaven, Germany (Manuscript received 30 April 2019, in final form 5 July 2019) 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 nonunique 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. 1. Introduction Consequently, arbitrary parameters may cause consid- erable confusion if they come without clear specification Physical oceanography, like any other field of science, through a name or a unit. For example, sound pressure is based on measurable and derived quantities that can level (loudness) is a parameter in acoustics, which be compared to each other and to the respective terms of needs a reference pressure. Since, for practical reasons, physical laws. To derive any understanding of processes the reference pressure used in underwater acoustics and gain, for example, insight into the ocean’s role in is different from that used in air acoustics, and even the climate system, these comparisons need to be un- though these references are internationally accepted ambiguous. Here, by ambiguity we do not mean un- standards, misunderstandings easily appear evoking certainty due to instrumental errors, to nonsynoptic even societal debates (Finfer et al. 2008; Slabbekoorn observations, to interpolation, or to model resolution et al. 2010). and parameterizations, but ambiguity due to an un- The notion of freshwater in the ocean is mostly in- avoidable arbitrary choice of reference. With ambigu- voked in recognition of salinity changes being caused ous comparisons, also analyses and conclusions are by dilution or concentration due to adding or removing ambiguous, and future scenarios can hardly be de- freshwater. This exchange of freshwater is an impor- veloped. As we will show, the concept of ‘‘freshwater in tant part of the hydrological cycle. In the nonoceanic the ocean’’ inevitably leads to this type of ambiguity and compartments of Earth, that is, soil, land ice, and at- thus is not useful in physical oceanography. mosphere, freshwater (which is simply termed ‘‘water’’ Calculations of quantities that depend on an arbitrary there) is an important quantity because the residence reference value yield arbitrary numbers by definition. times, except for ice sheets and glaciers, and the total amount of water are comparatively small and changes Denotes content that is immediately available upon publica- of the water exchange are crucial. In contrast, in the tion as open access. ocean the amount of water is large and changes are fairly small. But since these changes determine the Corresponding author: Ursula Schauer, [email protected] distribution of salt concentration, the freshwater flux DOI: 10.1175/JPO-D-19-0102.1 Ó 2019 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses). Unauthenticated | Downloaded 09/29/21 04:58 AM UTC 2310 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 49 into and out of the ocean has a huge impact on ocean is evaluated over a vertical cross section area sec to processes. which the velocity component u? is perpendicular and Salinity itself is a key feature of the ocean. Together dl and dz are the respective horizontal and the vertical with temperature, it determines the density of ocean line elements along the boundary. water and thereby influences almost all dynamical pro- The freshwater content of ocean water is, however, cesses, ranging from the large-scale overturning circu- uniquely defined as lation to double diffusive mixing at the centimeter scale. Salinity varies greatly in the World Ocean, but this 5 2 23 FW 1 10 SA , (3) variability is not a consequence of sinks and sources 21 of salt, as indicated by the small ratio between salt with SA the absolute salinity in g kg (IOC/SCOR/IAPSO 2 input [O(1012)kgyr 1] and salt content in the ocean 2010). This definition gives the mass relation, hence, [O(1019) kg], but of freshwater (e.g., Talley et al. 2011). the mass of freshwater in an ocean volume V. The fresh- Precipitation, input from land by rivers, glaciers and water mass VrFW (r for density) comprises almost the groundwater, and loss by evaporation create large dif- entire mass of ocean water. ferences in salinity on various spatial and temporal Consequently, the parameters defined by Eqs. (1) and scales. These salinity differences lead to ocean currents, (2) describe only fractions of the freshwater defined but on the other hand, they are also subject to ocean by Eq. (3). Accordingly, we will call these terms currents as water masses with a surplus or a deficit of ‘‘freshwater fraction’’ and ‘‘freshwater fraction trans- freshwater are transported to other regions. port’’ throughout this paper. This also differentiates the In the first combined analysis of freshwater input and latter from ‘‘true’’ freshwater inflow or removal through ocean salinities, Knudsen (1900) inferred the steady precipitation, evaporation, and continental runoff. Note state ocean circulation between a partially enclosed that Treguier et al. (2014) suggested using the termi- basin and the adjacent ocean from salt and mass bal- nology of ‘‘freshwater anomaly’’, but we find ‘‘anomaly’’ ances, including river runoff. This concept applies to any misleading and also inconsistent with definition (3), fixed volume of the ocean, also the global ocean (e.g., because anomaly usually refers to a deviation from Talley 2008). a mean. In the context of changes in the ocean, research also Freshwater fractions first appeared in the context of addresses changes of salinity. In the 1970s and 1980s, regional analyses, but ultimately the nature of ocean and when large-scale salinity anomalies were first investi- climate science is global. This global nature requires that gated they were still described as such, namely as ‘‘Great all analyses and results are formulated in a way that they Salinity Anomalies’’ (Dickson et al. 1988; Belkin et al. can be integrated in a global perspective easily and 1998). Only a few recent publications continue to pro- seamlessly. Results that depend on a locally determined vide salt budgets (Wijffels et al. 1992; Mauritzen et al. reference cannot satisfy this requirement. 2012; Treguier et al. 2014). In one of the first publications on freshwater fraction Since the 1990s, research papers often presented transports, Aagaard and Carmack (1989) provided es- terms of a freshwater budget instead (e.g., Rahmstorf timates of freshwater budgets of the Arctic Ocean and of 1996; Serreze et al. 2006; Yang et al. 2016; Holliday et al. the Nordic Seas. These budgets included the exchange 2018). These terms are then called ‘‘freshwater content’’ between the two basins and with the North Pacific and in the ocean as well as ‘‘freshwater transports,’’ for ex- the North Atlantic. Based on the same volume trans- ample, through individual sections and gateways. Such ports through the connecting passages Fram Strait and freshwater content Vff and freshwater transport fff in Barents Sea Opening and the same salinities in the the ocean is then defined as passages, Aagaard and Carmack (1989) estimated dif- ferent freshwater fraction transports depending on S 2 S 5 ref whether they calculated them for the Nordic Seas or for Vff V , (1) Sref the Arctic Ocean because they based these transports on different reference salinities Sref. For example, they and calculated the freshwater transport by southward flow in 3 21 ðð ! Fram Strait to be 820 km yr and at the same time to S 2 S 3 21 f 5 ref be also 1160 km yr . From a physical point of view, ff u? dl dz, (2) sec Sref such an ambiguity of results is not acceptable because it does not allow closing an overall budget. The root of the with an ocean volume V with salinity S, and an arbi- problem is the requirement of a reference salinity for trarily chosen reference salinity Sref. The double integral computing the freshwater fraction terms. Unauthenticated | Downloaded 09/29/21 04:58 AM UTC SEPTEMBER 2019 S C HAUER AND LOSCH 2311 Treguier et al. (2014) strongly recommended using addressed with simulations of a global configuration salinity in global analyses and to avoid analyzing ‘‘fresh- with an Arctic focus of the finite element sea ice ocean water in the ocean’’ as defined by Eqs. (1) and (2) because model FESOM (Wekerle et al.
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