Saltier or not?

Gridding sparse ocean observations onto a very high (in this case, 1-by-1 degree latitude x longitude) resolution is prone to producing some apparent structures that are simply artifacts of mathematical interpolation, even when isopycnal methods are utilised (this is common for gridding of data). On the other hand, the budgeting of salinity implicit in the ocean model used by Hatun et al. may not properly account for river run-off (freshens the water), transport from the Pacific, the Canadian Archipelago, the East Greenland current, or melting processes. The ocean model used by Hatun et al. has a northern latitude limit of 78N, where an artificial boundary is imposed with the salinity, temperatures and velocities all prescribed at that boundary by results from another model. The salt transport at this boundary is not well-known. If the prescribed salt transport is not correct, then the salt budget of the model will not represent reality.

Hatun et al. examined the possibilities that [i] a change in rain falling over the ocean (freshens the water) and evaporation (increases the salinity by removing water and leaving salt behind), [ii] increased salinity in the sub-tropical gyre (in the main part of the North Atlantic), [iii] increased salinity in the sub-polar gyre, or [iv] dynamical changes in the relative contributions from the two gyres could explain the high salinities in the in-flow regions. Of these processes, they concluded that it was the latter that was responsible for the high salinity in the region where Atlantic water flows into the northern oceans. This explanation involves a displacement of an oceanic front and hence a change in the circulation structure. Comparisons with observations in these regions show good agreement between the model and the observations. Thus, although their conclusions about the salinity within the inflow region being closely related to the dynamics of the sub-polar gyration circulation, the model may not give a representative account of the total salt content in the entire northern oceans. But their focus was on the increase in salinity in a certain region, not a general decrease, and hence they did not examine as other factors such as river-run off and melting. Finally, the results by Curry & Mauritzen focused on longer time scales for which they had collected ocean observations, while only model results were provided for long-term evolution by Hatun et al. Even if the model agreed well with respect to saline anomalies in the inflow region, it has not yet been established whether it provides representative values for the absolute salinity in the entire ocean basin.

Given the uncertainties and caveats implicit these two studies, their conclusions regarding trends in salinity of the northern oceans may not be as inconsistent as they might appear. The use of the term “record-high” in the paper by Hatun et al. may be misleading, as this only refers to a limited region since the 1960s (southwest off Iceland) or a very short interval (one decade) and doesn’t reflect the general degree of salinity in the entire basin over a longer period. The long-term observations suggest that the last decade has been a mere blip in a long-term trend towards fresher conditions in the northen oceans. 1995 was when the salt content in the norther oceans was at its lowest. It is hard to see how the relationship suggested by Hatun et al. can explain how the 19000 cubic km of fresh water reported by Curry & Mauritzen could be removed. However, these papers are probably not the final word on this.

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