Saltier or not?

Northern oceans ocean basin

The northern ocean region.

There has been increasing evidence over the last 50 years that the waters of the Subpolar and Nordic Seas have indeed become fresher. What Curry & Mauritzen (2005) did was to quantify when, where, how fast, and in what quantities this freshwater entered these seas. It appears from their analysis that the amount of freshwater added in recent decades was much larger than previously assumed. They also estimated how long it would take before the deep return current in the high latitudes would cease if the freshening continued at more or less the same rate, and arrived at an estimate around 100-200 years. This suggests the possibility that a slowing of the conveyor belt in the forseeable future could be a real possibility, and not just a theoretical curiosity.

Hatun et al. agree that large areas in the northern oceans are freshening. Furthermore, Curry & Mauritzen state that the salinity was lowest in the mid-1990s and that the seas have become more salty since then. Both also describe an outstanding event in the late 1960s early 1970s where a region of very low salinity was observed (known as “the Great Salinity Anomaly” – GSA). The GSA can also be seen in Fig. 2 of Hatun et al., and is a well-known feature in Atlantic oceanography. Curry & Mauritzen did find that the freshening trend peaked in the 1990s, an indication that warm and salty water from the south is “tending to counteract” the freshening influence. So there is no inconsistency with the Hatun et al. paper.

But does that mean that the freshening of the subpolar gyre has ceased? No: The northern oceans are significantly fresher than they were in the 1960s – the extra 1.8 meter of freshwater in the Nordic Seas has lost 10 cm, and the extra 3 meters of freshwater in the Subpolar Gyre has, in an upper limit, lost 1 meter. The last timeframe in the Sub-polar Gyre was not published in Curry & Mauritzen’s paper because they had very little data from the western Sub-polar Gyre in that period – the volume budget would therefore be biased towards the salty eastern atlantic, where the warm, salty subtropical waters reside. The (upper limit) 1 meter estimate is therefore expected to be reduced when more data from the western Sub-polar Gyre enters the database.

The different focus in the two studies may simply be giving descriptions of the same situation, even while some media reports portrayed the more recent paper as proof against the possibility of freshening of the northern oceans. Curry & Mauritzen analysed the salinity throughout the depth of the ocean. They observe an accumulation of fresh water in the entire column of the sub-polar ocean basins, especially at intermediate depths. They also find indications of higher salinities in the region where the Atlantic waters flow into the northern oceans. Hatun et al. focus on near-surface salinity, as it is near the surface the ocean circulation is stongest. However, part of their analysis also include observations from a transsect penetrating down to 800m at the Rockall Trough. The analysis of Curry & Mauritzen was based on 3-dimensional gridded oceanic observations whereas that of Hatun et al. focused on three inflow points into the Arctic Ocean. Hatun et al. also used altimeter data (local sea level height measurements from satellite observations) to diagnose the norther oceans gyre circulation. The altimeter data provides a measure of the heat and salt content of the water, but because the water density is a complicated non-linear function of temperature and salinity, it is difficult to invert the measurements to infer the salinity. Nevertheless, the local height profiles give indications of the currents that arise from sea level height differences.

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