Saltier or not?

In a recent (of Sept. 16, 2005) publication in Science, Hatun et al. find that record-high salinities have been observed over the past decade in the region where water from the Atlantic flows into the northern oceans. They combine an analysis of observations with simulations using an ocean model, concluding that the salinity of the inflow to the northern oceans is controlled by ocean dynamics and the circulation in the sub-polar gyre. The observations by Hatun et al. may suggest that at the moment the warm and salty waters from the south are especially warm and salty.

In another publication paper in Science from June 17th 2005, on the other hand, Curry & Mauritzen conclude that as a whole the northern North Atlantic has become significantly fresher (less salty) in recent decades. The latter study was based entirely on observations (hydrographic data between Labrador and Europe in the past 50 years). The recent evidence for salinification provided by Hatun et al. has been interpreted by some as being inconsistent with the evidence for high-latitude North Atlantic freshening found in previous reports. So what is really happening? Is the salinity increasing or decreasing? And can the two recent Science studies be consistent with each other?

Northern Europe experiences a mild climate relative to other regions of the same latitude. For instance, Oslo is at approximately the same latitude as the southern tip of Greenland, but has a substantially more mild climate. Though the atmosphere plays an important role here too, the heat transported by the ocean is a key factor responsible for the mild conditions in many parts of Northern Europe. A component of this transport is tied to a current system where salty surface water near the Arctic loses heat, becomes denser, and thus sinks into the deep ocean. This sinking is believed to be part of a large-scale ocean circulation known as the ‘global conveyor belt’, sometimes referred to as the thermohaline circulation because the circulation is driven by density variations related to variations in temperature and salinity.

Many of the surface currents of the world oceans (i.e., the ocean ‘gyres’ which appear as rotating horizontal current systems in the upper ocean) are driven by the wind, however, the sinking in the Arctic is related to the buoyancy forcing (effects that change either the temperature or salinity of the water, and hence its buoyancy). The sinking is mainly driven by the saltiness of the water, which is affected by evaporation of fresh water from the surface or, particularly in the Arctic, freezing seawater which leaves salt behind in the water beneath the ice.

Theory and modelling suggest that if the sinking of the salty surface waters in the North Atlantic slowed down or stopped, there would be a reduction in the heat transport by the ocean, which would have implications for the climate of northern Europe. Projections of potential future climates indicate that this may occur in response to increasing greenhouse gas forcing, though the degree of the change is highly variable from one model simulation to another.

Freshening of the ocean can result from numerous factors–the melting of ice, freshwater discharge from rivers, or increased precipitation at high latitudes. The salinity levels of the northern ocean region are also influence by the inflow of warm and salty water from lower latitudes in the Atlantic Ocean. Thus, the salinity of the water is the result of a delicate balance between multiple competing influences.

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