Monday, 4 February 2013

Change the pitch with a pinch of salt

The global balance of salinity of sea water is about stable. It is balanced by run-off from continents, evaporation, precipitation as well as inflow from hydrothermal vents and other interactions with the lithosphere. Salinity of different oceans and seas vary a bit reflecting mainly the regional balance of evaporation and run-off. Salinity of sea-water varies typically between 33 grams  and 37 grams per litre with a typical household average value of 35 grams per litre. However particular seas, like the Baltic Sea may deviate much from average salinity. Salinity of the open ocean is neither homogeneous nor fixed, and small variations are important for ocean dynamics and water flows. 

Surface salinity - see: Asian run-off, Arctic run-off etc.
Small variations of salinity are a markers for water-masses in the ocean, and tell us much about origin of these waters, their path and their fate. Early oceanographic research from the beginning of the 20th century discovered that already: The relative high salinity [1] of the North-Atlantic Ocean stems from waters of the Mediterranean Sea. 

Through the Straits of Gibraltar flows a mighty bottom current of very salty water [2] that is formed by the high evaporation in the eastern Mediterranean Sea. The  outflow at the bottom of the Gibraltar Strait is balanced by a surface flow of less saline waters of the North-Atlantic Ocean into the Mediterranean Sea. The relatively salty waters of the North-Atlantic move northward through the eastern North Atlantic touching on their westward flank relatively fresher waters flowing out of the Arctic Ocean. The mighty Siberian rivers discharge into the Arctic Ocean and cause a freshening of its surface waters. The high salinity of the North-Atlantic surface water is a key-parameter for the formation of  deep water in the Greenland Sea when sea-ice is formed in the fresher surface waters.

Deposits formed as Mediterranean was drying up
Throughout geological times salinity of the ocean varied reflecting geological or climatic change. A dramatic feature known for the Mediterranean Sea was its drying out a bit more than 5 Million years ago, in the so called “Messinian salinity crisis”, when the Strait of Gibraltar was closed - followed by the "Zanclean flood" once the straits opened again. Beyond such very dramatic events on a geological time scale, slight variations of sea-water salinity are a habitual play in the oceans. They are part of the “marine weather” below the surface. They are less well known to the general public than the known salinity changes in coastal zones in response to modified river run-off caused by human activity such as building the Assuan Dam on the Nile or water withdraw from the Colorado river or from China's big rivers. Human activity modifying the "marine weather" in the ocean would be of an impact of a higher magnitude than local changes of continental run-off. 

Inflow, evaporation and outflow
That anthropogenic climate change impacts on the physics and chemistry of the global ocean is established: rising sea-surface temperatures or climbing sea levels and ocean acidification by the absorption of excess carbon-dioxide  are known features.  They are not, however, the only potentially important shifts observed over recent decades. Drawing on observations from 1955 to 2004, [researchers] found that oceans' salinity changed throughout the study period, that changes were independent of known natural variability, and that shifts were consistent with the expected effects of anthropogenic climate change.” [*] This conclusion was drawn recently from studying salinity in the top 700m of the global ocean between 60°N and 60°S. 

A suite of computer models of the general water circulation in the ocean and atmosphere was used to assess the salinity changes that were shown in the observations since 1955. It came evident that the known natural cycles could not explain the observations. However the observed changes were consistent with what anthropogenic climate change should cause by shifting the hydrological cycle of evaporation, precipitation and run-off from continents. Theoretically, that is little surprising because higher sea-surface temperatures leading to higher heat content should modify evaporation and thus influence the global water cycle, and so in turn the salinity of the sea-water. However, that this impact can be tracked in changing salinities is new. At least I would have guessed that any impact cannot be discerned from the natural variability. 

Various estimate of ocean heat content in surface layer
Thus we might now conclude: salinity of sea water is balanced by run-off from continents, evaporation, inflow from hydrothermal vents and other interactions with the lithosphere. But that recently human-induced climate change has  started to influence this balance. Human activity being a noticeable co-driver of hydrological cycles up to shifting salinity of the world ocean, a bit along the line of action: ...increase Greenhouse gases, increase sea-surface temperatures and heat-content, modify hydrological cycles and shift salinities of sea-water...

[1] of about 36.5 grams per litre sea-water
[2] of about 38.5 grams per litre sea-water
[*] quote from: “Global ocean salinity changing due to anthropogenic climate change”, EOS, Transaction of the American Geophysical Union, Vol. 94(5) p.60 2013; reporting on research of Pierce et al. "The fingerprint of human-induced changes in the ocean's salinity and temperature fields" published in Geophysical Research Letters 39(21) 2012.
[#] from not identified source, picture found by Google  - the presentation is incomplete: (1) the out-flowing waters turn north flowing along continental margin, (2) the flow is structured by eddies that are spinning of to migrate south-westward 
[****] from: