365 Days of Climate Awareness 184 – Global Ocean Salinity

The salt content of the ocean is considered to be constant, roughly 35 g salt per kilogram of water. This is commonly represented as 35 ppt (parts per thousand) or, more recently, 35 PSU (Practical Salinity Units). There are roughly 5 x 10¹⁹ g of salt in the ocean. Current input from rivers and hydrothermal vents is infinitesimal by comparison: roughly 1.7 x 10¹¹ g/year, which seems like a massive amount, until you realize that at that rate it would take about 300 million years to double the salt content of the ocean. The current rate of increase globally is well below our instruments’ ability to detect, and in fact, global ocean salinity appears, via sediment cores, to have been consistent for the past several hundred million years, if not more.

2005 mean global ocean salinity map.  Larger Version

That does not mean that regional and local salinity never change. Advection—the physical movement of dissolved salt in flowing water—plus rain and river input, and evaporation all have large roles in determining local and regional salinity patterns worldwide. Generally speaking, ocean salinities vary between 30-38 PSU. Water is fresher near large fresh inputs, such as major rivers or regions where rain dominates; water is saltier in evaporation basins. The world’s most famous example is the Dead Sea, bordered by Israel and Jordan and the terminus of the Jordan and other rivers, which has a salinity of 337 PSU. Nearby the eastern Mediterranean has salinities over 39 PSU, and the Red Sea over 40.

Salinity patterns worldwide are generally consistent, though variation certainly occurs on many timescales. While the sun’s heat tends to dominate in the tropical and near-tropical latitudes, atmospheric dynamics lead to the waters of southeastern Asia and west of Central America being comparatively fresh. The Gulf of Alaska and the Arctic Ocean, dominated by precipitation, tend to be quite fresh (33 PSU or lower) Meanwhile the northern Atlantic, where the Gulf Stream and North Atlantic Current lead, is a subpolar evaporation basin with salinities around 36 PSU.

Ocean salinity changes, 1960 — 2005, upper 2000 m.  Larger Version

In recent decades, change in regional salinity has been due mostly to increase in the global hydrological (water) cycle. As the thermal energy of both ocean and atmosphere increase, evaporation, atmospheric water content, and precipitation all increase. This has tended to exacerbate the existing differences in salinity across the world ocean: fresh regions grow fresher, salty regions saltier.

These patterns are borne out by the in-situ and satellite measurements of recent decades. A survey of salinity data from 1960 – 2005 shows significant increases in salinity in the temperate Atlantic and northern Indian oceans, as well as the Mediterranean and Red Seas. Meanwhile most of the Pacific, and the oceans near both poles have freshened considerably.

Tomorrow: changes in atmospheric water content.

Be brave, and be well.