Physical and chemical changes in the ocean over basin-wide zones and decadal or longer time-scales: perspectives on current and future conditions - Ocean Sustainability in the 21st Century - page 83

3.3 Salinity and fresh water
Ocean storage and transport offresh water are critical elements ofthe global climate, includ-
ing the water cycle (e.g. Schanze et al ., 2010 ) and anthropogenic climate change (e.g. Held
andSoden, 2006 ).Understandingoftheocean'sfreshwaterbudgethasincreasedgreatlyover
the past decade due to the Argo Project, an array of profiling floats that measures temperat-
ure and salinity all year round in the upper 2 km of the ice-free global ocean (Roemmich and
Gilson, 2009 ) . Ocean salinity and hence freshwater content are changing on gyre and basin-
wide scales, and observed changes can have major implications for water mass composition
and circulation patterns.
Mixedlayersalinity andevaporation minusprecipitation (E−P)arewellcorrelated over
muchoftheoceansothatnear-surfacesalinityactsasaproxyraingauge(Schmitt, 2008 ;Yu,
2011 ) . As such, surface forcing has led to an intensification of the global hydrological cycle
over the past 50 years, shown by increasing salinity at the sea surface in areas dominated
by evaporation and decreasing surface salinity in areas dominated by precipitation (Durack
and Wijffels, 2010 ). These surface-forced changes also extend to subsurface levels. Changes
of salinity are of global scale with similar patterns in the basins: subtropical waters show
an increase; sub-polar surface and intermediate waters have freshened in the Atlantic and
the Pacific; the intermediate water at around 1000 m depth in the Southern Hemisphere has
freshened in both the Atlantic and Pacific parts. In the Northern Hemisphere, a freshening
has occurred in the North Pacific, with an attendant decrease in the oxygen content, indic-
ating less ventilation than before; but in the North Atlantic the intermediate layer, at about
900-1200 m, has become saltier due to increased outflow from the Mediterranean, which is
becoming saltier (cf. Curry et al ., 2003 ) . A decrease in salinity has also been observed in
northern seas.
In areas with increasing evaporation there are increasing salinity trends; in higher lat-
itude areas there is generally freshening due to more precipitation and higher run-off there.
The Atlantic is becoming saltier over much of the water column. The increasing difference