Changes in ocean salinity Due to global warming and increased glacier melt,
thermohaline circulation patterns may be altered by increasing amounts of freshwater released into oceans and, therefore, changing ocean salinity. Thermohaline circulation is responsible for bringing up cold, nutrient-rich water from the depths of the ocean, a process known as
upwelling. Seawater consists of fresh water and salt, and the concentration of salt in seawater is called salinity. Salt does not evaporate, thus the precipitation and evaporation of freshwater influences salinity strongly. Changes in the water cycle are therefore strongly visible in surface salinity measurements, which has already been known since the 1930s. The advantage of using surface salinity is that it is well documented in the last 50 years, for example with
in-situ measurement systems as
ARGO. Another advantage is that oceanic salinity is stable on very long time scales, which makes small changes due to anthropogenic forcing easier to track. The oceanic salinity is not homogeneously distributed over the globe, there are regional differences that show a clear pattern. The tropic regions are relatively fresh, since these regions are dominated by rainfall. The subtropics are more saline, since these are dominated by evaporation, these regions are also known as the 'desert latitudes'. and a high amount of fresh meltwater entering the Arctic Ocean. The long-term observation records show a clear trend: the global salinity patterns are amplifying in this period. This means that the high saline regions have become more saline, and regions of low salinity have become less saline. The regions of high salinity are dominated by evaporation, and the increase in salinity shows that evaporation is increasing even more. The same goes for regions of low salinity that are become less saline, which indicates that precipitation is intensifying only more. This spatial pattern is similar to the spatial pattern of evaporation minus precipitation. The amplification of the salinity patterns is therefore indirect evidence for an intensifying water cycle. To further investigate the relation between ocean salinity and the water cycle, models play a large role in current research.
General Circulation Models (GCMs) and more recently Atmosphere-Ocean General Circulation Models
(AOGCMs) simulate the global circulations and the effects of changes such as an intensifying water cycle. A metric to capture the difference in salinity between high and low salinity regions in the top 2000 meters of the ocean is captured in the SC2000 metric. An
instrument carried by the
SAC-D satellite Aquarius, launched in June 2011, measured global sea surface
salinity. Between 1994 and 2006, satellite observations showed an 18% increase in the flow of freshwater into the world's oceans, partly from melting ice sheets, especially Greenland and partly from increased precipitation driven by an increase in global ocean evaporation.
Salinity evidence for changes in the water cycle Essential processes of the water cycle are precipitation and evaporation. The local amount of
precipitation minus
evaporation (often noted as P-E) shows the local influence of the water cycle. Changes in the magnitude of P-E are often used to show changes in the water cycle. But robust conclusions about changes in the amount of precipitation and evaporation are complex. About 85% of the earth's evaporation and 78% of the precipitation happens over the ocean surface, where measurements are difficult. Precipitation on the one hand, only has long term accurate observation records over land surfaces where the amount of rainfall can be measured locally (called
in-situ). Evaporation on the other hand, has no long time accurate observation records at all. There have been limited changes in regional monsoon precipitation observed over the 20th century because increases caused by global warming have been neutralized by cooling effects of anthropogenic aerosols. Different regional climate models project changes in monsoon precipitation whereby more regions are projected with increases than those with decreases. Convection-permitting models (CPMs) are able to better simulate the diurnal cycle of tropical convection, the vertical cloud structure and the coupling between moist convection and convergence and soil moisture-convection feedbacks in the
Sahel. The benefits of CPMs have also been demonstrated in other regions, including a more realistic representation of the precipitation structure and extremes. A convection-permitting (4.5 km grid-spacing) model over an Africa-wide domain shows future increases in dry spell length during the wet season over western and central Africa. The scientists concludes that, with the more accurate representation of convection, projected changes in both wet and dry extremes over Africa may be more severe. In other words: "both ends of Africa's weather extremes will get more severe". == Impacts on water management aspects ==