Arctic Ocean Freshwater Changes over the Past 100 Years and Their Causes

Abstract

Recent observations show dramatic changes of the Arctic atmosphere-ice-ocean system. Here the authors demonstrate, through the analysis of a vast collection of previously unsynthesized observational data, that over the twentieth century the central Arctic Ocean became increasingly saltier with a rate of freshwater loss of 239 Ϯ 270 km 3 decade Ϫ1. In contrast, long-term freshwater content (FWC) trends over the Siberian shelf show a general freshening tendency with a rate of 29 Ϯ 50 km 3 decade Ϫ1. These FWC trends are modulated by strong multidecadal variability with sustained and widespread patterns. Associated with this variability, the FWC record shows two periods in the 1920s-30s and in recent decades when the central Arctic Ocean was saltier, and two periods in the earlier century and in the 1940s-70s when it was fresher. The current analysis of potential causes for the recent central Arctic Ocean salinification suggests that the FWC anomalies generated on Arctic shelves (including anomalies resulting from river discharge inputs) and those caused by net atmospheric precipitation were too small to trigger long-term FWC variations in the central Arctic Ocean; to the contrary, they tend to moderate the observed long-term centralbasin FWC changes. Variability of the intermediate Atlantic Water did not have apparent impact on changes of the upper-Arctic Ocean water masses. The authors' estimates suggest that ice production and sustained draining of freshwater from the Arctic Ocean in response to winds are the key contributors to the salinification of the upper Arctic Ocean over recent decades. Strength of the export of Arctic ice and water controls the supply of Arctic freshwater to subpolar basins while the intensity of the Arctic Ocean FWC anomalies is of less importance. Observational data demonstrate striking coherent long-term variations of the key Arctic climate parameters and strong coupling of long-term changes in the Arctic-North Atlantic climate system. Finally, since the high-latitude freshwater plays a crucial role in establishing and regulating global thermohaline circulation, the long-term variations of the freshwater content discussed here should be considered when assessing climate change and variability.

Time and space variability of freshwater content, heat content and seasonal ice melt in the Arctic Ocean from 1991 to 2011

“…Johnson and Polyakov (2001) tracked the salinification to originate in the Laptev Sea where salinity increased by almost 1.5 ppt by 1995. Later this anomaly was transported into the Makarov and Amundsen basins (Polyakov et al, 2008). Although the origins of salinification has been debated (Swift et al, 2005), extent and magnitude of the salinification presented by Johnson and Polyakov compare favourably with salinity increase of 1-1.5 ppt in the Amundsen and Makarov basins.…”

Mixing, heat fluxes and heat content evolution of the Arctic Ocean mixed layer

“…The large summer FWC can reflect a general increase in the Arctic fresh water inventory, due to an increasing discharge from Arctic rivers (Peterson et al, 2002;Rawlins et al, 2009) and changes in net precipitation vs. evaporation (Rawlins et al, 2009) decade. However, Polyakov et al (2008) found that changes in both net precipitation vs. evaporation and river discharge were too small to trigger changes in FWC in the central Arctic basin and argued that these changes were related to local freezing/melting and freshwater draining from the Arctic in response to wind. For the western Amundsen Basin, some of the increase in FWC can be exported from the Canadian Basin, where a significant freshening has been observed over the recent years Polyakov et al, 2008).…”

“…However, Polyakov et al (2008) found that changes in both net precipitation vs. evaporation and river discharge were too small to trigger changes in FWC in the central Arctic basin and argued that these changes were related to local freezing/melting and freshwater draining from the Arctic in response to wind. For the western Amundsen Basin, some of the increase in FWC can be exported from the Canadian Basin, where a significant freshening has been observed over the recent years Polyakov et al, 2008). The observed saltier winter mixed layer can reflect an increase in the ice export from the Arctic (Smedsrud et al, 2008), i.e.…”

“…Since the late 1970's, the atmospheric warming of the Arctic (Comiso, 2003) has been accompanied by a warming of the Arctic Ocean (Grotefendt et al, 1998) and a decrease in both sea ice extent (Comiso et al, 2008;Parkinson and Cavalieri, 2008;Stroeve et al, 2008) and thickness Kwok and Rothrock, 2009;Haas et al, 2008). In the same period, the salinity content of the Arctic shelves and the Canadian basin has decreased as opposed to an increase in the Eurasian basin Polyakov et al, 2008). Due to an earlier onset of melting and later onset of freezing, the melting season in the central Arctic has increased by 10 days since 1979 (Markus et al, 2009), which reflects a shift towards a more seasonal ice cover (Kinnard et al, 2008;Nghiem et al, 2007).…”

Towards the Marine Arctic Component of the Pan-Eurasian Experiment

“…Over most of the year, clouds heat the sea ice surface via enhanced downward longwave radiation (Shupe and 20 Intrieri, 2004), and precipitation brings freshwater to the Arctic Ocean both directly (exceeding evaporation) and, above all, via river discharge from the surrounding continents. These high freshwater fluxes are the main reason for a lower salinity in the Arctic Ocean compared to other oceans, and are crucial for the stratification, sea ice, chemical budget, and circulation patterns of the Arctic Ocean (Rudels, 2012;Polyakov et al, 2008). Due to increased precipitation, the river discharge has increased by approximately 30% in the period 2000-2010 (Haine et al, 2015) compared to 1979(Serreze et al, 2006.…”