Description: 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 ± 270 km3 decade−1. In contrast, long-term (1920–2003) freshwater content (FWC) trends over the Siberian shelf show a general freshening tendency with a rate of 29 ± 50 km3 decade−1. 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 central-basin 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.

Description: Analysis of modern and historical observations demonstrates that the temperature of the intermediate-depth (150–900 m) Atlantic water (AW) of the Arctic Ocean has increased in recent decades. The AW warming has been uneven in time; a local 1°C maximum was observed in the mid-1990s, followed by an intervening minimum and an additional warming that culminated in 2007 with temperatures higher than in the 1990s by 0.24°C. Relative to climatology from all data prior to 1999, the most extreme 2007 temperature anomalies of up to 1°C and higher were observed in the Eurasian and Makarov Basins. The AW warming was associated with a substantial (up to 75–90 m) shoaling of the upper AW boundary in the central Arctic Ocean and weakening of the Eurasian Basin upper-ocean stratification. Taken together, these observations suggest that the changes in the Eurasian Basin facilitated greater upward transfer of AW heat to the ocean surface layer. Available limited observations and results from a 1D ocean column model support this surmised upward spread of AW heat through the Eurasian Basin halocline. Experiments with a 3D coupled ice–ocean model in turn suggest a loss of 28–35 cm of ice thickness after 50 yr in response to the 0.5 W m−2 increase in AW ocean heat flux suggested by the 1D model. This amount of thinning is comparable to the 29 cm of ice thickness loss due to local atmospheric thermodynamic forcing estimated from observations of fast-ice thickness decline. The implication is that AW warming helped precondition the polar ice cap for the extreme ice loss observed in recent years.

Description: The water mass structure of the Arctic Ocean is remarkable, for its intermediate (depth range ~150–900 m) layer is filled with warm (temperature 〉0°C) and salty water of Atlantic origin (usually called the Atlantic Water, AW). This water is carried into and through the Arctic Ocean by the pan-Arctic boundary current, which moves cyclonically along the basins’ margins (Fig. 1). This system provides the largest input of water, heat, and salt into the Arctic Ocean; the total quantity of heat is substantial, enough to melt the Arctic sea ice cover several times over. By utilizing an extensive archive of recently collected observational data, this study provides a cohesive picture of recent large-scale changes in the AW layer of the Arctic Ocean. These recent observations show the warm pulse of AW that entered the Arctic Ocean in the early 1990s finally reached the Canada Basin during the 2000s. The second warm pulse that entered the Arctic Ocean in the mid-2000s has moved through the Eurasian Basin and is en route downstream. One of the most intriguing results of these observations is the realization of the possibility of uptake of anomalous AW heat by overlying layers, with possible implications for an already-reduced Arctic ice cover.

Description: The large scale features of the vertical thermohaline structures in the Arctic Ocean during 2007-2009 were considered to reveal the changes in temperature and salinity in comparison with the historical data. In general, the main features of vertical thermohaline structures have remained unchanged over the Arctic and demonstrate the frontal barrier area between Eurasian and American basins. However, the unique summer atmospheric forcing in 2007 resulted in the considerable thermohaline changes in the surface layer. The large areas of positive and negative anomalies in temperature and salinity have been formed over the Arctic Ocean. The volumetric changes in the water masses with different temperature and salinity gradation have been also observed. The volume of Atlantic Waters with temperatures above 0°C and salinity above 34,6 increased at 22% in 2007 in comparison with 1970-1979 mean. The changes in the deeper layers habe also been revealed. Thus, the volume of intermediate waters with temperatures ranged from -0,4 to 0°C and salinities above 34,6 decreased at 30% in 2007. The most saline and cold bottom waters became warmer and fresher.

Description: Oceanographic studies during IPY 2007/2009 provided new information on spatial variability of hydrographic parameters. Detailed pattern of irregularities in the Atlantic Water (AW) layer was documented in the Nansen Basin. Spatial scales of temperature distribution and the depth of the upper boundary of AW were estimated. In the Canadian Basin spatial variations of temperature were less pronounced. During IPY 2007/2008 the area occupied by AW has increased. According to our estimations the positive temperature anomaly in some regions was as high as 1,5°C, which is about 70% of temperature maximum in 1950-1959. The upper boundary of AW (zero degree isotherm) rose by 40-120 m around the Mendeleyev Ridge and in the Amundsen Basin. At the same time, in the Canada Basin and in the western Fram Strait the AW thickness decreased by similar value. Heat content of the AW layer around the major part of the Arctic Ocean exceeded mean climatic value, except for the compact area north of Franz Josef Land, where small negative anomaly was observed. Throughout 2008 mean temperature and maximum temperature in the AW layer were higher than mean climatic values. At the same time, the state of AW layer in the inflow region, east of Fram Strait along the continental margin to the Laptev Sea, substantially changed in comparison with 2007. Mean and maximum temperature of AW dropped by 0,25/0,5°C. Heat content and the Thickness of AW layer have also decreased. Basing on the obtained results, we conclude that during 2008/2009 there was a neneral reverse trend in AW parameters towards mean climatic results.

Description: Historical hydrographic data (1940s–2010) show a distinct cross-slope difference of the lower halocline water (LHW) over the Laptev Sea continental margins. Over the slope, the LHW is on average warmer and saltier by 0.2°C and 0.5 psu, respectively, relative to the off-slope LHW. The LHW temperature time series constructed from the on-slope historical records are related to the temperature of the Atlantic Water (AW) boundary current transporting warm water from the North Atlantic Ocean. In contrast, the on-slope LHW salinity is linked to the sea ice and wind forcing over the potential upstream source region in the Barents and northern Kara Seas, as also indicated by hydrodynamic model results. Over the Laptev Sea continental margin, saltier LHW favors weaker salinity stratification that, in turn, contributes to enhanced vertical mixing with underlying AW.