ALBERT

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 diffusive layering (DL) form of double-diffusive convection cools the Atlantic Water (AW) as it circulates around the Arctic Ocean. Large DL steps, with heights of homogeneous layers often greater than 10 m, have been found above the AW core in the Eurasian Basin (EB) of the eastern Arctic. Within these DL staircases, heat and salt fluxes are determined by the mechanisms for vertical transport through the high-gradient regions (HGRs) between the homogeneous layers. These HGRs can be thick (up to 5 m and more) and are frequently complex, being composed of multiple small steps or continuous stratification. Microstructure data collected in the EB in 2007 and 2008 are used to estimate heat fluxes through large steps in three ways: using the measured dissipation rate in the large homogeneous layers; utilizing empirical flux laws based on the density ratio and temperature step across HGRs after scaling to account for the presence of multiple small DL interfaces within each HGR; and averaging estimates of heat fluxes computed separately for individual small interfaces (as laminar conductive fluxes), small convective layers (via dissipation rates within small DL layers), and turbulent patches (using dissipation rate and buoyancy) within each HGR. Diapycnal heat fluxes through HGRs evaluated by each method agree with each other and range from ~2 to ~8 W m−2, with an average flux of ~3–4 W m−2. These large fluxes confirm a critical role for the DL instability in cooling and thickening the AW layer as it circulates around the eastern Arctic Ocean.