Ursula Schauer

One more step toward a warmer Arctic

This study was motivated by a strong warming signal seen in mooring-based and oceanographic survey data collected in 2004 in the Eurasian Basin of the Arctic Ocean. The source of this and earlier Arctic Ocean changes lies in interactions between polar and sub-polar basins. Evidence suggests such changes are abrupt, or pulse-like, taking the form of propagating anomalies that can be traced to higher-latitudes. For example, an anomaly found in 2004 in the eastern Eurasian Basin took ∼1.5 years to propagate from the Norwegian Sea to the Fram Strait region, and additional ∼4.5–5 years to reach the Laptev Sea slope. While the causes of the observed changes will require further investigation, our conclusions are consistent with prevailing ideas suggesting the Arctic Ocean is in transition towards a new, warmer state.

Impact of eastern Arctic shelf waters on the Nansen Basin intermediate layers

The Eurasian shelves supply water to the Nansen Basin intermediate layers in two ways: as buoyancy-driven plumes of dense winter water and as permanent inflow of the Barents Sea branch of Atlantic Water. While the plumes are local and seasonal phenomena, the Barents Sea flow is part of the large-scale circulation. Both interact with Atlantic Water, which enters the Arctic Ocean through Fram Strait and moves as a subsurface boundary current eastward along the continental slope. During the Polarstern cruise ARK IX/4 in summer 1993, the Fram Strait branch was observed as a narrow core within tens of kilometers of the Barents Sea shelf edge. Here, several patches of cold, low-salinity water spread across the slope down to about 500 m depth. Their origin is assumed to be the northern Barents Sea. They mix with the warm, saline Fram Strait branch water (FSBW), so that the core properties of the latter become modified downstream. In the eastern Nansen Basin the Fram Strait branch is displaced toward the inner basin by inflow of the Barents Sea Branch of Atlantic Water (BSBW). This inflow appears as a broad (200 km) wedge extending from 200 to 1300 m depth. BSBW is colder and less saline than water of the Fram Strait branch, and it is less dense and less stratified than the ambient water. Both branches appear to undergo vigorous mixing while spreading eastward, so that any eastward continuation of the boundary flow transports about 50% BSBW and 50% FSBW above 600 m and about 80% BSBW and 20% FSBW below that level toward the Canadian Basin. According to available observations, the Barents and Kara Seas are the only source areas for shelf waters ventilating the Nansen Basin below the halocline, and these waters constitute a freshwater input rather than a salt input. Winter shelf water from the Laptev Sea cannot contribute to layers deeper than the upper halocline.

Direct measurements of volume transports through Fram Strait

Heat and freshwater transports through Fram Strait are understood to have a significant influence on the hydrographic conditions in the Arctic Ocean and on water mass modifications in the Nordic seas. To determine these transports and their variability reliable estimates of the volume transport through the strait are required. Current meter moorings were deployed in Fram Strait from September 1997 to September 1999 in the framework of the EU MAST III Variability of Exchanges in the Northern Seas programme. The monthly mean velocity fields reveal marked velocity variations over seasonal and annual time scales, and the spatial structure of the northward flowing West Spitsbergen Current and the southward East Greenland Current with a maximum in spring and a minimum in summer. The volume transport obtained by averaging the monthly means over two years amounts to 9.5 ± 1.4 Sv to the north and 11.1 ± 1.7 Sv to the south (1 Sv = 106 m3s-1). The West Spitsbergen Current has a strong barotropic and a weaker baroclinic component; in the East Greenland Current barotropic and baroclinic components are of similar magnitude. The net transport through the strait is 4.2 ± 2.3 Sv to the south. The obtained northward and southward transports are significantly larger than earlier estimates in the literature; however, within its range of uncertainty the balance obtained from a two year average is consistent with earlier estimates.

The Arctic Ocean Boundary Current along the Eurasian slope and the adjacent Lomonosov Ridge: Water mass properties, transports and transformations from moored instruments

Year-long (summer 1995 to 1996) time series of temperature, salinity and current velocity from three slope sites spanning the junction of the Lomonosov Ridge with the Eurasian continent are used to quantify the water properties, transformations and transport of the boundary current of the Arctic Ocean. The mean flow is cyclonic, weak (1 to 5 cm s−1), predominantly aligned along isobaths and has an equivalent barotropic structure in the vertical. We estimate the transport of the boundary current in the Eurasian Basin to be . About half of this flow is diverted north along the Eurasian Basin side of the Lomonosov Ridge. The warm waters (>1.4°C) of the Atlantic layer are also found on the Canadian Basin side of the ridge south of 86.5°N, but not north of this latitude. This suggests that the Atlantic layer crosses the ridge at various latitudes south of 86.5°N and flows southward along the Canadian Basin side of the ridge. Temperature and salinity records indicate a small (0.02 Sv), episodic flow of Canadian Basin deep water into the Eurasian Basin at , providing a possible source for an anomalous eddy observed in the Amundsen Basin in 1996. There is also a similar flow of Eurasian Basin deep water into the Canadian Basin. Both flows probably pass through a gap in the Lomonosov Ridge at 80.4°N. A cooling and freshening of the Atlantic layer, observed at all three moorings, is attributed to changes (in temperature and salinity and/or volume) in the outflow from the Barents Sea the previous winter, possibly caused by an observed increased flow of ice from the Arctic Ocean into the Barents Sea. The change in water properties, which advects at along the southern edge of the Eurasian Basin, also strengthens the cold halocline layer and increases the stability of the upper ocean. This suggests a feedback in which ice exported from the Arctic Ocean into the Barents Sea promotes ice growth elsewhere in the Arctic Ocean. The strongest currents recorded at the moorings (up to ) are related to eddy features which are predominantly anticyclonic and, with a few exceptions, are of two main types: cold core eddies, confined to the upper 100–300 m, probably formed on the shelf, and warm core eddies of greater vertical extent, probably related to instabilities of an upstream front.

Variation of Measured Heat Flow Through the Fram Strait Between 1997 and 2006

The northernmost extension of the Atlantic-wide overturning circulation consists of the flow of Atlantic Water through the Arctic Ocean. Two passages form the gateways for warm and saline Atlantic Water to the Arctic: the shallow Barents Sea and the Fram Strait which is the only deep connection between the Arctic and the World Ocean. The flows through both passages rejoin in the northern Kara Sea and continue in a boundary current along the Arctic Basin rim and ridges (Aagaard 1989; Rudels et al. 1994). In the Arctic, dramatic water mass conversions take place and the warm and saline Atlantic Water is modified by cooling, freezing and melting as well as by admixture of river run-off to become shallow Polar Water, ice and saline deep water. The return flow of these waters to the south through the Fram Strait and the Canadian Archipelago closes the Atlantic Water loop through the Arctic. In the past century the Arctic Ocean evidenced close relation to global climate variation. Global surface air, upper North Atlantic Waters and Arctic intermediate waters showed coherently high temperatures in the middle of the last century and also in the past decades (Polyakov et al. 2003; Polyakov et al. 2004; Delworth and Knutson 2000). A likely candidate for this tight oceanic link is the flow through the Fram Strait. Through the Barents/Kara Sea, only the upper layer (200 m) of Atlantic Water can pass – thereby loosing much of its heat to the atmosphere – while the Fram Strait (sill depth 2,600 m) is deep enough to enable the through-flow of Atlantic Water at intermediate levels.

The release of brine‐enriched shelf water from Storfjord into the Norwegian Sea

Brine-enriched water masses are formed through surface cooling, freezing, and subsequent convective mixing and can be accumulated at the bottom of Arctic shelves. Time series from moored instruments over 1 year (1991/1992) in the northwestern Barents Sea reveal the flow of such water from a generation area in the coastal polynya in the Storfjord of Svalbard toward the western shelf edge. A volume of the order of 1012 m3 of cold, brine-enriched shelf water was released from this site into the Norwegian Sea during 5 months. The salinity of almost the entire water mass (96%) ranged between 34.8 and 35.1 practical salinity units. The source water of the observed outflow was provided by the East Spitsbergen Current advecting Arctic Water during summer and early winter and a mixture of Arctic and Atlantic Water during late winter. Owing to the broad salinity range, brine-enriched shelf water from the northwestern Barents Sea acts as a freshwater source for intermediate and as a salt source for deep waters of the Norwegian and Nansen Basins.

Evolution of the Arctic Ocean boundary current north of the Siberian shelves

Abstract The Arctic Mediterranean Sea is the most important source for the North Atlantic Deep Water, and the Arctic Ocean, often neglected in this respect, may provide a significant amount of the overflow waters crossing the Greenland–Scotland Ridge. Warm water from the south enters the Arctic Ocean through two main passages, Fram Strait and the Barents Sea, and the inward flowing boundary current that overlies the Eurasian continental slope of the Arctic Ocean supplies heat to the Arctic Ocean and exerts a dominant influence over its internal temperature and salinity characteristics. Major transformations of the inflow occur in the Barents Sea and as the two inflow branches meet in the boundary current north of the Kara Sea their characteristics are different. Lateral mixing between the two branches dominates the further transformations of the Atlantic and intermediate layers occurring in the Eurasian Basin. Ice formation, brine rejection and dense water formation on the shelves and subsequent convection down the slope lead to transformation of the boundary current that crosses the Lomonosov Ridge, and determine the properties of the Canadian Basin water column. Changes in the inflow characteristics of the boundary current will gradually, but slowly, affect also the intermediate and deep-water characteristics of the water column in the interior of the Canadian Basin. In the Eurasian Basin the influences of the shelf processes and pure slope convection are smaller and the water mass characteristics are mostly determined by advection and mixing of the two inflows. Only in the deepest part of the water column does slope convection appear to dominate the water mass transformations.

Double-diffusive layering in the Eurasian Basin of the Arctic Ocean

The central basins of the Arctic Ocean, below the surface mixed layer and remote from peripheral boundary currents, comprise an extremely low energy oceanic environment. Water masses having distinctly different Θ–S characteristics are organised throughout the central basins in extensive layers, consistent with occurrence of double-diffusive convection. In the Eurasian Basin, these structures can be explained by invoking formation along the narrow frontal region associated with the confluence of Fram Strait and Barents Sea waters north of the Kara Sea, and subsequent advection by the main circulation field. The presence of features in the interior of the basins requires a combination of processes that could include self-induced migration, through double-diffusive convection, as well as advection, across the central regions having weak horizontal gradients in temperature and salinity.

Arctic Ocean basin liquid freshwater storage trend 1992–2012

Freshwater in the Arctic Ocean plays an important role in the regional ocean circulation, sea ice, and global climate. From salinity observed by a variety of platforms, we are able, for the first time, to estimate a statistically reliable liquid freshwater trend from monthly gridded fields over all upper Arctic Ocean basins. From 1992 to 2012 this trend was 600±300 km3 yr−1. A numerical model agrees very well with the observed freshwater changes. A decrease in salinity made up about two thirds of the freshwater trend and a thickening of the upper layer up to one third. The Arctic Ocean Oscillation index, a measure for the regional wind stress curl, correlated well with our freshwater time series. No clear relation to Arctic Oscillation or Arctic Dipole indices could be found. Following other observational studies, an increased Bering Strait freshwater import to the Arctic Ocean, a decreased Davis Strait export, and enhanced net sea ice melt could have played an important role in the freshwater trend we observed.

The North Atlantic current and its associated hydrographic structure above and eastwards of the mid-atlantic ridge

Abstract Based on CTD data sets obtained in 1981–1984, XBT profiles, and long-term current meter moorings, the large-scale circulation field of the northeastern Atlantic north of the Azores was investigated. The mean volume transport through a standard meridional CTD section between 40°N and 52°N along the eastern flank of the Mid-Atlantic Ridge (MAR) was estimated to be 30 ± 9 Sv, with the North Atlantic Current (NAC) transporting 26 Sv. The NAC was found to be composed of clearly defined current branches (jets), that appear in temperature-salinity diagrams as a modal structure of the Central Water. Whereas the northernmost current branch (subarctic front) was found to be topographically fixed at the Gibbs Fracture Zone, the number, intensity and T - S structure of the remaining current branches, as well as their path over the MAR, are subject to intense variability. From 2 years of observations the branches were found to continue into the basins east of the MAR. They appeared as mesoscale features in a region of increased eddy kinetic energy and are interpreted to result from baroclinic instability. No indications of a branch of the NAC moving south, i.e. a recirculation as part of the North Atlantic subtropical gyre, were found.