Eberhard Fahrbach

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.

Antarctic climate change and the environment

Abstract The Antarctic climate system varies on timescales from orbital, through millennial to sub-annual, and is closely coupled to other parts of the global climate system. We review these variations from the perspective of the geological and glaciological records and the recent historical period from which we have instrumental data (∼the last 50 years). We consider their consequences for the biosphere, and show how the latest numerical models project changes into the future, taking into account human actions in the form of the release of greenhouse gases and chlorofluorocarbons into the atmosphere. In doing so, we provide an essential Southern Hemisphere companion to the Arctic Climate Impact Assessment.

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.

How much deep water is formed in the Southern Ocean

Three tracers are used to place constraints on the production rate of ventilated deep water in the Southern Ocean. The distribution of the water mass tracer PO4* (“phosphate star”) in the deep sea suggests that the amount of ventilated deep water produced in the Southern Ocean is equal to or greater than the outflow of North Atlantic Deep Water from the Atlantic. Radiocarbon distributions yield an export flux of water from the North Atlantic which has averaged about 15 Sv over the last several hundred years. CFC inventories are used as a direct indicator of the current production rate of ventilated deep water in the Southern Ocean. Although coverage is as yet sparse, it appears that the CFC inventory is not inconsistent with the deep water production rate required by the distributions of PO4* and radiocarbon. It has been widely accepted that the major part of the deep water production in the Southern Ocean takes place in the Weddell Sea. However, our estimate of the Southern Ocean ventilated deep water flux is in conflict with previous estimates of the flux of ventilated deep water from the Weddell Sea, which lie in the range 1–5 Sv. Possible reasons for this difference are discussed.

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.

Long-term temperature trends in the deep waters of the Weddell Sea

Warming of the deep water in the Weddell Sea has important implications for Antarctic bottom water formation, melting of pack ice, and the regional ocean–atmosphere heat transfer. In order to evaluate warming trends in the Weddell Sea, a historical data set encompassing CTD and bottle data from 1912 to 2000 was analyzed for temporal trends in the deep water masses: warm deep water (WDW) and Weddell Sea deep water (WSDW). The coldest WDW temperatures were primarily associated with the Weddell Polynya of the mid-1970s. Subsequent warming occurred at a rate of B0.01270.0071 Cy r � 1 from the 1970s to 1990s. This warming was comparable to the global, average surface water warming observed by Levitus et al. (Science 287 (2000) 2225), to the warming of the WSBW in the central Weddell Sea observed by Fahrbach et al. (Filchner–Ronne Ice Shelf Program, Report No. 12, Alfred-Wegener-Institut, Bremerhaven, Germany, 1998a, p. 24), and to the surface ice temperature warming from 1970 to 1998 in the Weddell Sea observed by Comiso (J. Climate 13 (2000) 1674). The warming was not compensated by an increase in salinity, and thus the WDW became less dense. The location of the warmest temperature was displaced towards the surface by B200 m from the 1970s to the 1990s. Although the average WSDW potential temperatures between 1500 and 3500 m were warmer in the 1990s than in the 1970s, high variability in the data prevented identification of a well-defined temporal trend. r 2002 Elsevier Science Ltd. All rights reserved.

Early spring phytoplankton blooms in ice platelet layers of the southern Weddell Sea, Antarctica

Abstract A dense diatom bloom growing in a shallow stratified layer maintained in position by loose ice platelets was found underlying pack-ice bordering the coastal polynyas of the Weddell Sea ice shelf south of 74°S in early spring well before the onset of seasonal melt. This rich bloom, which covered ca 20,000 km 2 , contrasted with the barrenness of the entire area between 74°S and the northern edge of the pack-ice at 58°S; its presence is explained by favourable conditions for accumulation of several decimetre-thick ice platelet layers under pack-ice of the southern shelf. Nutrient exhaustion and mass sinking of diatom chains were observed in this layer. Centric diatoms suspended in interstitial water dominated this bloom, which contrasted strongly with the flora of attached pennates typical of ice platelet layers underlying fast ice. Superblooms have been described previously from the southern Weddell Sea, although their developmental dynamics were not known at the time. We provide explanations for several perplexing features of this superbloom and show that they are significant in enhancing productivity of the Weddell Sea.

The Antarctic coastal current in the southeastern Weddell Sea

Between January and March 1989 during EPOS leg 3, a hydrographic survey was carried out in the southeastern Weddell Sea on transects across the continental shelf and slope off Kapp Norvegia and Halley Bay. This data set represents oceanographic conditions during Antarctic summer. Winter observations were obtained during the Winter Weddell Gyre Study in September and October 1989. During summer the water in the surface layer is relatively warm and of low salinity. In the area of Halley Bay exceptionally warm conditions were encountered with sea surface temperatures of nearly + 1°C. Over the upper continental slope a frontal zone separates Eastern Shelf Water from Antarctic Surface Water in the near surface layer and from Warm Deep Water in the deeper layers. The horizontal pressure gradient associated with the front produces the high velocity core of the Antarctic Coastal Current. In winter Antarctic Surface Water is replaced by colder Winter Water of higher salinity. Measurements from current meters moored off Kapp Norvegia and Vestkapp are used to describe the mean features of the current field and its fluctuations. At Kapp Norvegia annual mean current speeds range from 10 to 20 cm/s. The geostrophic current shear indicates that the speed of the current core decreases towards Halley Bay. The currents show significant seasonal variations with strong interannual differences. These compare well with the variations of the wind field observed at the Georg von Neumayer Station. Superimposed are higher frequency fluctuations with an energetic range between 5 and 15 days which is found in the wind measurements as well. A considerable part of the current velocity variance is due to the tides. The oceanographic conditions are strongly influenced by the local bottom topography. A topographic rise at the shelf edge off Kapp Norvegia reduces horizontal advection and allows a patch of cold Winter Water to be preserved into the summer. In contrast, a patch of Warm Deep Water was found on the shelf of Halley Bay. This illustrates rather heterogeneous conditions in the near bottom layers due to differences in the exchange rate with the open ocean as well as with the near surface layers.

Saline outflow from the Arctic Ocean: Its contribution to the deep waters of the Greenland, Norwegian, and Iceland seas

AbstractSince 1985 various investigators have proposed that Norwegian Sea deep water (NSDW) is formed by mixing of warm and saline deep water from the Arctic Ocean with the much colder and fresher deep water formed by convection in the Greenland Sea (GSDW). We here report on new observations which suggest significant modification and expansion of this conceptual model. We find that saline outflows from the Arctic Ocean result in several distinct intermediate and deep salinity maxima within the Greenland Sea; the southward transport of the two most saline modes is probably near 2 Sv. Mixing of GSDW and the main outflow core found over the Greenland slope, derived from about 1700 m in the Arctic Ocean, cannot by itself account for the properties of NSDW. Instead, the formation of NSDW must at least in part involve a source which in the Arctic Ocean is found below 2000 m. The mixing of various saline outflows is diapycnal. While significant NSDW production appears to occur in northern Fram Strait, large amounts of saline Arctic Ocean outflow also traverse the western Greenland Sea without mixing and enter the Iceland Sea. During the past decade, deep convection in the Greenland Sea has been greatly reduced, while deep outflow from the Arctic Ocean appears to have continued, resulting in a markedly warmer, slightly more saline, and less dense deep regime in the Greenland sea.

Structure and transports of the East Greenland Current at 75°N from moored current meters

The East Greenland Current runs from 80°N to 60°N from the Fram Strait to the Denmark Strait via the Nordic Seas. It transports waters from the Arctic and the Nordic Seas into the Atlantic and also acts as a western-intensified southward return flow for waters recirculating within the Greenland Sea Gyre, itself an area important for deep water formation. Data from current meters moored across the current at 75°N in 1994–1995 show a large seasonal variation in the current. The annual mean transport is 21±3 Sv (taking 9°W as the eastern boundary), varying from 11 Sv in summer to 37 Sv in winter (errors approximately ±5 Sv). No significant seasonal signal has been observed in the Fram or Denmark Straits, suggesting that the seasonal transport is confined within the Greenland Sea. Using temperature and velocity data, we split the flow at 75°N into two parts, a mainly wind-driven circulation (annual mean of order 19 Sv), which is trapped within the Greenland Sea Gyre and exhibits a large seasonal cycle, transporting, predominantly, the waters of the Greenland Sea, and a steadier throughflow probably thermohaline-driven (of order 8 Sv in the annual mean), with very little seasonal variation. Data from previous years, 1987–1994, indicate the interannual variability of the current is low. Assuming a spatially coherent structure to the current, we extend the time series of the transport back to 1991, and suggest it may be possible to monitor the total transport with one suitably placed mooring.