Gerd Rohardt

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.

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 occurrence of ice platelets at 250 m depth near the Filchner Ice Shelf and its significance for sea ice biology

Abstract Large single-crystal ice platelets were collected at 250 m depth in the vicinity of the Filchner Ice Shelf. They were probably formed in supercooled water streaming out from under the ice shelf as supported by hydrographic observations. The significance of large platelets rising from greater depths for the biological processes in sea ice along the Antarctic ice shelves is discussed.

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.

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.

Suppression of bottom water formation in the southeastern Weddell sea

Abstract The lack of bottom water formation in the southeastern Weddel Sea is investigated on the basis of CTD, current meter, and oxygen isotope data obtained in 1986 during the Winter Weddell Sea Project and in summer 1989 during the European Polastern Study. The principal underlying factor in suppressing the formation of bottom water is the narrow continental shelf in the region. This leads to two consequences not obtained in the western Weddel Sea: (1) the coastal polynya is able to extend out well over deep water and over th eswift-moving Antarctic Coastal Current, which acts to inhibit the acculmulation of salt released freezing in the polynya; and (2) the upper portions of Warm Deep Water come into close proximity with the glacial ice shelf floating above the continental shelf, thus providing heat for melting at the base of the ice shelf. Budgets for heat and salt derived from the winter data, along with measurements of δ18O, indicate that this melting occurs at rates more than sufficient to compensate the combined effects of brine released by freezing in the polynya and the upward flux of salt from the Warm Deep Water. As a result, the Eastern Sshelf Water cannot acquire the salt concentrations needed fopr the formation of bottom water.

Flow of bottom water in the northwestern Weddell Sea

The Weddell Sea is known to feed recently formed deep and bottom water into the Antarctic circumpolar water belt, from whence it spreads into the basins of the world ocean. The rates are still a matter of debate. To quantify the flow of bottom water in the northwestern Weddell Sea data obtained during five cruises with R/V Polarstern between October 1989 and May 1998 were used. During the cruises in the Weddell Sea, five hydrographic surveys were carried out to measure water mass properties, and moored instruments were deployed over a time period of 8.5 years to obtain quasi-continuous time series. The average flow in the bottom water plume in the northwestern Weddell Sea deduced from the combined conductivity-temperature-depth and moored observations is 1.3±0.4 Sv. Intensive fluctuations of a wide range of timescales including annual and interannual variations are superimposed. The variations are partly induced by fluctuations in the formation rates and partly by current velocity fluctuations related to the large-scale circulation. Taking into account entrainment of modified Warm Deep Water and Weddell Sea Deep Water during the descent of the plume along the slope, between 0.5 Sv and 1.3 Sv of surface-ventilated water is supplied to the deep sea. This is significantly less than the widely accepted ventilation rates of the deep sea. If there are no other significant sources of newly ventilated water in the Weddell Sea, either the dominant role of Weddell Sea Bottom Water in the Southern Ocean or the global ventilation rates have to be reconsidered.

Weddell Sea iceberg drift: Five years of observations

Since 1999, 52 icebergs have been tagged with GPS buoys in the Weddell Seato enable monitoring of their position. The chosen icebergs were of small tomedium size, with a few icebergs larger than 10 km associatedwith the calving of icebergs A38 and A43 from the Ronne Ice Shelf.The majority of icebergs were tagged off Neumayer Station (8E, 70S).It was found that smaller bergs with edges shorter than 200 m had the shortestlife cycle (< 0.5 yr). Iceberg and thus freshwater export out of theWeddell Sea was found to be highly variable. In one year the majority of buoysdeployed remained in the Weddell Sea, constituting about 40 % of the NCEP P-Efreshwater input, whereas in other years all of the tagged icebergs were exported.The observed drifts of icebergs and sea-ice showed a remarkably coherent motion.The analysis of an iceberg - sea-ice buoy array in the western Weddell Seaand an iceberg array in the eastern Weddell Sea showed a coherent sea-iceiceberg drift in sea-ice concentrations above 86 %. Dynamic kinematic parameter(DKP) during the course of coherent movement were low and deviations from the meancourse associated with the passage of low-pressure system. The length scale ofcoherent movement was estimated to be less than 250km; about half the value found forthe Arctic Ocean.

Winter distribution and overwintering strategies of the Antarctic copepod species Calanoides acutus, Rhincalanus gigas and Calanus propinquus (Crustacea,Calanoida) in the Weddell Sea

During the Winter Weddell Gyre Study in September–October 1989, the horizontal and vertical distribution, stage composition and feeding condition of the three antarctic copepod species Calanoides acutus, Rhincalanus gigas and Calanus propinquus were studied. The data indicate that C. acutus and R. gigas have the bases of their distributional ranges (sensu Makarov et al. 1982) in the Antarctic Circumpolar Current (ACC) and in the Warm Deep Water (WDW) entering the Weddell Gyre (WG). C. propinquus lived mainly in the cold WG south of the ACC. C. acutus overwintered mainly in the WG as stage IV copepodites (C). The species mainly inhabited the layers below the Tℴmax stratum and down to 2000 m, but C V and females occurred slightly higher than C III and IV. Males prevailed over females and were confined to a rather narrow layer between 500 and 1000 m. Feeding experiments suggested all deep-living stages to be resting. However, if this species spawns in late autumn the younger C I–II can stay in the Winter Water (WW). R. gigas inhabited mainly the Tℴmax stratum. In the eastern part of the WG, R. gigas breed in the WDW in autumn and hibernate as C I–III and C V–VI in the first and second winter, respectively. In the ACC zone, however, its life cycle is different and winter breeding of overwintered adults occurs. Most of the C. propinquus population overwintered in the WG as C III–V, inhabiting the WW. In the upper water layers in the interior of the WG, C III dominated with upto 18,000 individuals 1,000 m3. Shallow living C. propinquus were in the active, feeding state. Persistence of active feeding zooplankton populations in the WW of the WG can be an important factor influencing processes of phytoplankton development and the particle flux.

A chlorofluoromethane and hydrographic section across Drake Passage: Deep water ventilation and meridional property transport

New hydrographic and nutrient data obtained on a section across Drake Passage (F/S Meteor January 1990, World Ocean Circulation Experiment Hydrographic Program section S1) are in close agreement with property sections reported previously. The chlorofluoromethanes CFM 11 and CFM 12 were measured in Drake Passage for the first time. CFM concentrations are found to decrease from the surface down into the Upper Circumpolar Deep Water, for which they confirm water renewal from the south. For the Lower Circumpolar Deep Water, in which CFM concentrations were above detection limit only south of the Polar Front, very little water renewal on the CFM time scale is implied. Nonvanishing CFM is again found in the Weddell Sea Deep Water and the Southeast Pacific Deep Water toward the bottom in the south, but recent ventilation for the latter water mass is rejected. CFM 11 and CFM 12 concentrations vary essentially in constant proportion down to very low concentrations, questioning the possibility of using CFM ratios as “age” markers. The observed ratios are shown to be a natural feature of the upwelling regime of the southern ocean. Property concentrations on isopycnal surfaces display large undulations, reaching down into the Upper Circumpolar Deep Water. Their extrema, due to varying contribution of young water of southern origin, are situated at the boundaries of the current bands of the Antarctic Circumpolar Current The feature is ascribed to property advection by rings and is taken to support previous claims that rings are an important transport mechanism across the Antarctic Circumpolar Current and that they might assist in maintaining its fronts.