James H. Swift

Long-Term Coordinated Changes in the Convective Activity of the North Atlantic

The North Atlantic is a peculiarly convective ocean. The convective renewal of intermediate and deep waters in the Labrador Sea and Greenland/Iceland Sea both contribute significantly to the production and export of North Atlantic Deep Water, thus helping to drive the global thermohaline circulation, while the formation and spreading of 18-degree water at shallow-to-intermediate depths off the US eastern seaboard is a major element in the circulation and hydrographic character of the west Atlantic. For as long as time-series of adequate precision have been available to us, it has been apparent that the intensity of convection at each of these sites, and the hydrographic character of their products have been subject to major interannual change, as shown by Aagaard (1968), Clarke et al (1990), and Meincke et al (1992) for the Greenland Sea, in the OWS BRAVO record from the Labrador Sea, (eg Lazier,1980 et seq.), and at the PANULIRUS / Hydrostation “S” site in the Northern Sargasso off Bermuda (eg Jenkins, 1982, Talley and Raymer, 1982). This paper reviews the recent history of these changes showing that the major convective centres of the Greenland- and Labrador Seas are currently at opposite convective extrema in our postwar record, with vertical exchange at the former site limited to 1000 m or so, but with Labrador Sea convection reaching deeper than previously observed, to over 2300 m. As a result, Greenland Sea Deep Water has become progressively warmer and more saline since the early ‘70’s due to increased horizontal exchange with the Arctic Ocean through Fram Strait, while the Labrador Sea Water has become progressively colder and fresher over the same period through increased vertical exchange; most recently, convection has become deep enough there to reach into the more saline NADW which underlies it, so that cooler, but now saltier and denser LSW has resulted.

Massive phytoplankton blooms under Arctic Sea ice

In midsummer, diatoms have taken advantage of thinning ice cover to feed in nutrient-rich waters. Phytoplankton blooms over Arctic Ocean continental shelves are thought to be restricted to waters free of sea ice. Here, we document a massive phytoplankton bloom beneath fully consolidated pack ice far from the ice edge in the Chukchi Sea, where light transmission has increased in recent decades because of thinning ice cover and proliferation of melt ponds. The bloom was characterized by high diatom biomass and rates of growth and primary production. Evidence suggests that under-ice phytoplankton blooms may be more widespread over nutrient-rich Arctic continental shelves and that satellite-based estimates of annual primary production in these waters may be underestimated by up to 10-fold.

Seasonal transitions and water mass formation in the Iceland and Greenland seas

Abstract The dense waters of the Iceland and Greenland sea gyres are not simply the product of a gradual transition between cold, relatively fresh polar waters on the west and warmer, saline Atlantic water on the east, but instead constitute a unique hydrographic region, bounded by the polar and arctic fronts, which we term the arctic domain. Although deep and bottom water is the best-known water mass formed in the arctic domain, the region also produces a spectrum of dense intermediate water types in winter. Our study concentrates upon water mass formation in the Iceland Sea, where the principal winter product is an intermediate water mass nearly as cold as the deep water, but slightly less saline and therefore always lying above the deep water. The intermediate water mass produced in greatest volume in the Greenland Sea is warmer and more saline, although of nearly the same density as that produced in the Iceland Sea. The principal difference between the seasonal transitions in the Greenland and Iceland seas is that the transition in the Greenland Sea involves slightly more saline water than does that in the Iceland Sea, due to a more pronounced contribution of cooled Atlantic water. Subsequent along-isopycnal mixing of the intermediate water masses produces water which needs only to undergo a final cooling stage to be transformed into new deep and bottom water.

The Arctic Waters

The classic study of the Nordic Seas (Fig. 1) is that of Helland-Hansen and Nansen (1909). They discussed their topic so thoroughly that an essentially correct, comprehensive discussion of the hydrography of the region could still be based largely upon their work. One reason for their success was that they were among the first to achieve in their determinations of hydrographic properties a degree of accuracy and precision nearly up to today’s standards. Also, their understanding of the circulation was considerably strengthened by their use of the geostrophic method. In fact, it was Sandstrom and Helland-Hansen (1903) who published the original paper describing the method, and the 1909 study was the first to apply it. Finally, the region is nicely scaled, both in its geography and in its range of hydrographic properties. Thus it was possible for Helland-Hansen and Nansen to do justice to the region with the methods at their disposal. However, they lacked sufficient winter data to study the formation of the dense water masses characteristic of the region; thus they did not fully appreciate the uniqueness and importance of these water masses.

River runoff, sea ice meltwater, and Pacific water distribution and mean residence times in the Arctic Ocean

Hydrographic and tracer data collected during ARK IV/3 (FS Polarstern in 1987), ARCTIC91 (IB Oden), and AOS94 (CCGS Louis S. St-Laurent) expeditions reveal the evolution of the near-surface waters in the Arctic Ocean during the late 1980s and early 1990s. Salinity, nutrients, dissolved oxygen, and δ 18 O data are used to quantify the components of Arctic freshwater: river runoff, sea ice meltwater, and Pacific water. The calculated river runoff fractions suggest that in 1994 a large portion of water from the Pechora, Ob, Yenisey, Kotuy, and Lena Rivers did not flow off the shelf closest to their river deltas, but remained on the shelf and traveled via cyclonic circulation into the Laptev and East Siberian Seas. River runoff flowed off the shelf at the Lomonosov Ridge and most left the shelf at the Mendeleyev Ridge. ARCTIC91 and AOS94 Pacific water fraction estimates of Upper Halocline Water, the traditionally defined core of the Pacific water mass, document a decrease in extent compared to historical data. The front between Atlantic water and Pacific water shifted from the Lomonosov Ridge location in 1991 to the Mendeleyev Ridge in 1994. The relative age structure of the upper waters is described by using the 3 H- 3 He age. The mean 3 H- 3 He age measured in the halocline within the salinity surface of 33.1 ± 0.3 is 4.3 ± 1.7 years and that for the 34.2 ± 0.2 salinity surface is 9.6 ± 4.6 years. Lateral variations in the relative age structure within the halocline and Atlantic water support the well-known cyclonic boundary current circulation.

Distribution of Atlantic and Pacific waters in the upper Arctic Ocean: Implications for circulation

The Atlantic and Pacific oceans provide source waters for the Arctic Ocean that can be distinguished by their differing nitrate and phosphate concentration relationships. Using these relationships, we estimate the amount of Atlantic and Pacific waters in the surface layer (top 30 m) of the Arctic Ocean. Atlantic source water is dominant in most of the Eurasian Basin and is present in significant amounts in the Makarov Basin north of the East Siberian Sea. Pacific source water is dominant in most of the Canadian Basin and is present in significant amounts in the Amundsen Basin north of Greenland. We deduce circulation patterns from the distributions of Atlantic and Pacific source waters in the surface layer of the Arctic Ocean and conclude that the flow within the surface layer differs from ice drift along the North American and European boundaries of the Polar Basin.

Waters of the Makarov and Canada basins

Abstract Hydrographic measurements from the 1994 Arctic Ocean Section show how the Makarov and Canada basins of the Arctic Ocean are related, and demonstrate their oceanographic connections to the Eurasian Basin. The inflow into the Makarov Basin consists largely of well-ventilated water within a broad band of densities from a boundary flow over the Siberian end of the Lomonosov Ridge. The boundary flow contains a significant component of dense shelf water likely originating in the Barents, Kara, and Laptev Seas. Earlier ice camp data show that the Canada Basin is relatively more isolated from this ventilation source. In the Canada Basin shelf sources influenced by Bering Sea water appear to add cold waters with high silicate concentrations to the halocline and deeper. In 1994 the halocline silicate maximum over the central Makarov Basin was absent, evidence of the recent displacement of the upper (S∼ 33.1) halocline water from the Chukchi-East Siberian Sea region by water from the Eurasian Basin. Much of the Makarov Basin water in and below the halocline is in fact from the Eurasian Basin, with admixture of waters from the Canada Basin suggested by their higher silicate concentrations. Mid-depth eddies may transport anomalous properties into the central Arctic and create property gradients or fronts in mid-depth and deep waters. The complex topography of the Mendeleyev Ridge-Chukchi Plateau region also may assist spreading of water from the boundary into the interior. Atlantic layer characteristics in 1994 differed from previous general depictions. In particular the core temperatures at the Chukchi-Mendeleyev boundary were at least 0.2°C warmer on average than indicated in earlier work. The recent warming at intermediate depth has resulted from inflow of Atlantic waters that have been cooled relatively little during their transit of the Norwegian Sea.

Changes in temperature and tracer distributions within the Arctic Ocean: results from the 1994 Arctic Ocean section

Abstract Major changes in temperature and tracer properties within the Arctic Ocean are evident in a comparison of data obtained during the 1994 Arctic Ocean Section to earlier measurements. (1) Anomalously warm and well-ventilated waters are now found in the Nansen, Amundsen and Makarov basins, with the largest temperature differences, as much as 1 °C, in the core of the Atlantic layer (200–400 m). Thus thermohaline transition appears to follow from two distinct mechanisms: narrow (order 100 km), topographically-steered cyclonic flows that rapidly carry new water around the perimeters of the basins; and multiple intrusions, 40–60 m thick, which extend laterally into the basin interiors. (2) Altered nutrient distributions that within the halocline distinguish water masses of Pacific and Atlantic origins likewise point to a basin-wide redistribution of properties. (3) Distributions of CFCs associated with inflows from adjacent shelf regions and from the Atlantic demonstrate recent ventilation to depths exceeding 1800 m. (4) Concentrations of the pesticide HCH in the surface and halocline layers are supersaturated with respect to present atmospheric concentrations and show that the ice-capped Arctic Ocean is now a source to the global atmosphere of this contaminant. (5) The radionuclide 129I is now widespread throughout the Arctic Ocean. Although the current level of 129I level poses no significant radiological threat, its rapid arrival and wide distribution illustrate the speed and extent to which waterborne contaminants are dispersed within the Arctic Ocean on pathways along which other contaminants can travel from western European or Russian sources.

Water masses and circulation in the Eurasian basin. Results from the Oden-91 expedition

The Oden 91 North Pole expedition obtained Oceanographic measurements on four sections in the Nansen and Amundsen basins of the Eurasian Basin and in the Makarov Basin of the Canadian Basin, thereby proving the feasibility of carrying out a typical Oceanographic program using an icebreaker in the Arctic Ocean. The data show greater spatial variability in water structure and circulation than was apparent from previous data. The results show that a clear front exists between the Eurasian and Canadian basins such that upper halocline water in the Canadian Basin is almost absent from the Eurasian Basin. The lower halocline water produced in the Barents-Kara Sea region permeates much of the Eurasian Basin and flows along the continental slope into the Canadian Basin. The deeper circulation is strongly influenced by topography. Three return flows of the Atlantic layer are identified, one over the Nansen-Gakkel Ridge, one over the Lomonosov Ridge, and a third flowing from the Canadian Basin. The slight differences observed in salinity and temperature characteristics of the deeper waters of the Nansen and Amundsen basins do not lead to an obvious explanation of their origin or flow pattern.

Connections among ice, runoff and atmospheric forcing in the Beaufort Gyre

During SHEBA, thin ice and freshening of the Arctic Ocean surface in the Beaufort Sea led to speculation that perennial sea ice was disappearing [McPhee et al., 1998]. Since 1987, we have collected salinity, δ18O and Ba profiles near the initial SHEBA site and, in 1997, we ran a section out to SHEBA. Resolving fresh water into runoff and ice melt, we found a large background of Mackenzie River water with exceptional amounts in 1997 explaining much of the freshening at SHEBA. Ice melt went through a dramatic 4–6 m jump in the early 1990s coinciding with the atmospheric pressure field and sea-ice circulation becoming more cyclonic. The increase in sea-ice melt appears to be a thermal and mechanical response to a circulation regime shift. Should atmospheric circulation revert to the more anticyclonic mode, ice conditions can also be expected to revert although not necessarily to previous conditions.