Humfrey Melling

Modification of halocline source waters during freezing on the Beaufort Sea shelf: evidence from oxygen isotopes and dissolved nutrients

Abstract During some, but not all winters, waters on the Mackenzie shelf of the Beaufort Sea become sufficiently saline to ventilate the halocline of the adjacent Canada Basin. This occurred in March 1988, at which time a survey of the temperature, salinity, dissolved nutrient and 18 O properties of the ventilating waters was completed. The regional hydrography of 1988 was very similar to that of 1981, when ventilation also occurred in this area. The δ 18 O-salinity properties of the cold, saline shelf waters revealed that in the winter of 1987–1988, ice was grown from water initially more saline by about 1.5 [psu] than is typical for the area. The higher initial salinity appears to have been a consequence of a two-stage conditioning of shelf waters by storms in the autumn of 1987. Since the amount of ice growth, and consequent salt rejection, over the winter of 1987–1988 was abnormally low, this conditioning played a crucial role in the formation of the ventilating water mass. Nutrient concentrations in ventilating waters were the same as those of waters unaffected by freezing. Thus significant regeneration of nutrients within the cold saline shelf waters did not occur during their 6-month period of formation. In consequence, the nutrient signatures carried into the arctic halocline by winter shelf waters from this area tended to erode, rather than to reinforce the nutrient maxima. For this reason they are not the dominant source of supply to the arctic halocline. Waters in the Chukchi and northern Bering Seas during the same period had δ 18 O values intermediate between those on the Mackenzie shelf and those in the arctic halocline. Thus winter shelf waters are supplied to the arctic halocline with a range of nutrient, temperature, salinity and δ 18 O properties. On average, the southern Canada Basin is an impressive net producer of sea ice. The net rate of production from waters in the upper 350 m in this area is about 2 m y −1 , approximately twice the net rate of production in the central Arctic Ocean.

Ocean circulation within the North Water polynya of Baffin Bay

Abstract The North Water Polynya is a recurrent polynya at the northern end of Baffin Bay. As part of the International North Water Project, recording current meters were moored within this polynya during 1997–98 to study the physical reasons for its existence. The data demonstrate that the North Water Polynya is dominated by a strong southward flow of cold water and ice from the Arctic Ocean. Although the West Greenland Current directs a modest flow of warmer water towards the polynya from the south‐east, this flow loses much of its heat south of the polynya through re‐circulation into and isopycnal mixing with the Arctic outflow. If an ice jam stops the inflow of ice from the north, the continued drift of ice southward ‘below’ the blockage is sufficient to create a large polynya without oceanic heating. However, upwelling near the Greenland coast can bring relatively warm water to the base of the turbulent surface layer where it is entrained via convection driven by brine growing from ice. The resulting flux of sensible heat supplies about one‐third of the heat loss at the surface and slows ice growth. Because the sensible heat flux is dependent upon freezing, it decreases as ice growth slows in spring and cannot, itself, generate ice‐free waters early in the year.

The formation of a haline shelf front in wintertime in an ice-covered arctic sea

Abstract During some winters, the water overlying Mackenzie shelf in the southeastern Beaufort Sea becomes quite saline (33–35) and at freezing temperature throughout. Although this water is found at the surface, its density is that of waters within the halocline of the arctic basin, and ventilation of the halocline occurs. The formation of sea ice is a necessary, but not a sufficient condition for the production of this water. Observations from the winter of 1980–1981 are presented which illustrate that a two-stage preconditioning of shelf waters is also required: first, surface waters of low salinity must be driven from the shelf by strong westerly winds; then, more saline water must upwell onto the shelf in response to prolonged easterly winds. A haline front forms on the outer shelf separating cold, saline shelf waters from slightly warmer, less saline slope waters. The characteristics of this front are controlled by the input of negative buoyancy from ice growth over the shelf, and by turbulent entrainment and mixing driven by under-ice convection. A simple model to predict the position, dimension and buoyancy contrast of this front is presented. The cross-isobath circulation is examined and found to be relatively ineffective at flushing dense waters from the shelf over a winter. Thus shelf waters accumulate almost all the salt expelled from growing sea ice over the same time. The magnitude of the contribution by this shelf to renewal of waters in the arctic halocline is estimated to average 0.04 × 10 6 m 3 s −1 over a period of years. Although small relative to the overall rate of renewal (1 × 10 6 m 3 s −1 ), this contribution is in proportion to the fraction of the arctic shelf area which this region represents.

Measurements of the Underside Topography of Sea Ice by Moored Subsea Sonar

Abstract A practical technology based on moored subsea instrumentation has been developed to measure the draft of polar pack ice. The technology exploits the complementary capabilities of an ice-profiling sonar designed and built for the application and of a commercially available acoustic Doppler sonar. The former instrument observes the zenithal range of sea ice passing through its single narrow sonar beam, while the latter observes the radial motion of the ice along its four inclined beams. The sequence of ranges obtained by the ice-profiling sonar is combined with supplementary observations of hydrostatic pressure to yield a sequence of ice draft versus time; the sequence of Doppler speeds provides ice velocity that can be integrated to obtain displacement; by combining the draft and displacement sequences the profile of draft versus position is obtained. The foremost practical problem in calibration is establishing the temporal variation in the zero-draft reference. The technology is well suited to u...

Ocean circulation and properties in Petermann Fjord, Greenland

The floating ice shelf of Petermann glacier interacts directly with the ocean and isthought to lose at least 80% of its mass through basal melting. Based on three opportunisticocean surveys in Petermann Fjord we describe the basic oceanography: the circulationat the fjord mouth, the hydrographic structure beneath the ice shelf, the oceanic heatdelivered to the under‐ice cavity, and the fate of the resulting melt water. The 1100 m deepfjord is separated from neighboring Hall Basin by a sill between 350 and 450 m deep.Fjord bottom waters are renewed by episodic spillover at the sill of Atlantic water from theArctic. Glacial melt water appears on the northeast side of the fjord at depths between200 m and that of the glacier’s grounding line (about 500 m). The fjord circulation isfundamentally three‐dimensional; satellite imagery and geostrophic calculations suggest acyclonic gyre within the fjord mouth, with outflow on the northeast side. Tidal flowsare similar in magnitude to the geostrophic flow. The oceanic heat flux into the fjordappears more than sufficient to account for the observed rate of basal melting. Cold,low‐salinity water originating in the surface layer of Nares Strait in winter intrudes farunder the ice. This may limit basal melting to the inland half of the shelf. The melt rate andlong‐term stability of Petermann ice shelf may depend on regional sea ice cover andfjord geometry, in addition to the supply of oceanic heat entering the fjord.

An overview of physical processes in the North Water

The presence of a polynya strongly affects biological processes by influencing underwater light levels, water-column stratification, the upwelling of nutrients, and the timing of production cycles. An overview of the existing literature on the oceanography, meteorology and sea-ice conditions of the North Water, a large recurring polynya in northern Baffin Bay, is presented. The North Water is influenced by a cold inflow of Arctic Ocean water from the north and, in some areas, a warmer inflow of Atlantic water from the south. Earlier observations and modeling studies suggested that both wind and ocean advection of sea ice, as well as heat input to the surface waters by either upwelling or mixing, were responsible for the formation and maintenance of the polynya. Recent data indicate the North Water to be primarily generated by the southward drift of ice by winds and currents, as opposed to melting ice by sensible heat input over large areas. Localized sensible heat effects occur in fall, winter and spring along the Greenland side of the polynya.

Oceanic thermal structure in the western Canadian Arctic

Recent hydrographic data (1981–1982) from the western Canadian Arctic Archipelago and adjacent areas of the Arctic Ocean are interpreted from the viewpoint of thermal energy transfer. Within the Archipelago, a warmer halocline than in the Arctic Ocean and a cooler Atlantic layer are identified. The warmer halocline is a consequence of the continued diffusion of heat from underlying Atlantic water without a significant downward penetration from the surface of cold (≤1.5°C) seawater with salinity increased consequent to ice growth. The cooler Atlantic layer is primarily attributable to an enhanced cooling of these waters in a narrow band over the continental slope and shelf of the southern Beaufort Sea prior to their inflow into the Archipelago. Rates of transport and vertical diffusion in this region are estimated. The significance of these findings in regional and Arctic oceanography is discussed.

Exchanges of Freshwater through the Shallow Straits of the North American Arctic

The shallow straits of the North American Arctic include Bering Strait, which connects the Pacific Ocean to the Arctic Ocean, and the myriad channels of the Canadian Arctic Archipelago, which connect the Arctic Ocean to the Atlantic. A net flux of freshwater passes from the North Pacific into the North Atlantic via these straits (plus Fram Strait), and thereby stabilizes a global imbalance in precipitation less evaporation between the two major ocean basins [0, 2]. At the same time, the fluctuating outflows of fresh Arctic waters into the Greenland Sea, via Fram Strait, and into the Labrador Sea, via the Canadian Arctic Archipelago, may control the occurrence of deep ocean convection in these areas [3, 4, 5, 6]. For these reasons, the freshwater fluxes through the shallow straits of the North American Arctic are important within the global climate system.

Freshwater and its role in the Arctic Marine System: Sources, disposition, storage, export, and physical and biogeochemical consequences in the Arctic and global oceans

The Arctic Ocean is a fundamental node in the global hydrological cycle and the oceans thermohaline circulation. We here assess the systems key functions and processes: (1) the delivery of fresh and low-salinity waters to the Arctic Ocean by river inflow, net precipitation, distillation during the freeze/thaw cycle, and Pacific Ocean inflows; (2) the disposition (e.g., sources, pathways, and storage) of freshwater components within the Arctic Ocean; and (3) the release and export of freshwater components into the bordering convective domains of the North Atlantic. We then examine physical, chemical, or biological processes which are influenced or constrained by the local quantities and geochemical qualities of freshwater; these include stratification and vertical mixing, ocean heat flux, nutrient supply, primary production, ocean acidification, and biogeochemical cycling. Internal to the Arctic the joint effects of sea ice decline and hydrological cycle intensification have strengthened coupling between the ocean and the atmosphere (e.g., wind and ice drift stresses, solar radiation, and heat and moisture exchange), the bordering drainage basins (e.g., river discharge, sediment transport, and erosion), and terrestrial ecosystems (e.g., Arctic greening, dissolved and particulate carbon loading, and altered phenology of biotic components). External to the Arctic freshwater export acts as both a constraint to and a necessary ingredient for deep convection in the bordering subarctic gyres and thus affects the global thermohaline circulation. Geochemical fingerprints attained within the Arctic Ocean are likewise exported into the neighboring subarctic systems and beyond. Finally, we discuss observed and modeled functions and changes in this system on seasonal, annual, and decadal time scales and discuss mechanisms that link the marine system to atmospheric, terrestrial, and cryospheric systems.

Toward Quantifying the Increasing Role of Oceanic Heat in Sea Ice Loss in the New Arctic

AbstractThe loss of Arctic sea ice has emerged as a leading signal of global warming. This, together with acknowledged impacts on other components of the Earth system, has led to the term “the new Arctic.” Global coupled climate models predict that ice loss will continue through the twenty-first century, with implications for governance, economics, security, and global weather. A wide range in model projections reflects the complex, highly coupled interactions between the polar atmosphere, ocean, and cryosphere, including teleconnections to lower latitudes. This paper summarizes our present understanding of how heat reaches the ice base from the original sources—inflows of Atlantic and Pacific Water, river discharge, and summer sensible heat and shortwave radiative fluxes at the ocean/ice surface—and speculates on how such processes may change in the new Arctic. The complexity of the coupled Arctic system, and the logistic and technological challenges of working in the Arctic Ocean, require a coordinated ...