Open Water Polynya Research Articles

Study of the structure and intensity of density currents in the shelf-slope region in the Antarctic

The research involves the examination of modeling outcomes regarding the density structure and baroclinic dynamics of Antarctic shelf waters (ASW) within the shelf-slope area, encompassing a wide range of extreme weather conditions. We used a small-scale non-hydrostatic Fluidity-ICOM model to understand the formation and persistence of quasi-stationary polynyas in the Antarctic, which play a role in enhancing the formation of ASW. The salt fluxes, or buoyancy, are calculated for different forms of ice formation, namely static ice formation in young ice-covered polynyas and dynamic intra-water ice formation, which is considered the most effective and occurs in open water polynyas. Based on the intensification of ASW formation and its spread, three distinct modes of propagation along the continental slope have been identified: non-wave or subcritical mode, vortex mode, and wave or supercritical mode, which is characterized by rapid flow. The classification into different modes is determined by the internal Froude number (Fr) estimates. At the moment when the most developed stage of near-bottom density currents are transformed on a slope, the spatial dimensions of meanders, eddies, or frontal waves were found to be similar in magnitude, as well as their thickness. This observation aligns with model calculations of the local baroclinic Rossby deformation radius (RdL) for these currents. These findings agree with comparable assessments of the baroclinic Rossby deformation radius (RdL) for the Antarctic Slope Front (ASF) in the Commonwealth Sea, which were based on field observations. Additionally, the calculated propagation velocities of density currents and the density gradients at their boundaries coincide with the data obtained from field measurements. By estimating the volumetric fluxes (qv) and specific fluxes (ql) of ASW along the continental slope near the Cape Darnley coastal polynya area in the Commonwealth Sea, we can determine the contribution of ASW cascading to the formation of bottom waters under different flux regimes. The precision and accuracy of the qv and ql estimates are ensured through small-scale calculations using the non-hydrostatic Fluidity-ICOM model. These calculations consider the occurrences of intensified ASW formation in open water polynyas. Numerical experiments have revealed that a four-fold increase in a spatial step X results in an underestimation of qv by approximately 30%. As a consequence, in large-scale and even mesoscale hydrostatic models, such underestimation of qv and ql may be unsatisfactory (several times lower).

The role of oceanic heat flux in reducing thermodynamic ice growth in Nares Strait and promoting earlier collapse of the ice bridge

Abstract. The ice bridge in Nares Strait is a well-known phenomenon that affects the liquid and solid freshwater flux from the Arctic Ocean through the strait and controls the downstream North Open Water polynya in northern Baffin Bay. Recently, the ice bridge has been in a state of decline, either breaking up earlier in the year or not forming at all and thereby increasing the sea ice export out of the Arctic Ocean. The decline in the ice bridge has been ascribed to thinner and therefore weaker ice from the Arctic Ocean entering Nares Strait; however, local forcing also affects the state of the ice bridge and thereby influences when it breaks up. Using a variety of remotely sensed data we examine the spatial patterns of sea ice thickness within the ice bridge, highlighting the presence of negative ice thickness anomalies on both the eastern and western sides of the strait and identifying a recurrent sensible heat polynya that forms within the ice bridge near Cape Jackson in northwestern Greenland. Using the sea ice–ocean model FESOM2, we then attribute these ice thickness anomalies to the heat from warmer subsurface waters of Pacific and Atlantic origin that reduce thermodynamic ice growth throughout winter on the western and eastern sides, respectively. The consequently weaker and thinner areas within the ice bridge are then suggested to promote instability and earlier breakup. This work provides new insight into the structure of the Nares Strait ice bridge and highlights that warming of the modified Atlantic and/or Pacific waters that enter the strait may contribute to its further decline.

Polar Bear Fossil and Archaeological Records from the Pleistocene and Holocene in Relation to Sea Ice Extent and Open Water Polynyas

Polar Bear Fossil and Archaeological Records from the Pleistocene and Holocene in Relation to Sea Ice Extent and Open Water Polynyas

Subpolar Southern Ocean Response to Changes in the Surface Momentum, Heat, and Freshwater Fluxes under 2xCO2

AbstractThe Antarctic subpolar Southern Ocean (sSO) has fundamental climate importance. Antarctic Bottom Water (AABW) originates in the sSO and supplies the lower limb of the meridional overturning circulation (MOC), occupying 36% of ocean volume. Climate models struggle to represent continental shelf processes that form AABW. We explore sources of persistent model biases by examining response of the sSO to perturbations in surface forcing in a global ocean–sea ice model (ACCESS-OM2) that forms AABW both on shelf and in open ocean. The sSO response to individual and combined perturbations of surface heat, freshwater, and momentum fluxes follows the WCRP CMIP6 FAFMIP-protocol. Wind perturbation (i.e., a poleward shift and intensification of the westerlies) is dominant, enhancing AABW formation and accelerating the global MOC. This occurs through upwelling of warm waters and inhibition of sea ice growth during winter, which triggers large open water polynya (OWP) events with associated deep convection. These events occur in the Weddell and Ross Seas and their variability is associated with availability of heat at midocean depths. These OWPs cease when the heat reservoir is depleted. Effects of surface warming and freshening only partially compensate changes from increasing winds on ocean stratification and depletion of AABW formation. These results indicate that overly convective models, such ACCESS-OM2, can respond to CO2-perturbed scenarios by forming too much AABW in OWP, which might not hold in models without OWPs. This might contribute to the large intermodel spread thermosteric sea level projections, being relevant to the interpretation of future projections by current climate models.

Southern Ocean polynyas in CMIP6 models

Abstract. Polynyas facilitate air–sea fluxes, impacting climate-relevant properties such as sea ice formation and deep water production. Despite their importance, polynyas have been poorly represented in past generations of climate models. Here we present a method to track the presence, frequency and spatial distribution of polynyas in the Southern Ocean in 27 models participating in the Climate Model Intercomparison Project Phase 6 (CMIP6) and two satellite-based sea ice products. Only half of the 27 models form open-water polynyas (OWPs), and most underestimate their area. As in satellite observations, three models show episodes of high OWP activity separated by decades of no OWP, while other models unrealistically create OWPs nearly every year. In contrast, the coastal polynya area is overestimated in most models, with the least accurate representations occurring in the models with the coarsest horizontal resolution. We show that the presence or absence of OWPs is linked to changes in the regional hydrography, specifically the linkages between polynya activity with deep water convection and/or the shoaling of the upper water column thermocline. Models with an accurate Antarctic Circumpolar Current transport and wind stress curl have too frequent OWPs. Biases in polynya representation continue to exist in climate models, which has an impact on the regional ocean circulation and ventilation that should be addressed. However, emerging iceberg discharge schemes, more adequate vertical grid type or overflow parameterisation are anticipated to improve polynya representations and associated climate prediction in the future.

Reconstruction of Sea Ice Concentration in Northern Baffin Bay Using Deuterium Excess in a Coastal Ice Core From the Northwestern Greenland Ice Sheet

AbstractVariations in the conditions of sea ice in the northern part of Baffin Bay and North Open Water polynya influence human activity in northwestern Greenland through oceanic circulation and heat balance between air and sea. To evaluate the impact of variations in sea ice conditions on the surrounding environment, it is important to understand the mechanism of sea ice variations over long periods. In this study, we estimated the age of the SIGMA‐A ice core collected northwestern Greenland Ice Sheet and researched the relationship between annual or seasonal deuterium excess (d‐excess) and seasonal sea ice concentration. We found that a temporal variation in the spring d‐excess in the ice core negatively correlated significantly with that of sea ice concentration in February–April in northern Baffin Bay from 1979–2005 (r = −0.61,p < 0.001). Using this relationship, we reconstructed the temporal variations in sea ice concentrations for 100 years from the ice core drilled in the northwestern Greenland Ice Sheet. The sea ice concentration in the early twentieth century was lower than that in the present. The decrease in sea ice concentration was consistent with analytical results for marine sediments obtained from Baffin Bay. We also suggested that the sea ice concentration was controlled by atmospheric conditions from the 1920s to 1940s based on examinations of correlations with the North Atlantic Oscillation index and air temperature in Ilulissat and by oceanographic conditions from 1945–1955, 1959–1969, and 1982–1992 based on the Atlantic Multidecadal Oscillation index and meridional heat transport to western Greenland.

Linking the Open Water Area of the North Open Water Polynya to Climatic Parameters using a Multiple Linear Regression Prediction Model

ABSTRACTThe North Open Water polynya is a recurring polynya and is a significant feature of northern Baffin Bay in the winter. An area of open water remains as the sea ice grows around it in the fall, and in the spring the polynya gradually expands as the thinner ice surrounding it melts. It is an important overwintering area for various bird and animal species. With climate change affecting the Arctic in particular, this polynya has seen significant transformation in terms of its area of open water. Our study analyzes this transformation and attempts to understand it by linking it to climatic parameters using a multiple regression model. Using data from ice analyses from Environment and Climate Change Canada’s Canadian Ice Service, a significant difference in the annual maximum polynya extent was observed before and after 1998. The main climate indices related to the extent of the polynya for the years prior to 1998 are surface wind speed and sea-ice concentration. For the years after 1998, the main indices are surface wind speed, sea surface temperature, and surface air temperature. This study indicates that the extent of open water in the polynya could be a good indicator of a new climate regime after 1998.

Enhanced bottom-ice algal biomass across a tidal strait in the Kitikmeot Sea of the Canadian Arctic

Sea ice algae are an important contributor of primary production in the Arctic ecosystem. Within the bottom-ice environment, access to nutrients from the underlying ocean is a major factor controlling production, phenology, and taxonomic composition of ice algae. Previous studies have demonstrated that tides and currents play an important role in driving the flux of nutrients to bottom-ice algal communities when biological demand during the spring bloom is high. In this study we investigate how surface currents under landfast first-year ice influence nutrient supply based on stoichiometric composition, algal chlorophyll a biomass and species composition during spring 2016, in Dease Strait, Nunavut. Stronger water dynamics over a shoaled and constricted strait dominated by tidal currents (tidal strait) supported turbulent flow more than 85% of the deployment duration in comparison to outside the tidal strait in an embayment where turbulent flow was only evidenced a small percentage (<15%) of the time. The system appeared to be nitrate-depleted with surface water concentrations averaging 1.3 μmol L–1. Increased currents were correlated significantly with a decrease in ice thickness and an increase in ice algal chlorophyll a. Furthermore, pennate diatoms dominated the ice algal community abundance with greater contribution within the strait where currents were greatest. These observations all support the existence of a greater nutrient flux to the ice bottom where currents increased towards the center of the tidal strait, resulting in an increase of bottom ice chlorophyll a biomass by 5–7 times relative to that outside of the strait. Therefore, expanding beyond the long identified biological hotspots of open water polynyas, this paper presents the argument for newly identified hotspots in regions of strong sub-ice currents but persistent ice covers, so called “invisible polynyas”.

Reprint of: Benthic and planktic community changes at the North Siberian margin in response to Atlantic water mass variability since last deglacial times

Abstract The eastward penetration of Atlantic-derived water (ADW) into the Eurasian Basin of the Arctic Ocean was investigated at the western Laptev Sea continental margin for the time since c. 17.6 ka. Using a high-resolution investigation of the lithology, geochemistry, planktic and benthic foraminifers, and ostracods on a sediment core from 270 m water depth major steps in the environmental evolution of the region are recognized. In general, ADW was continuously present in the study area. Between 17.6 and 15.4 ka ADW manifested itself through open-water polynyas and associated upwelling events. Comparison between the Laptev Sea and northern Svalbard shelf using Cassidulina neoteretis allows assuming an unmodified subsurface inflow of ADW within its northern branch between 15.4 and 13.2, which was strongest after 14.7-ka and in line with the overall climate amelioration. A local freshwater event at 13 ka followed by shelf flooding and the establishment of a freshened shelf water mass resulted in an off-shelf displacement of ADW from the studied site as suggested by the disappearance of C. neoteretis between 12 and 7 ka. As evidenced by an abundance peak in Nonion labradoricum , the sea-ice marginal zone was located at the site around 12–11 ka but then shifted northward during the early Holocene warming. Enhanced ADW inflow since 7 ka correlated with climate cooling and southward retreat of the seasonal drift-ice margin. The inflow of ADW during mid–late Holocene differed from deglacial times because of the combined influence of northern and eastern ADW branches.

Benthic and planktic community changes at the North Siberian margin in response to Atlantic water mass variability since last deglacial times

The eastward penetration of Atlantic-derived water (ADW) into the Eurasian Basin of the Arctic Ocean was investigated at the western Laptev Sea continental margin for the time since c. 17.6 ka. Using a high-resolution investigation of the lithology, geochemistry, planktic and benthic foraminifers, and ostracods on a sediment core from 270 m water depth major steps in the environmental evolution of the region are recognized. In general, ADW was continuously present in the study area. Between 17.6 and 15.4 ka ADW manifested itself through open-water polynyas and associated upwelling events. Comparison between the Laptev Sea and northern Svalbard shelf using Cassidulina neoteretis allows assuming an unmodified subsurface inflow of ADW within its northern branch between 15.4 and 13.2, which was strongest after 14.7-ka and in line with the overall climate amelioration. A local freshwater event at 13 ka followed by shelf flooding and the establishment of a freshened shelf water mass resulted in an off-shelf displacement of ADW from the studied site as suggested by the disappearance of C. neoteretis between 12 and 7 ka. As evidenced by an abundance peak in Nonion labradoricum, the sea-ice marginal zone was located at the site around 12–11 ka but then shifted northward during the early Holocene warming. Enhanced ADW inflow since 7 ka correlated with climate cooling and southward retreat of the seasonal drift-ice margin. The inflow of ADW during mid–late Holocene differed from deglacial times because of the combined influence of northern and eastern ADW branches.

Evidence of reactive iodine chemistry in the Arctic boundary layer

Although it has recently been established that iodine plays an important role in the atmospheric chemistry of coastal Antarctica, where it occurs at levels which cause significant ozone (O3) depletion and changes in the atmospheric oxidising capacity, iodine oxides have not previously been observed conclusively in the Arctic boundary layer (BL). This paper describes differential optical absorption spectroscopy (DOAS) observations of iodine monoxide (IO), along with gas chromatographic measurements of iodocarbons, in the sub‐Arctic environment at Kuujjuarapik, Hudson Bay, Canada. Episodes of elevated levels of IO (up to 3.4 ± 1.2 ppt) accompanied by a variety of iodocarbons were observed. Air mass back trajectories show that the observed iodine compounds originate from open water polynyas that form in the sea ice on Hudson Bay. A combination of long‐path DOAS and multiaxis DOAS observations suggested that the IO is limited to about 100 m in height. The observations are interpreted using a one‐dimensional model, which indicates that the iodocarbon sources from these exposed waters can account for the observed concentrations of IO. These levels of IO deplete O3 at rates comparable to bromine oxide (BrO) and, more importantly, strongly enhance the effect of bromine‐catalyzed O3 depletion in the Arctic BL, an effect which has not been quantitatively considered hitherto. However, the measurements and modeling results indicate that the effects of iodine chemistry are on a much more localized scale than bromine chemistry in the Arctic environment.

Mass balance of the Prince of Wales Icefield, Ellesmere Island, Nunavut, Canada

This paper estimates the mass balance of the Prince of Wales Icefield, Ellesmere Island, Canada, averaged over four decades, from measurements of surface mass balance (SMB) and iceberg calving. Shallow ice core net accumulation measurements and annual mass balance stake measurements are used in conjunction with a digital elevation model and knowledge of the location of the dominant moisture source for precipitation over the ice cap to interpolate and extrapolate spatial patterns of SMB across the Prince of Wales Icefield. The contribution of iceberg calving to the mass balance is calculated from estimates of (1) the annual volume of ice discharged at the major tidewater glacier termini and (2) the annual volume loss or gain due to terminus fluctuations. Two different approaches to determining the SMB conclude that the SMB of the ice field is approximately in balance (average equals −0.1 ± 0.4 km3 w.e. a−1, where w.e. means water equivalent) largely because of its proximity to the main year‐round moisture source that is the Smith Sound portion of the North Open Water polynya. Iceberg calving is a highly significant component of mass loss (−1.9 ± 0.2 km3 w.e. a−1) and is sufficient to make the overall mass balance of the ice field averaged over the period 1963–2003 clearly negative (−2 ± 0.45 km3 w.e. a−1, equivalent to a mean‐specific mass balance across the ice field of −0.1 m w.e. a−1). The Prince of Wales Icefield contributes ∼0.005 mm a−1 to global eustatic sea level rise.

Stratigraphic analysis of an ice core from the Prince of Wales Icefield, Ellesmere Island, Arctic Canada, using digital image analysis: High‐resolution density, past summer warmth reconstruction, and melt effect on ice core solid conductivity

High‐resolution (1 mm) stratigraphic information was derived from digital image analysis of an ice core from the Prince of Wales (POW) Icefield, Central Ellesmere Island, Canada. Following careful image processing, a profile of ice core transmitted light was derived from the greyscale images and used to reconstruct high‐resolution density variations for the unfractured sections of the core. Images were further classified into infiltration and glacier ice using an automatic thresholding procedure, and were converted to a high‐resolution melt percentage index. The mean annual melt percentage over the last 580 years was 9%, and melting occurred in 8 years out of 10. Melting obliterated most of the original depositional sequence, and seasonal density cycles were mostly unrecognizable. The ice core solid conductivity was greater and more variable in melt features than in glacier ice, owing to washout of strong acids by meltwater (elution) and chemical enrichment upon refreezing. This hindered the identification of acid volcanic layers and further compromised dating by annual layer counting. Comparison of the melt record with those from other Arctic ice caps shows that the melt‐temperature relationship on POW Icefield is site‐specific. We speculate that this is due to the peculiar position of the icefield, which rests on the periphery of the Baffin Bay maritime climate zone, and to the proximity of the North Open Water polynya, which controls snow accumulation variability on the icefield and affects the melt percentage index.

On the link between SAR‐derived sea ice melt and development of the summer upper ocean mixed layer in the North Open Water Polynya

We examine the ability of SAR‐derived landfast sea ice thermodynamic state surrounding the North Water Polynya to predict the timing of a shallow, stably stratified summer ocean mixed layer in the region. Radarsat‐1 ScanSAR is used to derive melt and pond onset dates over landfast sea ice surrounding the polynya, describing its thermodynamic evolution. Salinity and temperature profiles, connected in time and space, are complemented by potential density profiles and mixed layer depths, which describe the evolution of the upper ocean mixed layer between 4 April and 21 July 1998. Surface salinity driven potential density stratification and summer mixed layer depths are temporally coincident. Sea ice–ocean connection is first described at the local scale, then regionally. At the local scale, a reduction in surface salinity occurs between the melt and pond onsets over landfast sea ice. At the polynya scale, interpolated pond onset date is highly effective at predicting the timing of a thin, stably stratified summer ocean mixed layer; this has consequences for the potential prediction of timing of maximum ecosystem productivity.