Heat Polynya Research Articles

Causes and evolution of winter polynyas north of Greenland

Abstract. During the 42-year period (1979–2020) of satellite measurements, four major winter (December–March) polynyas have been observed north of Greenland: one in December 1986 and three in the last decade, i.e., February of 2011, 2017, and 2018. The 2018 polynya was unparalleled in its magnitude and duration compared to the three previous events. Given the apparent recent increase in the occurrence of these extreme events, this study aims to examine their evolution and causality, in terms of forced versus natural variability. The limited weather station and remotely sensed sea ice data are analyzed combining with output from the fully coupled Regional Arctic System Model (RASM), including one hindcast and two ensemble simulations. We found that neither the accompanying anomalous warm surface air intrusion nor the ocean below had an impact (i.e., no significant ice melting) on the evolution of the observed winter open-water episodes in the region. Instead, the extreme atmospheric wind forcing resulted in greater sea ice deformation and transport offshore, accounting for the majority of sea ice loss in all four polynyas. Our analysis suggests that strong southerly winds (i.e., northward wind with speeds greater than 10 m s−1) blowing persistently over the study region for at least 2 d or more were required over the study region to mechanically redistribute some of the thickest Arctic sea ice out of the region and thus to create open-water areas (i.e., a latent heat polynya). To assess the role of internal variability versus external forcing of such events, we carried out and examined results from the two RASM ensembles dynamically downscaled with output from the Community Earth System Model (CESM) Decadal Prediction Large Ensemble (DPLE) simulations. Out of 100 winters in each of the two ensembles (initialized 30 years apart: one in December 1985 and another in December 2015), 17 and 16 winter polynyas were produced north of Greenland, respectively. The frequency of polynya occurrence had no apparent sensitivity to the initial sea ice thickness in the study area pointing to internal variability of atmospheric forcing as a dominant cause of winter polynyas north of Greenland. We assert that dynamical downscaling using a high-resolution regional climate model offers a robust tool for process-level examination in space and time, synthesis with limited observations, and probabilistic forecasts of Arctic events, such as the ones being investigated here and elsewhere.

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.

Oceanic conditions in the Barrow Coastal Polynya revealed by a 10-year mooring time series

From 2009 to 2021, continuous mooring observations were conducted within the Barrow Coastal Polynya (BCP) on the northeastern Chukchi Shelf. With a 10-year mooring record (2009–2019), satellite-derived sea-ice production (SIP) data, and atmospheric reanalysis, we confirm that the BCP is a wind-driven hybrid latent and sensible heat polynya forced by northeasterly winds, and we analyze oceanic conditions and their recent changes associated with winter processes such as freeze onset, SIP, and winter-water production. The decade-long record of SIP and warm water upwelling in the BCP is dominated by interannual variability with no discernable long-term trend. In the 2015/2016 season, the SIP reached a record high for the time period studied. Water temperature and salinity prior to the freeze-up period are directly associated with SIP and winter-water production. Although wintertime SIP-related salt input is necessary, autumn salinity is, in some cases, more critical in determining winter water characteristics. The 10-year BCP mooring captures the regional changes in oceanic conditions. Summer-to-autumn ocean temperatures have been increasing as of late, especially since 2016. The prolonged presence of warmer water suggests a direct influence on freeze onset over the Chukchi Shelf and enhanced heat transport into the Arctic basin. In some cases, the upwelled Atlantic-origin warm water also delays freeze onset in the BCP. The 10-year mooring record provides evidence of recent extreme oceanic warming combined with remote and local connections from the Bering to the Chukchi Sea. This collection of data improves understanding of the impact of oceanic changes in Chukchi coastal and inner shelf waters, and regional change in the Pacific Arctic in a rapidly changing climate.

The thermodynamic and dynamic control of the sensible heat polynya in the western Cosmonaut Sea

A prominent feature in the west Cosmonaut Sea is the reoccurrence of a sensible heat polynya — the western Cosmonaut Sea Polynya (wCSP), which normally exists in late austral autumn and early winter. The Southern Ocean Sate Estimate (SOSE) reanalysis product is employed to investigate the thermodynamic and dynamic processes controlling the formation and evolution of a real wCSP event in 2009, when the polynya properties revealed from satellite observations are reasonably simulated by SOSE. An oceanic heat budget analysis was conducted for the surface layer above the thermocline, and the results reveal that heat advection is the major term contributing to the surface heat content variation in this area. The precursor of wCSP — an embayment — was formed during cyclonic atmospheric circulations. The negative wind stress curl from a cyclone induced upwelling that brought the warm circumpolar deep water to the surface and presumably melted ice. Meanwhile, the location of the cyclone relative to the wCSP created weaker easterlies over the northern boundary of polynya and stronger easterlies over the southern boundary of polynya. This leads to difference in the meridional oceanic heat transports across the northern and southern boundaries, resulting in net positive meridional oceanic heat transport into the polynya. Both the vertical heat advection and the meridional heat advection contribute significantly to increased heat content in the surface layer of wCSP, causing sea-ice melt and formation of the embayment. The cyclonic wind field promotes convergence of sea ice over the northern area of the embayment and results in the polynya. The formation of wCSP is likely related to the strength and position of Southern Annular Mode on interannual scale, and may potentially affect the biological productivity by controlling the availability of light in the mixed layer.

The ice factory of Hudson Bay

Within the dynamic seasonal ice cover of Hudson Bay, the Kivalliq Polynya is a large latent heat polynya that forms throughout winter in the northwest as a result of strong northwesterly offshore surface winds. Polynyas are known to be physically, biologically, and geochemically important and contribute to the regional ice mass balance; however, the Kivalliq Polynya has yet to be characterized in terms of spatiotemporal variability and ice production. Using a thin ice algorithm applied to the 16-year record of daily AMSR-E and AMSR-2 passive microwave observations, we examine the interannual variability in the spatial and temporal characteristics of the polynya throughout winter (December–April) over the period 2002–2019. Our study reveals that the polynya is present in some form almost every day but that its daily area is highly variable. On average, 182 km3 of new ice is produced in the Kivalliq Polynya during winter, or approximately 20% of the end of winter ice volume in Hudson Bay. Daily ice production is found to be significantly correlated with the daily polynya area, though large, episodic events can increase annual cumulative ice production during a year of otherwise small polynyas. Annual cumulative ice production is also found to be significantly correlated with seasonally averaged offshore wind speeds, which explain 47.3% of the variance in winter ice production and drive a 46 km3 increase in ice production for every 1.0 m s–1 increase in offshore winds. Ultimately, the highly variable yet persistent Kivalliq Polynya is shown to be driven by offshore winds and significantly contributes to the regional ice mass balance.

A wind‐driven, hybrid latent and sensible heat coastal polynya off Barrow, Alaska

AbstractThe nature of the Barrow Coastal Polynya (BCP), which forms episodically off the Alaska coast in winter, is examined using mooring data, atmospheric reanalysis data, and satellite‐derived sea‐ice concentration and production data. We focus on oceanographic conditions such as water mass distribution and ocean current structure beneath the BCP. Two moorings were deployed off Barrow, Alaska in the northeastern Chukchi Sea from August 2009 to July 2010. For sea‐ice season from December to May, a characteristic sequence of five events associated with the BCP has been identified; (1) dominant northeasterly wind parallel to the Barrow Canyon, with an offshore component off Barrow, (2) high sea‐ice production, (3) upwelling of warm and saline Atlantic Water beneath the BCP, (4) strong up‐canyon shear flow associated with displaced density surfaces due to the upwelling, and (5) sudden suppression of ice growth. A baroclinic current structure, established after the upwelling, caused enhanced vertical shear and corresponding vertical mixing. The mixing event and open water formation occurred simultaneously, once sea‐ice production had stopped. Thus, mixing events accompanied by ocean heat flux from the upwelled warm water into the surface layer played an important role in formation/maintenance of the open water area (i.e., sensible heat polynya). The transition from a latent to a sensible heat polynya is well reproduced by a high‐resolution pan‐Arctic ice‐ocean model. We propose that the BCP, previously considered to be a latent heat polynya, is a wind‐driven hybrid latent and sensible heat polynya, with both features caused by the same northeasterly wind.

A Heuristic Model of Dansgaard–Oeschger Cycles. Part I: Description, Results, and Sensitivity Studies

Abstract A simple model for studying the Dansgaard–Oeschger (D-O) cycles of the last glacial period is presented, based on the T. Dokken et al. hypothesis for D-O cycles. The model is a column model representing the Nordic seas and is composed of ocean boxes stacked below a one-layer sea ice model with an energy-balance atmosphere; no changes in the large-scale ocean overturning circulation are invoked. Parameterizations are included for latent heat polynyas and sea ice export from the column. The resulting heuristic model was found to cycle between stadial and interstadial states at times scales similar to those seen in the proxy observational data, with the presence or absence of perennial sea ice in the Nordic seas being the defining characteristic for each of these states. The major discrepancy between the modeled oscillations and the proxy record is in the length of the interstadial phase, which is shorter than that observed. The modeled oscillations were found to be robust to parameter changes, including those related to the ocean heat flux convergence (OHFC) into the column. Production of polynya ice was found to be an essential ingredient for such sustained oscillatory behavior. A simple parameterization of natural variability in the OHFC enhances the robustness of the modeled oscillations. The authors conclude by discussing the implications of such a hypothesis for the state of the Nordic seas today and its state during the Last Glacial Maximum and contrasting the model to other hypotheses that invoke large-scale changes in the Atlantic meridional overturning circulation for explaining millennial-scale variability in the climate system. An extensive time-scale analysis will be presented in the future.

Polynya dynamics drive primary production in the Larsen A and B embayments following ice shelf collapse

The climate‐driven collapses of the Larsen A and B ice shelves have opened up new regions of the coastal Antarctic to the influence of sea ice resulting in increases in seasonal primary production. In this study, passive microwave remote sensing of sea ice concentration and satellite imagery of ocean color are employed to quantify the magnitude of and variability in open water area and net primary productivity (NPP) in the Larsen embayments between 1997 and 2011. Numerical model output provides context to analyze atmospheric forcing on the coastal ocean. Following ice shelf disintegration the embayments function as coastal, sensible heat polynyas. The Larsen A and B are as productive as other Antarctic shelf regions, with seasonally averaged daily NPP rates reaching 1232 and 1127 mg C m−2 d−1 and annual rates reaching 200 and 184 g C m−2 yr−1, respectively. A persistent cross‐shelf gradient in NPP is present with higher productivity rates offshore, contrasting with patterns observed along the West Antarctic Peninsula. Embayment productivity is intimately tied to sea ice dynamics, with large interannual variability in NPP rates driven by open water area and the timing of embayment opening. Opening of the embayment is linked to periods of positive Southern Annular Mode and stronger westerlies, which lead to the vertical deflection of warm, maritime air over the peninsula and down the leeward side causing increases in surface air temperature and wind velocity. High productivity in these new polynyas is likely to have ramifications for organic matter export and marine ecosystem evolution.

Ice production in Storfjorden (Svalbard) estimated from a model based on AMSR‐E observations: Impact on water mass properties

Storfjorden, which hosts a latent heat polynya, is a well known region of dense water formation. This Brine‐enriched Shelf Water (BSW) displays substantial year to year variability in its properties, which is partly linked to interannual variations in ice production. Here we have developed a model based on high‐resolution AMSR‐E satellite sea‐ice concentration data, available between 2002 and 2011, and atmospheric forcing to estimate the ice production in the polynya and associated salt release. The average modeled ice production for the epoch 2002–2011 is 47 km3 per year, corresponding to a salt release of 1200 × 109 kg. The two most anomalous winters were 2004–2005 (salt deficit of −367 × 109 kg) and 2007–2008 (salt excess of 398 × 109 kg). Available observations of BSW properties are relatively scarce during this period and are here augmented with data collected in March 2007 from an ice‐tethered mooring to the northwest of the fjord. BSW was found up to the surface, with maximum salinity and density of 35.27 and 28.4 kg m−3, respectively, at 55 m. In addition, supercooled water was found down to 10 m under relatively mild atmospheric conditions. It is shown to have formed a week before, during an intense frazil ice formation episode, exceeding 2 km3 of frazil ice according to the model. Although observations remain too few to robustly assess the relation between ice production and BSW properties, there is suggestion of a direct impact for most anomalous years. The exceptional ice production in 2007–2008 is most likely the cause of the very saline BSW in 2008 and strong plume of dense water toward Fram Strait reported by other authors. Anomalous ice production appears predominantly driven by the duration of the freezing season and anomalous opening of the polynya.

Modeling interannual dense shelf water export in the region of the Mertz Glacier Tongue (1992-2007)

[1] Ocean observations around the Australian-Antarctic basin show the importance of coastal latent heat polynyas near the Mertz Glacier Tongue (MGT) to the formation of Dense Shelf Water (DSW) and associated Antarctic Bottom Water (AABW). Here, we use a regional ocean/ice shelf model to investigate the interannual variability of the export of DSW from the Adélie (west of the MGT) and the Mertz (east of the MGT) depressions from 1992 to 2007. The variability in the model is driven by changes in observed surface heat and salt fluxes. The model simulates an annual mean export of DSW through the Adélie sill of about 0.07 ± 0.06 Sv. From 1992 to 1998, the export of DSW through the Adélie (Mertz) sills peaked at 0.14 Sv (0.29 Sv) during July to November. During periods of mean to strong polynya activity (defined by the surface ocean heat loss), DSW formed in the Adélie depression can spread into the Mertz depression via the cavity under the MGT. An additional simulation, where ocean/ice shelf thermodynamics have been disabled, highlights the fact that models without ocean/ice shelf interaction processes will significantly overestimate rates of DSW export. The melt rates of the MGT are 1.2 ± 0.4 m yr−1 during periods of average to strong polynya activity and can increase to 3.8 ± 1.5 m/yr during periods of sustained weak polynya activity, due to the increased presence of relatively warmer water interacting with the base of the ice shelf. The increased melting of the MGT during a weak polynya state can cause further freshening of the DSW and ultimately limits the production of AABW.

Creation and tidal advection of a cold salinity front in Storfjorden: 1. Polynya dynamics

[1] Hydrographical measurements from the Storfjorden polynya document the presence of an abrupt front in near-freezing water dividing saline water recently created by a polynya event, from less saline water originating further south. This event occurred days before the survey with estimated heat flux ∼400 W m−2 over the polynya. Brine-enriched shelf water (BSW) is observed downslope toward deeper parts of Storfjorden, and BSW from earlier polynya events overflows the sill. Current measurements from a nearby sound, Freemansundet, document tidal currents exceeding 80 cm s−1 that displaced the front back and forth beneath the measurement site on fast ice ∼400 m from the polynya edge. Front displacement of ∼12 km is documented and mainly due to the M2 component superimposed on a mean residual current of 0.28 m s−1 into the sound induced by southerly wind during the survey. Complex topography imposes baroclinic tidal currents with strong vertical shear in the fast ice-covered sound, and with significant cross-channel flow. Supercooling events indicated in the hydrographical time series, and likely enhanced frazil ice production, are associated with double-diffusive turbulent mixing when the salinity front passes. In this way, these measurements indicate a novel ice production process along the edge of tidally induced latent heat polynyas where salinity fronts are generated. Turbulence increases (decreases) during flood (ebb) due to the destabilization (stabilization) of the water column when the salinity front passes the measurement site. Double-diffusive turbulent mixing related to tidal advection of salinity front below fast ice is pursued in a companion paper.

Comparing Winter and Summer Simulated Estuarine Circulations in Foxe Basin, Canada

Foxe Basin is an Arctic inland sea located at the north of the Hudson Bay system (Canada). The characteristics of its estuarine circulation in winter differ from those in summer, mainly because of the sea-ice formation and cover. Using Lagrangian tracers and flux calculations from simulated data through a section at the outlet of the basin, we have shown that one-third of all the ice produced in winter is eventually exported toward the Labrador Sea. Foxe Basin is also a place where, in winter, latent heat polynyas to the west coexist with sensible heat polynyas around southwestern Foxe Peninsula, the latter resulting from an upwelling of intermediate water approximately 0.6°C above the freezing point of seawater. Sea ice affects circulation in the basin by adding friction at the surface layer and by shielding the sea surface from the atmosphere, which leads to an average reduction of 11% in the surface currents. Sea ice also extracts approximately 1.3 × 1014 kg of fresh water from the sea surface. Divided by Foxe Basin's surface area (0.2 × 1012 m2) and by the freshwater density, this mass represents a mean layer of 0.65 m of fresh water. The salt transport anomaly versus depth at the outlet of the basin and the mean vertical buoyancy flux were also examined; this confirmed previous assumptions that the estuarine circulation forms a positive (summer)–negative (winter) couple. We suggest that the sensitivity of this couple to climate change may be used as a proxy to evaluate the impact of global warming in Foxe Basin. RÉSUMÉ [Traduit par la rédaction] Le bassin de Foxe est une mer intérieure arctique située au nord du système de la baie d'Hudson (Canada). Les caractéristiques de sa circulation estuarienne en hiver diffèrent de celles de l’été, principalement à cause de la formation et de la couverture de glace de mer. Au moyen de traceurs lagrangiens et de calculs de flux basés sur des données simulées à travers une section à la sortie du bassin, nous avons montré que le tiers de toute la glace produite en hiver est éventuellement exporté vers la mer du Labrador. Le bassin de Foxe est aussi un endroit où, en hiver, des polynies de chaleur latente à l'ouest coexistent avec des polynies de chaleur sensible autour du sud-ouest de la péninsule de Foxe, ces dernières étant produites par des remontées d'eau intermédiaire dont la température est approximativement 0,6 °C au-dessus du point de congélation de l'eau de mer. La glace de mer influe sur la circulation dans le bassin en ajoutant du frottement dans la couche de surface et en faisant écran entre la surface de la mer et l'atmosphère, ce qui provoque une réduction de 11% dans les courants de surface. La glace de mer extrait aussi environ 1,3 × 1014 kg d'eau douce de la surface de la mer. Divisée par la superficie du bassin de Foxe (0,2 × 1012 m2) et par la masse volumique de l'eau douce, cette masse représente une couche moyenne de 0,65 m d'eau douce. Nous avons aussi examiné l'anomalie de transport de sel en fonction de la profondeur à la sortie du bassin et le flux vertical moyen de flottabilité; cela a confirmé des hypothèses précédentes selon lesquelles la circulation estuarienne forme un couple positif (été) – négatif (hiver). Nous croyons que la sensibilité de ce couple au changement climatique peut servir d'indicateur pour évaluer l'impact du réchauffement planétaire dans le bassin de Foxe.

Climate patterns and phytoplankton dynamics in Antarctic latent heat polynyas

Seasonal coherence between satellite‐derived phytoplankton parameters (chlorophyllaconcentration (Chl) and phytoplankton bloom initiation time (BIT)), environmental variables (sea ice concentration, surface solar radiation, and wind speed), and climate patterns (El Niño 3.4, the Southern Annular Mode, the Pacific South America pattern, the semiannual oscillation, and the wave‐3 stationary pattern) was investigated in four latent heat polynyas (Amundsen Sea, western Ross Sea, Dumont d'Urville, and Prydz Bay) using data corresponding to 1998–2006 phytoplankton growing seasons. In general, polynyas in the western sector (i.e., Amundsen Sea and western Ross Sea) had a greater sea ice cover, lower solar radiation levels, higher and more variable Chl, and more variable and delayed (i.e., high BIT) phytoplankton blooms. Differences in Chl and BIT were mainly attributed to differences in water stratification and interannual variability of sea ice concentration caused by ice shelf calving events in the Ross Sea. Changes in solar radiation reaching the sea surface played an important role in determining phytoplankton blooms in the western Ross Sea and Prydz Bay. Stronger winds tend to benefit development of phytoplankton blooms in polynyas having more stratified waters. Sensitivity of phytoplankton to climate variability in polynyas under investigation was highly influenced by the polynya size during summer (e.g., Chl in Dumont d'Urville and BIT in the western Ross Sea). Also, the response of Chl to the same climate pattern changed with the polynya's location (e.g., correlation between Chl and El Niño 3.4 in Amundsen Sea (positive) and Dumont d'Urville (negative)).

Internal waves and vertical mixing in the Storfjorden Polynya, Svalbard

[1] Latent heat polynya are preferential sites for dense water formation through brine release. The combined action of winds and heat loss leading to sea ice fracture followed by the reactivation of sea ice formation drives this process of dense water formation. The impact of high-frequency dynamics, and especially internal wave breaking, on this process is investigated here, with the analysis of hydrographic observations collected from a drifting mooring in Storfjorden, Svalbard. The main frequency components of the barotropic tide are first revealed by the analysis of a 23-day time series of pressure data. Interestingly the same frequency peaks are isolated by a spectral analysis of isopycnal displacements, suggesting a tidal forcing for these baroclinic waves. The hypothesis of local generation in the Storfjorden is next examined and numerous sites of potential generation are identified specifically for semi-diurnal and sixth-diurnal frequencies. Diurnal baroclinic waves, however, do not appear to be locally generated, but rather popagate cyclonically along the shelf as internal Kelvin waves. The characteristic of these waves and their energy flux are then inferred. A parameterization of dissipation based on the potential energy of the internal wavefield is next proposed and an upper bound for dissipation rate within overturning regions is provided. The most vigorous mixing is associated with brine release events (i.e. strong vertical mixing through thermohaline convection) with eddy diffusivities of up to 10−3 m2 s−1. The background mixing (likely associated with internal wave breaking), however, displays values within [10−6, 10−4] m2 s−1. This background mixing contributes about O(0.1) W m−2 to the vertical heat flux with a net heating in the bottom layer and cooling in the upper layer. Local heating and cooling rates of the order of 0.025°C month−1 are obtained which is significant with respect to the 0.1°C temperature difference between the bottom and surface layers.

Analysis of a dense water pulse following mid-winter opening of polynyas in western Foxe Basin, Canada

A recent study has shown that Foxe Basin's dense waters originate from coastal latent heat polynyas and each year replace 2/3rd of the basin's deep waters by propagating southeastwards in Foxe Channel as a gravity current. The formation mechanisms in 2004 of these dense waters are examined here. Strong meteorological events occurring in mid-winter over the domain are responsible for the simultaneous opening of two large polynyas at Lyon Inlet and along Melville Peninsula's eastern coast while a third important and recurrent polynya opens earlier at Hall Beach (northwestern Foxe Basin). Large sea-atmosphere heat exchanges take place in these polynyas, leading to the production of 21.2 × 10 12 kg of sea-ice and 1.53 × 10 12 m 3 of dense water. The ice production rate is on average five to six times higher in the polynyas than in the rest of the basin. Following the topography, the dense waters formed at Hall Beach and along Melville Peninsula cascade into Foxe Channel, while those produced at Lyon Inlet sink directly in the channel through deep convection. The two mechanisms synchronize and combine together when Lyon Inlet and Melville Peninsula polynyas open up. The heat exchanges, sea-ice and brine production rates estimated with a 21-year near-climatology are similar to those found in 2004. The results also show that the produced dense waters can overflow into Hudson Bay.

Excess of bottom-released methane in an Arctic shelf sea polynya in winter

Latent heat polynyas are regions generating strong ice formation, convection and extensive water mass formation. Here we report on the effects of these processes on resuspension of sediments and subsequent methane release from the seafloor and on the resulting excess methane concentration in surface water on a polar shelf during winter. The study is based on measurements of concentration and δ 1 3C values of methane, water temperature, salinity, light transmission and sea ice data collected in March 2003 in Storfjorden, southern Svalbard. In winter, strong and persistent northeasterly winds create polynyas in eastern Storfjorden and cause ice formation. The resulting brine-enriched water cascades from the Storfjordbanken into the central depression thereby enhancing the turbulence near the seafloor. A distinct benthic nepheloid layer was observed reflecting the resuspension of sediments by the cascading dense bottom water. High concentrations of 13C-depleted methane suggest submarine discharge of methane with the resuspended sediments. As the source of the submarine methane, we propose recent bacterial methanogenesis near the sediment surface because of extremely high accumulation rates of organic carbon in Storfjorden. Convective mixing transports newly released methane from the bottom to the sea surface. This eventually results in an excess concentration in surface water with respect to the atmospheric equilibrium, and a sea-air flux of methane during periods of open water. When a new ice cover is formed, methane becomes trapped in the water column and subsequently oxidized. Thus, the residual methane is strongly enriched in 13C in relation to the δ 13 C CH 4 signature of atmospheric methane. Our results show that latent heat polynyas may induce a direct pathway for biogases like methane from sediments to the atmosphere through coupling of biogeochemical and oceanographic processes. Extrapolating these processes to all Arctic ocean polynyas, we estimate a transfer of CH 4 between 0.005 and 0.02 Tg yr −1. This is not a large contribution but the fluxes from the polynyas are 20–200 times larger than the ocean average and the methane evasion process in polynyas is certainly one that can be altered under climate change.

A numerical study of the western Cosmonaut polynya in a coupled ocean–sea ice model

Employing results from a 0.4°, 40‐level fully global, coupled ocean–sea ice model, we investigated the role of physical processes emanating from atmosphere, ocean, and ice in the initiation, maintenance, and termination of a sensible heat polynya with a focus on the western Cosmonaut polynya that occurred during May–July 1999. The Cosmonaut polynya first appeared in early May 1999 in the form of an ice‐free embayment, transformed into an enclosed polynya on 5–9 July, and disappeared by late July, when the ice from the surrounding regions began to encircle the embayment. Except for the differences in ice concentrations, the time of appearance, size, and shape of the Cosmonaut polynya simulated by the model are in approximate agreement with the Special Sensor Microwave/Imager (SSM/I) observations. Between May and July 1999 the Cosmonaut Sea experienced two synoptic storms, both lasting ∼5 days. Followed by the passage of the first storm on 12–19 June, there was a remarkable growth in the size of the embayment by 21 × 103 km2. Associated with this, the sea surface temperature (SST) rose by 0.15°C, the upward heat flux jumped from 5 to 94 W m−2, and a net freshwater flux into the ocean increased by 2 cm d−1. By running the model simulation with a 20% wind speed increase, it is demonstrated that the twofold increase in SST and upward heat flux increased the embayment area by 15 × 103 km2 and decreased the ice concentration by approximately 10% from the control run. A similar, but somewhat weaker wind event that took place on 30 June to 10 July had less influence on the embayment area although the upward heat flux (65 W m−2) was comparable to the first event. By examining the vertical displacement of the −1.6°C isotherm depth prior to, during, and after these two storms, we demonstrate that the impetus provided by these storms was able to raise the −1.6°C isotherm depth by 30 m through wind‐driven mixing, making sufficient oceanic heat input from beneath the mixed layer available to prevent freezing and/or delay ice formation while ice in the adjacent regions continued to grow. A sudden shift in the ice drift direction from southwest to northeast (3 July) followed by the second storm, accompanied by large air‐sea temperature differences, caused the enclosure of the embayment, subsequent formation of the polynya, and its termination.

Are polynyas self‐sustaining?

In this paper, 441 Conductivity Temperature Depth (CTD) casts from the North Water (NOW) Polynya study were used to calculate geostrophic currents between the 10 and 200 dbar surface during April, May and June 1998. Results for April and May indicated a surface intensified southward flow of 10 to 15 cm s–1 with a small return flow along the Greenland coast in agreement with inferred currents described by Melling et al. (2001) and surface ice drifts found by Wilson et al. (2001). Southward transports at this time were 0.4–0.55 Sv in April and May. In June, however, surface currents diminished markedly: southward transports declined to 0.1–0.35 Sv, coincident with a decrease in directly measured winds over the polynya and in the surface barometric pressure difference between Grise Fjord and the Carey Islands that was used as a surrogate for the local north wind speed. There was no evident decrease in air pressure difference between Resolute and Grise Fjord, indicative of the strength of the north wind over the eastern Arctic in general. The results are consistent with present thinking that the NOW Polynya is primarily a latent heat polynya, forced by dominant north winds. The idea, broached here, is that the polynya creates its own microclimate which sustains the polynya's ice‐free condition after its initial formation. The mechanism is identified by an anomalous low pressure region associated with surface buoyancy flux in the polynya and is pursued through the application of a simple geostrophic adjustment model that suggests two self‐sustaining mechanisms. Firstly, the frontal intrusion of the cold ambient terrestrial air mass drives a significant surface wind that transports frazil ice to the edge of the polynya before it can congeal. Secondly, rotation at these high latitudes restricts the penetration of the front into the polynya, essentially insulating the centre from freezing temperatures.

Relationship between synoptic forcing and polynya formation in the Cosmonaut Sea: 2. Regional climate model simulations

In part I of this series [Arbetter et al., 2004], a relationship was found in the observations between synoptic atmospheric systems and polynya formation in the Cosmonaut Sea region of Antarctica. In this study, we use a regional coupled atmosphere‐sea ice model to investigate the role of atmospheric forcing of the polynyas in this area. The model successfully simulates both latent and sensible heat polynyas which are found in the region. In particular, a deep‐ocean polynya is formed coincident with the passage of an atmospheric synoptic scale system. The divergence in the sea ice is found to be highly correlated with the atmospheric divergence. We conclude that the initial formation of a polynya can be caused by the interaction of the atmospheric synoptic scale and the continental katabatic wind regimes. While we cannot directly evaluate the role of the ocean using the present model simulations, we performed experiments with different levels of prescribed basal heat flux. These experiments highlight a polynya which could be initiated by the winds and maintained by the oceanic heat. This leads to the concept of a wind‐driven sensible heat polynya, not typical of other deep‐ocean polynyas such as the Weddell polynya of the 1970s.

A model for the closure of a two-dimensional coastal latent heat polynya

This paper presents a numerical model for the closure of a two-dimensional island polynya, assuming that the pile-up depth H of consolidated new ice at the polynya edge (during opening) and at the coast (during closing) is constant. The polynya closing time T is found to be relatively insensitive to ice drift orientation, and the quotient | U |/| u | , where u and U are the frazil and consolidated ice velocities, respectively. Also, T is weakly dependent on the island length D, except when both the onset of closure occurs significantly before the opening polynya steady-state is reached, and also D≲ L a, the alongshore adjustment length scale. However, T is found to be sensitive to F/ F o, where F and F o are the constant frazil ice production rates during polynya closing and opening, respectively. We exploit the parameter dependence of T as a function of F, F o, D, u , U and H to derive an approximate expression for the closing time that is accurate to within ∼8%, assuming that the polynya closes from an initial area that is close to its steady-state area.