Polynya Formation Research Articles

Ekman-driven salt transport as a key mechanism for open-ocean polynya formation at Maud Rise.

Open-ocean polynyas formed over the Maud Rise, in the Weddell Sea, during the winters of 2016-2017. Such polynyas are rare events in the Southern Ocean and are associated with deep convection, affecting regional carbon and heat budgets. Using an ocean state estimate, we found that during 2017, early sea ice melting occurred in response to enhanced vertical mixing of heat, which was accompanied by mixing of salt. The melting sea ice compensated for the vertically mixed salt, resulting in a net buoyancy gain. An additional salt input was then necessary to destabilize the upper ocean. This came from a hitherto unexplored polynya-formation mechanism: an Ekman transport of salt across a jet girdling the northern flank of the Maud Rise. Such transport was driven by intensified eastward surface stresses during 2015-2018. Our results illustrate how highly localized interactions between wind, ocean flow and topography can trigger polynya formation in the open Southern Ocean.

STUDYING POLYNYA OCCURRENCE TERMS AND DURATION ON THE SHIPPING ROUTES IN THE LAPTEV SEA

The dynamics of the development of the Lena polynya formed on the shipping routes of the Northern Sea Route in the Laptev Sea and its influence on the conditions of navigation in the Arctic are studied. During the period of winter-spring navigation in the Laptev Sea, the Great Siberian polynya, which is located near the fast ice border and creates good opportunities for winter navigation on the Northern Sea Route, is formed. This problem is important to ensure year-round navigation in the Eastern Sector of the Northern Sea Route. The hypothesis that the terms of a polynya occurrence can significantly affect the average rate of its growth is tested in the study. When checking, the results of the GLORYS12v1 global reanalysis and standard methods of mathematical statistics are used. On the example of the polynya section located on the estuary seashore of the largest navigable channel of the Lena River Delta - Bykovskaya, the validity of the hypothesis is confirmed. Steady trends in the change in the polynya formation terms towards earlier dates in the modern period are revealed. A high probability of improving the conditions of navigation in this region in 2020-2040 is noted. At the same time, with further warming of the Siberian climate, the opposite process, namely, the freezing of a polynya that formed too early, which will lead to a cooling of the climate in the region and a deterioration in navigation conditions, is possible. Comparing the results obtained with other studies that are devoted to the construction of general routes for ships in various ice conditions, it is possible to identify trends that the actual routes during independent navigation pass along the forming polynyas, regardless of the physical principle of their formation, then the choice of general routes for ships in the areas of polynya distribution will be promote more intensive and safe shipping. What requires accurate forecasts of its size and position. The relevance of the research results for forecasting year-round navigation, both for independent navigation of ships with a high ice class, and in the development of plans for using icebreaking escort of ships, is noted.

On the Intermittent Occurrence of Open‐Ocean Polynyas in a Multi‐Century High‐Resolution Preindustrial Earth System Model Simulation

AbstractA 500‐year‐long high‐resolution Community Earth System Model simulation under preindustrial radiative forcing produces Maud Rise Polynyas (MRPs) and Weddell Sea Polynya (WSP) events intermittently from decadal (MRP) to multidecadal (WSP) timescales. This periodicity is correlated with the variation of a regional Southern Annular Mode (SAM) index (evaluated regionally instead of circum‐Antarctic) with corresponding variations of precipitation and winds over the Weddell Sea. A negative index causes the upper‐ocean salinity to increase over multiple decades. Ultimately, brine rejection during seasonal sea‐ice formation superimposed on the multidecadal increase raises the upper‐ocean salinity beyond the tipping point for triggering deep convection that leads to polynyas. The initiation of polynya events is thus controlled by surface properties while the location of initiation is determined by bathymetric features. The persistent Taylor column effect, which is well represented by the high‐resolution model topography of the Maud Rise seamount, preconditions this region for MRP initiation. Therefore, MRPs form more frequently than WSPs in the simulation. When the upper‐ocean salinity is high, deep convection in the MRP region tends to be stronger, in which case MRPs are also more likely to grow into WSPs. Once WSPs emerge, they affect the regional atmospheric circulation and associated variables. We propose a regional coupled ocean‐atmosphere mechanism to explain both the periodic emergence of polynyas and the periodic variation of the regional SAM index. Although the temperature and salinity of Weddell Deep Water show upward trends due to model drift, these density‐compensating changes do not affect the frequency of polynya formation.

A study of the timing and duration of icebreaks on shipping routes in the Laptev Sea

The dynamics of the development of the Lena polynya forming in the Laptev Sea and its impact on navigation conditions in the Arctic, in the Laptev Sea, is investigated. The hypothesis is tested that the timing of polynya may have a significant impact on its average growth rate. The results of the GLORYS12v1 global reanalysis and standard methods of mathematical statistics were used in this test. The validity of the hypothesis was confirmed by the example of the section of the polynya located at the mouth of the major navigable channel of the Lena delta Bykovskaya. Stable tendencies of changing the dates of the polynya formation towards earlier dates in the modern period have been revealed. A high probability of improvement of navigation conditions is observed in this region in 2020-2040. However, with further warming of the Siberian climate, the opposite process is possible - the freezing of the polynya that formed too early, which will lead to a colder climate in the region and deterioration of navigation conditions. The selection of general routes for ships in the areas where the polynya is spreading will contribute to more intensive and safer navigation. For this purpose, accurate forecasts of its size and position are required. The relevance of research results for ship traffic forecasting and plans for the use of icebreaker escorts for vessels is noted.

Ice, wind, and water: Synoptic-scale controls of circulation in the Chukchi Sea

A composite dataset of 27 moorings across the Chukchi Sea and Bering Strait in 2013–14, along with satellite sea ice concentration data, weather station data, and atmospheric reanalysis fields, are used to explore the relationship between the circulation, ice cover, and wind forcing. We find a clear relationship between northeasterly winds along the northwest coast of Alaska and reversed flow along the length of Barrow Canyon and at a mooring site ∼ 100 km upstream on the northeast shelf. Atlantic Water is frequently upwelled into the canyon during the fall and winter, but is only able to reach the head of Barrow Canyon after a series of long upwelling events. A pair of empirical orthogonal function (EOF) analyses of ice cover reveal the importance of inflow pathways on the pattern of freeze-up and melt-back, and shed light on the relative influence of sensible heat and wind forcing on polynya formation. An EOF analysis of 25 mooring velocity records reveals a dominant pattern of circulation with coherent flow across the shelf, and a secondary pattern of opposing flow between Barrow Canyon and Bering Strait. These are related to variations in the regional wind field.

Thinner Sea Ice Contribution to the Remarkable Polynya Formation North of Greenland in August 2018

In August 2018, a remarkable polynya was observed off the north coast of Greenland, a perennial ice zone where thick sea ice cover persists. In order to investigate the formation process of this polynya, satellite observations, a coupled ice-ocean model, ocean profiling data, and atmosphere reanalysis data were applied. We found that the thinnest sea ice cover in August since 1978 (mean value of 1.1 m, compared to the average value of 2.8 m during 1978−2017) and the modest southerly wind caused by a positive North Atlantic Oscillation (mean value of 0.82, compared to the climatological value of −0.02) were responsible for the formation and maintenance of this polynya. The opening mechanism of this polynya differs from the one formed in February 2018 in the same area caused by persistent anomalously high wind. Sea ice drift patterns have become more responsive to the atmospheric forcing due to thinning of sea ice cover in this region.

Multidecadal polynya formation in a conceptual (box) model

Abstract. Maud Rise polynyas (MRPs) form due to deep convection, which is caused by static instabilities of the water column. Recent studies with the Community Earth System Model (CESM) have indicated that a multidecadal varying heat accumulation in the subsurface layer occurs prior to MRP formation due to the heat transport over the Weddell Gyre. In this study, a conceptual MRP box model, forced with CESM data, is used to investigate the role of this subsurface heat accumulation in MRP formation. Cases excluding and including multidecadal varying subsurface heat and salt fluxes are considered, and multiple polynya events are only simulated in the cases where subsurface fluxes are included. The dominant frequency for MRP events in these results, approximately the frequency of the subsurface heat and salt accumulation, is still visible in cases where white noise is added to the freshwater flux. This indicates the importance and dominance of the subsurface heat accumulation in MRP formation.

Can we reconstruct the formation of large open-ocean polynyas in the Southern Ocean using ice core records?

Abstract. Large open-ocean polynyas, defined as ice-free areas within the sea ice pack, have only rarely been observed in the Southern Ocean over the past decades. In addition to smaller recent events, an impressive sequence occurred in the Weddell Sea in 1974, 1975 and 1976 with openings of more than 300 000 km2 that lasted the full winter. These big events have a huge impact on the sea ice cover, deep-water formation, and, more generally, on the Southern Ocean and the Antarctic climate. However, we have no estimate of the frequency of the occurrence of such large open-ocean polynyas before the 1970s. Our goal here is to test if polynya activity could be reconstructed using continental records and, specifically, observations derived from ice cores. The fingerprint of big open-ocean polynyas is first described in reconstructions based on data from weather stations, in ice cores for the 1970s and in climate models. It shows a signal characterized by a surface air warming and increased precipitation in coastal regions adjacent to the eastern part of the Weddell Sea, where several high-resolution ice cores have been collected. The signal of the isotopic composition of precipitation is more ambiguous; thus, we base our reconstructions on surface mass balance records alone. A first reconstruction is obtained by performing a simple average of standardized records. Given the similarity between the observed signal and the one simulated in models, we also use data assimilation to reconstruct past polynya activity. The impact of open-ocean polynyas on the continent is not large enough, compared with the changes due to factors such as atmospheric variability, to detect the polynya signal without ambiguity, and additional observations would be required to clearly discriminate the years with and without open-ocean polynya. Thus, it is reasonable to consider that, in these preliminary reconstructions, some high snow accumulation events may be wrongly interpreted as the consequence of polynya formation and some years with polynya formation may be missed. Nevertheless, our reconstructions suggest that big open-ocean polynyas, such as those observed in the 1970s, are rare events, occurring at most a few times per century. Century-scale changes in polynya activity are also likely, but our reconstructions are unable to precisely assess this aspect at this stage.

Multidecadal preconditioning of the Maud Rise polynya region

Abstract. In this paper, we consider Maud Rise polynya formation in a long (250-year) high-resolution (ocean 0.1∘, atmosphere 0.5∘ horizontal model resolution) of the Community Earth System Model. We find a dominant multidecadal timescale in the occurrence of these Maud Rise polynyas. Analysis of the results leads us to the interpretation that a preferred timescale can be induced by the variability of the Weddell Gyre, previously identified as the Southern Ocean Mode. The large-scale pattern of heat content variability associated with the Southern Ocean Mode modifies the stratification in the Maud Rise region and leads to a preferred timescale in convection through preconditioning of the subsurface density and consequently to polynya formation.

Weddell Sea Phytoplankton Blooms Modulated by Sea Ice Variability and Polynya Formation

AbstractSeasonal sea ice retreat is known to stimulate Southern Ocean phytoplankton blooms, but depth‐resolved observations of their evolution are scarce. Autonomous float measurements collected from 2015–2019 in the eastern Weddell Sea show that spring bloom initiation is closely linked to sea ice retreat timing. The appearance and persistence of a rare open‐ocean polynya over the Maud Rise seamount in 2017 led to an early bloom and high annual net community production. Widespread early ice retreat north of Maud Rise in 2017, however, had a similar effect, suggesting that the polynya most impacted the timing of bloom initiation. Still, higher productivity rates at Maud Rise relative to the surrounding region are observed in all years, likely supported by flow‐topography interactions. The longer growing season in 2017–2018 also allowed for separation of distinct spring and fall bloom signals, the latter of which was primarily subsurface and associated with mixed‐layer deepening.

Holocene changes in sea-ice cover and polynya formation along the eastern North Greenland shelf: New insights from biomarker records

Understanding the processes controlling the natural variability of sea ice in the Arctic, one of the most dynamic components of the climate system, can help to constrain the effects of future climate change in this highly sensitive area. For the first time, a high-resolution biomarker study was carried out to reconstruct past sea-ice variability off eastern North Greenland. This area is strongly influenced by cold surface waters and drift ice transported via the East Greenland Current, meltwater pulses from the outlet glaciers of the Northeast Greenland Ice Stream and the build-up of landfast ice. The well-dated Holocene sedimentary section of Core PS93/025 provides insights into variations of the sea-ice conditions (regional and local sea-ice signal), oceanic and atmospheric circulation and the biotic response to these changes. These biomarker records show a reduced to variable sea-ice cover during the early Holocene between 10.2 and 9.3 ka, followed by a steady increase in sea-ice conditions during the mid Holocene. During the last 5–6 ka, sea-ice conditions remained more stable representing a seasonal to marginal sea-ice situation. Based on our biomarker records, stable sea-ice edge conditions, with a fully developed polynya situation occurred since the last 1 ka.

Landfast ice controls on sea-ice production in the Cape Darnley Polynya: A case study

Antarctic coastal polynyas serve as crucially-important sea-ice “factories” and are (in certain cases) of global significance as sites of Antarctic Bottom Water (AABW) formation e.g., the Cape Darnley Polynya (CDP) in East Antarctica. As such, understanding change and variability in their behaviour, and the factors responsible, is a high priority in climate (change) science. One such important factor that is both poorly understood and quantified is their linkage with adjacent landfast sea ice (fast ice). Here, we present a first detailed analysis that identifies and quantifies key linkages between the dynamics/behaviour of the CDP and fast ice. By forming a barrier to the incursion of pack ice at the eastern boundary, fast ice immediately upstream of the CDP (associated with the distribution of grounded icebergs) plays a crucially-important role in the polynya formation, maintenance and variability. This satellite data-based study builds upon earlier conceptual work on polynya-fast ice feedback mechanisms in the Mertz Glacier region by showing that statistical estimation of the rates of sea ice production (and hence AABW formation) in the CDP is significantly improved through inclusion of two key fast-ice metrics, compared to use of atmospheric covariates alone. These two metrics are the meridional extent of fast ice upstream of the polynya and total fast ice area within the region of high sea ice production. The work highlights the importance of accurate representation of the coupled Antarctic coastal polynya-fast ice system in global climate models.

Nordic Seas polynyas and their role in preconditioning marine productivity during the Last Glacial Maximum

Arctic and Antarctic polynyas are crucial sites for deep-water formation, which helps sustain global ocean circulation. During glacial times, the occurrence of polynyas proximal to expansive ice sheets in both hemispheres has been proposed to explain limited ocean ventilation and a habitat requirement for marine and higher-trophic terrestrial fauna. Nonetheless, their existence remains equivocal, not least due to the hitherto paucity of sufficiently characteristic proxy data. Here we demonstrate polynya formation in front of the NW Eurasian ice sheets during the Last Glacial Maximum (LGM), which resulted from katabatic winds blowing seaward of the ice shelves and upwelling of warm, sub-surface Atlantic water. These polynyas sustained ice-sheet build-up, ocean ventilation, and marine productivity in an otherwise glacial Arctic desert. Following the catastrophic meltwater discharge from the collapsing ice sheets at ~17.5 ka BP, polynya formation ceased, marine productivity declined dramatically, and sea ice expanded rapidly to cover the entire Nordic Seas.

Circumpolar polynya regions and ice production in the Arctic: results from MODIS thermal infrared imagery from 2002/2003 to 2014/2015 with a regional focus on the Laptev Sea

Abstract. High-resolution MODIS thermal infrared satellite data are used to infer spatial and temporal characteristics of 17 prominent coastal polynya regions over the entire Arctic basin. Thin-ice thickness (TIT) distributions (≤ 20 cm) are calculated from MODIS ice-surface temperatures, combined with ECMWF ERA-Interim atmospheric reanalysis data in an energy balance model for 13 winter seasons (2002/2003 to 2014/2015; November to March). From all available MODIS swath data, daily thin-ice thickness composites are computed in order to derive quantities such as polynya area and total thermodynamic (i.e., potential) ice production. A gap-filling approach is applied to account for cloud and data gaps in the MODIS composites. All polynya regions combined cover an average thin-ice area of 226.6 ± 36.1 × 103 km2 in winter. This allows for an average total winter-accumulated ice production of about 1811 ± 293 km3, whereby the Kara Sea region, the North Water polynya (both 15 %), polynyas on the western side of Novaya Zemlya (20 %), as well as scattered smaller polynyas in the Canadian Arctic Archipelago (all combined 12 %) are the main contributors. Other well-known sites of polynya formation (Laptev Sea, Chukchi Sea) show smaller contributions and range between 2 and 5 %. We notice distinct differences to earlier studies on pan-Arctic polynya characteristics, originating in some part from the use of high-resolution MODIS data, as the capability to resolve small-scale (> 2 km) polynyas and also large leads are increased. Despite the short record of 13 winter seasons, positive trends in ice production are detected for several regions of the eastern Arctic (most significantly in the Laptev Sea region with an increase of 6.8 km3 yr−1) and the North Water polynya, while other polynyas in the western Arctic show a more pronounced variability with varying trends. We emphasize the role of the Laptev Sea polynyas as being a major influence on Transpolar Drift characteristics through a distinct relation between increasing ice production and ice area export. Overall, our study presents a spatially highly accurate characterization of circumpolar polynya dynamics and ice production, which should be valuable for future modeling efforts of atmosphere–ice–ocean interactions in the Arctic.

Thermohaline instability and the formation of glacial North Atlantic super polynyas at the onset of Dansgaard‐Oeschger warming events

AbstractLate Quaternary rapid warming events inferred on the basis of oxygen isotopic data from Greenland ice cores are the most prominent characteristic of millennial‐scale Dansgaard‐Oeschger oscillations. In a coupled climate model simulation which has accurately reproduced this oscillatory behavior for the first time, we show that formation of a glacial North Atlantic super polynya characterizes the initial stage of transition from cold stadial to warm interstadial conditions. The winter polynya forms within the otherwise sea ice‐covered North Atlantic as a consequence of the onset of a thermohaline convective instability beneath an extensive stadial sea ice lid. Early in the stadial period, the tendency for thermal convective instability of an extensive warm pool beneath the sea ice lid is strongly inhibited by a stabilizing vertical salinity gradient, which gradually diminishes until a thermohaline convective instability occurs that leads to polynya formation and the rapid retreat of North Atlantic sea ice cover.

Abyssal Ocean Warming and Salinification after Weddell Polynyas in the GFDL CM2G Coupled Climate Model

AbstractThe role of Weddell Sea polynyas in establishing deep-ocean properties is explored in the NOAA Geophysical Fluid Dynamics Laboratory’s (GFDL) coupled climate model CM2G. Using statistical composite analysis of over 30 polynya events that occur in a 2000-yr-long preindustrial control run, the temperature, salinity, and water mass changes associated with the composite event are quantified. For the time period following the composite polynya cessation, termed the “recovery,” warming between 0.002° and 0.019°C decade−1 occurs below 4200 m in the Southern Ocean basins. Temperature and salinity changes are strongest in the Southern Ocean and the South Atlantic near the polynya formation region. Comparison of the model results with abyssal temperature observations reveals that the 1970s Weddell Polynya recovery could account for 10% ± 8% of the recent warming in the abyssal Southern Ocean. For individual Southern Ocean basins, this percentage is as little as 6% ± 11% or as much as 34% ± 13%.

The role of wind forcing from operational analyses for the model representation of Antarctic coastal sea ice

An accurate representation of Antarctic coastal sea ice in Earth System Models is important as it affects the rate of Antarctic Bottom Water formation. Here we investigate the effect of using relatively high-resolution (∼20 km) near-surface wind fields of two different operational analyses (from the European Centre for Medium-Range Weather Forecasts (ECMWF), and the Antarctic Mesoscale Prediction System (AMPS)) to force a sea-ice - ocean model. We focus on the wind impact on coastal polynya formation in winter. Even though the horizontal resolution of both analyses is about the same, striking differences emerge in simulated coastal sea ice and thus the representation of coastal polynyas and net freezing rates. In AMPS, offshore winds are up to 10 m s−1 stronger than those of ECMWF, and mostly overestimated compared to weather station data. This leads to major differences in simulated coastal ice drift, concentration, and thickness, as well as discrepancies in the annual net freezing rate of up to 5 m per year. To identify the reasons for the discrepancies in the coastal wind fields of the analyses we scrutinized their orography, boundary-layer stratification, sea-ice boundary conditions, large-scale atmospheric circulations, data assimilation and initialization schemes, and their parameterization of subgrid-scale orographic effects. The final factor explained most of the discrepancies along steep coastlines, whereas the differences in boundary-layer stratification are mainly a consequence of the differences in surface winds. The data assimilation and initialization procedure biased the AMPS wind fields toward those of the NCEP analyses, which probably overestimate offshore winds as a result of coarse-resolution orography. The sea-ice boundary conditions and the large-scale circulations turned out to have a minimal effect.

Zooplankton in the areas of polynya formation in the seas of the Arctic Ocean

The published data, as well as the authors’ own data, on the distribution, composition, biomass, production, and lifecycles of zooplankton organisms in the areas of potential formation of Arctic polynyas, are discussed in this article. In many cases, there is a lack of materials for the periods when polynyas actually exist (e.g., winter or early spring); thus, the available data for the summer and fall seasons are analyzed. The copepods Oithona similis, Calanus hyperboreus, C. glacialis, C. finmarchicus, Metridia longa, Triconia borealis, Pseudocalanus spp., and Microcalanus pygmaeus dominate the most of the areas in terms of abundance and biomass. Some specific estuarine communities that are dominated by Limnocalanus macrurus, Pseudocalanus major, and Drepanopus bungei are formed in waters that are intensely freshened by river runoff. Common species develop in polynyas faster than in adjacent waters that are covered by ice. In the summer the mean total zooplankton biomass in the areas of polynyas varies from 0.3 (Barrow Strait) to 9.3 gC/m2 (Lena Polynya). The maximum values of zooplankton secondary production are typical for coastal and estuarine zones. The mean daily zooplankton production during the summer season rarely exceeds 50 mgC/(m2 day).

Circumpolar Mapping of Antarctic Coastal Polynyas and Landfast Sea Ice: Relationship and Variability

Abstract Sinking of dense water from Antarctic coastal polynyas produces Antarctic Bottom Water (AABW), which is the densest water in the global overturning circulation and is a key player in climate change as a significant sink for heat and carbon dioxide. Very recent studies have suggested that landfast sea ice (fast ice) plays an important role in the formation and variability of the polynyas and possibly AABW. However, they have been limited to regional and case investigations only. This study provides the first coincident circumpolar mapping of Antarctic coastal polynyas and fast ice. The map reveals that most of the polynyas are formed on the western side of fast ice, indicating an important role of fast ice in the polynya formation. Winds diverging from a boundary comprising both coastline and fast ice are the primary determinant of polynya formation. The blocking effect of fast ice on westward sea ice advection by the coastal current would be another key factor. These effects on the variability in sea ice production for 13 major polynyas are evaluated quantitatively. Furthermore, it is demonstrated that a drastic change in fast ice extent, which is particularly vulnerable to climate change, causes dramatic changes in the polynyas and possibly AABW formation that can potentially contribute to further climate change. These results suggest that fast ice and precise polynya processes should be addressed by next-generation models to produce more accurate climate projections. This study provides the boundary and validation data of fast ice and sea ice production for such models.

Adélie penguins coping with environmental change: results from a natural experiment at the edge of their breeding range

We investigated life history responses to extreme variation in physical environmental conditions during a long-term demographic study of Adélie penguins at 3 colonies representing 9% of the world population and the full range of breeding colony sizes. Five years into the 14-year study (1997-2010) two very large icebergs (spanning 1.5 latitude degrees in length) grounded in waters adjacent to breeding colonies, dramatically altering environmental conditions during 2001 - 2005. This natural experiment allowed us to evaluate the relative impacts of expected long-term, but also extreme, short-term climate perturbations on important natural history parameters that can regulate populations. The icebergs presented physical barriers, not just to the penguins but to polynya formation, which profoundly increased foraging effort and movement rates, while reducing breeding propensity and productivity, especially at the smallest colony. We evaluated the effect of a variety of environmental parameters during breeding, molt, migration and wintering periods during years with and without icebergs on penguin breeding productivity, chick mass, and nesting chronology. The icebergs had far more influence on the natural history parameters of penguins than any of the other environmental variables measured, resulting in population level changes to metrics of reproductive performance, including delays in nesting chronology, depressed breeding productivity, and lower chick mass. These effects were strongest at the smallest, southern-most colony, which was most affected by alteration of the Ross Sea Polynya during years the iceberg was present. Additionally, chick mass was negatively correlated with colony size, supporting previous findings indicating density-dependent energetic constraints at the largest colony. Understanding the negative effects of the icebergs on the short-term natural history of Adélie penguins, as well as their response to long-term environmental variation, are important