A Study of the North Water Polynya Ice Arch using Four Decades of Satellite Data

Polynyas are sections of the polar ocean that remain relatively ice-free during winter, imparting significant physical and biological impact on the region. The North Water polynya (NOW) situated between Ellesmere Island and Greenland is the largest recurring Arctic polynya. Historically, the NOW forms every season when Arctic Ocean floes moving southward through Nares Strait become congested and form an ice arch that defines the northern border of the polynya. This blockage usually forms during winter and breaks down in spring. It is conjectured that the polynya is maintained by latent heat of fusion from the continuous formation of new ice as floes are swept southward from the ice arch by wind and ocean currents. Analysis of four decades of satellite imagery indicates a growing instability in the location of the ice arch, challenging previous models of polynya maintenance. A linear trend of the data indicates the number of days of Nares Strait blockage has decreased 2.1 days/year between 1979 and 2019 with wide interannual variations. Prior to 2007, ice arches blocked Nares Strait an average of 177 days/year compared to 128 days/year since that time. The overall trend of reduced ice arch duration is a contributing factor to the dramatic loss of multiyear ice in the Arctic basin.

Polynyas are areas of the polar ocean that are covered by open-water and thin ice under climatic conditions that would normally dictate thick ice cover. These regions are generally classified as latent or sensible heat polynyas. For a latent heat polynya, ice is removed from the area by winds and/or ocean currents as quickly as it forms. Heat loss to the atmosphere is balanced by latent heat of fusion released by the continual formation of ice. For a sensible heat polynya, oceanic heat provided by the vertical mixing of warmer water from depth due to wind-induced upwelling, or through advection from ocean currents, prevents the formation of ice. Polynyas impart significant oceanographic, climatic and biological impact on the environment. They dominate the regional heat budget in winter with an ocean-to-atmosphere heat flux approximately two orders of magnitude higher than the surrounding pack ice 1 . The continuous formation of ice in polynyas leads to salt rejection and the subsequent formation of water masses and local currents. Interannual changes in polynya characteristics may contribute to the variability of the polar climate, reflecting large-scale climatic changes 2 . Polynyas constitute key habitats for seabirds and marine mammals [3][4][5] , serving as areas of high productivity and biodiversity 6,7 . As a result, polynyas are considered oceanographic 'windows' that allow an evaluation of the state of the polar marine ecosystem 8,9 .
The North Water polynya (NOW) situated between Ellesmere Island and Greenland in northern Baffin Bay is the largest recurring polynya in the Canadian Arctic (Fig. 1a) and one of the most biologically productive marine areas in the Arctic [10][11][12][13] . Historically, pack ice transported southward from the Arctic Ocean through Nares Strait becomes congested and forms a blockage, or ice arch, across the narrow head of Smith Sound. The formation of ice arches is common in the narrow water passages of the Canadian Arctic Archipelgo (CAA) 14 and this process is considered a critical aspect of the NOW's formation and maintenance 2,8,[15][16][17][18][19][20] . The Smith Sound ice arch usually forms in winter or early spring at which point ice is continuously swept southward from the blockage by prevailing winds and ocean currents [21][22][23] . The subsequent formation of new ice releases latent heat, which is an important mechanism for maintaining the polynya 24 . Other factors contributing to keeping the area relatively free of ice include strong tidal fluctuations 25 and potential sensible heat influx through the upwelling of warm water near the Greenland coast 13,26 . While the Smith Sound ice arch sharply defines the northern limit of the polynya (Fig. 1b), the southern boundary is characterized by diffuse pack ice in northern Baffin Bay. The NOW is not fully ice-free in winter 1 with an estimated 50% of Smith Sound covered with ice greater than 30 cm thick in February and March 27 . Since the mid-1990s there has been an expansion of open water in the polynya during winter and West Greenland Current (2.0 °C) flows northward along the Greenland coast with a surface current speed of 3 to 5 cm s −1 until crossing the southern portion of the NOW to join the Nares Strait outflow and form the southward moving Baffin Current 24 . Nares Strait, which averages 30 to 50 km in width and extends 500 km from the Arctic Ocean to the NOW, transports an annual average area of 40,000 km 2 of ice from the Arctic Basin into Baffin Bay 30 with multiyear ice contributing a significant proportion to this amount 31 . Multiyear ice with drafts exceeding 5 m constitute up to 16% of the observed sea ice traversing Nares Strait, draining the last reservoir of old multiyear ice in the Lincoln Sea north of Ellesmere Island 32 . The formation and duration of ice arches in the Nares Strait system impacts the rate of ice loss from this reservoir, which is part of a region that climate models predict will be the last to lose perennial ice cover in the Arctic Ocean known as the Last Ice Area 33 .
The establishment of bird colonies and human presence in the Thule, Greenland region indicate that the NOW was established about 4,500 years ago 34 . The diverse ecological system of the NOW made it possible for the prehistoric Thule Inuit to settle and their descendants survive in the region since 1250 AD 34,35 . Historical references to the NOW extend back to the voyages of William Baffin from 1612 to 1618 in which he discovered a great "open sea" south of what is now known as Kane Basin 36 . In 1861 Isaac Hayes, an American Arctic explorer, wintered at the narrowest section of Smith Sound where his sailing progress was stopped by the ice arch 37 . Air ice reconnaissance in the 1950s and 1960s allowed the Smith Sound ice arch to be drawn on charts with some precision 38 . The first scientific journal to feature the NOW emphasized that the most striking aspect of the polynya was the "extreme" stability of the "ice bridge" that defines the northern border 39 . The consistency of the Smith Sound ice arch from 1968 to 1979 is illustrated by ice charts created during this era in which it averages a seasonal latitude of 79.2 ± 0.5°N. (Fig. 2).
Beginning in October 1978, the availability of consistent satellite imagery of the Arctic in visible and thermal infrared (IR) wavelengths permitted accurate tracking of the annual evolution of the NOW ice arch. In recent years, satellite data has revealed atypical ice configurations in the NOW. In 2007 the Smith Sound ice arch failed to consolidate (Fig. 1c), which allowed a record area (87,000 km 2 ) and volume (254 km 3 ) of Arctic Ocean ice to pass through Nares Strait in a one-year period 31 . In 2009 the Smith Sound ice arch failed to form again, but a different ice arch at the northern terminus of Nares Strait (Fig. 1d) prevented ice floes from entering Nares Strait between 08 January and 12 July. Northern ice arches are relatively common in Nares Strait, but they historically precede the formation of the Smith Sound ice arch by 10 to 30 days 31 . The anomalous 2009 northern ice arch persisted for 184 days and caused historically low ice cover in the NOW, leading to unprecedented sea surface temperatures in the The changing ice arch dynamics of the NOW impacts the state of multiyear ice in the Arctic Ocean as well as the delicate ecosystem of the region and the people who have come to rely on this Arctic oasis. This research analyzes the NOW ice arches using visible and thermal IR satellite imagery from 1979 to 2019. The location and duration of ice arches over the past forty years are investigated and trends in the data examined.

Methods
Advanced Very High Resolution Radiometry (AVHRR) imagery was chosen for this study owing to its long heritage, continuity, availability and ease of processing. The AVHRR sensor is capable of imaging the Earth in visible and thermal IR wavelengths with a spatial resolution of 1.1 km at nadir. The first AVHRR was launched in October 1978 on the Television Infrared Observation Satellite -Next Generation (TIROS-N), which was put into service by the National Oceanic and Atmospheric Administration (NOAA). The follow-on to TIROS-N was NOAA-6 in 1979, at which point the NOAA spacecraft took on numerical designations upon reaching operational status 41 Table 1 lists the parameters and variations of the AVHRR as well as the timeline of satellites that have flown with the sensor.
All satellites equipped with an AVHRR are in low Earth polar orbits that pass over the Arctic approximately 14 times per day. The sensor has a swath of approximately 2,900 km, allowing the NOW region to be imaged five to seven passes in a 24-hour period. The 1.1 km spatial resolution at nadir degrades to about 8 km toward the edge of the swath but is still sufficient to define the ice arch structures. File formats offering the best spatial resolution are High Resolution Picture Transmission for the NOAA series and Full Resolution Area Coverage for the MetOp series. For earlier satellite images only Global Area Coverage format was available offering 4 km resolution at nadir. The reduced spatial resolution was found to be adequate for analyzing ice arch structures. All AVHRR data utilized in this study was retrieved on-line from NOAA's Comprehensive Large Array-data Stewardship System for various AVHRR-equipped satellites from 1978 to 2019 (https://www.bou.class.noaa.gov/saa/products/ welcome).
The time and location of the formation and break down of the southern ice arch (Smith Sound) and northern ice arch (head of Nares Strait) was determined for each season from 1979 to 2019 by visibly examining individual AVHRR images for each day. The term 'season' is used since the formation of an ice arch sometimes occurred in the previous calendar year of the ice arch break down. The darkness of Arctic winter requires the use of thermal IR imagery to determine ice arch formation, while visible wavelengths are superior for observing the break down of these structures. An ice arch was determined to be fully formed when it achieved the characteristic shape for the season, whereas the break down date was signified by the initial collapse of the arch structure. Figure 3 is an example of ice arch formation and break down. The unique ice arch shape for each year is maintained throughout the season, which helps to identify the presence of the formation under thin cloud or amidst thin ice. There  www.nature.com/scientificreports www.nature.com/scientificreports/ is commonly extensive cloud cover in the Arctic maritime environment during the warmer months 42,43 , which made satellite observation of the ice arch break down difficult for some seasons. However, the high number of satellite passes per day allowed the date of the event to be confidently assessed to within two days. The development of ice arches in winter was relatively straight forward owing to the reduced cloud cover and excellent contrast between ice and water provided by the thermal IR imagery. Since the assessment of ice arch formation and break down may differ between researchers and clouds can mask the exact day of formation, a ± 2-day uncertainty is attached to these timings to account for potential discrepancies. Incidences in which ice arches formed for a few days but did not completely solidify are not included in the results. The goal was to determine the continuous number of days that Nares Strait was blocked by ice arches for each season and the time frame of the blockage. The latitude of the ice arch was designated by the most northern point of the structure, which usually occurs at the center. Although 'arch' is the generic term for the structure, there are years when it goes straight across the channel like an 'ice bridge' . The following is a decadal and overall summary of the data in this study.

Results
1980 to 1989. During the 1980s the southern ice arch formed each year in a confined region just north of Smith Sound with an average latitude of 78.9 ± 0.2° (Fig. 6a) which is similar to the pattern shown by ice charts between 1968 and 1979 (Fig. 2). The northern ice arch formation preceded the southern ice arch on four occasions (1981,1983,1984,1988). Historically, in years that the northern ice arch forms the southern ice arch consolidates within 30 days, which is followed by the eventual freezing of Nares Strait. In 1984 the northern ice arch preceded the southern ice arch by a record 93 days. The average continuous ice arch blockage of Nares Strait for the decade was 221.5 days per season, ranging between 90 days (1982) and 310 days (1980). 1990 to 1999. The 1990s experienced the lowest decadal continuous ice arch blockage of Nares Strait, averaging 126.5 days per season, with a range of 10 days (1993) to 229 days (1992). The location of the southern ice arch is more variable than observed in previous decades (Fig. 6b) and is highlighted by an ice arch in 1995 that formed www.nature.com/scientificreports www.nature.com/scientificreports/ at approximately the mid point of Nares Strait 240 km north of the normal position. The edge of this anomalous ice arch retreated northward during May before collapse. The northern ice arch formed first in three seasons during the decade (1994,1997,1998) with the 1994 formation preceding the southern ice arch by 92 days. The 1990s are highlighted by the first known significant variation in the ice arch location and featured a year (1993) in which Nares Strait was only blocked for 10 days by a fully consolidated ice arch. The 1990s also marked a sudden expansion in polynya size during winter (November to March), which appears to be linked to warmer temperatures brought about by a transition to a prevailing more negative phase of the North Atlantic Oscillation 28 .  1°N (Fig. 6c). The northern ice arch formed first four times during this time frame (2000,2001,2004,2006) preceding the southern ice arch formation by 3 to 24 days. During the 2007 season no ice arches solidified in Nares Strait for the first time in recorded history and contributed to record low ice coverage in the Arctic Ocean that year 31 . In 2008 the southern ice formed in the northern section of Kane Basin, approximately 100 km north of the normal position. This was followed by the formation of only the northern arch in 2009. This ice configuration, which lasted for 184 days, had never been observed before and led to a significant warm water Overall 1979 to 2019. Figure 7a shows the number of days ice arches continuously blocked Nares Strait for each season from 1979 to 2019. There is considerable variation between seasons, but a linear trend reveals that the number of ice arch days is decreasing by 2.1 days/year. The variability of the data leads to a low R 2 value (0.104), which means that despite the downward trend of ice arch days it is statistically difficult to predict what will happen in any particular year in the future. The average continuous blockage of Nares Strait between 1979 and 2019 is 161 days per season. Prior to 2007 this value was 177 days compared to 128 days since 2007, which represents the beginning of an era of frequent ice arch anomalies.
The time of year for initial ice arch formation and the date of ice arch break down is shown in Fig. 7b. The data reveals high interannual variability in the formation of the ice arch with more consistency shown in the break down of the structure. A linear trend indicates that the ice arch is forming 0.8 days/year later and breaking down 1.1 days/year earlier. The R 2 values for these trends are 0.039 and 0.112 respectively, indicating a low statistical significance for predicting the future as a result of the high variability of the data. Figure 7c summarizes the location of both the northern and southern ice arches. Except for 1995 when an ice arch formed part way up Nares Strait, the location of the southern ice arch shows good consistency from 1976 to 2006. After the 2007 season in which no ice aches formed there is high variability in the location of ice arches, including four seasons of only a northern ice arch. There are 13 years in which a northern ice arch consolidated prior to the southern ice arch formation. The delay between the two ice arches averages 30.7 days, ranging from 3 to 93 days, with the location of the northern ice arch varying from 81.1° N to 83.5° N. The data suggests a temporal relationship between the northern and southern ice arch formation when both exist but the complicated nature of arching, which is a function of locally complex ocean and meteorological conditions, makes it difficult to determine a precise geophysical connection 31 .

Discussion
Literature on the maintenance of the NOW indicates that the formation of the southern ice arch is a critical aspect of the polynya's existence, but this does not appear to be the case. Since 2007 there have been five seasons when the southern ice arch did not fully consolidate, but the NOW has remained relatively ice free during all these years. Indeed, during the 2007 season when no ice arches consolidated in Nares Strait, the NOW reached record levels of winter polynya size 28 . The data suggests that other factors contributing to polynya maintenance play a more dominant role than the presence of the southern ice arch. For example, orographically channeled winds through Nares Strait accelerate through Smith Sound and lead to increased ice advection 44,45 . Additionally, the diurnal divergence and convergence of the pack ice in the NOW as a result of tides allow the escape of oceanic heat with the creation of leads in the ice 25   www.nature.com/scientificreports www.nature.com/scientificreports/ traverse the length of Nares Strait 40,46-48 . Delays of up to 93 days for the southern ice arch formation were observed prior to 2009, but eventually Nares Strait would freeze completely once the southern ice arch formed. This leads to the conclusion that the freezing of Nares Strait is linked to the formation of the southern ice arch.
The amount of Arctic Ocean ice loss during any particular year is complex, depending on the removal of ice by wind, ocean currents and in situ melting. These driving factors are affected by both large-scale and regional meteorological conditions. The trend toward younger, thinner ice in the Arctic basin makes it more vulnerable to melting as well as break up and export out of the region 49 . Arctic ice is generally transported southward through Fram Strait, the CAA and Nares Strait. The mean annual outflow through Fram Strait is 706,000 km 2 50 , which is significantly higher than yearly averages for Nares Strait (40,000 km 2 ) 30 and the CAA (23,000 km 2 ) 51 . However, unlike Fram Strait the ice transported through Nares Strait is dominated by thick multiyear ice from north of Ellesmere Island and Greenland 52 . The Last Ice Area is losing ice mass at double the rate of the Arctic Ocean as a whole, which is augmented by reduced ice arch blockages in Nares Strait 33 . The ice arches have been observed to contain thick multiyear ice held together with thin younger ice 31 and it is suggested that ice arches in the region are less stable in the absence of thick ice 52 . Consequently, the observed temporal and spatial variability of the Nares Strait ice arches may be a function of the thinning and depletion of the ice reservoir north of Ellesmere Island.
Arctic amplification is the observation that near surface air temperatures are increasing more rapidly in the Arctic than the global average 53,54 leading to a significant diminishment of ice in the Arctic Ocean over the past four decades. Physically and biologically, polynyas are considered a model for scientist to predict the Arctic system response to climate change 9 . After many years of stability, the iconic shape of the NOW as defined by the southern ice arch is no longer a certainty, reflecting the significant changes taking place in the Arctic icescape as a result of Arctic amplification. In 2009 the northern ice arch resulted in a virtually ice-free Baffin Bay in July, which led to unprecedented summertime sea surface temperatures largely as a result of decreased albedo and 24-hour solar insolation, affording insight to a seasonally ice-free Arctic Ocean 40 . Increased energy imparted to the ocean