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This study provides the first estimation of sea ice-melt amount in the Sea of Okhotsk based on spring hydrographic data accumulated for nearly a hundred years. Just after sea ice melts completely, a low-salinity layer appears on the ocean surface, overlying the layer of Winter Water at the freezing point. The integration of the salinity decrease from Winter Water should correspond to the total ice-melt amount. We developed an algorithm to extract the profiles that clearly show the salinity deficit and converted the salinity deficit to the ice-melt amount from all available data. The climatological map shows that ice-melt amount decreases toward the ice edge and exhibits large values around the northern Sakhalin Island, reflecting the ice thickness distribution. In the southern area (south of 48°N), where sea ice is transported from the north, the average ice-melt amount is estimated to be ~ 71 cm in thickness. It is clearly shown that the ice-melt amount has decreased by ~ 30% in the southern area since the 1990s. These changes possibly affect the regional climate through the decreased latent heat of sea ice and potentially affect biological production through weakened stratification caused by decreased ice melt. We also suggested that ice-melt amount did not show a significant trend during the 1930s–1970s, implying that our methodology could extract information on sea ice before the era of satellite observations. [ABSTRACT FROM AUTHOR]

The Sea of Okhotsk is a marginal sea that plays a major role in the ventilation of the North Pacific, being the key location where Dense Shelf Water (DSW) forms at the surface and sinks to the intermediate layer. The Okhotsk Sea Intermediate Water (OSIW) is a key water mass because it includes large amounts of DSW, outflows to the North Pacific, and supplies the ocean with the micronutrient iron. OSIW has been warming over the past few decades, which is attributed to a decreasing trend in DSW production. The acquisition of numerous hydrographic data after 2000 in the Kuril Basin, especially dissolved oxygen from profiling floats, offers an opportunity to better quantify the water mass composition of OSIW, and the changes in OSIW properties and DSW volume. Here, we used all available hydrographic records and a mapping technique specially adapted to polar and sub‐polar regions to revisit the Sea of Okhotsk water properties and document their long‐term changes. Our analysis revealed that the volume of heavier DSW (potential density above 26.9 kg.m−3) has decreased over the past three decades by 3,600 km3, or 15% of the volume present before 1990. This decline is nearly entirely compensated for by an increase in lighter DSW. This shift toward lighter DSW is possibly a sign of the weakening of the intermediate overturning circulation starting in the Okhotsk Sea. Additionally, we found that dense Soya Current Water only accounts for about 1% of OSIW, against the 5% previously estimated. Plain Language Summary: The water of the Sea of Okhotsk is a mix of several water masses which includes the Dense Shelf Water (DSW), a water mass generated in winter via sea ice formation. When sea ice forms, the surface water salinity and density increase, and the cold DSW is generated as the surface water sinks to the bottom of the continental shelf while micronutrients, especially iron, are also incorporated. DSW mixes with other water masses and becomes the Okhotsk Sea Intermediate Water (OSIW) before being exported to the North Pacific where it plays an important role for both the climate and the biological productivity. In this study, we used all the temperature, salinity, and dissolved oxygen data acquired since 1930 to evaluate how global warming may have affected the composition of OSIW. We found that, over the past century, the amount of denser DSW has strongly decreased whereas that of lighter DSW has increased, which is consistent with the decline in sea ice production in the Sea of Okhotsk. This also suggests that the ventilation of the Sea of Okhotsk and the circulation of the intermediate layer of the North Pacific is weakening, which could indirectly affect the ocean's biological productivity. Key Points: The composition of the Okhotsk Sea Intermediate Water (OSIW) is reevaluated using an updated data set and a new mapping techniqueOSIW properties have changed in the long‐term (≥50 years) scale, with a decrease (increase) in the amount of heavy (light) Dense Shelf WaterThis suggests that the North Pacific intermediate overturning is weakening, which could affect the ocean's biological productivity [ABSTRACT FROM AUTHOR]

The Labrador Sea and contiguous Baffin Bay play an important role in the formation of the upper layer of the North Atlantic Deep Water, an essential component of the Atlantic Meridional Ocean Circulation. The hydrography of these two seas is strongly influenced by the melting of sea-ice and glacier-ice, which has likely been affected by long-term climate changes. In this study, we use historical data of ocean temperature and salinity from 1950 to 2022 to estimate the summer freshwater volume (SFV) in Baffin Bay and the Labrador Sea, establish climatologies, and assess the impact of multi-decadal climate change. The SFV climatology (1956 km3)and the summer freshwater budget (2286 km3)estimated from various components are in good agreement. Sea ice and glacial melt account for 37 % and 26 % of the freshwater budget, respectively. SFV climatologies before and after 1995 reveal an increase in Baffin Bay (+226 km3) because of enhanced glacier melting, and a decline (-112 km3) in the Labrador Sea because of recent sea ice volume decreases. The time series of Labrador Sea SFV and total freshwater content are uncorrelated at the multi-decadal scale possibly because the influx of freshwater from the Beaufort Sea dominates the long-term variability. [ABSTRACT FROM AUTHOR]

The article presents a study on dominant frazil ice production in the Cape Darnley polynya (CDP) which leads to Antarctic Bottom Water (ABW) formation. It discusses the process of Antarctic Bottom Water (AABW) formation, mapping of sea ice production and active-frazil area in the CDP, change in salinity, and implications of the dominant underwater frazil ice.

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.dsr.2005.11.002 Byline: Jun Ono (a)(b), Kay I. Ohshima (b), Genta Mizuta (a), Yasushi Fukamachi (b), Masaaki Wakatsuchi (b) Abstract: To clarify the characteristics of tidal currents and heights over Kashevarov Bank (KB) in the Sea of Okhotsk, continuous mooring observation with ADCP and CT sensors was carried out near the summit of KB from 20 September 1999 to 20 June 2000. The velocity, sea-surface height, and bottom-water temperature were dominated by the diurnal signal with fortnightly modulation. The velocity amplitude reached [approximately equal to]1.4ms.sup.-1 during the spring tide. The K.sub.1 and O.sub.1 tidal current ellipses are clockwise with a nearly circular shape and nearly barotropic. To interpret the physical mechanism of the amplified diurnal currents over KB, a seamount-trapped wave (SMTW) model was applied to KB. The frequency of SMTW approximated to KB is close to the diurnal tidal frequencies for the first-gravest mode of first azimuthal wavenumber. The structure and properties of the SMTW mode are consistent with the observations. It is proposed that the amplified diurnal currents over KB are caused by the resonance with SMTW. The observed fortnightly variability in tidal currents and bottom- and surface-temperatures suggests that both the cold spot in summer and the low sea-ice concentration in winter over KB are due to the horizontal and vertical mixing with the surrounding subsurface water, caused by strong tidal currents. Author Affiliation: (a) Graduate School of Environmental Earth Science, Hokkaido University, Sapporo, Japan (b) Institute of Low Temperature Science, Hokkaido University, Sapporo, Japan Article History: Received 13 December 2004; Revised 20 September 2005; Accepted 14 November 2005

Long-term quantification of sea ice production in coastal polynyas (thin sea ice areas) is an important issue to understand the global overturning circulation and its changes. The Special Sensor Microwave Imager (SSM/I), which has nearly 30 years of observation, is a powerful tool for that purpose owing to its ability to detect thin ice areas. However, previous SSM/I thin ice thickness algorithms differ between regions, probably due to the difference in dominant type of thin sea ice in each region. In this study, we developed an SSM/I thin ice thickness algorithm that accounts for three types of thin sea ice (active frazil, thin solid ice, and a mixture of two types), using the polarization and gradient ratios. The algorithm is based on comparison with the ice thickness derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) for 22 polynya events off the Ross Ice Shelf, off Cape Darnley, and off the Ronne Ice Shelf in the Southern Ocean. The algorithm can properly discriminate the ice type in coastal polynyas and estimate the thickness of thin sea ice (≤20 cm) with an error range of less than 6 cm. We also confirmed that the algorithm can be applied to other passive microwave radiometers with higher spatial resolution to obtain more accurate and detailed distributions of ice type and thickness. The validation of this algorithm in the Arctic Ocean suggests its applicability to the global oceans. [ABSTRACT FROM AUTHOR]

This study presented an extended time series of ice production in Antarctic coastal polynyas for 20 years of 2002–21 using high spatial resolution satellite data from passive microwave sensors: the Advanced Microwave Scanning Radiometer for EOS (AMSR-E) and the Advanced Microwave Scanning Radiometer 2 (AMSR2). For this, we reconstructed the previously presented thin ice thickness algorithms and re-estimated ice production by replacing atmospheric input data for the heat flux calculations with the newly released ECMWF Reanalysis v5 (ERA5). The consistency of ice production between AMSR-E and AMSR2, whose operation periods do not overlap, was confirmed from comparisons with ice production by the Special Sensor Microwave/Imager (SSM/I) and the Special Sensor Microwave Imager Sounder (SSMIS) with relatively coarse spatial resolution. The open ocean area, which cannot be detected from the thin ice thickness algorithm, was defined using sea-ice concentration (SIC) from AMSR-E and AMSR2. The satellite derived SIC has been suggested to be underestimated in the new thin ice (polynya) area. The effect of the underestimation on the ice production estimation was evaluated. The underestimation of SIC from three major algorithms was also investigated from comparisons with the thin ice thickness presented in this study. The coastal polynya dataset covering 20 years with the higher spatial resolution presented in this study is vital for climate-change-related studies. [ABSTRACT FROM AUTHOR]

Landfast sea ice (fast ice) is an important component of the Antarctic nearshore marine environment, where it strongly modulates ice sheet–ocean–atmosphere interactions and biological and biogeochemical processes, forms a key habitat, and affects logistical operations. Given the wide-ranging importance of Antarctic fast ice and its sensitivity to climate change, improved knowledge of its change and variability in its distribution is a high priority. Antarctic fast-ice mapping to date has been limited to regional studies and a time series covering East Antarctica from 2000 to 2008. Here, we present the first continuous, high-spatio-temporal resolution (1 km, 15 d) time series of circum-Antarctic fast-ice extent; this covers the period March 2000 to March 2018, with future updates planned. This dataset was derived by compositing cloud-free satellite visible and thermal infrared imagery using an existing methodology, modified to enhance automation and reduce subjectivity in defining the fast-ice edge. This new dataset has wide applicability and is available at 10.26179/5d267d1ceb60c. The new algorithm presented here will enable continuous large-scale fast-ice mapping and monitoring into the future. [ABSTRACT FROM AUTHOR]

We created, for the first time, a map of sea ice production in the Bering Sea, based on thin-ice thickness data from the Advanced Microwave Scanning Radiometers (AMSR-E and AMSR2) with a heat flux calculation. We used the AMSR-E thin-ice algorithm developed for the Arctic Ocean with some modification. We provided a 16-yr data set of ice production from the 2002/2003 to 2018/2019 seasons, excepting the 2011/2012 season. It is found that the Anadyr polynya has by far the highest sea ice production (average of 93 km3/yr) and accounts for more than 30% of all polynya ice production in the Bering Sea. The combined ice production in the Anadyr, Anadyr Strait, and St. Lawrence polynyas becomes the second-largest ice production during the AMSR-E period in the Northern Hemisphere. It is considered that the high ice production in the Anadyr polynya produces cold, saline, nutrient-rich water, so-called Anadyr Water, which would contribute to the formation of the cold halocline layer and high biological productivity. The ice production in the Anadyr polynya shows very large year-to-year variability. The record low ice extent year of the 2017/2018 season is also the lowest ice production year; the production is only one tenth of the highest value, observed during the 2015/2016 season. The high sensitivity of the wind direction and strength to the location of the Aleutian Low causes this large variability. We also built reconstruction schemes of ice production in the polynyas, using the offshore wind and air temperature, by multiple linear regression. [ABSTRACT FROM AUTHOR]

High-frequency (HF) ocean radar has the potential to observe sea ice, but this application has not been thoroughly investigated. In this article, we explore sea ice observation with HF radar (HFR) and develop a corresponding signal processing procedure. We derived sea ice radial velocities from raw HFR data and determined angular positions using the MUltiple SIgnal Classification (MUSIC) algorithm. Sea ice velocities were evaluated by comparison with measurements from moored acoustic Doppler current profilers (ADCPs) and a drifting buoy. The root-mean-square (rms) differences between the sea ice velocities obtained by HFR and the ADCPs decreased with increasing radar signal-to-noise ratio (SNR), implying that a high SNR was associated with accurate measurement of sea ice velocity. These results agree with previous validations of HFR ocean current observations. The sea ice pattern mapped by HFR is consistent with the simultaneous C-band microwave radar observations. In particular, the HFR captured the approach of the sea ice edge toward the coast. Sharp attenuation of the HF signal over high concentration sea ice resulted in missing and/or low-SNR data at ranges beyond the front edge of pack ice, in qualitative agreement with model estimates of attenuation. [ABSTRACT FROM AUTHOR]

Arctic sea ice incorporates and transports sediment, releasing it back into the water column during the melting season. This process constitutes an important aspect of marine sediment transport and biogeochemical cycling. Sediment incorporation into sea ice is considered to occur mainly through underwater interaction between frazil ice and resuspended sediment, referred to as suspension freezing. However, harsh environmental conditions have greatly limited field observations of this phenomenon. Analysis of mooring data from a coastal polynya in the northeastern Chukchi Sea, in conjunction with coastal ice radar and meteorological data, indicates that suspension freezing is a key mechanism for sediment entrainment into sea ice. During polynya episodes, acoustic backscatter data obtained by an Acoustic Doppler Current Profiler showed the presence of frazil ice from the surface down to 20‐ to 25‐m depth, coinciding with in situ and potential supercooling. Underwater frazil ice persisted over 1 week under windy, turbulent water column conditions. A combination of the turbidity and Acoustic Doppler Current Profiler backscatter data revealed upward sediment dispersion associated with strong currents during the polynya episodes. The fact that frazil ice and resuspended sediment were detected at the same depth and time strongly suggests the interaction between ice crystals and sediment particles, that is, suspension freezing. Plain Language Summary: Sea ice incorporates, transports, and releases particulate matter. These processes constitute an important aspect of the biology, biogeochemical cycling, and pollutant transport in polar oceans. Seafloor sediments serve as the most important source of such particulate matter; however, the process of sediment incorporation into sea ice remains poorly explored. We conducted a year‐long study of sediment resuspension and entrainment processes, using underwater sensors deployed in the Chukchi Sea. During winter, wind‐driven offshore transport of sea ice created area of open water and newly grown thin ice that persisted for several days, so‐called coastal polynya or flaw lead system. Our sensors recorded small ice crystals, so‐called frazil ice, that formed in the water column when water temperatures were below freezing point (supercooling). During some of these episodes, sediment was resuspended from the seafloor and dispersed upward by the strong currents, bringing it into water depths at which frazil ice was encountered. Such conditions provide for opportunities that allow frazil ice crystals or aggregates to capture resuspended sediment, a process referred to as suspension freezing. Based on this study, we propose that suspension freezing commonly occurs in shallow Arctic polynyas, serving as a key process of sediment incorporation into sea ice. Key Points: ADCP data provided evidence of frazil ice penetrating to ~25 m in the water column during supercooling episodes in a coastal polynyaThe combination of acoustic and optical measurements revealed underwater frazil ice‐sediment interaction at water depths of 10–25 mSuspension freezing can often occur in shallow polynyas and is suggested to be the dominant process of sediment incorporation into sea ice [ABSTRACT FROM AUTHOR]

The quantification of sea ice production in coastal polynyas is a key issue to understand the global climate system. In this study, we directly compared Advanced Microwave Scanning Radiometer-EOS (AMSR-E) data with the sea ice thickness distribution obtained from a mooring observation during the winter of 2003 off Sakhalin in the Sea of Okhotsk to evaluate the algorithm for estimation of sea ice thickness in coastal polynyas. By using thermal ice thickness as a target physical quantity, we found that the obtained relationship between the polarization ratio (PR) and ice thickness can provide an appropriate AMSR-E algorithm to estimate thin ice thickness, irrespective of the uniform or nonuniform ice thickness field. The relationship between the PR value and thermal ice thickness is likewise consistent with the local PR–thickness relationship that is observed at individual ice floes. This is because both the PR value and thermal ice thickness are more sensitive to thinner ice. Furthermore, we evaluated the method for detection of active frazil in a coastal polynya by comparing with the mooring data, and subsequently modified it to classify the coastal polynya into three thin ice types, namely, active frazil, thin solid ice, and mixed ice (mixture of active frazil and thin solid ice). The improved algorithm successfully represents the thermal ice thickness even for a relatively small-scale polynya off Sakhalin and is expected to be useful for better quantification of sea ice production in the global ocean owing to its high versatility. [ABSTRACT FROM AUTHOR]

Precise knowledge of wintertime sea ice production in Arctic polynyas is not only required to enhance our understanding of atmosphere‐sea ice‐ocean interactions but also to verify frequently utilized climate and ocean models. Here, a high‐resolution (2‐km) Moderate Resolution Imaging Spectroradiometer (MODIS) thermal infrared satellite data set featuring spatial and temporal characteristics of 17 Arctic polynya regions for the winter seasons 2002/2003 to 2017/2018 is directly compared to an akin low‐resolution Advanced Microwave Scanning Radiometer‐EOS (AMSR‐E) passive microwave data set for 2002/2003 to 2010/2011. The MODIS data set is purely based on a 1‐D energy‐balance model, where thin‐ice thicknesses (≤ 20 cm) are directly derived from ice‐surface temperature swath data and European Centre for Medium‐Range Weather Forecasts Re‐Analysis‐Interim atmospheric reanalysis data on a quasi‐daily basis. Thin‐ice thicknesses in the AMSR‐E data set are derived empirically. Important polynya properties such as areal extent and potential thermodynamic ice production can be estimated from both pan‐Arctic data sets. Although independently derived, our results show that both data sets feature quite similar spatial and temporal variations of polynya area (POLA) and ice production (IP), which suggests a high reliability. The average POLA (average accumulated IP) for all Arctic polynyas combined derived from both MODIS and AMSR‐E are 1.99×105 km2 (1.34×103 km3) and 2.29×105 km2 (1.31×103 km3), respectively. Narrow polynyas in areas such as the Canadian Arctic Archipelago are notably better resolved by MODIS. Analysis of 16 winter seasons provides an evaluation of long‐term trends in POLA and IP, revealing the significant increase of ice formation in polynyas along the Siberian coast. Plain Language Summary: The term "polynya" essentially describes the appearance of "holes" in an otherwise closed sea ice cover, that are kept free of thick sea ice due to strong physical forcing mechanisms such as wind or warmer ocean temperatures. The size of such a polynya can sometimes exceed several tens of thousands of square kilometers, indicating a high physical and biological importance. The contrast between the relatively warm ocean and the cold atmosphere creates a flow of heat to the atmosphere that causes intense freezing at the ocean surface. Gaining a more detailed knowledge about the total amount of new sea ice that is formed in these polynyas during the Arctic winter is highly important to understand the large‐scale characteristics of the Arctic sea ice cover. Using different kinds of satellite data, this study follows the aim to analyze the difference in Arctic‐wide polynya monitoring between two independent and previously published data sets. Our results show that both data sets show highly similar estimations of polynya size and new sea ice production, with the overall Arctic‐wide difference being in the range of 2–15%. The results of this study enhance the confidence when using the data sets for other applications and when analyzing long‐term developments and trends of Arctic polynyas. Key Points: Overall high agreement between independent MODIS and AMSR‐E data sets on observed Arctic polynya characteristicsHigh‐resolution MODIS data set beneficial in narrow coastal areas and the evaluation of long‐term trends in polynya area and ice productionAnalysis of 16 winter seasons reveals significantly increasing ice formation in polynyas along the Siberian coast [ABSTRACT FROM AUTHOR]

Abstract: Water properties and formation processes of Alaskan Coastal Winter Water (ACWW) over the eastern Chukchi shelf along the Alaska coast, the so‐called Barrow Canyon pathway, are examined using data from moorings, atmospheric reanalysis, satellite‐derived sea‐ice production (SIP), and a numerical tracer experiment. Along this pathway, Pacific Winter Water (PWW) can be modified to produce ACWW through SIP accompanied by production of cold, saline polynya water in the coastal polynyas, upwelling of warm Atlantic Water (AW), and mixing processes on the shelf. Three different types of ACWW are formed: (i) a mixture of AW and PWW, (ii) a mixture of AW and polynya water, and (iii) hypersaline polynya water. The northeasterly winds, correlated with the north‐south atmospheric pressure gradient between Beaufort High and Aleutian Low, are common triggers of polynya SIP episodes and AW upwelling in the Barrow Coastal Polynya (BCP). Due to the dual impact of northeasterly winds, PWW modification processes in the BCP are more complicated than what occurs elsewhere in the Chukchi Polynya. The impact of AW upwelling on the ACWW formation is most prominent in the BCP, usually centered along the coast. All types of ACWW are thought to be basically transported westward or northwestward with the Chukchi slope current and/or Beaufort Gyre and finally contribute to maintenance of the lower halocline layer especially over the Chukchi Borderland, Northwind Ridge, and southern Canada Basin. Even in the Pacific sector of the Arctic Ocean, ACWW properties are strongly influenced by both Atlantic‐origin and Pacific‐origin waters. [ABSTRACT FROM AUTHOR]

Rare CO2 flux measurements from Arctic pack ice show that two types of ice contribute to the release of CO2 from the ice to the atmosphere during winter and spring: young, thin ice with a thin layer of snow and older (several weeks), thicker ice with thick snow cover. Young, thin sea ice is characterized by high salinity and high porosity, and snow-covered thick ice remains relatively warm (>-7.5 °C) due to the insulating snow cover despite air temperatures as low as -40 °C. Therefore, brine volume fractions of these two ice types are high enough to provide favorable conditions for gas exchange between sea ice and the atmosphere even in mid-winter. Although the potential CO2 flux from sea ice decreased due to the presence of the snow, the snow surface is still a CO2 source to the atmosphere for low snow density and thin snow conditions. We found that young sea ice that is formed in leads without snow cover produces CO2 fluxes an order of magnitude higher than those in snow-covered older ice (+1.0±0.6 mmol C m-2day-1 for young ice and +0.2 ±0.2 mmolCm-2 day-1 for older ice). [ABSTRACT FROM AUTHOR]

We show that young, snow-covered ice has a potential for sea-ice-to-air CO2 release during winter and spring in the Arctic Ocean north of Svalbard. Young thin sea ice was characterized by high salinities and thus porosity, while the surface of thicker sea ice was relatively warm (> -7.5 °C), due to a thick insulating snow cover, even though air temperatures were as low as -40 °C. During these conditions, brine volume fractions of sea ice were high, providing potentially favorable conditions for gas exchange between sea ice and overlying air even in mid-winter. Although the potential CO2 flux through the sea ice decreased due to the presence of the snow, the snow surface still is a CO2 source to the atmosphere for low snow density and thin snow conditions. Especially young ice formed in leads, without snow cover, is important for the CO2 flux from the ice pack as the fluxes are an order of magnitude higher than for snow-covered older ice. [ABSTRACT FROM AUTHOR]

Time series ice-draft data were obtained from moored ice-profiling sonar (IPS), in the coastal northeastern Chukchi Sea during 2009/10. Time series data show seasonal growth of sea-ice draft, occasionally interrupted by coastal polynya. The sea-ice draft distribution indicates a slightly lower abundance of thick, deformed ice compared with the eastern Beaufort Sea. In January, a rapid increase in the abundance of thick ice coincided with a period of minimal drift indicating compaction again the coast and dynamical thickening. The overall mean draft and corresponding derived thickness are 1.27 and 1.38 m, respectively. The evolution of modal ice thickness observed can be explained mostly by thermodynamic growth. The derived ice thicknesses are used to estimate heat losses based on ERA-interim data. Heat losses from the raw, 1 s IPS data are ~50 and 100% greater than those calculated using IPS data averaged over spatial scales of ~20 and 100 km, respectively. This finding demonstrates the importance of subgrid-scale ice-thickness distribution for heat-loss calculation. The heat-loss estimate based on thin ice data derived from AMSR-E data corresponds well with that from the 1 s observed ice-thickness data, validating heat-loss estimates from the AMSR-E thin ice-thickness algorithm. [ABSTRACT FROM AUTHOR]

Sinking of the dense water plays a significant role in the global climate system by driving thermohaline (overturning) circulation and biogeochemical cycles. Deep water convection occurs also in the Japan Sea, and the convection has been considered to be mainly caused by intense cooling of the sea surface. Another possible cause of the convection is brine rejection associated with high sea-ice production in a coastal polynya (thin sea-ice) area in the northern Japan Sea. We have developed an algorithm which detects the thin ice area and estimates the thickness using passive microwave satellite data. Based on a heat flux calculation with the satellite-derived ice thickness, the total sea-ice production in winter (December–March) averaged over 2002/03–2010/11 is estimated to be 4.27 × 10 10 m 3 . This indicates that the ice production was underestimated by about half in a previous study in which the polynya was unrealistically treated as a low ice concentration area. The main determinant factor for the total ice production is the surface air temperature in early winter (December–January), which shows a rapid warming trend of 0.7 °C/decade for this 40-years. Based on a linear regression approach, the total ice production is estimated to have decreased by ~5%/decade due to air temperature warming. If brine rejection due to the ice production contributes to the deep water formation in the Japan Sea, this is consistent with the fact that the deep water formation has been decreasing for the last 50–100 years. [ABSTRACT FROM AUTHOR]

Antarctic coastal polynyas are very high sea-ice production areas. The resultant large amount of brine rejection leads to the formation of dense water. The dense water forms Antarctic bottom water, which is the densest water in the global overturning circulation and a key player in climate change as a significant sink for heat and carbon dioxide. In this study, an algorithm was developed that uses Advanced Microwave Scanning Radiometer 2 (AMSR2) data (2012–present) to detect polynya area and estimates thin ice thickness by a method similar to that used to develop the algorithm for Advanced Microwave Scanning Radiometer for EOS (AMSR-E) data. Landfast sea-ice areas were also detected using AMSR2 data. Ice production in the polynyas was estimated by a heat flux calculation using AMSR2 sea-ice data. In four major polynyas, AMSR2 ice production was compared with AMSR-E (2003–2011) ice production through comparison of them with Special Sensor Microwave Imager (SSM/I) and Special Sensor Microwave Imager/Sounder (SSMIS) ice production. The comparison confirmed that the ice production from AMSR-E/2 data, which have higher spatial resolution than SSM/I-SSMIS data, can be used to analyze time series covering more than 10 years. For example, maps of annual ice production based on AMSR-E/2 data revealed detailed changes of the Mertz Polynya, where the ice production decreased significantly after the Mertz Glacier Tongue calving in 2010. Continuous monitoring of the coastal polynyas by the AMSR series sensors is essential for climate-change-related analyses in the Antarctic Ocean. [ABSTRACT FROM PUBLISHER]

Sea-ice thickness in the Sea of Okhotsk is estimated for 2004–2008 from ICESat derived freeboard under the assumption of hydrostatic balance. Total ice thickness including snow depth (h t o t ) averaged over 2004–2008 is 95 cm. The interannual variability of h t o t is large; from 77.5 cm (2008) to 110.4 cm (2005). The mode of h t o t varies from 50–60 cm (2007 and 2008) to 70–80 cm (2005). Ice thickness derived from ICESat data is validated from a comparison with that observed by Electromagnetic Induction Instrument (EM) aboard the icebreaker Soya near Hokkaido, Japan. Annual maps of h t o t reveal that the spatial distribution of h t o t is similar every year. Ice volume of 6.3 × 1011 m3 is estimated from the ICESat derived h t o t and AMSR-E derived ice concentration. A comparison with ice area demonstrates that the ice volume cannot always be represented by the area solely, despite the fact that the area has been used as a proxy of the volume in the Sea of Okhotsk. The ice volume roughly corresponds to that of annual ice production in the major coastal polynyas estimated based on heat budget calculations. This also supports the validity of the estimation of sea-ice thickness and volume using ICESat data. [ABSTRACT FROM AUTHOR]

Under the Japanese Antarctic Research Expedition (JARE) program, sea-ice thickness has been routinely monitored off Lützow–Holm Bay (East Antarctica) during the summer (mid-December to early January) since 2000/01, using an electromagnetic induction (EM) instrument onboard the icebreaker Shirase . Analysis of these data over a 10-year period, combined with visual observations using a simplified form of the ASPeCt (Antarctic Sea ice Processes and Climate) protocol, suggests a strong interannual variability in sea-ice thickness in this region. For the repeat pack-ice observation area, where the sea-ice thickness averaged over the nine seasons is ∼1.9 m, mean thicknesses of observed sea-ice in 2010/11 and 2011/12 are exceptionally large, at ∼3.3 and ∼5.8 m, respectively. This result is strongly related to regional patterns of sea ice dynamics. Ice convergence caused by anomalous northerly winds was particularly high in 2011/12, suggesting that the extremely thick ice observed in that season resulted largely from sea-ice deformation processes (including pressure ridging). Longer-term analysis of data from the past 34 years confirms that sea-ice conditions and thickness off Lützow–Holm Bay in summer are determined mainly by the large-scale pattern of atmospheric pressure in December. [ABSTRACT FROM AUTHOR]

Formation of supercooled water and frazil ice was studied in the Chukchi Sea coastal polynya off Barrow, Alaska, USA, in winter 2009/10, using moored salinity/temperature sensors and Ice Profiling Sonar (IPS) data along with satellite data. Oceanographic data from two moorings revealed episodic events of potential supercooling at 30-40 m depth, including the possibility of in situ super-cooling, while the polynya was open. We identified frazil ice-like signals in the IPS data down to 5-15 m depth, associated with large heat loss and windy, turbulent conditions in an active polynya. This likely represents the first IPS observation of frazil ice in the marine environment. On the day of the maximum signal of frazil ice, spaceborne synthetic aperture radar shows streaks of high backscatter within the polynya, indicating active frazil ice formation just downwind of the mooring sites. In addition, the longer-term potential supercooling that persisted for 1-3 weeks occurred twice despite the absence of polynya activity at the mooring sites. These events occurred during periods dominated by the northeastward current. A series of coastal polynyas had formed southwest of the mooring sites prior to these events. Thus, the water masses with potential supercooling were likely advected from these polynyas. [ABSTRACT FROM AUTHOR]

Accurately measuring and monitoring the thickness distribution of thin ice is crucial for accurate estimation of ocean-atmosphere heat fluxes and rates of ice production and salt flux in ice-affected oceans. Here we present results from helicopter-borne brightness temperature (TB) measurements in the Southern Ocean in October 2012 and in the Sea of Okhotsk in February 2009 carried out with a portable passive microwave (PMW) radiometer operating at a frequency of 36 GHz. The goal of these measurements is to aid evaluation of a satellite thin-ice thickness algorithm which uses data from the spaceborne Advanced Microwave Scanning Radiometer-Earth Observing System AMSR-E) or the Advanced Microwave Scanning Radiometer-II (AMSR-II). AMSR-E and AMSR-II TB agree with the spatially collocated mean TB from the helicopter-borne measurements within the radiometers' precision. In the Sea of Okhotsk in February 2009, the AMSR-E 36 GHz TB values are closer to the mean than the modal TB values measured by the helicopter-borne radiometer. In an Antarctic coastal polynya in October 2012, the polarization ratio of 36 GHz vertical and horizontal TB is estimated to be 0.137 on average. Our measurements of the TB at 36 GHz over an iceberg tongue suggest a way to discriminate it from sea ice by its unique PMW signature. [ABSTRACT FROM AUTHOR]

Data from the Seasonal Ice Zone Observing Network (SIZONet) acquired near Barrow, Alaska, during the 2009/10 ice season allow novel comparisons between measurements of ice thickness and velocity. An airborne electromagnetic survey that passed over a moored Ice Profiling Sonar (IPS) provided coincident independent measurements of total ice and snow thickness and ice draft at a scale of 10 km. Once differences in sampling footprint size are accounted for, we reconcile the respective probability distributions and estimate the thickness of level sea ice at 1.48 ± 0.1 m, with a snow depth of 0.12 ± 0.07 m. We also complete what we believe is the first independent validation of radar-derived ice velocities by comparing measurements from a coastal radar with those from an under-ice acoustic Doppler current profiler (ADCP). After applying a median filter to reduce high-frequency scatter in the radar-derived data, we find good agreement with the ADCP bottom-tracked ice velocities. With increasing regulatory and operational needs for sea-ice data, including the number and thickness of pressure ridges, coordinated observing networks such as SIZONet can provide the means of reducing uncertainties inherent in individual datasets. [ABSTRACT FROM AUTHOR]

At several locations around Antarctica, dense water is formed as a result of intense sea ice formation. When this dense water becomes sufficiently denser than the surrounding water, it descends the continental slope and forms Antarctic Bottom Water (AABW). This study presents the AABW formation off the coast of Cape Darnley [Cape Darnley Bottom Water (CDBW)] in East Antarctica, using a nonhydrostatic model. The model is forced for 8 months by a temporally uniform surface salt flux (because of sea ice formation) estimated from Advanced Microwave Scanning Radiometer for Earth Observing System (EOS; AMSR-E) data and a heat budget calculation. The authors reproduce AABW formation and associated periodic downslope flows of dense water. Descending pathways of dense water are largely determined by the topography; most dense water flows into depressions on the continental shelf, advects onto the continental slope, and is steered downslope to greater depths by the canyons. Intense sea ice formation is the most important factor in the formation of AABW off Cape Darnley, and the existence of depressions is of only minor importance for the flux of CDBW. The mechanism responsible for the periodic downslope flow of dense water is further analyzed using an idealized model setup. The period of dense water outflow is regulated primarily by the topographic beta effect. [ABSTRACT FROM AUTHOR]

The Sea of Okhotsk is the southernmost sea-ice zone with sizable ice. It is widely believed that the prevailing northwesterly wind and the southward East Sakhalin Current (ESC) are the two main factors that drive the southward drift of sea ice. However, the relative contributions of these factors have not been understood. In this paper, by using the current and ice-drift data measured with the moored Acoustic Doppler Current Profiler, a 3-D ocean model simulation, objective analysis data of the wind, and satellite sea-ice data, we examine to what degree and how the ice drift is determined by the wind and ocean current. From a linear regression of the observed ice drift, ocean current, and wind, the wind-forced component of the ice drift was best fitted when sea ice is assumed to move with a speed of 1.6% of the geostrophic wind with a turning angle of 17.6° to the left of the wind. Such a relationship was adopted as the wind-drift component for all sea-ice pixels detected from Special Sensor Microwave Imager data. For the ocean-forced component of the ice drift, we adopted the current at 20m depth from a numerical model simulation that reproduces well the ESC and its variability. We then evaluated the sea-ice drift over 46-54°N during 1998-2005. For the southward drift of sea ice, the contribution of the wind component is found to be larger than the oceanic component, although the ocean contribution becomes larger, typically comparable to the wind contribution, near the coast and in the northern region where the ESC is stronger. We estimated the average annual cumulative southward ice transport to be 3.0±0.9×1011m3 at 53°N. This ice transport is comparable to the annual discharge of the Amur River. The ratio of wind to oceanic components in the transport is estimated to be ~1.2-1.8. We also conducted ice-drift simulations based on the modeled current velocity and the assumed wind drift of 1.5% geostrophic wind with a turning angle of 15° to the left. The simulations reproduce well the ice drift north of 47°N but not south of 47°N, likely due to the poor representation of the current system at the latter, underevaluation of the wind factor near the ice edges, and the neglect of ice formation and melt. [ABSTRACT FROM AUTHOR]

A recently constructed hydrographic dataset from the Okhotsk Sea reveals a prominent freshening to depths of ~500m during the past four decades, with the maximum in the northwestern part of the sea. Averaged over the sea, this freshening corresponds to an input of 0.55m of freshwater. This leads to a decrease in density of the intermediate water and deepening of the isopycnals, with the maximum deepening at 26.8 σθ of ~60m averaged over the sea. The intermediate water is significantly warmed along the pathway of dense shelf water (DSW). A simple box model shows that DSW production has decreased by ~30% during the past four decades. We propose that the freshening and DSW reduction are caused by the weakening of salt/freshwater redistribution through sea ice decline as well as by the increase of excess precipitation over evaporation. Since the overturning in the North Pacific originates from the Okhotsk Sea through the DSW, these changes possibly weaken the shallow overturning of the North Pacific. [ABSTRACT FROM AUTHOR]

Overturning in the North Pacific extending to the intermediate layer (about 200-800m depth) originates from the sinking of dense shelf water (DSW) formed by sea ice production in the Okhotsk coastal polynyas. It has been suggested that this overturning has weakened during the past 50years. The purpose of this study is to clarify the long-term variability of sea ice production in the polynyas and to discuss its linkage with DSW formation and the overturning. First, we have developed a thin ice thickness retrieval algorithm using Special Sensor Microwave/Imager (SSM/I) data for the Sea of Okhotsk, and have estimated the ice production for 21years (1988-2008) by calculating heat flux from SSM/I-derived ice thickness. From a comparison with atmospheric variables, it is suggested that interannual variability of sea ice production in the polynyas can be explained mainly by three atmospheric parameters: autumn air temperature northwest of the sea, winter air temperature north of the sea, and late winter offshoreward wind speed north of the sea. By using these parameters from atmospheric reanalysis data, the annual ice production for the 34years period from 1974 to 2008 is reconstructed from a multiple regression coefficient analysis. The reconstructed ice production shows a significant decreasing trend of ~11.4% over 34years, which is mainly explained by the warming of autumn air temperature. It is also found that the variation in the annual total ice production corresponds well with the potential temperature variation in Okhotsk Sea Intermediate Water (OSIW). This first observational evidence of a linkage between the annual total ice production and OSIW supports a hypothesis that decreasing ice production in the Okhotsk coastal polynyas, at least in part, has led to weakening of the overturning in the North Pacific. [ABSTRACT FROM AUTHOR]

Wind factor, the ratio of sea ice drift speed to surface wind speed, is a key factor for the dynamics of sea ice and is generally about 2%%. In some coastal oceans, however, the wind factor tends to be larger near the coast. This study proposes the enhancement mechanism of the sea ice drift caused by the dynamical coupling between sea ice and a coastal ocean. In a coastal ocean covered with sea ice, wind-forced sea ice drift excites coastal trapped waves (shelf waves) and generates fluctuating ocean current. This ocean current can enhance sea ice drift when the current direction is the same as that of the wind-driven drift. The authors consider a simplified setting where spatially uniform oscillating wind drifts sea ice parallel to the coast. When a barotropic long shelf wave is assumed for the ocean response, sea ice drifts driven by wind and ocean are obtained analytically. The ratio of ocean-driven to wind-driven sea ice drifts is used for the evaluation of the oceanic contribution to the enhancement of sea ice drift. The enhancement is mostly determined by the characteristics of the shelf waves, and sea ice drift is significantly enhanced close to the coast with lower-frequency wind forcing. Comparison with the observation off the Sakhalin coast shows that the degree of enhancement of sea ice drift and its characteristic such that larger enhancement occurs near the coast are mostly consistent with our theoretical solution, suggesting that this mechanism is present in the real ocean. [ABSTRACT FROM AUTHOR]

Abstract: To clarify the generation and dissipation mechanisms of diurnal coastal-trapped waves (CTWs) over the Sakhalin shelf, a series of numerical experiments were conducted using a three-dimensional tidal model of the Okhotsk Sea with density stratification. The tidal model used has good reproduction owing to the careful fitting to the recent observations. The numerical experiments suggested that diurnal CTWs are primarily (~60%) generated by the conversion of tidal energy at the northern corner of the Sakhalin shelf, and further amplified by vorticity generation due to the water column oscillation from Sakhalin Bay and the influence of Kashevarov Bank. From the observations, it was found that diurnal CTWs are effectively dissipated by the strong spin-down due to bottom friction. The conventional turbulent closure model cannot reproduce the observed damping of diurnal CTWs, which raises a caution in modeling the tidal fields in high-latitude regions where diurnal CTWs exist. To resolve this underestimation of the damping, the vertical eddy viscosity was parameterized using its dependence on the observed major axis length of the diurnal tidal current ellipses, which improves the model reproduction on the damping of diurnal CTWs. The model also suggests that the spin-down effects due to friction associated with the sea-ice cover play an important role in the tidal current reduction in the region where diurnal CTWs exist, as the observations suggested. [Copyright &y& Elsevier]

Small sea ice crystals, so-called frazil ice, are created during initial ice formation in the water column. Frazil ice can be suspended under turbulent conditions, and it can scavenge particulate matter through interaction with resuspended sediment. This study examined the suspension depth of frazil ice based on backscatter data from 150-kHz Acoustic Doppler Current Profilers (ADCPs) moored in the pelagic regions east of Sakhalin Island in the Sea of Okhotsk with bottom depths from 800 to 1700 m during 1998–2000. ADCP backscatter strength revealed dominant signals from the diurnal vertical migration of zooplankton. In addition, the backscatter was enhanced by suspended frazil ice that extended down the water column from the surface. Frazil ice was detected several times at each mooring site during sea-ice seasons. These events occurred when the mooring sites were on the edge of the sea ice regions and the wind speed was high, typically exceeding 8 m/s. The estimated mean suspension depth of frazil ice was 35 m as long as valid ADCP backscatter data from depths greater than ~15 m were used. The maximum suspension depth reached ~100 m, which was much deeper than that previously reported in various sea-ice regions as frazil ice originating from the atmospheric heat absorption at the surface. These results suggest that frazil ice comes into contact with the bottom and scavenges seafloor sediment in shallower regions, resulting in incorporation of sediment-origin macronutrients into sea ice, which would play an important role in bio-related material cycle. • ADCP backscatter often detected suspended frazil ice around sea ice edge. • Suspended frazil ice generally penetrates to a depth of ~35 m in the water column. • Suspended frazil ice can reach down to the maximum of ~100 m. • Suspended frazil ice typically appears when the wind speed exceeds 8 m/s. • Frazil ice can contact and scavenge seafloor sediment in regions shallower than ~35 m. [ABSTRACT FROM AUTHOR]

Abstract: From July 1998 to June 2000, the first long-term mooring measurements were carried out off the east coast of Sakhalin. Using these data, we examined the characteristics of the tidal heights and currents. The tidal heights and currents are dominated by the diurnal variability with fortnightly modulation over the northern part of the shelf. The and tidal current ellipses are clockwise with their major axes along the isobaths and their signal propagates with the coast on the right with phase speeds of 3.4 and , respectively. The diurnal tidal currents are almost uniform in the vertical direction except for the bottom Ekman layer. The thickness of the bottom Ekman layer caused by the diurnal tidal currents is larger in the region of stronger tidal currents, reaching 20–30m over the northern part of the shelf. The diurnal tidal currents over the northern part of the shelf can be explained by the first-mode diurnal coastal-trapped waves (CTWs). The diurnal CTWs are almost independent of the seasonal variability of density stratification and contribute significantly to diurnal currents, but only slightly to sea-surface heights. The diurnal tidal currents over the southern part of the shelf are significantly smaller than those over the northern part. This is because the diurnal CTWs cannot exist south of from the dispersion relation. The diurnal tidal currents are significantly reduced over the northern shelf, where the diurnal CTWs exist, during the high sea-ice concentration periods. From this result, we propose the scenario that the CTWs are damped by the spin-down effect due to the Ekman layer that would occur underneath the sea ice. [Copyright &y& Elsevier]

Antarctic coastal polynyas are important areas of high sea ice production and dense water formation, and thus their detection including an estimate of thin ice thickness is essential. In this paper, the authors propose an algorithm that estimates thin ice thickness and detects fast ice using Defense Meteorological Satellite Program (DMSP) Special Sensor Microwave Imager (SSM/I) data in the Antarctic Ocean. Detection and estimation of sea ice thicknesses of <0.2 m are based on the SSM/I 85- and 37-GHz polarization ratios (PR85 and PR37) through a comparison with sea ice thicknesses estimated from the Advanced Very High Resolution Radiometer (AVHRR) data. The exclusion of data affected by atmospheric water vapor is discussed. Because thin ice and fast ice (specifically ice shelves, glacier tongues, icebergs, and landfast ice) have similar PR signatures, a scheme was developed to separate these two surface types before the application of the thin ice algorithm to coastal polynyas. The probability that the algorithm correctly distinguishes thin ice from thick ice and from fast ice is ∼95%, relative to the ice thicknesses estimated from AVHRR. Although the standard deviation of the difference between the thin ice thicknesses estimated from the SSM/I algorithm and AVHRR is ∼0.05 m and thus not small, the estimated ice thicknesses from the microwave algorithm appear to have small biases and the accuracies are independent of region and season. A distribution map of thin ice occurrences derived from the SSM/I algorithm represents the Ross Sea coastal polynya being by far the largest among the Antarctic coastal polynyas; the Weddell Sea coastal polynyas are much smaller. Along the coast of East Antarctica, coastal polynyas frequently form on the western side of peninsulas and glacier tongues, downstream of the Antarctic Coastal Current. [ABSTRACT FROM AUTHOR]

In the Antarctic Ocean, sea ice melts mostly by warming of the ocean mixed layer through heat input (mainly solar radiation) in open water areas. A simplified ice–upper ocean coupled model is proposed in which sea ice melts only by the ocean heat supplied from the air. The model shows that the relationship between ice concentration (i.e., fraction, C) and mixed layer temperature (T) converges asymptotically with time (C–T relationship), which agrees with observed C–T plots during summer in the sector 25°–45°E. This relationship can be used for estimating the bulk heat transfer coefficient between ice and ocean by fitting to observations, and a value of 1.2 × 10-4 m s-1 is obtained. The model shows that the ratio of the heat used for melting to the heat input through open water is inclined to be determined as a function of ice concentration. For typical conditions in the Antarctic ice melt season, the ratio ranges mostly between 0.7 and 0.9. When the model is extended to two dimensions in the meridional direction, with the inclusion of wind forcing, it approximately reproduces the meridional retreat of the Antarctic sea ice. This two-dimensional model can describe the open water–albedo feedback effect, which partly explains the year-to-year variation of the sea-ice retreat in the Antarctic Ocean. [ABSTRACT FROM AUTHOR]

Hydrographic observations under sea ice were conducted in the southwestern part of the Sea of Okhotsk for 4 years from 1996 to 1999. Every year a cold mixed layer with near-freezing temperature was distributed from the sea surface to a depth of 150–300 m near the shelf break under sea ice. The thicknesses of the mixed layer and sea ice were largest in 1997. While the depth of the mixed layer was considerably deeper than that of dichothermal water, which is identified as a temperature minimum from spring to fall in this region, the density of water in the mixed layer was equal to or less than that of dichothermal water. It is shown that deepening of isopycnals due to the alongshore component of the wind stress is essential for thickening of the mixed layer. In 1997 the nearly northerly winds, which are usually directed offshore, were more parallel to the coast than those in the other years. Thickening of the mixed layer in 1997 is attributed to this wind condition. The air temperature and wind indicate that the sea-ice production rate was low in 1997, whereas the wind direction was favorable for rafting and ridging. Thus the change in wind direction is proposed as an important factor in determining the thickness of both the mixed layer and the sea ice in this region. [Copyright &y& Elsevier]

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. • Studies the Cape Darnley Polynya, a globally-important producer of bottom water • Details links between Antarctic landfast sea ice and coastal polynya ice production • Defines fast ice metrics which describe variability in polynya size and production • Fast ice metrics are as important as atmospheric quantities for this polynya. • Provides an inter-comparison between passive microwave and MODIS fast ice maps [ABSTRACT FROM AUTHOR]

Knowledge of wintertime sea-ice production in polynyas is an important aspect ofatmosphere – sea-ice – ocean interactions in high latitudes, and a precise quantification canbe for instance utilized to verify frequently used climate and ocean models. This studyfeatures a high-resolution (2km) MODIS thermal infrared satellite data set with spatial andtemporal characteristics of 17 coastal polynya regions over the entire Arctic basin for2002/2003 to 2017/2018, as well as a similar data set based on lower resolution (6.25km)AMSR-E passive microwave satellite data for a nine-year overlapping comparison period (upto 2010/2011). The MODIS data set is relying on a 1D energy balance model, where quasi-dailythin-ice thickness composites (up to 20cm) are directly derived from ice-surfacetemperature swath-data and ERA-Interim atmospheric reanalysis data. Dedicated cloudscreening and spatial/temporal interpolation techniques are therein applied to effectivelyaccount for sensor-specific drawbacks. In case of the AMSR-E data set, thin-icethicknesses are derived using an empirical approach that utilizes a characteristicpolarization ratio (PR) – ice thickness relationship. In both data sets, the daily pan-Arcticmapping of thin-ice thicknesses allows for a long-term derivation of important polynyaproperties such as polynya area (POLA) and potential thermodynamic ice production(IP). It shows that the average POLA (average accumulated IP) for all Arctic polynyascombined is 1.99 x 105 km2 (1.34 x 103 km3) when derived from MODIS data and 2.23 x105 km2 (1.29 x 103 km3) in case of AMSR-E data. Although the two data sets areindependently derived, they show quite similar spatial and temporal variations of POLA andIP, which suggests a high reliability of both data sets. Hence, despite all methodicaldifferences, both data sets are to a large degree coherent in terms of capturing the generalspatial and temporal characteristics of Arctic polynyas for the overlapping 9-yr period.Emerging positive trends in POLA and IP over the long 16-yr period of the MODIS data setare mainly visible in the Eastern Arctic and are potentially related to large-scaleatmospheric modes and/or changing characteristics of the Transpolar drift system. [ABSTRACT FROM AUTHOR]

Landfast sea ice is an important seasonal feature along most Arctic coastlines, such as that of the Chukchi Sea near Barrow, Alaska. Its stability throughout the ice season is determined by many factors but grounded pressure ridges are the primary stabilizing component. Landfast ice breakouts occur when these grounded ridges fail or unground, and previously stationary ice detaches from the coast and drifts away. Using ground-based radar imagery from a coastal ice and ocean observatory at Barrow, we have developed a method to estimate the extent of grounded ridges by tracking ice motion and deformation over the course of winter and have derived ice keel depth and potential for grounding from cumulative convergent ice motion. Estimates of landfast ice grounding strength have been compared to the atmospheric and oceanic stresses acting on the landfast ice before and during breakout events to determine prevailing causes for the failure of stabilizing features. Applying this approach to two case studies in 2008 and 2010, we conclude that a combination of atmospheric and oceanic stresses may have caused the breakouts analyzed in this study, with the latter as the dominant force. Preconditioning (as weakening) of grounded ridges by sea level variations may facilitate failure of the ice sheet leading to breakout events. [ABSTRACT FROM AUTHOR]

Over the past few decades, sea ice retreat during summer has been enhanced in the Pacific sector of the Arctic basin, likely due in part to increasing summertime heat flux of Pacific-origin water from the Bering Strait. Barrow Canyon, in the northeast Chukchi Sea, is a major conduit through which the Pacific-origin water enters the Arctic basin. This paper presents results from 6 repeat high-resolution shipboard hydrographic/velocity sections occupied across Barrow Canyon in summer 2010. The different Pacific water masses feeding the canyon – Alaskan coastal water (ACW), summer Bering Sea water (BSW), and Pacific winter water (PWW) – all displayed significant intra-seasonal variability. Net volume transports through the canyon were between 0.96 and 1.70 Sv poleward, consisting of 0.41–0.98 Sv of warm Pacific water (ACW and BSW) and 0.28–0.65 Sv of PWW. The poleward heat flux also varied strongly, ranging from 8.56 TW to 24.56 TW, mainly due to the change in temperature of the warm Pacific water. Using supplemental mooring data from the core of the warm water, along with wind data from the Pt. Barrow weather station, we derive and assess a proxy for estimating heat flux in the canyon for the summer time period, which is when most of the heat passes northward towards the basin. The average heat flux for 2010 was estimated to be 3.34 TW, which is as large as the previous record maximum in 2007. This amount of heat could melt 315,000 km 2 of 1-meter thick ice, which likely contributed to significant summer sea ice retreat in the Pacific sector of the Arctic Ocean. [ABSTRACT FROM AUTHOR]

Abstract: From December to June 2002–2003, sea-ice and oceanic data were obtained from moorings near Sakhalin in the west central Okhotsk Sea. Ice draft measured by sonar reveals distinct periods of thin and thick ice. Thin-ice periods in January–March corresponded to offshore ice movement and increasing seawater salinity. The measured change in salinity corresponds well with that derived from heat-flux calculations using the observed ice thickness. Brine rejection from ice growing in a coastal polynya off northern Sakhalin is responsible for much of the observed salinity increase. The simultaneous observation of dense shelf water () suggests that this region is one possible source. The periods of thick-ice incursion are likely indicative of heavily deformed pack formed further north and drifting south with the current. The mean draft (1.95 m), thick-ice ratio, and keel frequency during these periods are close to values observed in the Beaufort Sea. Freshwater transport estimated from the observed ice thickness and velocity is larger than that of the Amur River discharge. [Copyright &y& Elsevier]