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Polar and subpolar oceans play a particularly important role in the global climate and its temporal changes, yet these regions are less well sampled than the rest of the global ocean. To better understand the physical or biogeochemical properties and their variabilities in these regions, accurate data mapping is crucial. In this paper, we introduce a mapping methodology that includes a water column shrinking and stretching constraint (SSC) based on the principle of conservation of potential vorticity. To demonstrate the mapping scheme efficiency, we map the ocean temperature in the southern Sea of Okhotsk, where the bottom topography comprises a broad and shallow shelf, a sharp continental slope, and a deep oceanic basin. Such topographic features are typical of polar and subpolar marginal seas. Results reveal that the SSC integrated (SSCI) mapping strongly reduces the mapping error in the broad and shallow shelf compared with a recently introduced topographic constraint integrated (TCI) mapping procedure. We also tested our mapping scheme in the Southern Ocean, which has a comparatively slanted shelf, a wider and gentler slope, and a deep and broad oceanic basin. We found that the SSCI and TCI methods are practically equivalent there. The SSCI mapping is thus an effective method to map the ocean's properties in various topographic environments and should be adequate in all polar and subpolar regions. Importantly, we introduced a standardized procedure for determining the decorrelation length scales—a necessary step prior to implementing any mapping scheme—in any topographic conditions. [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.

The western subarctic Pacific exhibits major biological productivity fed by the Oyashio Current and its two source waters: Western Subarctic Water, which supplies nutrients from the subarctic Pacific, and cold Okhotsk Sea Intermediate Water (OSIW), which supplies iron from the Sea of Okhotsk. We created seasonal climatologies of water properties to understand how the long-term trend (~ 50 years) and 18.6-year tidal cycle affect the Oyashio Intermediate Water (OYW). We found that over the trend, decreased OSIW outflow due to weakening of North Pacific overturning modifies OYW in winter. Meanwhile, OSIW outflow increases (decreases) in strong (weak) tide years. We predict that the opposite effects of the trend and strong tide will lead to stagnation of OYW properties until the mid-2020s, followed by accelerated warming until the mid-2030s (weak tide). A predicted 1 °C increase in OYW temperature and 50% decrease in OSIW content between 1960 and 2040 potentially have significant impact on biological productivity and carbon drawdown in the North Pacific. [ABSTRACT FROM AUTHOR]

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]

Atime series analysis of the sea surface height anomaly (SSHa) was conducted in the Kuril Basin of the Sea of Okhotsk. The mapping of the satellite-derived SSHa data was optimized to mitigate the effects of sea ice on the SSHa field during winter and early spring. Complex empirical orthogonal functions (CEOFs) were then used to analyze the SSHa field, revealing that the first three modes account for 55% of the signal variance. Mode 1 mainly represents the coherent variability trapped over the shelves all along the coastal regions and the Kuril Islands. Both seasonal and interannual variations are strongly correlated with the alongshore wind stress and can be well explained by an arrested topographic wave. Mode 3 is a propagating mode that exhibits trains of southwestward-propagating, high-amplitude anomalies. One possible mechanism for this is firstmode baroclinic Rossby waves, whose energy propagates from the Kuril Straits toward the Kuril Basin. However, mode 3 can be better interpreted as barotropic Rossby normal modes generated in the deep Kuril Basin. Mode 2 is a standing mode that may encompass the baroclinic variability in the basin. The monthly mean of the SSHa in the Kuril Basin is primarily governed by variability in mode 1, with mode 2 contributing to a lesser extent, and mode 3 being insignificant. [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]

Abstract The seasonal variability of the hydrography and anticyclonic circulation in the Kuril Basin of the Sea of Okhotsk, from which the North Pacific ventilation originates, is studied from bi-monthly climatologies of temperature, salinity, dynamic height and current velocity. The subsurface hydrography of the Kuril Basin is characterized by the dominant role of fresh and cold waters transported by the East Sakhalin Current during winter and spring, and relatively warm and saline waters flowing with the Soya Warm Current. At the intermediate layer, the influence of dense shelf water is maximum from May to August, and this water mixes with surrounding waters in around half a year to form a well homogenized Okhotsk Sea Intermediate Water, the source of North Pacific ventilation. The anticyclonic circulation typical of the Kuril Basin exhibits a strong seasonality, being absent in winter and showing a maximum amplitude in summer. Its formation is mainly related to the dynamic response of isopycnals to wind forcing. A dynamic height maximum appears along the coastal regions from Sakhalin Island to the southern Kuril Straits in winter, likely migrates towards the Kuril Basin in March–April and forms the anticyclonic system between May and October. The dynamic height – and thus the flow field – is mainly determined by isopycnal displacement in and around the Kuril Basin, whereas advection of water masses has a negligible effect. Surface intensification and decay of the anticyclonic circulation seems to be related to other processes such as heat exchange with the atmosphere. Highlights • The anticyclonic circulation in the Kuril Basin exhibits a strong seasonality. • Its formation is related to a dynamic response of isopycnals to wind stress changes. • Isopycnal displacement is mainly responsible for dynamic height changes there. • Dense Shelf Water is found in May–August and Soya Current Water in July–October. • They mix to form a homogenized Okhotsk Sea Intermediate Water within half a year. [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]

Volume, heat, and salt transports through the Soya Strait are estimated based on measurements from high-frequency ocean radars during 2003-15 and all available hydrographic data. The baroclinic velocity structure derived from the climatological geopotential anomaly is combined with the sea surface gradient obtained from radar-derived surface velocities to estimate the absolute velocity structure. The annual-mean volume, heat, and salt transports are 0.91 Sv (1 Sv ≡ 106 m3 s−1), 25.5 TW, and 31.15 × 106 kg s−1, respectively. The volume transport exhibits strong seasonal variations, with a maximum of 1.41 Sv in August and a minimum of 0.23 Sv in January. The seasonal amplitude and phase roughly correspond to those of the Tsushima-Korea Strait. Time series of the monthly transport is presented for the 12 yr, assuming that the baroclinic components are the monthly climatological values. In cold seasons (November to April), the monthly volume transport is strongly correlated with the sea level difference between the Japan and Okhotsk Seas, and an empirical formula to estimate the transport from the sea level difference is introduced. It is likely that the sea level setup by the wind stress along the east coast of Sakhalin determines the sea level difference, which explains the seasonal and interannual wintertime variations of transport through the strait. The annual flux of water through the Soya Strait with a density greater than 26.8 σ θ, a potential source of Okhotsk Sea Intermediate Waters, is estimated to be 0.18 Sv. [ABSTRACT FROM AUTHOR]

A new modelling framework using Lagrangian particle tracking has been developed to assess dynamic and thermodynamic effects of underwater frazil ice. This frazil-ice model treats a Lagrangian particle as a bulk cluster of many frazil crystals, and calculates the thermodynamic growth of each particle and the corresponding budget of latent heat and fresh water. The effective density and viscosity of sea water depend on the mass fraction of underwater frazil ice, and hence affect ocean convection. An idealized experiment using our model successfully reproduces the formation of underwater frazil ice and its transition to grease ice at the surface. Because underwater frazil ice does not reduce the atmosphere/ocean heat exchange, surface heat flux and net sea-ice production in the experiment with frazil ice are relatively high compared with the experiment where surface cooling directly leads to columnar growth of a solid ice cover which effectively insulates the heat flux. These results suggest that large-scale sea-ice models which do not take account of the effects of frazil ice might underestimate atmosphere/ocean heat exchange, particularly at times of active new ice formation. [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]

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. [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 Southern Ocean allows circumpolar structure and the Antarctic coastline plays a role as a waveguide for oceanic Kelvin waves. Under the cyclic conditions, the horizontal wavenumbers and frequencies for circumpolarly propagating waves are quantized, with horizontal wavenumbers 1, 2, and 3, corresponding to periods of about 32, 16, and 11 h, respectively. At these frequencies, westward-propagating signals are detected in sea level variation observed at Antarctic coastal stations. The occurrence frequency of westward-propagating signals far exceeds the statistical significance, and the phase speed of the observed signal agrees well with the theoretical phase speed of external Kelvin waves. Therefore, this study concludes that the observed, westward-propagating sea level variability is a signal of the external Kelvin waves of wavenumbers 1, 2, and 3 around Antarctica. A series of numerical model experiments confirms that Kelvin waves around Antarctica are driven by surface air pressure and that these waves are excited not only by local forcing over the Southern Ocean, but also by remote forcing over the Pacific Ocean. Sea level variations generated over the Pacific Ocean can travel to the western side of the South American coast and cross over Drake Passage to the Antarctic continent, constituting a part of the Kelvin waves around Antarctica. [ABSTRACT FROM AUTHOR]

We investigated the seasonal and interannual variations of the sea level in and around the Sea of Okhotsk and their causes, based on tide gauge and satellite altimeter data. The sea level all along the coastal region of the Sea of Okhotsk is found to be dominated by the seasonal variation with a maximum in winter and a minimum in summer, which cannot be explained by the annual cycle of atmospheric heat flux and pressure. This sea level variation appears to reflect ocean current variations. Both the Arrested Topographic Waves (ATWs) caused by alongshore wind stress and the Sverdrup transport by wind stress curl show corresponding seasonal variations. Seasonal amplitude of the sea level is relatively large along Sakhalin Island with a tendency of a larger amplitude toward the south. This meridional dependence is consistent with the ATWs, but not with the Sverdrup transport in the Sea of Okhotsk. Seasonal variation of the geostrophic current velocity expected from the sea level variation is comparable to that of the observed nearshore current and is consistent with the theoretical ATW transport. It is also revealed that, on an interannual timescale, the wintertime sea level fluctuates quite coherently all around the Sea of Okhotsk and further along the East Kamchatka and Oyashio coasts in the North Pacific. The altimeter data clearly show that this coherent sea level variation is trapped over the coastal and continental shelf regions with depths shallower than 1000m. The wintertime sea levels have a higher correlation with the ATW transport than with the Sverdrup transport in the Sea of Okhotsk and the upstream East Kamchatka coast. All these suggest that the interannual sea level variation along the coastal and shelf regions in winter, as well as the seasonal variation, is mainly caused by the ATWs (coastal trapped current forced by the alongshore wind stress). The wintertime Sverdrup transport, raised by the previous studies, is the secondary contributor to these variations. [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]

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]

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]

Abstract: Strong vertical mixing in the Kuril Straits is believed to be an important contribution to the ventilation of the intermediate layer and water mass transformation in the North Pacific, especially for density greater than . Furthermore, a recent modeling study has suggested that the vertical profile of vertical diffusivity is a key factor in the determination of thermohaline circulation in the North Pacific. Here we report the distribution of vertical diffusivity in the Bussol' Strait, the main conduit of water exchange and a possible central site of strong mixing in the Kuril Straits. Our analysis is based on a set of highly densed CTD observations, with a total of 127 casts across the strait in 2001. Vigorous density inversions occurred in the strait with the largest vertical displacement being over 250m. We estimated the vertical diffusivity coefficient from the Thorpe scale for all the CTD data. The vertical average of estimated from all the casts is 60×10−4 m2 s−1. Overall, is relatively small in the upper 300m (density range approximately ), whereas it is relatively large below a depth of 500m (density range of ), with a maximum at the depths of 1100–1700m. The distributions of and the amplitude of the diurnal tidal current are similar, suggesting that the mixing is caused by the strong diurnal tidal current. The amplification of the diurnal (tidal) current over slopes near the bottom causes the maximum at depths of . We also introduce an empirical relationship between and the amplitude of the diurnal tidal current. The vertical diffusivity is one order of magnitude larger at the spring tide than at the neap tide, suggesting that there is extremely large variability of tidal mixing with the fortnightly modulation. In the intermediate layer at densities of , large values ( ) corresponds well to the colder and less-saline water mass characterized in the Bussol' Strait, confirming that water mass transformation occurs locally in the strait through strong diapycnal mixing. [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]

The formation of Antarctic Bottom Water-the cold, dense water that occupies the abyssal layer of the global ocean-is a key process in global ocean circulation. This water mass is formed as dense shelf water sinks to depth. Three regions around Antarctica where this process takes place have been previously documented. The presence of another source has been identified in hydrographic and tracer data, although the site of formation is not well constrained. Here we document the formation of dense shelf water in the Cape Darnley polynya (65°-69° E) and its subsequent transformation into bottom water using data from moorings and instrumented elephant seals (Mirounga leonina). Unlike the previously identified sources of Antarctic Bottom Water, which require the presence of an ice shelf or a large storage volume, bottom water production at the Cape Darnley polynya is driven primarily by the flux of salt released by sea-ice formation. We estimate that about 0.3-0.7×106 m3 s−1 of dense shelf water produced by the Cape Darnley polynya is transformed into Antarctic Bottom Water. The transformation of this water mass, which we term Cape Darnley Bottom Water, accounts for 6-13% of the circumpolar total. [ABSTRACT FROM AUTHOR]

Sea ice formation, its transport, and its melting cause the redistribution of heat and salt, which plays an important role in the climate and biogeochemical systems. In the Sea of Okhotsk, a heat and salt flux dataset is created in which such sea ice processes are included, with a spatial resolution of ~12.5 km. The dataset is based on a heat budget analysis using ice concentration, thickness, and drift speed from satellite observations and the ECMWF Interim Re-Analysis (ERA-Interim) data. The salt flux calculation considers both salt supplied to the ocean from sea ice production and freshwater supplied when the ice melts. This dataset will be useful for the validation and boundary conditions of modeling studies. The spatial distribution of the annual fluxes shows a distinct contrast between north and south: significant ocean cooling with salt supply is shown in the northern coastal polynya region, while ocean heating with freshwater supply is shown in the south. This contrast suggests a transport of freshwater and negative heat by ice advection. The annual fluxes also show ocean cooling with freshwater supply in the Kashevarov Bank (KB) region and the central and eastern Sea of Okhotsk, suggesting the effect of warm water advection. In the ice melt season, relatively prominent ice melting is shown in the coastal polynya region, probably due to large solar heating of the upper ocean. This indicates that the polynya works as a 'meltwater factory' in spring, contrasting with its role as an 'ice factory' in winter. In the coastal polynya region, the spatial distribution of phytoplankton bloom roughly corresponds with the ice melt region. [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: This paper examines the seasonal variation in the in- and outflow of the Okhotsk Sea with the Pacific, mainly based on profiling float and satellite altimeter data. The motions of the floats at depths of 500-750m show that the waters of the Okhotsk tend to flow out to the Pacific through the southern Kuril Straits, mostly Bussol’ Strait, in winter seasons (November-March). Based on data from the floats exiting the Sea, the mean residence time of the intermediate water in the Kuril Basin is estimated as about one year. Isopycnal analysis from the profiling data shows that, in the mid-eastern area of the Okhotsk Sea, potential temperature of the intermediate water with σθ=26.6-27.2 increases by 0.2-0.5°C in every spring, suggesting the inflow from the Pacific in the preceding season. These suggest that the in- and outflow predominantly occurs during winter, with the inflow at the northern straits and outflow at the southern straits. The surface current field inferred from satellite altimeter data also shows that the inflow to the Sea of Okhotsk through the northern straits is strengthened in winter, linked with strengthening of the East Kamchatka Current (EKC). From yearly anomalies of the altimeter data, the winter inflow to the Okhotsk through the northern straits is significantly correlated with the EKC and Sverdrup transport over the North Pacific. Further, interannual variations in property of Okhotsk Sea Intermediate Water appear to be related with the EKC and wind stress over the North Pacific in winter, possibly through the change in inflowing Pacific water transport. These suggest that the seasonal and interannual variations of the in- and outflow are, to some extent, controlled by the large scale wind stress over the North Pacific, being consistent with Island Rule qualitatively. [Copyright &y& Elsevier]

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]

Coastal sea level variation around Antarctica is characterized by a coherent (circumpolarly in-phase) fluctuation, correlated with the Antarctic Oscillation (AAO). This study addresses the dynamics of the wind-driven sea level variation around Antarctica. A realistic barotropic numerical model reproduced well the observed sea level around Antarctica. From numerical model experiments, the authors demonstrate that the forcing responsible for the coastal sea level is the wind stress at the coastal boundary. Both the dominant coherent signal and westward propagating signals are identified in the model, and these signals are trapped over the shelf and slope around Antarctica. As a mechanism of these trapped signals, the authors consider analytical solutions of the oceanic response to alongshore wind stress over the shelf and slope in the circumpolar domain. In these solutions, besides the shelf wave mode, a wavenumber-zero mode appears and characterizes the coastal dynamics around Antarctica. At periods from 10 to 200 days, the coherent sea level can be explained quantitatively by the solution of this wavenumber-zero mode with a 5–10-day damping time scale. The spectral peaks of the westward propagating signals can be explained by the resonance of the shelf wave mode. The wavenumber-zero mode can respond to the wavenumber-zero forcing at any frequency and the degree of response increases with decreasing frequency. In addition, the wavenumber-zero component of wind stress, corresponding to the AAO variation, is a dominant forcing. Therefore, the coherent sea level variation around Antarctica is preferably generated and becomes a dominant feature in the circumpolar domain, particularly at lower frequencies. [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]

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 during the spring tide. The and 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. [Copyright &y& Elsevier]

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]

It is proposed that the cyclonic gyre over the northern half-basin of the Okhotsk Sea is driven by the wind stress curl and that a major part of the East Sakhalin Current (ESC) can be regarded as its western boundary current. Both from the high-resolution ECMWF and Comprehensive Ocean–Atmosphere Dataset (COADS) data, the annual mean wind stress curl is positive over the sea. When the Sverdrup streamfunction is calculated by excluding the shallow shelves, the streamfunction shows a cyclonic pattern over the central basin, which is roughly consistent with the geopotential anomaly distribution from all the available hydrographic data. Profiling floats suggest that the cyclonic gyre extends to at least a depth of 500 m: a relatively intense southward flow (ESC) with an average speed of approximately 10 cm s[sup -1] near the western boundary and slow northward flow with an average speed of approximately 2 cm s[sup -1] in the east. Climatological data show that along zonal sections at 50°–53°N isopycnal surfaces gradually rise from the east to west and sharply drop near the western boundary, suggesting the Sverdrup balance. This feature persists throughout the year. The integrated northward baroclinic transport of 3.5 Sv along 53°N is comparable to the Sverdrup transport of 3.7 Sv, calculated from the annual mean wind stress. Sverdrup balance appears to hold roughly in the baroclinic field in 50°–53°N. A flat-bottom numerical model forced by realistic wind stress reproduces well the cyclonic gyre, with the observed baroclinic features. In the south, the anticyclonic circulation in the Kuril Basin cannot be explained by the wind stress curl inside the Okhotsk Sea in this simplified model. [ABSTRACT FROM AUTHOR]

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 (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]

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]

This study investigated a method for creating a climatological dataset with improved reproducibility and reliability for the Southern Ocean. Despite sparse observational sampling, the Southern Ocean has a dominant physical characteristic of a strong topographic constraint formed under weak stratification and strong Coriolis effect. To increase the fidelity of gridded data, the topographic constraint is incorporated into the interpolation method, the weighting function of which includes a contribution from bottom depth differences and horizontal distances. Spatial variability of physical properties was also analyzed to estimate a realistic decorrelation scale for horizontal distance and bottom depth differences using hydrographic datasets. A new gridded dataset, the topographic constraint incorporated (TCI), was then developed for temperature, salinity, and dissolved oxygen, using the newly derived weighting function and decorrelation scales. The root-meansquare (RMS) of the difference between the interpolated values and the neighboring observed values (RMS difference) was compared among available gridded datasets. That the RMS differences are smaller for the TCI than for the previous datasets by 12%-21% and 8%-20% for potential temperature and salinity, respectively, demonstrates the effectiveness of incorporating the topographic constraint and realistic decorrelation scales. Furthermore, a comparison of decorrelation scales and an analysis of interpolation error suggests that the decorrelation scales adopted in previous gridded datasets are 2 times or more larger than realistic scales and that the overestimation would increase the interpolation error. The interpolation method proposed in this study can be applied to other high-latitude oceans, which are weakly stratified but undersampled. [ABSTRACT FROM AUTHOR]

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

Chlorofluorocarbon (CFC) and sulfur hexafluoride (SF6) were used to investigate the timescale of Antarctic Bottom Water (AABW) that spreads off Cape Darnley (CD) in East Antarctica. The age of the AABW was estimated based on the observed SF6/CFC-12 ratio while taking into account tracer dilution by Lower Circumpolar Deep Water. Along the western canyons off CD and the ~ 3000 to 3500 m isobaths, the bottom water age was < 5 years, reflecting the spread of newly formed CD Bottom Water. Higher ages of ~ 8 years obtained for areas east of CD and > 20 years in the northwestern offshore region indicate inflows of AABW through the Princess Elizabeth Trough and Weddell Sea Deep Water, respectively. This study determined the age distribution in the region off CD, where three different types of AABW spread. [ABSTRACT FROM AUTHOR]

Landfast sea ice (fast ice) is an important though poorly understood component of the cryosphere on the Antarctic continental shelf, where it plays a key role in atmosphere–ocean–ice-sheet interaction and coupled ecological and biogeochemical processes. Here, we present a first in-depth baseline analysis of variability and change in circum-Antarctic fast-ice distribution (including its relationship to bathymetry), based on a new high-resolution satellite-derived time series for the period 2000 to 2018. This reveals (a) an overall trend of -882±824 km 2 yr -1 (-0.19±0.18 % yr -1) and (b) eight distinct regions in terms of fast-ice coverage and modes of formation. Of these, four exhibit positive trends over the 18-year period and four negative. Positive trends are seen in East Antarctica and in the Bellingshausen Sea, with this region claiming the largest positive trend of +1198±359 km 2 yr -1 (+1.10±0.35 % yr -1). The four negative trends predominantly occur in West Antarctica, with the largest negative trend of -1206±277 km 2 yr -1 (-1.78±0.41 % yr -1) occurring in the Victoria and Oates Land region in the western Ross Sea. All trends are significant. This new baseline analysis represents a significant advance in our knowledge of the current state of both the global cryosphere and the complex Antarctic coastal system, which are vulnerable to climate variability and change. It will also inform a wide range of other studies. [ABSTRACT FROM AUTHOR]

In order to clarify the structure and seasonal variability of the flow field near the western boundary of the Sea of Okhotsk, long-term mooring measurements were carried out from 1998 to 2000 in this region. In most of the mooring period a persistent southward flow (the East Sakhalin Current) was observed, which extends from the surface to a depth around 1000 m. The speed of this southward flow clearly changed seasonally. The peak monthly mean speed along 53°N at a depth of 200 m attained a maximum of 37 ± 9 cm s[sup -1] in January and a minimum of 10 ± 8 cm s[sup -1] in July. Three different cores of intense flow were identified in the southward flow. The first core was centered over the continental slope and had rather large vertical extent, reaching the bottom on the slope. The second core was trapped over the shelf near the surface and was observed from October to November. This core was associated with less saline surface water affected by the Amur River discharge. The third core was intensified toward the bottom on the slope. The spatial and temporal distribution of this bottom-intensified core coincided with that of dense shelf water, which is formed over the broad shelf in the north. The intensity of this core damped within a few hundred kilometers from the northern end of Sakhalin probably because of strong mixing of dense shelf water with surrounding waters. The total transport of the southward flow at 53°N was 6.7 × 10[sup 6] m[sup 3] s[sup -1] in the annual average, varying from a maximum of 12.3 × 10[sup 6] m[sup 3] s[sup -1] in February and a minimum of 1.2 × 10[sup 6] m[sup 3] s[sup -1] in October. Most of the transport was maintained by the first core of the southward flow. [ABSTRACT FROM AUTHOR]

• Circumpolar distribution of age of Antarctic Bottom Water (AABW) was revealed. • Young age of AABW reflected the spread of newly formed AABW. • Anthropogenic CO 2 (C ant) content of AABW in the Southern Ocean (SO) was estimated. • C ant amount in AABW was estimated to be 15% of total C ant in the whole SO. • Atlantic Ocean sector of the SO is the largest reservoir of the C ant in AABW. Antarctic Bottom Water (AABW) sequesters anthropogenic CO 2 (C ant) over the long-term and impacts the global carbon cycle. In this study, data of chlorofluorocarbon and sulfur hexafluoride observed from 2005 to 2020 in the Southern Ocean (SO) were used to investigate the age and C ant content of AABW. In the coastal regions of the Cosmonaut, Cooperation, D'Urville, and Somov Seas and near the Pacific-Antarctic Ridge, the age of AABW was < 5 years, reflecting the spread of newly formed AABW. Higher C ant inventory in AABW was detected in the Atlantic Ocean sector of the SO such as the Weddell and Lazarev Seas (29–55 ± 19–39 mol m−2) and the Cosmonaut and Cooperation Seas (35 ± 14 mol m−2) than those in the other regions. The amount of C ant in AABW in this region (7 ± 1–2 PgC) was estimated to be 78 % of the total C ant in AABW of the SO, suggesting the Atlantic Ocean sector as a major player to C ant sequestration in the abyssal ocean. The high C ant inventory in the Cosmonaut and Cooperation Seas is attributed to the presence of new AABW. The high C ant inventory also corresponded to the thick layer and high formation rate of AABW. Similar to the Cosmonaut and Cooperation Seas, the high C ant inventory in the Weddell and Lazarev Seas reflects the thick layer and high formation rate of AABW. The total amount of C ant in AABW in the SO (9 ± 1–2 PgC that would be the upper limit) was roughly estimated to be 15 % of the total C ant in the whole water column in the SO. This study highlights that AABW formation, transport, and storage processes play important roles in the total C ant sequestration in the SO. [ABSTRACT FROM AUTHOR]

This paper examines underwater frazil ice formation in a coastal polynya. In a coastal polynya, the seawater near the ocean surface can become supercooled due to intense atmospheric cooling. Supercooled water sinks deeper under turbulent conditions, resulting in frazil ice formation in the water column. For the winter of 2002–2003, mooring measurement with an Acoustic Doppler Current Profiler (ADCP), an Ice-Profiling Sonar (IPS) and a conductivity-temperature recorder was conducted off the east coast of Sakhalin in the Sea of Okhotsk with the water depth of 33 m. IPS-derived ice draft data showed several polynya periods lasting for 1–2 weeks, associated with strong offshore winds. During these polynya periods, the ADCP acoustic backscatter strength was enhanced throughout the water column, accompanying in-situ and/or potential supercooling. This is evidence of underwater frazil ice formation associated with supercooling. Our data indicate that ice divergence/convergence, caused by the combination of wind and tidal forcing, results in a diurnal cycle of underwater frazil ice formation. According to a heat budget analysis, the contribution of frazil ice production to the total ice production during the winter is estimated to be at least 60% of mass. Based on the mooring data, this study shows that frazil ice formation occurs throughout the water column in a polynya, resulting in effective ice production. • Frazil ice appeared throughout the water column associated with supercooling. • Open water was maintained over two weeks when frazil ice was suspended in the polynya. • Frazil ice production contributed to more than half of the total ice production. • Underwater frazil ice showed diurnal variability caused by wind and tidal forcing. [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]

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]

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]

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]

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]