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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]

Material transport by sea ice plays an important role in the biological production of spring blooms. We conducted backward-tracking simulations to estimate the origins of the sea ice in the southern part of the Sea of Okhotsk and examine the relationship between the origin of sea ice and biological production, the concentration of iron, and the frazil ice fraction. The simulation of the sea ice melted in areas, where the net community production (NCP) was estimated shows that the sea ice that melted in areas with high NCP tended to originate in coastal areas, mostly the Terpenia Bay and Sakhalin polynyas, where ice production is high and sea ice can incorporate the seafloor sediments. This result supports the hypothesis that the incorporation of sedimentary particles into sea ice and their release during ice melt are key factors in high biological production. The simulation of the sea ice floes sampled by the icebreaker Soya off Hokkaido coast suggests that the samples collected over the shelf with depths of less than 500 m or so were formed over the shelf around the southern tip of Sakhalin Island, whereas the samples, including that with high iron concentration, collected over the shelf slope at greater depths were likely transported from the northern area up to Terpenia Bay but not further north. The simulation also suggests that the sea ice floes with a high fraction of frazil ice could be formed anywhere in the southern part of the Sea of Okhotsk. [ABSTRACT FROM AUTHOR]

In the southwestern Sea of Okhotsk, a large spring phytoplankton bloom occurs after the sea ice melts. A suggested scenario is that sea ice with materials such as iron-containing sediment or ice algae is transported from the north and melted with release of them, inducing a prominent bloom. We hypothesize that sea ice containing materials that could enhance spring bloom originates from active coastal polynyas in the upstream region. To verify this hypothesis and identify which coastal areas generate sea ice that is further transported to the bloom area, we simulated the transport of sea ice produced in the coastal polynyas by a particle-tracking method. Sea ice production and drift velocity were derived from satellite microwave data. For regions near the coast, where ice drift data derived from the satellite are inaccurate, we combined ice drift data derived from the wind using the wind factor and the turning angle obtained from mooring data. Further, we used the apparent wind factor that expresses enhancement of the alongshore component of ice drift by the ocean current. The simulations suggest that most of the sea ice that melts in the western Kuril Basin originates from the Terpenia Bay and Sakhalin polynyas. The area where high net community production occurs after the sea ice melts corresponds well to the area where sea ice originating from these polynyas melts. The simulation of frazil ice suggests the importance of melt ice originating from the Terpenia Bay polynya with a higher rate of frazil ice production. [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]

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]

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]

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]