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We processed the sea surface height measured by the SAR (Synthetic Aperture Radar)/Interferometric Radar Altimeter (SIRAL) on board CryoSat-2 (CS-2) and successfully estimated the monthly dynamic ocean topography (DOT) of the Arctic Ocean. The CS-2 monthly DOT showed the interannual and monthly variability of the Beaufort Gyre (BG) during winter between 2010/2011 and 2014/2015. The northward flow at the western edge of the BG was primarily estimated over the Chukchi Borderland (CBL). However, the BG extended across the CBL, and the northward flow was estimated over the Mendeleev Ridge in the winter of 2012/2013. Our analyses revealed a significantly variable BG in response to changes in the sea surface stress field. Our analysis indicated that (1) sea ice motion, driven by wind fields, acts as a driving force for the BG when sea ice motion was intensified during winter and (2) sea ice motion can also act as an inhibiting force for the BG when sea ice motion is weakened during winter. In addition, the relationship between the DOT, steric height, and ocean bottom pressure implied that the DOT during winter responded to varying wind stresses through baroclinic and barotropic adjustments. According to a tracer experiment, we inferred that in the winter of 2012/2013, the Pacific-origin water carried into the BG through the Barrow Canyon was transported to the northern shelf and shelf break of the Chukchi Sea rather than the CBL, which is where the Pacific-origin water had been transported in the other years of the observation period.

We examined to what degree a simplified polynya model can explain a real polynya based on satellite-derived sea-ice data. In the model, the polynya area, defined as the frazil ice production region, is determined by a balance between the offshore consolidated ice drift and frazil ice production. We used daily polynya area, ice production, and ice drift data derived from AMSR-E. The study area is the Ross Ice Shelf Polynya (RISP), which has the highest sea-ice production in the Southern Ocean. As a modification of the original model to apply the available satellite data set, we introduced the lag time by which produced frazil ice is transported and accumulated at the polynya edge. The model represents a half (48–60%) of the polynya variability when using a lag time of 1.5 days. The frazil ice collection depth at the polynya edge, a key parameter in the model, is estimated to be ∼16 cm. The expansion of the RISP is achieved by ice divergence, and the contraction is achieved mostly by ice production. Both the wind and the remaining components (mainly regarded as the ocean current component) in the ice divergence are larger in the western part of the RISP, which explains the larger extent there. In the one-dimensional frame, assuming that the frazil ice produced within the RISP transforms into consolidated ice with a thickness of 16 cm, the frazil ice production (∼1.7 × 103 m2 d−1) within the RISP approximately balances the export (∼1.6 × 103 m2 d−1) of consolidated thin ice from the RISP edge.

An intercomparison of four low-resolution remotely sensed ice-drift products in the Arctic Ocean is presented. The purpose of the study is to examine the uncertainty in space and time of these different drift products. The comparison is based on monthly mean ice drifts from October 2002 to December 2006. The ice drifts were also compared with available buoy data. The result shows that the differences of the drift vectors are not spatially uniform, but are covariant with ice concentration and thickness. In high (low) ice-concentration areas, the differences are small (large), and in thick (thin) ice-thickness areas, the differences are small (large). A comparison with the drift deduced from buoys reveals that the error of the drift speed depends on the magnitude of the drift speed: larger drift speeds have larger errors. Based on the intercomparison of the products and comparison with buoy data, uncertainties of the monthly mean drift are estimated. The estimated uncertainty maps reasonably reflect the difference between the products in relation to ice concentration and the bias from the buoy drift in relation to drift speed. Examinations of distinctive features of Arctic sea ice motion demonstrate that the transpolar drift speed differs among the products by 13% (0.32 cm s−1) on average, and ice drift curl in the Amerasian Basin differs by up to 24% (3.3 × 104 m2 s−1). These uncertainties should be taken into account if these products are used, particularly for model validation and data assimilation within the Arctic.

Using daily sea ice data derived from satellite-borne sensors and atmospheric data, we examined processes controlling the variation of sea ice extent in the Northern Hemisphere. The daily ice motion field was computed from imagery of the Defense Meteorological Satellite Program (DMSP) Special Sensor Microwave Imager (SSM/I) for seven winters (December to April) from 1991/1992 to 1997/1998, by employing the maximum cross-correlation method. In order to examine mechanisms of the temporal variation of the ice extent, we analyzed 50 specified lines across the daily ice edge. Although a high correlation between the ice motion and the geostrophic wind speed was observed in all the ice edge areas, the degree for correlation between the speed of the ice edge displacement and the wind speed varied with region. The degree for response of the ice edge speed to the wind speed largely depended upon that of the ice edge speed to the ice motion. The following mechanisms controlling the variation of ice extent for regions in the Northern Hemisphere were anticipated. In the Barents Sea, Bering Sea, and the Sea of Okhotsk the ice extent advances by wind-driven ice advection and the daily scale variation of the ice extent were also controlled by the variation in wind speed. In contrast, the ice extent in the Labrador Sea and the Greenland Sea seemed to be considerably affected by oceanographic factors such as the location of the thermal front and was not related to the variation in wind speed. The regional difference of the variation mechanism was also reflected in the interannual variation in maximum ice extent.

This paper presents maps showing the relationship between sea-ice motion derived from Defense Meteorological Satellite Program (DMSP) Special Sensor Microwave Imager (SSM/I) imagery and the geostrophic wind in the Northern Hemisphere for seven winters (1991/92–1997/98). The sea-ice motion is highly correlated with the geostrophic wind except for some coastal regions. Overall the direction of the ice motion is nearly parallel to the wind. An obvious contrast between seasonal ice zones and the interior of the Arctic is observed in the speed reduction factor (ratio of ice speed to wind speed), which is probably caused by the spatial variation of internal ice stress. Surface ocean currents are also derived by subtracting the wind effect from the ice motion.

Using Defense Meteorological Satellite Program (DMSP) special sensor microwave imager (SSM/I) sea-ice data and meteorological data, we investigate processes controlling the advance and retreat of sea ice in the Sea of Okhotsk. For this investigation we apply a new algorithm to the SSM/I data, which is capable of calculating ice concentration and distinguishing sea-ice types; this algorithm is confirmed by comparison with National Oceanic and Atmospheric Administration (NOAA) advanced very high resolution radiometer (AVHRR) imagery. Our investigations show that there exists a locality for the areas of new-ice formation, and the ice formation near the ice edge always occurs as a recovery after sea ice has retreated suddenly. We also reveal that the speed of ice-edge advance fairly coincides with that predicted by 2% of the geostrophic wind. Furthermore, there exists a good correspondence between the times when the overall advance and retreat of ice cover begin and the decrease and increase of air temperature begin. We conclude that the advance of the ice cover in the Sea of Okhotsk occurs primarily through ice advection by wind and that the interannual variation in the maximum sea-ice extent can be interpreted by that in wind and air temperature conditions.

[1] This study investigated mechanisms for the intraseasonal variability of sea-ice concentration in the Antarctic, using Complex Empirical Orthogonal Function (CEOF) analysis of daily sea-ice concentration data during the period 1992 through 2001 derived from images of the Defense Meteorological Satellite Program (DMSP) Special Sensor Microwave Imager (SSM/I). The first CEOF mode clearly showed that the large amplitudes of sea-ice concentration occur in the marginal sea-ice zone of the western Antarctic. The first mode also revealed the existence of eastward propagating phases with a period of 10-20 days in the western Antarctic. Regression analysis of meridional wind velocity onto the temporal coefficient of the first CEOF mode showed that the spatial phase of the meridional wind velocity precedes that of sea-ice concentration by about 90 degrees, indicating that the maximum change of sea-ice concentration occurs at the maximum wind velocity. From data analyses of ice-drifting velocity and simple sea-ice model results, it is suggested that thermodynamical effects such as sea-ice production are likely to contribute dominantly to the intraseasonal variability of sea-ice concentration in the marginal sea-ice zone of the western Antarctic.

[1] Using a moored ice-profiling sonar along with a moored acoustic Doppler current profiler, a total spatial section of draft across 3334 km of sea ice was obtained in the southwestern Sea of Okhotsk near Hokkaido in the winters of 1999–2001. Using this draft data set, the average draft and keel statistics are discussed in this sea for the first time. The mean draft was 0.60 m, which corresponds to the thickness of 0.71 m, over the three winters with the range of 0.49–0.72 m for each winter. The classification of level and deformed ice reveals a small range of the monthly mean level ice draft (0.18–0.27 m) and the dominance of the deformed ice in terms of volume (80%). The mean draft varied with the areal ratio of the deformed ice fairly well. These results suggest that dynamic processes such as ridging and rafting are important for the evolution of draft in the region of observation. The observed draft probability density distribution and keel statistics show that the thick ice ratio and keel frequency are lower than the similar data in polar regions and closer to those observed in Davis Strait west of Greenland. Along with the ice concentration and speed derived from the satellite data the southward ice transport to the southwestern Sea of Okhotsk is estimated on the basis of the observed sea ice thickness. The estimated ice transport ranged from 15 to 70 km3 in each winter. The heat and freshwater transport associated with the ice transport ranged from −3.9 × 1017 to −1.8 × 1018 J and from 12 to 57 km3, respectively.

[1] This paper examines the production and reduction of sea ice area in the Sea of Okhotsk. Analyses were carried out for 10 winters (December–April) from 1991/1992 to 2000/2001 using daily data of sea ice velocity and concentration, derived from images of the Defense Meteorological Satellite Program (DMSP) Special Sensor Microwave Imager (SSM/I). First, we mapped sea ice divergence, which was calculated from the daily ice velocity field. In the case of offshore winds, high divergence was observed along the coastline, while there existed a broad convergence area with inshore winds. Next, we examined local changes in an area of sea ice. As in the case of ice divergence, ice production and reduction are strongly controlled by wind direction. We found that a large amount of sea ice was produced in coastal areas under offshore wind conditions. The geographical distribution of the ice production area agrees well with that of the well-known coastal polynya areas. Our estimation shows that 8.7 × 105 km2 yr−1 of sea ice is produced in the northern coastal area, and 3.7 × 105 km2 yr−1 in the Sakhalin coastal area. In contrast, 7.4 × 105 km2 yr−1 of the ice area disappeared in Shantarskiy Bay. This reduction of ice area is mainly caused by dynamic deformation of existing ice forming rafted ice or pressure ridges. This dynamic deformation plays an important role, not only in the reduction of Okhotsk sea ice area, but also in the thickening of ice.