Photosynthetic carbon fixation capability of diatom communities in the southern Sea of Okhotsk from winter to spring as estimated using their rbcL genes

Determining dominant phytoplankton assemblages and their controlling factors in the winter sea-ice-covered southern sea of okhotsk and the spring open water through a multiple analytical approach.

Evidence of sea ice-driven terrigenous detritus accumulation and deep ventilation changes in the southern Okhotsk Sea during the last 180 ka

The Sea of Okhotsk, between northern Japan and Siberia, is the southernmost sea region of the Northern Hemisphere with seasonal sea ice cover. During early spring, occasional rapid reduction in sea ice cover observed in the southern Sea of Okhotsk can lead to anomalously early disappearance of sea ice along the Hokkaido coast (northern Japan). This study investigated the atmospheric and oceanic processes leading to rapid reduction in sea ice in the southern Sea of Okhotsk during early spring. We detected six events of subseasonal continued reduction in sea ice cover in March between 1993 and 2019. Strong southerly winds with marked thermal advection and moisture transport over northern Japan were observed in five of the six events. The passage of extratropical cyclones brought warm moist air from the south, which led to drifting and melting of sea ice by changing the surface wind speed and surface heat budget in the southern Sea of Okhotsk. The perturbation in sensible heat flux attributable to warm air advection contributed substantially to sea ice reduction, whereas longwave radiation played a secondary role. In addition to the atmospheric processes, an enhanced Soya Current also transports heat from the Sea of Japan, possibly contributing to the sea ice melt. The initial sea ice reduction leads to further sea ice loss through ice–albedo feedback, resulting in prolonged sea ice reductions for 1–2 weeks.

Recent observations have revealed that dynamical thickening is dominant in the growth process of Sea ice in the Southern Sea of Okhotsk. That indicates the importance of understanding the nature of thick deformed ice in this area. The objective of the present paper is to establish a Ship-based method for observing the thickness of deformed ice with reasonable accuracy. Since February 2003, one of the authors has engaged in the core Sampling using a Small basket from the icebreaker Soya. Based on these results, we developed a new model which expressed the internal Structure of pack ice in the Southern Sea of Okhotsk, as a one-dimensional multilayered Structure. Since 2004, the electromagnetic (EM) inductive Sounding of Sea-ice thickness has been conducted on board Soya. By combining the model and theoretical calculations, a new algorithm was developed for transforming the output of the EM inductive instrument to ice + Snow thickness (total thickness). Comparison with total thickness by drillhole observations Showed fair agreement. The probability density functions of total thickness in 2004 and 2005 Showed Some difference, which reflected the difference of fractions of thick deformed ice.

Interannual to decadal variability of summer sea surface temperature in the Sea of Okhotsk recorded in the shell growth history of Stimpson's hard clams (Mercenaria stimpsoni)

Absorption and fluorescence of chromophoric dissolved organic matter (CDOM) in sea ice and surface waters in the southern Sea of Okhotsk was examined. Sea-water CDOM had featureless absorption increasing exponentially with shorter wavelengths. Sea ice showed distinct absorption peaks in the ultraviolet, especially in younger ice. Older first-year sea ice had relatively flat absorption spectra in the ultraviolet range. Parallel factor analysis (PARAFAC) identified five fluorescent CDOM components, two humic-like and three protein-like. Sea water was largely governed by humic-like fluorescence. In sea ice, protein-like fluorescence was found in considerable excess relative to sea water. The accumulation of protein-like CDOM fluorescence in sea ice is likely a result of biological activity within the ice. Nevertheless, sea ice does not contribute excess CDOM during melt, but the material released will be of different composition than that present in the underlying waters. Thus, at least transiently, the CDOM introduced during sea-ice melt might provide a more labile source of fresher protein-like DOM to surface waters in the southern Sea of Okhotsk.

Although satellite data are useful for obtaining ice-thickness distribution for perennial sea ice or in stable thin-sea-ice areas, they are still largely an unresolved issue for the seasonal ice zone (SIZ). We address this problem using L-band synthetic aperture radar (SAR). In the SIZ, ice-thickness growth is closely related to deformation, so surface roughness is expected to correlate with ice thickness. L-band SAR, suitable for detecting such surface roughness, is a promising tool for obtaining thickness distribution. This idea was supported by an airborne polarimetric and interferometric SAR (Pi-SAR) validation. To extend this result to spaceborne L-band SAR with coarser resolution, we conducted in situ measurements of ice thickness and surface roughness in February 2008 in the southern Sea of Okhotsk with an icebreaker in coordination with the Advanced Land Observing Satellite (ALOS)/Phased Array-type L-band SAR (PALSAR) orbit. A helicopter-borne laser profiler was used to improve the estimation of surface roughness. It was found that backscatter coefficients (HH) correlated well with ice thickness (R = 0.86) and surface roughness (R = 0.70), which confirms the possibility of determining ice-thickness distribution in the SIZ. the interannual variation of PALSAR-derived ice-thickness distribution in the southern Sea of Okhotsk is also discussed.

The lowest latitude sea ice in the world (excluding coastal freezing) is in the southern Sea of Okhotsk (south of 46°N), where it has significant impacts on freshwater input and primary production. This region is subject to climate change, and accordingly the monitoring of sea ice conditions is important. However, the interannual variability of the region's sea ice is poorly understood due to its logistical challenges. Sea ice observations have been conducted in this region every winter for the period 1996–2020. The interannual variability of the ice conditions and the likely factors responsible for it were investigated using visual observations following the international ASPeCt protocol, combined with satellite SSM/I‐SSMIS ice concentration data (1988–2020). AMSR‐derived ice drift data sets and ERA5 meteorological reanalysis data sets were also analyzed to examine the effects of dynamic and thermodynamic processes. Our analysis revealed that (a) sea ice area in this region varies differently from that in the central and northern Sea of Okhotsk, where decreasing trends are reported, (b) sea ice volume has remarkable interannual variation and the peaks appeared much to more affected by dynamically deformed ice than freezing conditions, and (c) prominently deformed ice can be explained by taking shear components into account based on sea ice rheology. These results suggest the importance of including the proper sea ice rheology in numerical sea ice models to reproduce the realistic sea ice volume and deformation processes, for all seasonal ice zones.

The heat budget over the ice-covered area of the southern Sea of Okhotsk is estimated from in situ meteorological and ice observation for four years, 1996−99. The data are from about 1 week in early February in each of four years. Ice-thickness distributions required for calculating the heat budget are quantitatively obtained from video analysis. A one-dimensional thermodynamical model is used to calculate the heat flux. The total heat flux is obtained by summing up the area-weighted heat flux of each ice-thickness category. In addition, to determine the characteristics of the heat budget in this region, we also calculated the heat budget in the northern Sea of Okhotsk using Special Sensor Microwave/Imager ice-extent data and European Centre for Medium-range Weather Forecasts meteorological data, and compared the results. Our investigations show the following characteristics in the southern Sea of Okhotsk: (1) Due to relatively thin ice thickness, the average turbulent heat flux is upward. (2) Thin ice and open water contribute significantly to the total turbulent heat flux. (3) Thermodynamic ice growth is limited to about 1 cm d−1 on average. (4) The heat budget is largely characterized by abundant solar radiation. The first, third and fourth results are characteristic of this region located at a relatively low latitude, while the second one is similar to that for polar regions.

Winter iron supply processes fueling spring phytoplankton growth in a sub-polar marginal sea, the Sea of Okhotsk: Importance of sea ice and the East Sakhalin Current

Glacial to deglacial ventilation and productivity changes in the southern Okhotsk Sea

Sea-ice distribution and atmospheric pressure patterns in southwestern Okhotsk Sea since the Last Glacial Maximum

The pelagic mollusk Clione is a naked pteropod with a sympatric two-species distribution in the southern Okhotsk Sea, Japan, consisting of the morphologically and genetically distinct C. elegantissima and C. okhotensis. Clione elegantissima appears in both winter and spring, and body length differs between the winter coastal population (WCP, January to March, 10–20 mm) and the spring offshore population (SOP, April to July; up to 30 mm). This body size difference and temporal-spatial separation of the populations suggests that the SOP is either a cryptic species or C. limacine drifted from the Subarctic Atlantic Ocean or an interspecies of C. elegantissima resulting from reproductive isolation. We investigated the taxonomic positions of both populations using morphological and genetic analyses and identified both as C. elegantissima with very high genetic similarity. We explain the occurrence of spatio-temporal isolated populations using the water mass dynamics in the Okhotsk Sea. Warm water entering the southern Okhotsk Sea around Japan through the Soya Straits is divided into the Soya Warm Water (SWW: June to November) and the Forerunner of the SWW (FSWW: March to May); cold water entering the Okhotsk Sea around Japan, east of Sakhalin Island, is divided into the East Sakhalin Current Water (ESCW: November to April). The Cold Water Belt (CWB) is frequently formed off the SWW during summer and autumn and comprises upwelling cold water originating from either subsurface water of the Japan Sea off Sakhalin or Okhotsk Sea bottom water. We present the temporal-spatial isolation mechanism of WCP and SOP per the SWW, FSWW, ESCW, and CWB dynamics.

Sea ice thickness data and sea ice samples were analyzed to examine the characteristics of the ice thickness distribution and ice texture, and to understand ice growth processes in the southern Sea of Okhotsk. Ice thickness data and samples were obtained aboard the icebreaker Soya in early February, the ice growth season. Ice thickness data, which were obtained with a video monitoring system installed on the side deck of the ship each winter from 1991 to 2000 except 1995, show that the average thickness ranges from 19 ± 7 to 55 ± 23 cm and that it matches the characteristics of a Poisson distribution. Ice structure analysis reveals that granular texture occupies about three quarters of the total ice thickness and that the ice exhibits a layered structure with unit thickness averaging 5 to 10 cm. Stratigraphy and stable isotopic composition of the ice indicate that snow ice accounts for 10% and frazil ice accounts for 64% of the total ice thickness. This suggests that dynamic ice thickening processes such as frazil ice growth and piling up are more significant than congelation growth. On the basis of these characteristics, which resemble more those of Antarctic than Arctic sea ice, we propose a conceptual model for the ice thickening process in this region. It is shown that this model can explain the shape of the ice thickness distribution well, and is analogous with the concept of the “pancake cycle” and multiple rafting of Antarctic sea ice growth and thickening.

A taxonomic review of the snailfish genus Careproctus (Liparidae) with a reduced pelvic disk-the Careproctus gilberti species group-from the western North Pacific recognized three new species from the southern Sea of Okhotsk, and confirmed the validity of Careproctus gilberti, Careproctus mederi, Careproctus ostentum, and Careproctus parvidiscus. One of the newly described species, Careproctus longibarbatus sp. nov., had been previously confused with C. mederi, but was readily distinguishable on the basis of counts of vertebrae (61-66 vs. 57-63), dorsal- (56-60 vs. 52-56) and anal-fin rays (50-54 vs. 47-52), as well as length of the pectoral fin lower lobe (extending past anus vs. not reaching anus) and color of the peritoneum (dark vs. pale with black dots). Careproctus barbatulus sp. nov. and Careproctus spinulosus sp. nov. were diagnosed by pectoral fin lower lobe length, anus position, and vertebral, and dorsal- and anal-fin ray counts. The validities of the three new species were also confirmed by mitochondrial sequence data. Careproctus mederi was redescribed based on specimens from the southern Sea of Okhotsk, representing the first Japanese, as well as southernmost record of the species. Furthermore, the full description of an unidentified Careproctus species, most similar to C. gilberti, from the Pacific coast of Hokkaido was provided. Significant morphological variation within the C. gilberti species group, possibly resulting from reproductive interference among sympatric species, was also discussed.