Ventilation of the intermediate layer in the Sea of Okhotsk is studied with regard to the ventilation of North Pacific Intermediate Water (NPIW). Measurements of chlorofluorocarbons (CFCs) from 1998 to 2000 reveal that the Okhotsk intermediate water is ventilated in two ways. The first consists of dense water formation in the polynyas that form on the northern shelves during winter. Measurements show that on the northwestern shelf, because of atmospheric cooling and brine rejection, the cold water that forms is dense enough to enter the intermediate layer. The CFC concentration in this dense shelf water (DSW) is high and almost saturated with respect to atmospheric CFCs, indicating that the DSW experiences active air‐water gas exchange during its formation. Away from the shelves, the CFC distribution shows that the DSW directly ventilates the upper level of the Okhotsk Sea Intermediate Water (OSIW). The second ventilation process is driven by the strong tidal mixing around the Bussol' Strait. The observed distribution of CFCs at the 27.4 σ θ level suggest that diapycnal mixing around the strait transports CFCs from the surface to this deeper layer, ventilating the OSIW. Combination of both processes means that the OSIW is more ventilated than Pacific water at the same density levels.
We examined the behavior of the sea ice in the Okhotsk Sea which formed over the deep Kuril Basin during the period 1978–82. When ice extended over the basin, we observed the formation of large eddies with diameters of order 200 km. We determined the size and duration of these eddies through use of the 37 GHz channel on the Nimbus 7 Scanning Multichannel Microwave Radiometer, and with the visible channel on the geostationary Himawari satellite. Within the ice cover, the satellite data show that these eddies produced open-water regions which persisted for 4–6 weeks, and that the eddies recurred year after year, even though their relative position changed. Comparison of eddy positions determined from satellite data with oceanographic positions shows that the oceanography drives the eddies. An estimate of heat loss from these eddies shows that the role of the ocean eddies is to keep the region ice-free until heat loss approaches zero, so that fluxes over the eddies primarily cool the water column without adding salt. Then as the atmosphere begins to warm in spring, the eddies tend to become ice-covered, so that melt water is introduced to their surface. Examination of the oceanography shows that the early summer water-column structure depends on the heat loss from the region during the preceding ice season, the amount of ice over the basin, and the total amount of ice formation in the Okhotsk Sea. During the heavy ice year of 1979, the upper 200–300 m were cooler, less saline, and highly oxygenated. This modification appears to be a local process, driven by eddy-induced mixing, local cooling, and ice melting. At 300–1200 m depths, water modification is caused by advection of water from outside the Kuril Basin. During heavy ice years with strong cooling, this water is more saline, colder, and richer in oxygen than during lighter ice years. The water modified in the basin can be traced into the North Pacific, where it cools and dilutes the surface water, and oxygenates the upper 200–400 m.
The seasonal variability of sea-ice cover in the Southern Ocean is examined using daily sea-ice concentration and ice velocity products for 2003–2009, derived from Advanced Microwave Scanning Radiometer for EOS (AMSR-E) data. This study quantitatively shows the contribution of (1) ice production/reduction within the sea ice, (2) ice production/reduction at the sea-ice edge and (3) zonal ice transport to the seasonal change of sea-ice area. Area of greatest ice production occurs along the coast of Ross Sea and East Antarctica from March to September. The contribution of zonal transport to the seasonal change of ice area is one order magnitude smaller than local ice production/reduction. Clear regional and seasonal differences are found in the large-scale processes named above. Generally, ice area increases due to ice production, both at the ice edge and within the pack in the autumn and winter. The most significant ice production at the ice edge occurred in the Weddell Sea; the ice production provides 56% of total increase of ice cover in this area. In contrast, moderate ice melting occurs at the ice edge through almost all months in the Indian Ocean sector.
Biogenic opal and ice‐rafted detritus (IRD) data from sediments in the Okhotsk Sea and the neighboring North Pacific revealed the remarkable reduction in opal production and southward advancement of sea‐ice covered area during the last glacial maximum, resulting also southward shift of high biological productive area in the northwestern North Pacific. It implies that the substantial reduction in outflux of CO 2 to the atmosphere in northwestern North Pacific and the pronounced increase in CO 2 sequestering in temperate North Pacific. This could be an additional CO 2 reduction mechanism of atmospheric CO 2 in the last glacial period.
Hydrographic and drifting buoy data from Japanese cruises show that the Antarctic Divergence in the Indian Ocean sector is composed of a street of cyclonic eddies. These eddies measure about 500 km in the zonal direction and 200 km in the meridional. Part of the eastward flowing Antarctic Circumpolar Current (ACC) meanders southward in the regions between the eddies. In the eddy regions, warm, saline Circumpolar Deep Water is upwelled into the shallow layers, while cold, dense coastal water advects into the deep layers; the advection occurs along the isobaths of ridges which extend north from the coast. The combination of the advection with the upwelling produces a water column denser than the surrounding water and leads to the formation and maintenance of the cyclonic eddies. Presence of the northward extending ridges approximately governs the location of eddy formation. The eddy formation recurs year after year, although eddy locations can vary somewhat. A polynya was observed to persistently occur and corresponded with one of the eddies in location, size, and form. The oceanographic observations also suggest that the primary meridional exchanges of heat and salt in the Antarctic are caused through the eddies and ACC meanders within the Antarctic Divergence.
In order to estimate sea ice albedo around the marginal sea ice zone of the southwestern Okhotsk Sea, we conducted the measurement of albedo aboard the ice breaker Soya in early February of 1996 and 1997. Using upward and downward looking pyranometers mounted at the bow of the ship, we obtained albedo data. We also measured ice concentration and thickness quantitatively by a video analysis. The observations show a good correlation between albedo and ice concentration. From a linear regression, sea ice albedo (ice concentration=100%) is estimated to be 0.64±0.03 at the 95% confidence level. The developed snow grains on sea ice due to sea water and/or solar radiation may be responsible for this somewhat lower value, compared with that over the snow-covered land fast ice in the polar region. Deviations of the observed values from this regression have a statistically significant correlation with solar zenith cosine at the 99% level, and with ice thickness at the 95% level. The linear regression formula which predicts albedo is also derived as the variables of ice concentration and solar zenith cosine. Although the regression coefficients are both statistically significant, the coefficient of ice concentration is much more significant in this formula than that of solar zenith cosine. The deviation of the observed albedo from this regression seems to be mainly caused by ice surface conditions rather than by ice thickness or cloud amount. All these results suggest that snow cover on sea ice plays an important role in determining the surface albedo.We also did albedo observations of dark nilas with snow-free surface, they were estimated as 0.10 and 0.12 for ice thickness of 1 to 1.5cm and 2 to 3cm, respectively.
