The time evolution of the ocean mixed layer in the seasonal sea ice zone of the Okhotsk Sea has been observed with profiling floats. The heat storage rate in the mixed layer estimated from the float data well coincides with the surface heat flux. Mixed layer deepening and the onset of ice formation can be well reproduced by a bulk mixed layer model, suggesting that onset of ice formation can be predicted by local atmospheric conditions. This explains the remarkably high correlation of the onset day of sea ice formation with the surface heat loss in the preceding fall. The anomalously large heat loss in the fall of 2000 led to the anomalously early ice formation. Strong stratification due to the Amur River fresh water flux is indispensable for ice formation through suppression of deep convection.
In order to clarify the distribution and formation of Okhotsk Sea Intermediate Water (OSIW) an isopycnal climatological data set based on all of the available historical observations is developed and examined. The isopycnal maps clearly show that there are two direct ventilation sources for OSIW: Dense Shelf Water (DSW) with cold and fresh properties influenced by sea ice formation in the northwest shelf region and Forerunner of Soya Warm Current Water (FSCW), which has warm and saline properties originating in the Japan Sea. The cold and fresh water extends southward from the northwest shelf to the shelf slope off Sakhalin Island. It suggests that DSW is transported southward by the East Sakhalin Current. The DSW enters the Kuril Basin and mixes with FSCW and Western Subarctic Water (WSAW) originating from the North Pacific to form OSIW. The annual mean production rate of DSW estimated from the total volume of DSW over the shelf in spring and summer is 0.67 Sv (1 Sv = 10 6 m 3 /s). The annual mean volume transport of FSCW is estimated to be 0.08 Sv on the basis of its cross‐section area and the current speed. Assumption of isopycnal mixing yields a mixing ratio of 1:1:0.1 among DSW, WSAW, and FSCW to form OSIW. We estimate the annual production rate of OSIW to be 1.4 Sv; the corresponding OSIW's renewal time is 7 years. Continued assumption of isopycnal mixing yields a mixing ratio of 0.6:0.4 between WSAW and OSIW to form Oyashio Intermediate Water (OYIW).
Three HF ocean radar stations were installed at the Soya/La Perouse Strait in the Sea of Okhotsk in order to monitor the Soya Warm Current. The frequency of the HF radar is 13.9 MHz, and the range and azimuth resolutions are 3 km and 5deg, respectively. The radar covers a range of approximately 70 km from the coast. It is shown that the HF radars clearly capture seasonal and short-term variations of the Soya Warm Current. The velocity of the Soya Warm Current reaches its maximum, approximately 1 m s -1 , in summer, and weakens in winter. The velocity core is located 20 to 30 km from the coast, and its width is approximately 50 km. The surface transport by the Soya Warm Current shows a significant correlation with the sea level difference along the strait, as derived from coastal tide gauge records. The cross-current sea level difference, which is estimated from the sea level anomalies observed by the Jason-1 altimeter and a coastal tide gauge, also exhibits variation in concert with the surface transport and along-current sea level difference.
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.
We investigate the offshore transport process of dense shelf water, using a three-dimensional, primitive equation model.We focus on the effects of bottom topography, in particular, inclination of a bottom slope from a continental shelf to a deep basin.For the numerical domain we use an idealized bottom topography in which the bottom deepens gradually from a shallow continental shelf region to a steep slope region.In the continental shelf region we use a salt flux which represents the typical brine rejection in a coastal polynya.Results of the numerical experiments show that dense shelf water is transported offshore by eddy flux and a dense plume.The transport by eddy flux occurs mainly over a continental shelf, while that by the dense plume occurs over a continental slope.A boundary between the regions where the above two processes are dominant corresponds locally to a shelf break.A salinity front is developed in the boundary over the shelf break, separating the dense shelf water from the offshore water.We also investigate the stability of the surface westward current over the shelf break front, using a simple analytical model.The analytical model investigation shows that shelf break topography plays an important role in determining a neutral point of the stability of the shelf break current and preventing dense shelf water from being transported farther offshore by eddy flux.We suggest that eddy activity on a continental shelf contributes not only to the development of the shelf break front but also to the water exchange between a continental shelf and a slope region.1.
Terrestrial plant‐derived n‐alkanes (C 25– C 35 ) were measured in three piston cores (PC1, PC2 and PC4) in the Sea of Okhotsk covering the last 30 kyrs. Down core profiles of the n‐alkane concentrations and mass accumulation rates (MAR) were characterized by deglacial maxima. In particular, cores PC2 and PC4, which were collected from the central and western Sea of Okhotsk, respectively, show a two‐step increase around the Meltwater Pulse events (MWP) 1A (14.5–13.5 kyr BP) and 1B (about 10 kyr BP). This finding was interpreted by the outflow of terrestrial organic matter from the submerged land shelf to the Sea of Okhotsk through the East Sakhalin Current. This study demonstrated that the sea level rise forced by global warming in the deglaciation period may have caused the enhanced transport of terrestrial organic matter in marginal seas.
Abstract The subarctic Pacific is a high‐nutrient low‐chlorophyll (HNLC) region in which phytoplankton growth is broadly limited by iron (Fe) availability. However, even with Fe limitation, the western subarctic Pacific (WSP) has significant phytoplankton growth and greater seasonal variability in lower trophic levels than the eastern subarctic Pacific. Therefore, differences in Fe supply must explain the west‐to‐east decrease in seasonal phytoplankton growth. The Fe flux to the euphotic zone in the WSP occurs at a “moderate” value, in that it is significantly higher than its value on the eastern side, yet it is not sufficient enough to cause widespread macronutrient depletion, that is, HNLC status is maintained. Although we recognize several Fe supply processes in the WSP, the mechanisms that account for this moderate value of Fe supply have not previously been explained. Here we demonstrate the pivotal role of tidal mixing in the Kuril Islands chain (KIC) for determining the moderate value. A basin‐scale meridional Fe section shows that Fe derived from sediments in the Sea of Okhotsk is discharged through the KIC into the intermediate water masses (~ 800 m) of the western North Pacific. The redistribution of this Fe‐rich intermediate water by intensive mixing as it crosses the KIC is the predominant process determining the ratio of micronutrient (Fe) to macronutrients (e.g., nitrate) in subsurface waters. This ratio can quantitatively explain the differences in surface macronutrient consumption between the western and eastern subarctic, as well as the general formation and biogeochemistry of HNLC waters of the subarctic North Pacific.
