winter oceanographic conditions in the...

451

Journal of Oceanography, Vol. 57, pp. 451 to 460, 2001

Keywords:⋅ Okhotsk Sea,⋅ sea ice,⋅ winter convection,⋅ ice-ocean interac-tion,

⋅ Soya WarmCurrent Water,

⋅ East SakhalinCurrent,

⋅ heat budget.

* Corresponding author. E-mail: [email protected]

Copyright © The Oceanographic Society of Japan.

Winter Oceanographic Conditions in the SouthwesternPart of the Okhotsk Sea and Their Relation to Sea Ice

KAY I. OHSHIMA1*, GENTA MIZUTA2, MOTOYO ITOH1, YASUSHI FUKAMACHI1,TATSURO WATANABE3, YASUSHI NABAE4, KOUKICHI SUEHIRO5 and MASAAKI WAKATSUCHI1

1Institute of Low Temperature Science, Hokkaido University, Sapporo 060-0819, Japan2Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060-0810, Japan3Japan Sea National Fisheries Research Institute, Fisheries Agency, Niigata 951-8121, Japan4Hydrographic Department of the Japan Coast Guard, Maizuru 624-8686, Japan5Hydrographic Department of the Japan Coast Guard, Kitakyushu 801-8507, Japan

(Received 22 March 2000; in revised form 16 October 2000; accepted 22 December 2000)

In the southwestern part of the Okhotsk Sea, oceanographic and sea-ice observationson board the icebreaker Soya were carried out in February 1997. A mixed layer ofuniform temperature nearly at the freezing point extending down to a depth of about300 m was observed. This is much deeper than has previously been reported. It issuggested that this deep mixed layer originated from the north (off East Sakhalin),being advected along the shelf slope via the East Sakhalin Current, accompanied withthe thick first-year ice (average thickness 0.6 m). This vertically uniform winter wa-ter, through mixing with the surrounding water, makes the surface water more saline(losing a characteristic of East Sakhalin Current Water) and the water in the 100–300 m depth zone less saline, colder, and richer in oxygen (a characteristic of theintermediate Okhotsk Sea water). The oceanographic structure and a heat budgetanalysis suggest that new ice zone, which often appears at ice edges, can be formedthrough preconditioning of thick ice advection and subsequent cooling by the latentheat release due to its melting.

tion in some parts of the sea when the water becomesdenser. Kitani (1973) suggested that cold, dense water isformed by brine rejection with sea ice formation in thenorthwestern continental shelf and that it flows downalong the sea bottom into the deep, then spreads into themid-depth layer. The ventilation of North Pacific Inter-mediate Water (NPIW) with the potential density of 26.8can be attributed to this process (Alfultis and Martin,1987; Talley and Nagata, 1995).

In the southwestern part of the Okhotsk Sea, salinewater from the Sea of Japan (Soya Warm Current Water:SWCW) is brought by the Soya Warm Current throughthe Soya Strait, with a maximum influx in summer and aminimum in winter (Aota and Kawamura, 1978; Aota,1984; Matsuyama et al., 1999). On the other hand, lesssaline water (East Sakhalin Current Water: ESCW) origi-nating from the influx of the Amur River dominates thesurface layer in this region in November and December(Itoh and Ohshima, 2000). In winter and early spring, thepotential density of SWCW reaches 26.8, which is muchhigher than elsewhere in the surface layer of the OkhotskSea (Takizawa, 1982; Talley, 1991). Watanabe andWakatsuchi (1998) proposed that this water share an im-

1. IntroductionThe Sea of Okhotsk is the southern limit of sea ice

extent in the Northern Hemisphere except for regions oflocal coastal freezing. This low southern ice limit is notcaused only by severe winter conditions with cold tem-peratures and strong winds from Siberia. The surface layerof the Okhotsk Sea consists of very fresh water due toinflux from the Amur River, which results in markedstratification around a depth of 50–100 m. Winter con-vection to greater depths is suppressed by this oceaniccondition, which allows sea ice to form at quite low lati-tudes (Fukutomi, 1950; Tabata, 1958).

The water down to several hundred meters in theOkhotsk Sea is much colder, fresher, and richer in oxy-gen than that in the North Pacific (e.g., Freeland et al.,1998; Watanabe and Wakatsuchi, 1998). This implies thatpart of the cold, fresh surface water might penetrate downto several hundred meters against the strong stratifica-

452 K. I. Ohshima et al.

portant role in the formation of the intermediate waterwith the potential density of 26.8.

Although the winter season is the key time for watermass formation and ventilation, in-situ observations dur-ing winter are very limited because of logistical difficul-ties associated with the existence of sea ice. We have notyet understood well what really occur in this sea duringwinter.

Sea ice in the Okhotsk Sea is generally advectedsouthward by the prevailing northerly or northwesterlywinds. The southward extension of sea ice is also a resultof the southward flow of the East Sakhalin Current alongthe Sakhalin coast (Watanabe, 1962, 1963a). The south-ern limb of the sea then turns to be a typical marginal icezone (MIZ), a key area for air-ice-ocean interaction. Inother MIZs, extensive air-ice-ocean observations havebeen undertaken in the context of several projects suchas the Marginal Ice Zone Experiment (MIZEX Group,1986). No attempt has so far been undertaken to makesuch observations in the Okhotsk Sea, however.

In February 1997, we conducted oceanographic ob-servations together with sea ice and atmospheric obser-vations in the southwestern part of this sea on board anicebreaker. The observational area covers the marginalice zone in this sea. We found that the winter mixed layerextends to a depth of around 300 m, which is much deeperthan has previously been reported and/or believed. In thispaper we describe the features of this deep mixed layer

and discuss its origin and its role on water mass modifi-cation. We also give a description of the Soya Warm Cur-rent Water in winter and the associated front, which hasbeen scarcely reported. An advantage of our observationsis that the ice, ocean, and atmospheric data were obtainedsimultaneously. Further, the observations in 1997 wereundertaken immediately after the advent of sea ice. Thisresults in relatively simple situation to consider ice-oceaninteraction. Hence we also discuss the ice-ocean interac-tive processes at the initial stage of ice cover.

2. Data and MethodsObservations were conducted from the icebreaker

Soya from 1 to 10 February 1997. Surveys of the upperocean (down to a depth of 200 m) of this area have beenroutinely conducted with a portable STD (salinity-tem-perature-depth unit) and XBTs (expendablebathythermographs) by the Hydrographic Department ofthe Japan Coast Guard since the 1980s (Ishii, 1991). In1997, with the cooperation of the Hydrographic Depart-ment and Hokkaido University, the observational programwas expanded to include water sampling to a depth of800 m using a calibrated conductivity-temperature-depthunit (CTD; Seabird SBE-19). Water samples were col-lected with Niskin bottles. The CTD salinity values werecalibrated using bottle samples. Dissolved oxygen con-centrations were determined by the Winkler method. Ourobservations afforded the first data on dissolved oxygen

Fig. 1. Sequence (every ten days) of sea ice distribution from 11 December 1996 to 30 January 1997, derived from Special SensorMicrowave Imager (SSM/I) on the Defense Meteorological Satellite Program (DMSP). Areas with ice concentration higherthan 0.3 are plotted. Solid and open circles indicate first-year ice and young ice, respectively. The algorithm by Kimura andWakatsuchi (1999) is used. Note that some of the grid points very close to land show false sea ice signals. In this paper, eastSakhalin coast is defined as the area labeled E, and Terpenia Bay is denoted by label T. The cross denotes the grid point wherethe wind vectors from ECMWF are shown in Fig. 7.

Winter Oceanographic Conditions of Southwestern Okhotsk Sea 453

and calibrated salinity under ice cover in this area. Be-cause of heavy ice conditions we could not carry out thewater sampling for some stations.

Figure 1 shows the time evolution of sea ice distri-bution from December 1996 to January 1997. Sea icebegan to cover the region off the east coast of Sakhalin inmid-December and reached the southern tip of Sakhalinin mid-January. Sea ice began to cover the observationalarea about 10 days prior to the observational period. Fig-ure 2 shows the positions of the CTD and XBT stationsand the ice edge locations during the observational pe-riod. The CTD surveys were carried out across the ice-covered area, supplemented with XBT surveys.

Sea ice conditions and ice thickness were also ob-served using a video monitoring system along with seaice floe sampling. The details of the sea ice analysis waspresented in Ukita et al. (2000). We also monitored airtemperature, relative humidity, wind, downward and up-ward shortwave radiation, and fractional cloud coveraboard the icebreaker to estimate the heat flux using bulkformulae.

3. ResultsDuring the observational period, most of the ice cov-

ered area was occupied by thick first-year ice with newice (grease or pancake ice) and/or brash ice at ice edges,where brash ice is a term used to describe the wreckageof ice floes with a size not more than 2 m across (WMO,1970). From the video measurements, the average thick-ness of sea ice in the first-year ice zone was estimated tobe about 0.6 m, including a thickness of snow cover ofabout 0.1 m (Toyota et al., 1999).

Figure 3 shows vertical sections of temperature, sa-linity, potential density, and dissolved-oxygen contentalong transects A and B marked in Fig. 2, which cross theice covered area (sea ice distributions are also indicatedat the top of Fig. 3). Density contours almost coincidewith those of salinity since density is mostly determinedby salinity in this low temperature range. The most re-markable feature along transect A (Fig. 3(a)) is that themixed layer, with temperatures nearly at the freezingpoint, extends to more than 300 m at Stn. 9, which is muchdeeper than has been previously reported. Figure 4 showsthe vertical profiles of temperature, salinity, and poten-tial density at Stn. 9, superimposed on those ofclimatological data around Stn. 9 in December. Fukutomi(1950) and Tabata (1958) considered that winter convec-tion is limited to a depth of 50–100 m in the Okhotsk Seabecause of the strong stratification. In fact, beneath thefresh and warm surface layer a near-freezing cold layer(called a dichothermal layer) usually exists in the OkhotskSea, extending to a depth of 100–150 m (Freeland et al.,1998; Watanabe and Wakatsuchi, 1998). This implies thatwinter convection can reach depths as great as 150 m(Yang and Honjo, 1996). The mixed layer of 300 m whichwe observed is even deeper yet.

Also along transect B (Fig. 3(b)), cold water (tem-perature less than –1.5°C) is found from the surface downto a depth of 300 m, although intermediate intrusions ofrelatively warm water occur within this layer. Fromtransect B, it is found that dissolved oxygen content isrelatively high and vertically uniform within the mixedlayer, suggesting a result of convection. Both transects Aand B reveal that the region of the deep mixed layer cor-responds to that of the shelf slope and the first-year icecover. The temperature structure obtained from bothtransects suggests that intrusions are prominent aroundthe front between the deep mixed layer and the warm off-shore water.

An interesting feature in Fig. 3(a) is that salinity inthe surface layer is greatly reduced at the ice edge (Stn.10). The surface salinity is also somewhat reduced aroundthe ice edge along transect B (Stns. 21 and 22). This fresh-ening is a result of ice melting, which will be discussedlater.

Fig. 2. Map of the observed area. The CTD and XBT stationsare denoted by solid and open circles, respectively. The iceedge locations of the first-year ice on 3 and 9 February arealso shown, derived from NOAA advanced very high reso-lution radiometer (AVHRR) images. Crosses indicate loca-tions of a large ice floe at 1900 LST on 3 February and at0700 LST on 4 February.


Fig. 3. Vertical sections of temperature (deg. Celsius), salinity, potential density, and dissolved oxygen (ml/l), along transects(a) A and (b) B denoted by solid lines in Fig. 2. Note that the vertical and horizontal scales are different between (a) and (b).Tick marks on the lateral lines correspond to observation points. Sea ice conditions along the transects are also shown at thetop. Here brash ice area indicated is defined by the area which has a concentration of brash ice more than 0.2. Observationaldates on transect A are 3–4 February for Stns. 4–11, 7 February for Stn. 20, 9 February for Stns. 26–30. Observational dateson transect B are 7–8 February. Temperatures less than –1.5° C and salinities less than 33.6 are shaded. The gap mark isinserted between Stns. 32 and 5 in the ice condition of (a) because there is a 5-day difference in the observational timebetween these stations.


Figures 5(a) and (b) show temperature distributionsat depths of 50 m and 100 m, respectively. These figuresalso indicate that very low temperature zone, correspond-ing to the region of the deep mixed layer, exists over theshelf slope and also the region of the first-year ice. Fig-ure 5(d) shows apparent oxygen utilization (AOU) at thesurface. A high AOU zone also corresponds to the regionof the deep mixed layer.

The coastal side of section A comprises relativelywarm, saline water with a sharp, inclined front (Fig. 3(a)).Horizontal distributions of temperature and salinity (Figs.5(a), (b) and (c)) show that this water comes from theSoya Strait and thus it is regarded as Soya Warm CurrentWater (SWCW) or water originating from SWCW(SWCW is defined as water having salinity greater than33.6 according to Itoh and Ohshima (2000)). Compari-

Fig. 4. Vertical profiles of temperature, salinity, and potential density at Stn. 9 (dashed lines), superimposed on those in Decem-ber (solid lines) derived from the climatological data set of Itoh and Ohshima (2000). Location of the climatological data is45.12°N, 143.98°E.

Fig. 5. Horizontal distributions of (a) temperature (deg. Celsius) at a depth of 50 m with the bottom contours (dotted curves),(b) temperature (deg. Celsius) at a depth of 100 m, with the ice edge location on 3 February (dashed curves), (c) salinity at adepth of 50 m, and (d) apparent oxygen utilization (AOU) at the surface.


son of the temperature distribution at 50 m with that at100 m (Figs. 5(a) and (b)) reveals that the influence ofSWCW extends farther downstream at greater depths. Thepotential density of SWCW observed near the Soya Straitin this cruise (not shown here) is 26.7 at most, which doesnot reach the potential density of NPIW (26.8). In thisseason of the year, the water with potential density of26.8 has not yet been produced from the surface.

Itoh and Ohshima (2000) showed that East SakhalinCurrent Water (ESCW: water having salinity less than 32.0according to their definition) comes to this region in No-vember and December and dominates over the shelf re-gion. In the salinity sections shown in Fig. 3, low salinitywater exists in the upper layer between SWCW and thedeep mixed-layer for transect A and close to the coast fortransect B. This fresh water is considered to be a remnantof ESCW. The salinity distribution at 50 m (Fig. 5(c))also shows that a low salinity zone can be identified be-tween the region of SWCW and the deep mixed-layer zone(see the 32.4 contours), corresponding to water influencedby ESCW.

Figure 6 shows the temperature and salinity (TS)diagram for several CTD observations. Thick curves de-note TS curves in the SWCW region (Stn. 26), the ESCWremnant region (Stn. 5), and the deep mixed layer region(Stn. 9). These curves are distinctly separated in the dia-gram. Thin curves denote TS curves of the offshore deeperstations. In deep layers (typically deeper than 300 m) allthe thin curves converge to the property of the OkhotskSea. In the upper layers they approach the characteristicof the deep mixed layer (Stn. 9) or the ESCW remnant

(Stn. 5) region, being mixed with water in the SWCWregion (Stn. 26). The diagram demonstrates that suchmixing is active in this region. Thick dashed curves de-note TS curves of the furthest offshore station (Stn. 20).The diagram suggests that the offshore warm saline wa-ter found at Stn. 20 (see Figs. 3 and 5) originates fromthe SWCW region.

The mean atmospheric conditions measured aboardthe icebreaker were: air temperature –5.4°C, relative hu-midity 0.75, fractional cloud cover 0.65, wind speed 4.3m s–1, and solar radiation 108 W m–2. Atmospheric con-ditions seem to be approximately uniform within the scaleof our observation area. According to European Centrefor Medium-range Weather Forecasts (ECMWF) data, theobservation period had representative atmospheric con-ditions for the period January and February 1997.

Using these values, heat budgets have been estimatedat water and sea ice surfaces (Table 1), where all theshortwave radiation is assumed to be absorbed at the sur-faces. We follow Maykut and Perovich (1987) for formu-lae for longwave radiation, sensible and latent heat flux.The bulk transfer coefficient for both sensible and latentheat flux is assumed to be 1.4 × 10–3 (Launiainen andVihma, 1994). The surface water temperature is assumedto be –1.7°C. The water and ice albedos are set to 0.1 and0.65, respectively (Toyota et al., 1999). The bulk thermalconductance of the combined ice-snow slab is set to 1.71W m–2°K–1, corresponding to that of a 50 cm-thick icecovered with 10 cm of snow. The net heat flux at the wa-ter surface is expressed as the sum of shortwave radia-tion, longwave radiation, sensitive and latent hear fluxes.At the ice surface the sum of these four components plusconductive heat flux in the sea ice is assumed to be zero.For the case where the conductive heat flux is upward,the net heat flux at the ice surface is regarded as the nega-tive value of that conductive heat flux in this study.

The net heat fluxes (Table 1) are slightly negative atboth water and ice surfaces. When the density of sea iceand the latent heat of fusion for sea ice are taken as 900kg m–3 and 0.276 MJ kg–1, respectively, the ice produc-tion rate becomes 0.53 cm day–1 at the water surface and0.24 cm day–1 at the ice surface, respectively. Atmospheric

Fig. 6. Temperature and salinity (TS) diagram at 10 differentstations. Thick curves denote TS curves in SWCW region(Stn. 26), ESCW remnant region (Stn. 5), and deep mixedlayer region (Stn. 9). Thick dashed curve denotes that ofthe most offshore station (Stn. 20). Thin curves denote thoseof the offshore deeper stations (Stns. 10, 11, 17, 18, 21, and22).

Table 1. Heat budgets at water and sea ice surfaces.

Water Sea ice

Net shortwave radiation (W m–2) 97 38Net longwave radiation (W m–2) –54 –36Sensible heat flux (W m–2) –29 3Latent heat flux (W m–2) –29 –12Net heat flux (W m–2) –15 –7Ice production rate (em day–1) 0.53 0.24


conditions allow new ice to be formed primarily in openwater areas, but does not permit the growth of thick ice.Thus, most of the first-year ice is considered to beadvected from the north.

On 3 and 4 February we had an opportunity to pur-sue a large ice floe near Stn. 9 under the condition of lowwind speed, and its half-day drift is toward the east-south-east with an average speed of 0.28 m s–1 (its start and endpositions are designated by crosses in Fig. 2). This sug-gests the existence of a southeastward flowing current, acontinuation of the East Sakhalin Current. A recentcurrentmeter mooring around Stn. 17 also showed theexistence of a southeastward mean current (Fukamachiet al., 1999).

4. Discussion

4.1 Deep mixed layerFresh water flux from the Amur River, which occurs

mostly in summer, leads to a prominent stratification inthe surface layer. This fresh surface water extends to thesouthwestern part of the Okhotsk Sea by reinforcementof the East Sakhalin Current in fall (Fig. 4; Watanabe,1963b). A remnant of this low salinity water (ESCW) canalso be seen in Figs. 3 and 5. The entrainment of deeperwater against this strong stratification and the generationof the deep mixed layer observed at Stn. 9 requires pri-marily convection by severe atmospheric cooling and saltrejection with the sea ice formation. The convection trans-ports cold and oxygen-rich water downward to form themixed layer (Fig. 3). At the same time the convectionentrains the deeper water with low dissolved-oxygen con-tent (high AOU) into the mixed layer. Thus, at the sur-face, AOU can be higher than in the surrounding regions.This is consistent with the fact that the deep mixed layerregion corresponds to that of high AOU at the surface(Fig. 5(d)).

As presented in Table 1, the net in-situ heat flux hasonly a small negative value (–15 W m–2: ocean loses heat)at the water surface, and thus local cooling is unlikely togenerate such a deep mixed layer. In the Sea of Okhotskthe air temperature decreases and wind speed increasestoward the northwest with a strong gradient (Wakatsuchiand Martin, 1990), and in the northwest shelf the heatflux has large negative values (typically –300 W m–2) lead-ing to active ice production (Alfultis and Martin, 1987;Martin et al., 1998).

According to Martin et al. (1998), active ice pro-duction occurs off the east Sakhalin coast and off TerpeniaBay (see the locations in Fig. 1) as well as on the north-west shelf. Actually, as can be seen in Fig. 1, new iceformation occurred off the east Sakhalin coast on Decem-ber 21 and January 10 and off Terpenia Bay on January20. We infer that the deep mixed layer water originates

from the north, probably off the east Sakhalin coast oroff Terpenia Bay. Considering the fact that the deep mixedlayer water exists over the shelf slope (Figs. 3 and 5), thewater column with the deep mixed layer is likely to beadvected to this region, trapped by the bottom topogra-phy, via the East Sakhalin Current. Here we assume thatthe current speeds are 0.1–0.3 m s–1 (these values arebased on our recent observations of current-meter moor-ings and surface drifters off the east Sakhalin coast; un-published data (2001)). As shown in Fig. 1, sea ice beganto cover the northern Sakhalin coast at the beginning ofDecember and extended to Terpenia Bay by the end ofDecember. Thus we assume that the thick mixed layerhad been formed in the beginning of January (one monthbefore our observation). Under these assumptions, the areawhere the thick layer originated would be the region fromthe east Sakhalin coast to Terpenia Bay.

The East Sakhalin Current also brings sea ice fromthe north, as inferred by Watanabe (1962, 1963a). Sinceno strong atmospheric disturbances occurred after sea icehad began to cover this region: the average wind speed atAbashiri (see Fig. 2 for the location) is 3.4 m s–1 duringthe period, the situation during the observational periodwas that the sea-ice distribution reflects the ocean cur-rent. This explains why the first-year ice zone correspondsto the region of the deep mixed layer.

It should be noted that the density in the layer deeperthan 100 m at Stn. 9 is lower than that of the surroundingregion (Figs. 3(a) and 4). One explanation for this is thatthe deepening of the mixed layer is not only caused bydensity inversion due to cooling and salt rejection but alsoby some mechanical forcing. The most likely mechanismis the downwelling caused by the onshore Ekman trans-port due to the prevailing northerly wind in winter. As

Fig. 7. Time series of surface wind vector off the east Sakhalincoast (see location in Fig. 1) from 1 December, 1996 to 28February, 1997. The vectors are represented by stick dia-gram. Data are derived from ECMWF data.


shown in Fig. 7, the northerly or northwesterly wind cer-tainly prevails off the east Sakhalin coast in this winter.Since downward displacement of isopycnal propagatesas Kelvin wave like signal, further discussion of thismechanism requires more information about the oceanicconditions along the east Sakhalin coast.

The deep mixed layer water is vertically uniform atStn. 9 on transect A, while on transect B its uniformity issomewhat lost and the mixing with the warm saline wa-ter originating from SWCW (see Fig. 6) seems to be moreprominent. Since transect B is located downstream oftransect A and also the complex topography (Fig. 5(a)),transect B can be more affected by the mixing, which maybe induced by topographic steering and tidal mixing, inaddition to such mechanisms as frontal instability, dou-ble diffusion, and others.

Advection of the vertically uniform water, along withthese processes, results in salinization in the upper layer(0–100 m deep) while freshening, cooling and enrichmentof oxygen in the layer below (100–300 m deep). Then thecharacteristic of ESCW (very low salinity water) is lostin winter in the surface layer. Even in the open ocean thewinter water can penetrate directly down to at least 300m with no downsloping processes, and then the water ata depth from 100 to 300 m is modified to colder, fresher,and oxygen-richer water (a characteristic of the interme-diate Okhotsk Sea water). However, the potential densityof the deep mixed-layer water is around 26.4, which doesnot reach that of NPIW (26.8). The water density is ex-pected to increase in later months, since the amount ofice (and, correspondingly, the amount of brine rejection)reaches a maximum in March (Martin et al., 1998) andsuch dense water is advected from the north with sometime lag. Observations in later seasons (March–April) willbe needed to detect ventilated water of potential density26.8.

4.2 Front of Soya Warm Current WaterWe made particularly tightly-spaced observations

around the front of SWCW, which also corresponds tothe ice edge (Figs. 2 and 3(a)). In this section we describesome features associated with this front, together with abrief theoretical consideration. Because of its higher den-sity, SWCW forms a sharp, inclined front with the lesssaline water offshore. This inclined feature can be ex-plained by the geostrophic adjustment, as follows.

Csanady (1978) investigated the two-dimensionalfrontal adjustment where an imaginary membrane sepa-rating heavier water from lighter water (density defect∆ρ) is withdrawn initially in a constant bottom (=H)ocean. His solution shows that the horizontal width ofthe frontal zone becomes 2.4Ri, where Ri is the baroclinicdeformation radius, defined as Ri = f–1(0.5gH∆ρ/ρ)1/2

( f: the Coriolis parameter, g: the acceleration of gravity,

ρ: water density). For a gently sloping bottom, Csanady’ssolution can be applied as an approximation, as can beinferred from Hsueh and Cushman-Roisin (1983). As Fig.3(a) shows, the width of the frontal zone is apparentlyabout 30 km. However, the transect line crosses the frontwith an orientation of about 30° (Figs. 4(a), (b) and (c)),and thus the width of the frontal zone (perpendicular tothe front) reduces to about 15 km. This coincides approxi-mately with the value of 12 km, estimated from Csanady’ssolution of 2.4Ri, where we use H = 100 m, and ∆ρ =0.5 × 10–3.

4.3 Interactive processes between ice and ocean at iceedgesIt is likely that the low salinity around the ice edges

(Fig. 3) is due to sea ice melt. Actually, ice melt was ob-

Fig. 8. Photographs around ice edges taken from the icebreakerSoya (a) near Stn. 18 on 4 February and (b) near Stn. 22 on7 February. In (a) thick ice floes were broken to pieces offloes with sizes less than a few meters and ice melting wasobserved visually. In (b) broken thick ice floes with sizesof a few meters were seen in the grease ice field.


served visually around the ice edge on transect A (Stn.10), where first-year ice floes existed together with melt-ing brash ice (Fig. 8(a)). The salinity of the top surfaceof 30 m at Stn. 10 drops by 0.3 (Fig. 3(a)), which is equiva-lent to a decrease in ice thickness of 0.18 m for bottommelting. On the other hand, grease ice and pancake icewere observed (Fig. 8(b)) around the ice edge on transectB (Stn. 22), which indicates the formation of new icethere. A similar situation was also observed around Stn.18, where the ice edge was located at the time of the CTDobservation.

These observations can be explained as follows. First,thick, first-year ice floes are advected from the north viathe mainstream of the southward current, while part ofthem are advected onto the offshore warm region, possi-bly by the wind. The first-year ice then melts there (cor-responding to Stn. 10: Fig. 8(a)), causing the surface-layerwater to freshen, cool, finally fall to the freezing pointdue to the release of latent heat. In this phase new ice canbe formed (corresponding to Stn. 22: Fig. 8(b)), since thenet heat budget at the water surface is slightly negative(Table 1). This is consistent with the conditions at Stn.22: broken, thick ice floes were also seen amid the greaseand pancake ice field (Fig. 8(b)).

Heat flux changes through a daily cycle. The typicalnet heat flux at the water surface reaches about 100W m–2 in the daytime while it drops to about –100W m–2 in the nighttime in this region. The associated dailychange in quantity of heat is about 4 M J m–2, which cor-responds to a temperature decrease of only 0.03 degreesfor a 30 m water column. Hence, it is unlikely that theoverall ice condition is much affected by the daily cycleof atmospheric conditions. Indeed, a new ice zone and/ora brash ice zone could be observed, regardless of the timeof day.

Our analysis suggests that, although there are someareas where new ice formation is dominant, local coolingalone is insufficient for ice formation. Preconditioningby thick ice advection and subsequent cooling appears tobe important for ice formation.

5. Concluding RemarksIt has been generally considered that the winter con-

vection is limited to a depth of 150 m in the Sea ofOkhotsk due to the strong stratification. However, we havediscovered that a winter mixed layer with temperaturesnearly at the freezing point extends to a depth of about300 m. This is probably caused not only by the convec-tive mixing due to cooling and ice formation but also bysome mechanical effects, possibly by Ekman convergenceat the Sakhalin coast due to the prevailing northerly wind.

The water column with the thick mixed layer isadvected along the shelf slope from the north via the EastSakhalin Current. At the southern limb of the sea, the

advected thick mixed layer gradually loses its uniformityby mixing with the surrounding water. In the surface layerthe characteristics of East Sakhalin Current Water (verylow salinity water) are lost in winter. On the other hand,the water in the 100–300 m depth zone becomes less sa-line, colder, and richer in oxygen, which is a characteris-tic of the intermediate Okhotsk Sea water. Although thewinter water can penetrate directly down to at least 300m depth, its potential density does not reach to that ofNPIW (26.8) in this season of the year.

From in-situ observations, we examined interactiveprocesses between ice and ocean in the initial stage ofice cover. In the southwestern part of the sea, thick first-year ice is primarily advected by the East Sakhalin Cur-rent. A part of the thick ice is advected onto the offshorewarm region, and melts there first, making the surface-layer water fresher, then cooling it down to the freezingpoint, finally leading to new ice formation at ice edges.In brief, for the formation of new ice zone, which oftenappears at ice edges, local cooling alone is insufficientand preconditioning by thick ice advection and subsequentcooling is important.

The present study is only based on a snapshot obser-vation from one cruise. The oceanographic and sea iceconditions might vary from year to year. General conclu-sions must await the accumulation of further observations.

AcknowledgementsWe express our appreciation to the captain and crew

of the icebreaker Soya for their support during the obser-vations. Thanks are extended to Kazuaki Kubo, KatsuhikoSatoh, Hiroki Shimomura of the Hydrographic Depart-ment of Japan Coast Guard, Masaaki Aota, TakenobuToyota, Daisuke Shimizu, Noriaki Kimura, SoheyNihashi, Shuji Ono, Jinro Ukita, and Akihisa Otsuki ofHokkaido University for their cooperation in observations.We would like to thank Sei-ichi Saitoh for the AVHRRdata, Takenobu Toyota for the photographs, NoriakiKimura for Fig. 1, Asako Hatsushika and ChikakoKusajima for their typing and drawing. Instructive com-ments from Jiayan Yang, the editor Moto Ikeda, and twoanonymous reviewers were very helpful in improving themanuscript.

The SSM/I data were provided by the National Snowand Ice Data Center (NSIDC), University of Colorado.Some figures were produced by GFD DENNOU Library.This work was supported by the International Coopera-tive Research Programme on Global Ocean ObservingSystem (1993–1997), the Grant-in-Aid for Scientific Re-search on Priority Areas (Nos. 08241201 and 09227201),and by the fund from Core Research for Evolutional Sci-ence and Technology (CREST), Japanese Science andTechnology Corporation.


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