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

Transcription

1 Journal of Oceanography, Vol. 57, pp. 451 to 460, 2001 Winter Oceanographic Conditions in the Southwestern Part of the Okhotsk Sea and Their Relation to Sea Ice KAY I. OHSHIMA 1 *, GENTA MIZUTA 2, MOTOYO ITOH 1, YASUSHI FUKAMACHI 1, TATSURO WATANABE 3, YASUSHI NABAE 4, KOUKICHI SUEHIRO 5 and MASAAKI WAKATSUCHI 1 1 Institute of Low Temperature Science, Hokkaido University, Sapporo , Japan 2 Graduate School of Environmental Earth Science, Hokkaido University, Sapporo , Japan 3 Japan Sea National Fisheries Research Institute, Fisheries Agency, Niigata , Japan 4 Hydrographic Department of the Japan Coast Guard, Maizuru , Japan 5 Hydrographic Department of the Japan Coast Guard, Kitakyushu , 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 observations on board the icebreaker Soya were carried out in February A mixed layer of uniform temperature nearly at the freezing point extending down to a depth of about 300 m was observed. This is much deeper than has previously been reported. It is suggested that this deep mixed layer originated from the north (off East Sakhalin), being advected along the shelf slope via the East Sakhalin Current, accompanied with the thick first-year ice (average thickness 0.6 m). This vertically uniform winter water, 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 m depth zone less saline, colder, and richer in oxygen (a characteristic of the intermediate Okhotsk Sea water). The oceanographic structure and a heat budget analysis suggest that new ice zone, which often appears at ice edges, can be formed through preconditioning of thick ice advection and subsequent cooling by the latent heat release due to its melting. Keywords: Okhotsk Sea, sea ice, winter convection, ice-ocean interaction, Soya Warm Current Water, East Sakhalin Current, heat budget. 1. Introduction The Sea of Okhotsk is the southern limit of sea ice extent in the Northern Hemisphere except for regions of local coastal freezing. This low southern ice limit is not caused only by severe winter conditions with cold temperatures and strong winds from Siberia. The surface layer of the Okhotsk Sea consists of very fresh water due to influx from the Amur River, which results in marked stratification around a depth of m. Winter convection to greater depths is suppressed by this oceanic condition, which allows sea ice to form at quite low latitudes (Fukutomi, 1950; Tabata, 1958). The water down to several hundred meters in the Okhotsk Sea is much colder, fresher, and richer in oxygen than that in the North Pacific (e.g., Freeland et al., 1998; Watanabe and Wakatsuchi, 1998). This implies that part of the cold, fresh surface water might penetrate down to several hundred meters against the strong stratifica- * Corresponding author. ohshima@lowtem.hokudai. ac.jp Copyright The Oceanographic Society of Japan. tion in some parts of the sea when the water becomes denser. Kitani (1973) suggested that cold, dense water is formed by brine rejection with sea ice formation in the northwestern continental shelf and that it flows down along the sea bottom into the deep, then spreads into the mid-depth layer. The ventilation of North Pacific Intermediate Water (NPIW) with the potential density of 26.8 can be attributed to this process (Alfultis and Martin, 1987; Talley and Nagata, 1995). In the southwestern part of the Okhotsk Sea, saline water from the Sea of Japan (Soya Warm Current Water: SWCW) is brought by the Soya Warm Current through the Soya Strait, with a maximum influx in summer and a minimum in winter (Aota and Kawamura, 1978; Aota, 1984; Matsuyama et al., 1999). On the other hand, less saline water (East Sakhalin Current Water: ESCW) originating from the influx of the Amur River dominates the surface layer in this region in November and December (Itoh and Ohshima, 2000). In winter and early spring, the potential density of SWCW reaches 26.8, which is much higher than elsewhere in the surface layer of the Okhotsk Sea (Takizawa, 1982; Talley, 1991). Watanabe and Wakatsuchi (1998) proposed that this water share an im- 451

2 portant role in the formation of the intermediate water with the potential density of Although the winter season is the key time for water mass formation and ventilation, in-situ observations during winter are very limited because of logistical difficulties associated with the existence of sea ice. We have not yet understood well what really occur in this sea during winter. Sea ice in the Okhotsk Sea is generally advected southward by the prevailing northerly or northwesterly winds. The southward extension of sea ice is also a result of the southward flow of the East Sakhalin Current along the Sakhalin coast (Watanabe, 1962, 1963a). The southern limb of the sea then turns to be a typical marginal ice zone (MIZ), a key area for air-ice-ocean interaction. In other MIZs, extensive air-ice-ocean observations have been undertaken in the context of several projects such as the Marginal Ice Zone Experiment (MIZEX Group, 1986). No attempt has so far been undertaken to make such observations in the Okhotsk Sea, however. In February 1997, we conducted oceanographic observations together with sea ice and atmospheric observations in the southwestern part of this sea on board an icebreaker. The observational area covers the marginal ice zone in this sea. We found that the winter mixed layer extends to a depth of around 300 m, which is much deeper than has previously been reported and/or believed. In this paper we describe the features of this deep mixed layer and discuss its origin and its role on water mass modification. We also give a description of the Soya Warm Current Water in winter and the associated front, which has been scarcely reported. An advantage of our observations is that the ice, ocean, and atmospheric data were obtained simultaneously. Further, the observations in 1997 were undertaken immediately after the advent of sea ice. This results in relatively simple situation to consider ice-ocean interaction. Hence we also discuss the ice-ocean interactive processes at the initial stage of ice cover. 2. Data and Methods Observations were conducted from the icebreaker Soya from 1 to 10 February Surveys of the upper ocean (down to a depth of 200 m) of this area have been routinely conducted with a portable STD (salinity-temperature-depth unit) and XBTs (expendable bathythermographs) by the Hydrographic Department of the Japan Coast Guard since the 1980s (Ishii, 1991). In 1997, with the cooperation of the Hydrographic Department and Hokkaido University, the observational program was expanded to include water sampling to a depth of 800 m using a calibrated conductivity-temperature-depth unit (CTD; Seabird SBE-19). Water samples were collected with Niskin bottles. The CTD salinity values were calibrated using bottle samples. Dissolved oxygen concentrations were determined by the Winkler method. Our observations 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 Sensor Microwave Imager (SSM/I) on the Defense Meteorological Satellite Program (DMSP). Areas with ice concentration higher than 0.3 are plotted. Solid and open circles indicate first-year ice and young ice, respectively. The algorithm by Kimura and Wakatsuchi (1999) is used. Note that some of the grid points very close to land show false sea ice signals. In this paper, east Sakhalin coast is defined as the area labeled E, and Terpenia Bay is denoted by label T. The cross denotes the grid point where the wind vectors from ECMWF are shown in Fig K. I. Ohshima et al.

3 and calibrated salinity under ice cover in this area. Because of heavy ice conditions we could not carry out the water sampling for some stations. Figure 1 shows the time evolution of sea ice distribution from December 1996 to January Sea ice began to cover the region off the east coast of Sakhalin in mid-december and reached the southern tip of Sakhalin in mid-january. Sea ice began to cover the observational area about 10 days prior to the observational period. Figure 2 shows the positions of the CTD and XBT stations and the ice edge locations during the observational period. The CTD surveys were carried out across the icecovered area, supplemented with XBT surveys. Sea ice conditions and ice thickness were also observed using a video monitoring system along with sea ice floe sampling. The details of the sea ice analysis was presented in Ukita et al. (2000). We also monitored air temperature, relative humidity, wind, downward and upward shortwave radiation, and fractional cloud cover aboard the icebreaker to estimate the heat flux using bulk formulae. Fig. 2. Map of the observed area. The CTD and XBT stations are denoted by solid and open circles, respectively. The ice edge locations of the first-year ice on 3 and 9 February are also shown, derived from NOAA advanced very high resolution radiometer (AVHRR) images. Crosses indicate locations of a large ice floe at 1900 LST on 3 February and at 0700 LST on 4 February. 3. Results During the observational period, most of the ice covered area was occupied by thick first-year ice with new ice (grease or pancake ice) and/or brash ice at ice edges, where brash ice is a term used to describe the wreckage of ice floes with a size not more than 2 m across (WMO, 1970). From the video measurements, the average thickness of sea ice in the first-year ice zone was estimated to be about 0.6 m, including a thickness of snow cover of about 0.1 m (Toyota et al., 1999). Figure 3 shows vertical sections of temperature, salinity, potential density, and dissolved-oxygen content along transects A and B marked in Fig. 2, which cross the ice covered area (sea ice distributions are also indicated at the top of Fig. 3). Density contours almost coincide with those of salinity since density is mostly determined by salinity in this low temperature range. The most remarkable feature along transect A (Fig. 3(a)) is that the mixed layer, with temperatures nearly at the freezing point, extends to more than 300 m at Stn. 9, which is much deeper than has been previously reported. Figure 4 shows the vertical profiles of temperature, salinity, and potential density at Stn. 9, superimposed on those of climatological data around Stn. 9 in December. Fukutomi (1950) and Tabata (1958) considered that winter convection is limited to a depth of m in the Okhotsk Sea because of the strong stratification. In fact, beneath the fresh and warm surface layer a near-freezing cold layer (called a dichothermal layer) usually exists in the Okhotsk Sea, extending to a depth of m (Freeland et al., 1998; Watanabe and Wakatsuchi, 1998). This implies that winter convection can reach depths as great as 150 m (Yang and Honjo, 1996). The mixed layer of 300 m which we observed is even deeper yet. Also along transect B (Fig. 3(b)), cold water (temperature less than 1.5 C) is found from the surface down to a depth of 300 m, although intermediate intrusions of relatively warm water occur within this layer. From transect B, it is found that dissolved oxygen content is relatively high and vertically uniform within the mixed layer, suggesting a result of convection. Both transects A and B reveal that the region of the deep mixed layer corresponds to that of the shelf slope and the first-year ice cover. The temperature structure obtained from both transects suggests that intrusions are prominent around the front between the deep mixed layer and the warm offshore water. An interesting feature in Fig. 3(a) is that salinity in the surface layer is greatly reduced at the ice edge (Stn. 10). The surface salinity is also somewhat reduced around the ice edge along transect B (Stns. 21 and 22). This freshening is a result of ice melting, which will be discussed later. Winter Oceanographic Conditions of Southwestern Okhotsk Sea 453

4 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 the top. Here brash ice area indicated is defined by the area which has a concentration of brash ice more than 0.2. Observational dates on transect A are 3 4 February for Stns. 4 11, 7 February for Stn. 20, 9 February for Stns Observational dates on transect B are 7 8 February. Temperatures less than 1.5 C and salinities less than 33.6 are shaded. The gap mark is inserted between Stns. 32 and 5 in the ice condition of (a) because there is a 5-day difference in the observational time between these stations. 454 K. I. Ohshima et al.

5 Fig. 4. Vertical profiles of temperature, salinity, and potential density at Stn. 9 (dashed lines), superimposed on those in December (solid lines) derived from the climatological data set of Itoh and Ohshima (2000). Location of the climatological data is N, 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 a depth of 50 m, and (d) apparent oxygen utilization (AOU) at the surface. Figures 5(a) and (b) show temperature distributions at depths of 50 m and 100 m, respectively. These figures also indicate that very low temperature zone, corresponding to the region of the deep mixed layer, exists over the shelf slope and also the region of the first-year ice. Figure 5(d) shows apparent oxygen utilization (AOU) at the surface. A high AOU zone also corresponds to the region of the deep mixed layer. The coastal side of section A comprises relatively warm, 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 the Soya Strait and thus it is regarded as Soya Warm Current Water (SWCW) or water originating from SWCW (SWCW is defined as water having salinity greater than 33.6 according to Itoh and Ohshima (2000)). Compari- Winter Oceanographic Conditions of Southwestern Okhotsk Sea 455

6 son of the temperature distribution at 50 m with that at 100 m (Figs. 5(a) and (b)) reveals that the influence of SWCW extends farther downstream at greater depths. The potential density of SWCW observed near the Soya Strait in this cruise (not shown here) is 26.7 at most, which does not reach the potential density of NPIW (26.8). In this season of the year, the water with potential density of 26.8 has not yet been produced from the surface. Itoh and Ohshima (2000) showed that East Sakhalin Current Water (ESCW: water having salinity less than 32.0 according to their definition) comes to this region in November and December and dominates over the shelf region. In the salinity sections shown in Fig. 3, low salinity water exists in the upper layer between SWCW and the deep mixed-layer for transect A and close to the coast for transect B. This fresh water is considered to be a remnant of ESCW. The salinity distribution at 50 m (Fig. 5(c)) also shows that a low salinity zone can be identified between the region of SWCW and the deep mixed-layer zone (see the 32.4 contours), corresponding to water influenced by ESCW. Figure 6 shows the temperature and salinity (TS) diagram for several CTD observations. Thick curves denote TS curves in the SWCW region (Stn. 26), the ESCW remnant region (Stn. 5), and the deep mixed layer region (Stn. 9). These curves are distinctly separated in the diagram. Thin curves denote TS curves of the offshore deeper stations. In deep layers (typically deeper than 300 m) all the thin curves converge to the property of the Okhotsk Sea. In the upper layers they approach the characteristic of the deep mixed layer (Stn. 9) or the ESCW remnant (Stn. 5) region, being mixed with water in the SWCW region (Stn. 26). The diagram demonstrates that such mixing is active in this region. Thick dashed curves denote TS curves of the furthest offshore station (Stn. 20). The diagram suggests that the offshore warm saline water found at Stn. 20 (see Figs. 3 and 5) originates from the SWCW region. The mean atmospheric conditions measured aboard the icebreaker were: air temperature 5.4 C, relative humidity 0.75, fractional cloud cover 0.65, wind speed 4.3 m s 1, and solar radiation 108 W m 2. Atmospheric conditions seem to be approximately uniform within the scale of our observation area. According to European Centre for Medium-range Weather Forecasts (ECMWF) data, the observation period had representative atmospheric conditions for the period January and February Using these values, heat budgets have been estimated at water and sea ice surfaces (Table 1), where all the shortwave radiation is assumed to be absorbed at the surfaces. We follow Maykut and Perovich (1987) for formulae for longwave radiation, sensible and latent heat flux. The bulk transfer coefficient for both sensible and latent heat flux is assumed to be (Launiainen and Vihma, 1994). The surface water temperature is assumed to be 1.7 C. The water and ice albedos are set to 0.1 and 0.65, respectively (Toyota et al., 1999). The bulk thermal conductance of the combined ice-snow slab is set to 1.71 W m 2 K 1, corresponding to that of a 50 cm-thick ice covered with 10 cm of snow. The net heat flux at the water surface is expressed as the sum of shortwave radiation, longwave radiation, sensitive and latent hear fluxes. At the ice surface the sum of these four components plus conductive 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 negative value of that conductive heat flux in this study. The net heat fluxes (Table 1) are slightly negative at both water and ice surfaces. When the density of sea ice and the latent heat of fusion for sea ice are taken as 900 kg m 3 and MJ kg 1, respectively, the ice production rate becomes 0.53 cm day 1 at the water surface and 0.24 cm day 1 at the ice surface, respectively. Atmospheric Table 1. Heat budgets at water and sea ice surfaces. Fig. 6. Temperature and salinity (TS) diagram at 10 different stations. Thick curves denote TS curves in SWCW region (Stn. 26), ESCW remnant region (Stn. 5), and deep mixed layer region (Stn. 9). Thick dashed curve denotes that of the most offshore station (Stn. 20). Thin curves denote those of the offshore deeper stations (Stns. 10, 11, 17, 18, 21, and 22). Water Sea ice Net shortwave radiation (W m 2 ) Net longwave radiation (W m 2 ) Sensible heat flux (W m 2 ) 29 3 Latent heat flux (W m 2 ) Net heat flux (W m 2 ) 15 7 Ice production rate (em day 1 ) K. I. Ohshima et al.

7 conditions allow new ice to be formed primarily in open water areas, but does not permit the growth of thick ice. Thus, most of the first-year ice is considered to be advected from the north. On 3 and 4 February we had an opportunity to pursue a large ice floe near Stn. 9 under the condition of low wind speed, and its half-day drift is toward the east-southeast with an average speed of 0.28 m s 1 (its start and end positions are designated by crosses in Fig. 2). This suggests the existence of a southeastward flowing current, a continuation of the East Sakhalin Current. A recent currentmeter mooring around Stn. 17 also showed the existence of a southeastward mean current (Fukamachi et al., 1999). 4. Discussion 4.1 Deep mixed layer Fresh water flux from the Amur River, which occurs mostly in summer, leads to a prominent stratification in the surface layer. This fresh surface water extends to the southwestern part of the Okhotsk Sea by reinforcement of the East Sakhalin Current in fall (Fig. 4; Watanabe, 1963b). A remnant of this low salinity water (ESCW) can also be seen in Figs. 3 and 5. The entrainment of deeper water against this strong stratification and the generation of the deep mixed layer observed at Stn. 9 requires primarily convection by severe atmospheric cooling and salt rejection with the sea ice formation. The convection transports cold and oxygen-rich water downward to form the mixed layer (Fig. 3). At the same time the convection entrains the deeper water with low dissolved-oxygen content (high AOU) into the mixed layer. Thus, at the surface, AOU can be higher than in the surrounding regions. This is consistent with the fact that the deep mixed layer region corresponds to that of high AOU at the surface (Fig. 5(d)). As presented in Table 1, the net in-situ heat flux has only a small negative value ( 15 W m 2 : ocean loses heat) at the water surface, and thus local cooling is unlikely to generate such a deep mixed layer. In the Sea of Okhotsk the air temperature decreases and wind speed increases toward the northwest with a strong gradient (Wakatsuchi and Martin, 1990), and in the northwest shelf the heat flux has large negative values (typically 300 W m 2 ) leading to active ice production (Alfultis and Martin, 1987; Martin et al., 1998). According to Martin et al. (1998), active ice production occurs off the east Sakhalin coast and off Terpenia Bay (see the locations in Fig. 1) as well as on the northwest shelf. Actually, as can be seen in Fig. 1, new ice formation occurred off the east Sakhalin coast on December 21 and January 10 and off Terpenia Bay on January 20. We infer that the deep mixed layer water originates from the north, probably off the east Sakhalin coast or off Terpenia Bay. Considering the fact that the deep mixed layer water exists over the shelf slope (Figs. 3 and 5), the water column with the deep mixed layer is likely to be advected to this region, trapped by the bottom topography, via the East Sakhalin Current. Here we assume that the current speeds are m s 1 (these values are based on our recent observations of current-meter moorings and surface drifters off the east Sakhalin coast; unpublished data (2001)). As shown in Fig. 1, sea ice began to cover the northern Sakhalin coast at the beginning of December and extended to Terpenia Bay by the end of December. Thus we assume that the thick mixed layer had been formed in the beginning of January (one month before our observation). Under these assumptions, the area where the thick layer originated would be the region from the east Sakhalin coast to Terpenia Bay. The East Sakhalin Current also brings sea ice from the north, as inferred by Watanabe (1962, 1963a). Since no strong atmospheric disturbances occurred after sea ice had began to cover this region: the average wind speed at Abashiri (see Fig. 2 for the location) is 3.4 m s 1 during the period, the situation during the observational period was that the sea-ice distribution reflects the ocean current. This explains why the first-year ice zone corresponds to the region of the deep mixed layer. It should be noted that the density in the layer deeper than 100 m at Stn. 9 is lower than that of the surrounding region (Figs. 3(a) and 4). One explanation for this is that the deepening of the mixed layer is not only caused by density inversion due to cooling and salt rejection but also by some mechanical forcing. The most likely mechanism is the downwelling caused by the onshore Ekman transport due to the prevailing northerly wind in winter. As Fig. 7. Time series of surface wind vector off the east Sakhalin coast (see location in Fig. 1) from 1 December, 1996 to 28 February, The vectors are represented by stick diagram. Data are derived from ECMWF data. Winter Oceanographic Conditions of Southwestern Okhotsk Sea 457

8 shown in Fig. 7, the northerly or northwesterly wind certainly prevails off the east Sakhalin coast in this winter. Since downward displacement of isopycnal propagates as Kelvin wave like signal, further discussion of this mechanism requires more information about the oceanic conditions along the east Sakhalin coast. The deep mixed layer water is vertically uniform at Stn. 9 on transect A, while on transect B its uniformity is somewhat lost and the mixing with the warm saline water originating from SWCW (see Fig. 6) seems to be more prominent. Since transect B is located downstream of transect A and also the complex topography (Fig. 5(a)), transect B can be more affected by the mixing, which may be induced by topographic steering and tidal mixing, in addition to such mechanisms as frontal instability, double diffusion, and others. Advection of the vertically uniform water, along with these processes, results in salinization in the upper layer (0 100 m deep) while freshening, cooling and enrichment of oxygen in the layer below ( m deep). Then the characteristic of ESCW (very low salinity water) is lost in winter in the surface layer. Even in the open ocean the winter water can penetrate directly down to at least 300 m with no downsloping processes, and then the water at a depth from 100 to 300 m is modified to colder, fresher, and oxygen-richer water (a characteristic of the intermediate Okhotsk Sea water). However, the potential density of the deep mixed-layer water is around 26.4, which does not reach that of NPIW (26.8). The water density is expected to increase in later months, since the amount of ice (and, correspondingly, the amount of brine rejection) reaches a maximum in March (Martin et al., 1998) and such dense water is advected from the north with some time lag. Observations in later seasons (March April) will be needed to detect ventilated water of potential density Front of Soya Warm Current Water We made particularly tightly-spaced observations around the front of SWCW, which also corresponds to the ice edge (Figs. 2 and 3(a)). In this section we describe some features associated with this front, together with a brief theoretical consideration. Because of its higher density, SWCW forms a sharp, inclined front with the less saline water offshore. This inclined feature can be explained by the geostrophic adjustment, as follows. Csanady (1978) investigated the two-dimensional frontal adjustment where an imaginary membrane separating heavier water from lighter water (density defect ρ ) is withdrawn initially in a constant bottom (=H) ocean. His solution shows that the horizontal width of the frontal zone becomes 2.4Ri, where Ri is the baroclinic deformation radius, defined as Ri = f 1(0.5gH ρ /ρ )1/2 ( f: the Coriolis parameter, g: the acceleration of gravity, 458 K. I. Ohshima et al. ρ: water density). For a gently sloping bottom, Csanady s solution can be applied as an approximation, as can be inferred from Hsueh and Cushman-Roisin (1983). As Fig. 3(a) shows, the width of the frontal zone is apparently about 30 km. However, the transect line crosses the front with an orientation of about 30 (Figs. 4(a), (b) and (c)), and thus the width of the frontal zone (perpendicular to the front) reduces to about 15 km. This coincides approximately with the value of 12 km, estimated from Csanady s solution of 2.4Ri, where we use H = 100 m, and ρ = Interactive processes between ice and ocean at ice edges It 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 icebreaker Soya (a) near Stn. 18 on 4 February and (b) near Stn. 22 on 7 February. In (a) thick ice floes were broken to pieces of floes with sizes less than a few meters and ice melting was observed visually. In (b) broken thick ice floes with sizes of a few meters were seen in the grease ice field.

9 served visually around the ice edge on transect A (Stn. 10), where first-year ice floes existed together with melting brash ice (Fig. 8(a)). The salinity of the top surface of 30 m at Stn. 10 drops by 0.3 (Fig. 3(a)), which is equivalent to a decrease in ice thickness of 0.18 m for bottom melting. On the other hand, grease ice and pancake ice were observed (Fig. 8(b)) around the ice edge on transect B (Stn. 22), which indicates the formation of new ice there. A similar situation was also observed around Stn. 18, where the ice edge was located at the time of the CTD observation. These observations can be explained as follows. First, thick, first-year ice floes are advected from the north via the mainstream of the southward current, while part of them are advected onto the offshore warm region, possibly by the wind. The first-year ice then melts there (corresponding to Stn. 10: Fig. 8(a)), causing the surface-layer water to freshen, cool, finally fall to the freezing point due to the release of latent heat. In this phase new ice can be formed (corresponding to Stn. 22: Fig. 8(b)), since the net 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 grease and pancake ice field (Fig. 8(b)). Heat flux changes through a daily cycle. The typical net heat flux at the water surface reaches about 100 W m 2 in the daytime while it drops to about 100 W m 2 in the nighttime in this region. The associated daily change in quantity of heat is about 4 M J m 2, which corresponds to a temperature decrease of only 0.03 degrees for a 30 m water column. Hence, it is unlikely that the overall ice condition is much affected by the daily cycle of atmospheric conditions. Indeed, a new ice zone and/or a brash ice zone could be observed, regardless of the time of day. Our analysis suggests that, although there are some areas where new ice formation is dominant, local cooling alone is insufficient for ice formation. Preconditioning by thick ice advection and subsequent cooling appears to be important for ice formation. 5. Concluding Remarks It has been generally considered that the winter convection is limited to a depth of 150 m in the Sea of Okhotsk due to the strong stratification. However, we have discovered that a winter mixed layer with temperatures nearly at the freezing point extends to a depth of about 300 m. This is probably caused not only by the convective mixing due to cooling and ice formation but also by some mechanical effects, possibly by Ekman convergence at the Sakhalin coast due to the prevailing northerly wind. The water column with the thick mixed layer is advected along the shelf slope from the north via the East Sakhalin Current. At the southern limb of the sea, the advected thick mixed layer gradually loses its uniformity by mixing with the surrounding water. In the surface layer the characteristics of East Sakhalin Current Water (very low salinity water) are lost in winter. On the other hand, the water in the m depth zone becomes less saline, colder, and richer in oxygen, which is a characteristic of the intermediate Okhotsk Sea water. Although the winter water can penetrate directly down to at least 300 m depth, its potential density does not reach to that of NPIW (26.8) in this season of the year. From in-situ observations, we examined interactive processes between ice and ocean in the initial stage of ice cover. In the southwestern part of the sea, thick firstyear ice is primarily advected by the East Sakhalin Current. A part of the thick ice is advected onto the offshore warm region, and melts there first, making the surfacelayer water fresher, then cooling it down to the freezing point, finally leading to new ice formation at ice edges. In brief, for the formation of new ice zone, which often appears at ice edges, local cooling alone is insufficient and preconditioning by thick ice advection and subsequent cooling is important. The present study is only based on a snapshot observation from one cruise. The oceanographic and sea ice conditions might vary from year to year. General conclusions must await the accumulation of further observations. Acknowledgements We express our appreciation to the captain and crew of the icebreaker Soya for their support during the observations. Thanks are extended to Kazuaki Kubo, Katsuhiko Satoh, Hiroki Shimomura of the Hydrographic Department of Japan Coast Guard, Masaaki Aota, Takenobu Toyota, Daisuke Shimizu, Noriaki Kimura, Sohey Nihashi, Shuji Ono, Jinro Ukita, and Akihisa Otsuki of Hokkaido University for their cooperation in observations. We would like to thank Sei-ichi Saitoh for the AVHRR data, Takenobu Toyota for the photographs, Noriaki Kimura for Fig. 1, Asako Hatsushika and Chikako Kusajima for their typing and drawing. Instructive comments from Jiayan Yang, the editor Moto Ikeda, and two anonymous reviewers were very helpful in improving the manuscript. The SSM/I data were provided by the National Snow and Ice Data Center (NSIDC), University of Colorado. Some figures were produced by GFD DENNOU Library. This work was supported by the International Cooperative Research Programme on Global Ocean Observing System ( ), the Grant-in-Aid for Scientific Research on Priority Areas (Nos and ), and by the fund from Core Research for Evolutional Science and Technology (CREST), Japanese Science and Technology Corporation. Winter Oceanographic Conditions of Southwestern Okhotsk Sea 459

10 References Alfultis, M. A. and S. Martin (1987): Satellite passive microwave studies of the Sea of Okhotsk ice cover and its relation to oceanic processes, J. Geophys. Res., 92, 13,013 13,028. Aota, M. (1984): Oceanographic structure of the Soya Warm Current. Bull. Coast. Oceanogr., 22, (in Japanese). Aota, M. and T. Kawamura (1978): Observation of oceanographic condition in the Okhotsk Sea coast of Hokkaido in winter. Teion Kagaku, 37, (in Japanese with English abstract). Csanady, G. T. (1978): Wind effects on surface to bottom fronts. J. Geophys. Res., 83, 4,633 4,640. Freeland, H. J., A. S. Bychkov, F. Whitney, C. Taylor, C. S. Wong and G. I. Yurasov (1998): WOCE section P1W in the Sea of Okhotsk, 1, Oceanographic data description. J. Geophys. Res., 103, 15,613 15,623. Fukamachi, Y., G. Mizuta, K. I. Ohshima, M. Itoh, M. Wakatsuchi and M. Aota (1999): Mooring measurements off Shiretoko Peninsula, Hokkaido in PICES Scientific Report No. 12, Fukutomi, T. (1950): Study of sea ice (the 4th report). A theoretical study on the formation of sea ice in the central part of the Okhotsk Sea. Teion Kagaku, 3, (in Japanese with English abstract). Hsueh, Y. and B. Cushman-Roisin (1983): On the formation of surface to bottom fronts over steep topography. J. Geophys. Res., 88, Ishii, H. (1991): Oceanic structure of the southwestern Okhotsk Sea. Kaiyo Monthly, 23, (in Japanese). Itoh, M. and K. I. Ohshima (2000): Seasonal variations of water masses and sea level in the southwestern part of the Okhotsk Sea. J. Oceanogr., 56, Kimura, N. and M. Wakatsuchi (1999): Processes controlling the advance and retreat of sea ice in the Sea of Okhotsk. J. Geophys. Res., 104, 11,137 11,150. Kitani, K. (1973): An oceanographic study of the Okhotsk Sea: particularly in regard to cold waters. Bull. Far Sea Fish. Res. Lab., 9, Launiainen, J. and T. Vihma (1994): On the surface heat fluxes in the Weddell Sea. p In The Polar Ocean and Their Role in Shaping the Global Environment, Geophysical Monograph 85, AGU, Washington, D.C. Martin, S., R. Drucker and K. Yamashita (1998): The production of ice and dense shelf water in the Okhotsk Sea polynyas. J. Geophys. Res., 103, 27,771 27,782. Matsuyama, M., M. Aota, I. Ogasawara and S. Matsuyama (1999): Seasonal variation of Soya Current. Umi no Kenkyu, 8, (in Japanese with English abstract). Maykut, G. A. and D. K. Perovich (1987): The role of shortwave radiation in the summer decay of a sea ice cover. J. Geophys. Res., 92, 7,032 7,044. MIZEX Group (1986): MIZEX East 83/84, The summer marginal ice zone experiment in the Fram Strait/Greenland Sea. Eos, 67, Tabata, T. (1958): On the formation and growth of sea ice especially on the Okhotsk Sea. p In Arctic Sea Ice, National Academy Press, Washington, D.C. Takizawa, T. (1982): Characteristics of the Soya Warm Current in the Okhotsk Sea. J. Oceanogr. Soc. Japan, 38, Talley, L. D. (1991): An Okhotsk Sea water anomaly: implications for ventilation in the North Pacific. Deep-Sea Res., 38, Suppl. 1, Talley, L. D. and Y. Nagata (1995): The Okhotsk Sea and Oyashio Region. PICES Scientific Report No. 2, 227 pp. Toyota, T., J. Ukita, K. I. Ohshima, M. Wakatsuchi and K. Muramoto (1999): A measurement of sea ice albedo over the southwestern Okhotsk Sea. J. Meteor. Soc. Japan, 77, Ukita, J., T. Kawamura, N. Tanaka, T. Toyota and M. Wakatsuchi (2000): Physical and stable isotopic properties and growth processes of sea ice collected in the southern Sea of Okhotsk. J. Geophys. Res., 105, 22,083 22,093. Wakatsuchi, M. and S. Martin (1990): Satellite observations of the ice cover of the Kuril Basin region of the Okhotsk Sea and its relation to the regional oceanography. J. Geophys. Res., 95, 13,393 13,410. Watanabe, K. (1962): Drift velocity of ice measured from air and separately computed value of their wind-induced and currents induced components; Study on sea ice in the Okhotsk Sea (II). Oceanogr. Mag., 14, Watanabe, K. (1963a): On the estimation of the origin and drifting speed of ice appearing off the coast of Hokkaido; Study on sea ice in the Okhotsk Sea (III). Oceanogr. Mag., 14, Watanabe, K. (1963b): On the reinforcement of the East Sakhalin Current preceding to the sea ice season off the coast of Hokkaido; Study on sea ice in the Okhotsk Sea (IV). Oceanogr. Mag., 14, Watanabe, T. and M. Wakatsuchi (1998): Formation of σ θ water in the Kuril Basin of the Sea of Okhotsk, as a possible origin of North Pacific Intermediate Water. J. Geophys. Res., 103, 2,849 2,865. World Meteorological Organization (WMO) (1970): Sea Ice Nomenclature. Terminology, Codes, and Illustrated Glossary. WMO/OMM/BMO 259, TP 145, Secr. World Meteorol. Organ., Geneva, Switzerland. Yang, J. and S. Honjo (1996): Modeling the near-freezing dichothermal layer in the Sea of Okhotsk and its interannual variations J. Geophys. Res., 101, 16,421 16, K. I. Ohshima et al.