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Winter mixed layer and its yearly variability under sea ice in the southwestern part of the Sea of Okhotsk
ARTICLE IN PRESS
Continental Shelf Research 24 (2004) 643–657
Winter mixed layer and its yearly variability under sea ice in the southwestern part of the Sea of Okhotsk Genta Mizutaa,*, Kay I. Ohshimab, Yasushi Fukamachib, Motoyo Itoha,1, Masaaki Wakatsuchib,c a
Graduate School of Environmental Earth Science, Hokkaido University, Sapporo, Japan b Institute of Low Temperature Science, Hokkaido University, Sapporo, Japan c Japan Science and Technology Corporation, Kawaguchi, Japan
Received 9 December 2002; received in revised form 7 January 2004; accepted 23 January 2004
Abstract Hydrographic observations under sea ice were conducted in the southwestern part of the Sea of Okhotsk for 4 years from 1996 to 1999. Every year a cold mixed layer with near-freezing temperature was distributed from the sea surface to a depth of 150–300 m near the shelf break under sea ice. The thicknesses of the mixed layer and sea ice were largest in 1997. While the depth of the mixed layer was considerably deeper than that of dichothermal water, which is identified as a temperature minimum from spring to fall in this region, the density of water in the mixed layer was equal to or less than that of dichothermal water. It is shown that deepening of isopycnals due to the alongshore component of the wind stress is essential for thickening of the mixed layer. In 1997 the nearly northerly winds, which are usually directed offshore, were more parallel to the coast than those in the other years. Thickening of the mixed layer in 1997 is attributed to this wind condition. The air temperature and wind indicate that the sea-ice production rate was low in 1997, whereas the wind direction was favorable for rafting and ridging. Thus the change in wind direction is proposed as an important factor in determining the thickness of both the mixed layer and the sea ice in this region. r 2004 Elsevier Ltd. All rights reserved. Keywords: Sea ice; Water masses; The Sea of Okhotsk (a marginal sea in the Northwestern Pacific); Hydrographic data
1. Introduction The Sea of Okhotsk is a marginal sea in the Northwestern Pacific and a seasonal ice zone, which is connected with the Japan Sea and the *Corresponding author. Graduate School of Environmental Earth Science, Hokkaido University, Kita 10, Nishi 5, Sapporo 060-0810, Japan. E-mail address: [email protected] (G. Mizuta). 1 Present address: Japan Marine Science and Technology Center, Yokosuka, Japan.
North Pacific with narrow straits (Fig. 1). Two major water masses exist in the southwestern part of the Sea of Okhotsk, with their distribution being markedly different with season. The Soya Warm Current Water (SWCW), which is warm and saline water originating from the Japan Sea through the Soya Strait, is dominant in summer. The Soya Warm Current weakens in winter and appears intermittently as a bottom trapped current (Aota, 1984). From November to December East Sakhalin Current Water (ESCW), which is less
0278-4343/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.csr.2004.01.006
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saline water affected by Amur River discharge, extends southward along Sakhalin Island. The ESCW forms a less saline layer from the surface to a depth around 50 m accompanied with a strong halocline underneath it (Watanabe, 1963). Fukutomi (1950) pointed out that a strong, shallow halocline due to ESCW tends to reinforce the production of sea ice by suppressing the vertical convection in winter. In fact water with near-freezing temperature, which is thought to be a remnant of winter convection, is found at a depth of 50–100 m from spring to fall in this region. This water is called dichothermal water (DTW). On the other hand, satellite images suggest that sea ice is drifted southward by northerlies and a southward current called the East Sakhalin Current (ESC) near the coast of Sakhalin Island (Parkinson and Gratz, 1983; Kimura and Wakatsuchi, 2000). Watanabe (1963) hypothesized that the thick sea ice found in the southwestern part of the Sea of Okhotsk originates from the northern part. By using heatflux calculation, Alfultis and Martin (1987) and
Martin et al. (1998) showed that a considerable amount of sea ice is produced in the northern part of the Sea of Okhotsk because of severe atmospheric condition there. A recent study based on direct current measurements shows that the ESC is most intense in winter (Mizuta et al., 2003). Surface-drifter measurements show that a part of the ESC reaches the southwestern end of the Sea of Okhotsk in October–January (Ohshima et al., 2002). Thus it is suggested that the ESC and ESCW are important to both the production and advection of sea ice observed in the southwestern part of the Sea of Okhotsk. However, there are no measurements of the current speed under sea ice in this region except in the shallow shelf region (Aota and Kawamura, 1978; Matsuyama et al., 1999). Hydrographic data are quite limited because of the presence of sea ice. Thus it is still not clear what kind of water mass really exists under sea ice in this region. Recently, Ohshima et al. (2001) conducted hydrographic observations in February 1997 in the southwestern part of the Sea of Okhotsk. They found that a mixed layer, in which the temperature is nearly at the freezing point, extends from the surface to a depth of 300 m under sea ice. The lower boundary of the mixed layer is much deeper than that of DTW or the less saline layer associated with ESCW. Because of its low temperature and large vertical extent, water in the mixed layer seems to originate from the northern part of the Sea of Okhotsk. As a possible candidate of the mechanism that maintains the large thickness of the mixed layer, they proposed a downwelling due to the Ekmantransport convergence at the east coast of Sakhalin forced by northerly winds. However, it is not clear whether the Ekman downwelling can account for the thickening of the mixed layer quantitatively. The yearly variability of the mixed-layer properties and their relation to sea ice are also still unknown. In order to answer these questions we analyze the hydrographic data obtained in the southwestern part of the Sea of Okhotsk in every February from 1996 to 1999. The purpose of this study is to investigate the yearly variability of the mixed-layer properties and to clarify the mechanism that maintains the thick mixed layer. Then we
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discuss the relation between the yearly variability of the thick mixed layer and that of sea ice. In the next section, we describe the data used in this study. After describing the ice condition in the observation area in Section 3, we examine the properties of the cold mixed layer and their relation to sea ice in Section 4. The yearly variability of the mixed layer and sea ice are compared with that of the atmospheric condition in Section 5. All these results are summarized in Section 6.
2. Data Hydrographic observation was conducted in early February from 1996 to 1999 by the icebreaker Soya of Japan Coast Guard, under cooperation of the Hydrographic and Oceanographic Department of the Japan Coast Guard and Hokkaido University. The observation period was 1–7 February 1996, 1–10 February 1997, 3–12 February 1998, and 3–12 February 1999. The observation area is indicated by a rectangle frame in Fig. 1. The temperature and salinity were measured by a CTD (SeaBird SBE 19) and XBTs (Tsurumi Seiki) from the surface to a depth of 800 m in maximum. Water samples were collected with Niskin bottles. The CTD salinities were calibrated against water samples. Except for 1996, post-cruise calibration of the CTD sensors was also performed. The dissolved-oxygen content was measured with the Winkler method. Sea-ice condition and thickness and meteorological parameters such as the temperature, relative humidity, wind, shortwave radiation, and fractional cloud cover were also measured on the icebreaker. Detailed results of those measurements are presented by Toyota et al. (1999) and Ukita et al. (2000).
3. Sea-ice condition Fig. 2 shows the ice concentration on 31 January observed by the Defense Meteorological Satellite Program (DMSP) Special Sensor Microwave Imager (SSM/I) in the 4 years. Sea ice was
Fig. 2. Distribution of ice concentration on 31 January observed by the SSM/I in (a) 1996, (b) 1997, (c) 1998, and (d) 1999. Crosses, open circles, solid circles indicate ice concentration 20–50%, 50–80%, and 80–100%, respectively.
distributed from the northern part to the southern part of the Sea of Okhotsk along the western boundary. The observation area was covered with sea ice 3, 7, 7, and 3 days before the start of the observation in the 4 years, respectively. Fig. 3 shows the sea-ice distribution during the observation period. This figure was redrawn from ice charts published by the Hydrographic Department of the Japan Coast Guard and the in-situ visual observations to supplement region without data in the ice chart. Since the ice condition changes day by day, several ice charts were combined so that the ice condition at each observation site reflects that at the observation time. Most of the observation area was covered with sea ice every year except for open waters in the western part near the
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Fig. 3. Distribution of sea ice in (a) 1996, (b) 1997, (c) 1998, and (d) 1999. Shading indicates ice covered area. The darker shade indicates the more developed ice. The sign ‘F’ denotes ice floe, ‘C’ denotes ice cake, ‘Y’ denotes young ice, ‘P’ denotes pancake ice, ‘Br’ denotes brash ice, and ‘New’ denotes new ice, which consists of grease ice (‘Gr’), and nilas (‘Ni’). The definition of these ices are based on that of the World Meteorological Organization (WMO) (1970). The sign ‘O’ denotes open water. In 1998, the ice distribution near the Soya Strait is for 11 February. Whereas in the other region observation date changes continuously from 4 to 10 February from the western to eastern part, respectively. Solid and open circles denote CTD and XBT stations, respectively.
Soya Strait, in the offshore part northeastward, and near the coast. Several types of sea ice were distributed in the observation area. The iceproduction rate based on the heat flux calculation is about 1 cm day1 in open water in this area (Toyota and Wakatsuchi, 2001; Ohshima et al., 2001). Thus it is most likely that ice floe, which is defined as the sea ice whose diameter is larger than 200 m, and probably ice cake, which is defined as the sea ice whose diameter is between 20 and 200 m, originate from the northern part of the Sea of Okhotsk. In 1997 most of sea ice observed in this area consisted of ice floe, whereas new ice, pancake ice, and ice cake occupied more than half of the area in the other years. In the northernmost part of the observation area the ice thickness in 1997 was more than 1 m, which is the largest among the 4 years. The mean sea-ice thickness
monitored by the video was 18.5, 54.9, 30.1, and 29.0 cm in 1996, 1997, 1998, and 1999, respectively (Toyota and Wakatsuchi, 2001). Thus sea ice was the thickest in 1997.
4. Results 4.1. General features of the temperature and salinity field Fig. 4 shows the distribution of surface temperature. The temperature was nearly at the freezing point in most part of the ice covered area every year (see Fig. 3). Relatively warm water was found every year near the Soya Strait. This water corresponds to SWCW coming from the Japan Sea. The temperature was higher than 1.5 C in
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Fig. 4. Contours of the surface temperature in (a) 1996, (b) 1997, (c) 1998, and (d) 1999. Shaded region indicates temperature less than 1.5 C. Dashed lines indicate the sections shown in Figs. 5 and 6.
the northeastern area, which was largest in 1998 and absent in 1999, and near the coast in 1997 and 1999. These distributions of water warmer than the freezing point generally coincided with that of open water. Figs. 5 and 6 show the distributions of temperature and salinity at the vertical crosssections along the dashed lines in Fig. 4, respectively. The temperature was vertically uniform near the surface to form a cold mixed layer under the sea ice (Fig. 5). Cold and warm waters were interleaved with each other at the offshore side of the cold mixed layer, for example, around stations 9608, 9710, and 9810. We define the depth, hT, of the mixed layer as the maximum depth at which the difference of the temperature from that at the surface does not exceed 0.2 C. Note that even if there is an intrusion of warm water at the mid depth of the cold mixed layer, the depth of the mixed layer can be deeper than the intrusion depth. We adopted this definition as a measure of
the thickness of the mixed layer unaffected by the intrusion. The mixed-layer thickness attained a maximum at the shelf break every year (dashed line in Fig. 5). The maximum thickness was largest in 1997 with the mixed layer extending to a depth of 300 m or more and reaching the bottom, whereas the maximum thickness was 150–200 m in the other years. The 1997 corresponds to the year when sea ice was the thickest. These depths of the mixed layer were larger than that of DTW, which is found from spring to fall at a depth of 50–100 m, or that of ESCW in late fall, which is found at a depth around 50 m. The salinity was rather uniform in the vertical direction in the thick mixed layer (Fig. 6). The salinity at the deepest part of the mixed layer was 32.8, 32.9, 32.8, and 32.7 in 1996, 1997, 1998, and 1999, respectively. Hence most saline water was found in 1997. At depths greater than 100 m, the isohalines deepened toward the coast. Since the density is almost uniquely determined from the
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Fig. 5. Contours of the temperature at the vertical cross-sections along the dashed lines in Fig. 4 in (a) 1996, (b) 1997, (c) 1998, and (d) 1999. Shading indicates the temperature less than 1.5 C. Bars at the top of each panel denote ice covered area. Ticks with and without numbers indicate CTD and XBT stations, respectively. Dashed lines indicate the lower boundary of the mixed layer.
salinity for the low temperature range, the inclination of the isohalines indicates a southward geostrophic flow in the upper layer, suggesting that mixed-layer water was advected from the north. Geostrophic velocity referenced to 500 db or the bottom was typically 10 cm s1 to the south on the slope. The dissolved-oxygen content was also uniform in the vertical direction within the mixed layer (not shown). The oxygen content in the mixed layer was slightly less than saturation, suggesting the presence of intense cooling or entrainment of deep water. Other water masses were also identified in these sections. Saline water existed around the Soya Strait near the bottom. Since the temperature was also high there (Fig. 5), this water is SWCW. The SWCW extended largely in 1999 and intruded
below cold mixed-layer water at the shelf break. On the other hand, less saline water existed on the shelf near the surface. Because of its low salinity this water seems to be a remnant of ESCW in late fall, which is defined as the water with salinity less than 32. Less saline water was also observed near the surface in the offshore area where the temperature was higher than the freezing point. The less saline feature was especially clear at the ice edge in 1997. This feature seems to indicate melting of sea ice (Ohshima et al., 2001). Such features were also observed in other years, though they were not so clear. The relations between the potential temperature and salinity obtained at all the CTD stations and regions of water masses defined by Aota (1970) and Takizawa (1982) are displayed in Fig. 7 for
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Fig. 6. Similar to Fig. 5 except for the salinity.
each year. Most water in the observation area was more saline than ESCW except for the least saline water found in the observation period. This water corresponds to the less saline water on the shelf in Fig. 6. Warm and saline water observed at the stations closest to the Soya Strait (9601, 9733, 9825, and 9901) is classified as SWCW. Some curves extend almost linearly from the cold and less saline range to the warm and saline range in 1997 and 1999. These curves clearly show that ESCW and SWCW are mixed diapycnally over the shelf. This mixed water was also observed in other years. The curves near the freezing temperature correspond to the cold mixed layer and less saline layer near the surface. The maximum salinity of cold mixed-layer water was 32.9. Although the thickness of the mixed layer was 150–300 m in maximum, the salinity and, accordingly, the
density of mixed-layer water was similar to that of DTW observed at shallower depths from spring to fall. Aota (1970) showed that even more saline water, which is around 33.2 and colder than 1 C, exists in the dichothermal layer in summer. Thus the density of cold mixed-layer water was higher than that of ESCW and equal to or lower than that of DTW. There are spiky features at the portion of curves in the density range less than 26.7. These spiky features indicate that water in and below the mixed layer were mixed isopycnally with relatively warm water, which is the mixture of SWCW and ESCW. The mixing is associated with the interleaving shown in Fig. 5. Cold mixed-layer water lost its original characteristics due to the mixing. The curves in the density range greater than 26.7 asymptote to a single curve. Kitani (1973) showed that cold water with a density of 26.8–27.0sy is formed in the northern part of the Sea of Okhotsk
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Fig. 7. Plots of the potential temperature, y, versus salinity, S, at all the CTD stations in (a) 1996, (b) 1997, (c) 1998, and (d) 1999. Solid and dashed curves indicate the y–S relation in the region where the bottom depth is larger and smaller than 200 m, respectively. The solid curve near the bottom indicates the freezing point. Rectangular regions denote SWCW, ESCW, and DTW after the definition by Aota (1970) and Takizawa (1982). Station numbers are displayed for some curves near the Soya Strait.
in winter. However, cold water in this density range was not observed in our stations. 4.2. Spatial and temporal variability of the cold thick mixed layer The horizontal distribution of the mixed-layer thickness defined in the previous subsection is displayed in Fig. 8. The mixed-layer thickness was
largest in 1997 not only along the section shown in Fig. 5 but also in the whole observation area. The thick mixed layer was distributed along the slope just offshore of the shelf break (dashed line in Fig. 8). The thickness was largest or attained a local maximum every year at almost the same site around (144 00 E, 45 100 N). The ice condition at this site changed from year to year. In 1997, this site was covered with the heaviest ice observed in
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Fig. 8. Contours of the mixed-layer thickness in (a) 1996, (b) 1997, (c) 1998, and (d) 1999. Dashed lines denote the isobath of 200 m, which roughly corresponds to the shelf break.
this year (Fig. 3). In 1998 this site was located near the ice edge and the ice condition was rather light there. Ice cake and young ice were observed there in 1996 and 1999, respectively. Thus bottom topography controlled the precise location of the thick mixed layer more strongly than sea ice, though the overall distribution of cold water coincided with ice cover. The vertical uniformity of the thick mixed layer weakened in the southeastern part of the observation area because of the intrusion of warm water from the offshore such as shown in Fig. 5 (see also Fig. 3 in Ohshima et al., 2001). Fig. 9 shows the scatter plot of isopycnal depths at CTD stations versus the horizontal distances between the stations and the 200 m isobath. Crosses and circles denote the depths of 26.5 and 26.7sy surfaces, respectively. These densities correspond to those of water with salinity about 32.9 and 33.2 at the freezing temperature, respectively. The 26.5sy surface, which was nearly coincident with or slightly deeper than the lower
boundary of the mixed layer, tended to deepen near the shelf break every year. A similar feature was also found for 26.7sy surface, which was deeper than the lower boundary of the mixed layer. Solid lines in Fig. 9 indicate depths of the isopycnals interpolated with a Gaussian weight, where we set the e-folding scale of the Gaussian to 30 km to smooth out small scale fluctuations. On the deeper stations, the depths of 26.5 and 26.7sy surfaces were not so different among the 4 years with their depths being about 110–140 m and 200– 250 m, respectively, whereas the isopycnal depths near the shelf break varied considerably. The isopycnals in 1997 were the deepest among the 4 years.
5. Discussion In the previous section we have shown that there was a thick mixed layer with near-freezing temperature under sea ice. The thick mixed layer
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Fig. 9. Scatter plots of depths of isopycnal surfaces at CTD stations versus the distance between the stations and the position of the 200 m isobath in (a) 1996 (b) 1997, (c) 1998, and (d) 1999. Isopycnal depths at the all CTD stations at which the bottom was deeper than 200 m are plotted. Crosses and circles denote 26.5 and 26.7sy surfaces, respectively. Solid lines indicate depths of the isopycnals interpolated with a Gaussian weight. Dotted lines in (a), (b), and (d) indicate the isopycnal depths in 1997.
was associated with deepening of isopycnals near the shelf break and the mixed layer was thickest in 1997 when the sea ice was thickest among the 4 years. In this section, we examine first whether or not the accumulation of mixed-layer water can be explained dynamically by winds. Then we discuss the relation of winds and the air temperature with the ice thickness. 5.1. Thick mixed layer Itoh and Ohshima (2000) compiled climatological data in the southwestern part of the Sea of Okhotsk and showed that depths of isohalines change seasonally (see their Fig. 6). The isohalines
are almost level from April to November, except that they shoal near the coast where SWCW is dominant. From December to February the isohalines deepen from the offshore toward the coast. Thus the deepening of isohalines and isopycnals occurs first and cold mixed-layer water appears later in this area. Itoh and Ohshima (2000) suggested that the deepening is associated with the convergence of the Ekman transport toward the coast by winds. This change in depth of the isopycnals propagates as coastally trapped waves and moves at a much higher speed than the water in the boundary current caused by the waves. So we examine the deepening of isopycnals due to the wave excited by winds.
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correspond to the 200 m isobath, for each year. We defined the sign of the wind stress so that positive values cause deepening of isopycnals at the shelf break. The wind stress was calculated from the wind speed, U10, at every 12 h by using a drag coefficient, CD, by Large and Pond (1981) in neutral stability,
Figs. 10a and b are the vector plots of the monthly-mean wind at 10 m in January calculated from the European Centre for Medium-Range Weather Forecasts (ECMWF) wind data for 1997 and that averaged for the other 3 years, respectively. In both figures northeasterly (northwesterly) winds prevailed in the northern (southern) part. The along-shore component of wind causes the Ekman transport directed toward the coast along the northern and western boundaries of the Sea of Okhotsk. The wind was more parallel to the coast in 1997 than that in the other years, especially near the southern end of Sakhalin, and in the northern part. Thus it is suggested that the Ekman transport and the corresponding deepening of isopycnals were large in 1997. Depths of the isopycnal surfaces at our observation sites are determined from the wind stress integrated along the isobaths from the site to the direction opposite to the propagation direction of the coastally trapped wave (e.g. Gill and Clarke, 1974). Table 1 shows the integration of the monthly-mean wind stress in January along the paths indicated by dashed lines in Fig. 10a, which
( CD ¼
for U10 o11 m s1 for U10 X11 m s1 :
60 -30
KAM
56
54
5 -1
-10
-10
54
-25
-10
-20 -15
NSH
-20
58
58
56
1:2 103 ð0:49 þ 0:065U10 Þ 103
We first neglected the change of the drag coefficient by the presence of sea ice for simplicity. The total integrated wind stress was positive every year due to the positive stress along southern Sakhalin (SSAK), northern Sakhalin (NSAK), and northern shelf (NSH). The integrated wind stress in 1997 was stronger than those in the other years, because the wind direction was more parallel to the coast in this year especially along SSAK and NSH (Fig. 10). We estimate the vertical displacement of the isopycnal surface associated with the wind stress in a two-layer model. For simplicity we assume that
-25
60
653
-25
-5
-5
52
52 NSAK 50
50
48
48
46
46
SSAK
-5
44
44 0
138 140 142 144 146 148 150 152 154 156
(a)
0
138 140 142 144 146 148 150 152 154 156
(b)
Fig. 10. Vector plots of the monthly mean wind at 10 m and contours of the temperature at 2 m in January for (a) 1997 and (b) those averaged for 1996, 1998, and 1999. A dashed line in (a), which is divided into four segments by dots, indicates the integration path used in Table 1.
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Table 1 The monthly mean wind stress in January integrated along the paths indicated by dashed lines in Fig. 10, which are the 200 m isobath in the southern Sakhalin (SSAK), northern Sakhalin (NSAK), northern shelf (NSH), and Kamchatka peninsula (KAM) R x h (m) r1=40 day Dz26.5 (m) h (m) Year t dx ð103 N m1 Þ
96 97 98 99 x, km
SSAK
NSAK
NSH
KAM
Total
10 15 7 5 368
101 73 69 61 924
47 168 88 129 1556
60 66 67 46 979
98 190 96 149 3826
59 90 53 69
94 180 72 97
r1=20 day
r1=N
Ice
52 69 44 52
62 121 61 95
93 137 87 100
At the bottom of the table x denotes the length of the integration path. The vertical displacement, h, of the isopycnal estimated from the stress with a two-layer model is listed in the 7th column. The displacement, Dz26.5, of the isopycnal surface sy=26.5 in Fig. 9 is also listed in the 8th column for comparison. The h calculated for r1=20 day and r1=N are listed in the 9th and 10th columns, respectively. The 11th column represents h calculated from the wind stress considering the change of the drag coefficient on sea ice.
the sea floor is flat. Then the vertical displacement, h, of the interface between the two layers is determined from the response of baroclinic Kelvin waves to the wind. After Gill and Clarke (1974), we have Z 0 x t ðx þ x0 ; t þ x0 =c1 Þ r=c1 x0 hðx; tÞ ¼ e dx0 ; ð1Þ r1 c21 N when the propagation speed, c1, of the baroclinic Kelvin wave is a constant. Here x is the curvilinear coordinate along the coast with its origin taken at the observation site, t the time, tx the alongshore component of the wind stress, and r a (linear) damping coefficient. c1 is expressed as sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi gðr2 r1 ÞH1 H2 ; ð2Þ c1 ¼ r2 ðH1 þ H2 Þ
Ohshima, 2000). Then the monthly mean wind stress in January, which is listed in Table 1, would be appropriate for the estimation of the isopycnal depth at the observation period, i.e. early February. We set r1=40 days, though there are some ambiguities for the value of r. This value corresponds to the damping coefficient for about 500 m depth in Chapman et al. (1986). Then waves coming from the North Pacific, which enter the Sea of Okhotsk turning around the southern end of Kamchatka peninsula, damp before they reach the observation area. With these assumptions, Eq. (1) reduces to a steady response of the twolayer model to the monthly-mean wind stress applied in the Sea of Okhotsk, Z 0 x 0 t ðx Þ r=c1 x0 hð0Þ ¼ e dx0 ; ð3Þ 2 x0 r1 c1
where H1 and H2 are the upper and lower layer thicknesses, respectively, r1 and r2 are the upper and lower layer densities, respectively, and g is the acceleration of gravity. Since the depth of the Sea of Okhotsk is typically 1000 m, except for the southernmost part, we set g=9.8 m s2, r1= 1026 kg m3, r2=1027 kg m3, H1=200 m, and H2=800 m. Then we have c1=1.24 m s1. Hence it takes 36 days for the baroclinic Kelvin wave to propagate from the southern end of Kamchatka peninsula to the observation area. We neglect the fluctuation of the wind whose time scale is shorter than 1 month, since deepening of isopycnals is observed in monthly-mean climatology (Itoh and
where x=x0 corresponds to the southern end of Kamchatka peninsula. The value of h calculated by Eq. (3) and the displacement of the 26.5sy isopycnal surface at the shelf break evaluated from Fig. 9 are listed in Table 1. Here the original position of the isopycnal is assumed to be the average isopycnal depths at all observation sites whose distance from the shelf break is 100 km or more among the 4 years. The simple two-layer model can reproduce deepening of isopycnals and its yearly variability, except that the values are smaller than the observational ones. To check the sensitivity of the model result to the damping coefficient, r, we also calculated h for r1=20 days
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and r1=N (Table 1). Values of h show the similar tendency from year to year for these values of r. We also estimated the change of the wind stress due to sea ice, taking the weighted mean of the stress on open water and sea ice with respect to their areas (R^ed and O’Brien, 1983) and neglecting the internal stress of sea ice for simplicity. We used daily ice concentration obtained by the SSM/ I and set the drag coefficient on sea ice to 2.5 103 after McPhee (1990). Values of h with the larger drag coefficient on sea ice and r1=40 days become closer to the observational ones (Table 1). Although ice concentration was different from year to year, yearly variability of h is similar in the cases with and without the sea ice effect. Therefore, we can conclude that the Ekman transport by northerlies accounts for the deepening of isopycnals and, accordingly, the thickening of the mixed layer in the observation area. We also calculated h for other months using the wind stress in a previous month with the sea-ice effect and r1=40 days. The value of h averaged in the 4 years increases from 34 m in December to 74 m and 105 m in January and February, respectively, and decreases to 72 m in March, 41 m in April, and 15 m in May. Thus in spring the cold mixed-layer water, which probably becomes DTW, spreads offshore at depths around 100 m. The internal Kelvin wave also changes the seasurface height, Z. Since Z={(r2r1)H2}/{r2(H1+ H2)}h B 0.8 103h, the seasonal variability of Z resembles that of the anomalous sea level observed in the Sea of Okhotsk and the North Pacific along the Hokkaido coast (Matsuyama et al., 1999). 5.2. Yearly variability of ice thickness By using satellite and hydrographic data, Gladyshev et al. (2000) pointed out that both ice production and dense water formation associated with brine rejection were less in 1997 than those in 1996. Their analysis showed that relatively high temperature in 1997 in the major polynyas, which are the sites of ice production, weakened the ice production. In fact the monthly-mean ECMWF temperature at 2 m in January 1997 was higher than that in the other years in the Sea of Okhotsk (Fig. 10). As candidates of major sources of sea
655
ice in the observation area, we consider three polynyas in the western half of the Sea of Okhotsk (Fig. 1), that is, the Northwest Shelf polynya (NW), Sakhalin Shelf polynya (SAK), and Terpenia Bay polynya (TER). Table 2 shows the monthly-mean temperature and the offshore component of the monthly-mean geostrophic wind in January, where these quantities are averaged along the dashed lines in Fig. 1. Temperatures in 1997 were the highest in the 4 years from 1996 to 1999. The offshore component of the wind also affects the ice production, because it drifts sea ice offshore and enhances new ice production in coastal polynyas (Martin et al., 1998). It is unlikely that ice drifted offshore more intensely in 1997, in which the wind direction was rather parallel to the coast (Fig. 10). The geostrophic wind is a better indicator of ice motion than the 10 m wind, because ice motion is directed to the right of the 10 m wind and roughly parallel to geostrophic wind (Thorndike and Colony, 1982). The offshore geostrophic wind was not strong but rather weak in 1997 compared with that in the other years (the right part of Table 2). Thus, although we observed thick ice in 1997, both the temperature and geostrophic wind indicate that ice production was less in 1997 than that in the other years. In fact the offshore ice extent near the western boundary of the Sea of Okhotsk was smallest in 1997 among the 4 years (Fig. 2). The thickening of sea ice is not only due to the thermodynamic processes but also due to Table 2 The monthly mean temperature, T2, in January from the ECMWF data averaged along the four lines in Fig. 1, which are the representative of the Northwest Shelf polynya (NW), the Sakhalin Shelf polynya (SAK), the Terpenia Bay polynya (TER), and the path of sea ice along the southern part of Sakhalin (SSA) Vg (m s1)
Year T2 ( C) NW 96 97 98 99
SAK
TER
SSA
NW SAK TER SSA
21.3 11.5 11.3 9.4 9.5 13.0 8.4 7.9 6.0 5.2 23.9 19.4 14.3 10.7 7.2 16.8 10.3 9.6 8.3 3.4
0.9 10.3 1.2 9.3 0.5 7.0 0.9 9.4
0.1 2.5 0.9 0.3
The mean geostrophic wind, Vg, normal to the lines is also listed, where positive values indicate the wind directed offshore.
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dynamical processes such as rafting and ridging. Toyota (1998) showed that first-year ice and young ice collected in our observation area in 1997 consisted of multiple layers of columnar structure and granular structure sandwiched each other. This indicates that several ice floes are piled on each other to produce the collected ice. Fukamachi et al. (2003) showed that deformed ice dominates in the total ice mass in our observation area based on data from a moored sonar. Thus rafting and ridging are important for the thickening of sea ice in the observation area. The wind direction in 1997 was more parallel to the coast and more favorable for rafting and ridging of sea ice converged toward the coast than the other years. Note that the offshore geostrophic wind is smallest in this year along SAK and SSA which is the path of sea ice along the southern part of Sakhalin (Table 2). Thus it is suggested this wind direction was important for the thickening of sea ice as well as the mixed layer.
year. A two-layer model in a flat ocean suggests that this wind stress is responsible for the deepening of the isopycnal surface and its yearly variability. Therefore, we can conclude that the thick mixed layer was formed by the Ekman transport by northerlies. Although the air temperature and the offshore component of the geostrophic wind indicate that the production rate of sea ice was low in 1997 compared with that in the other years, relatively thick ice was observed in this year in the observation area. It is most likely that rafting and ridging of sea ice were responsible to this thickening of sea ice. In this year the wind direction, which was responsible for the thickening of the mixed layer, was also favorable for rafting and ridging at the coast. Thus the change of the wind direction is proposed as a factor in determining the thicknesses of both the mixed layer and sea ice in the southwestern part of the Sea of Okhotsk.
Acknowledgements 6. Summary We conducted hydrographic observations in the southwestern part of the Sea of Okhotsk in early February from 1996 to 1999. A thick mixed layer with near-freezing temperature, which extended to a depth of 150–300 m, was observed along the shelf break under sea ice. The thickness of the mixed layer was largest in 1997 when the ice condition was the heaviest among the 4 years. Although the mixed layer extended to a depth deeper than that of DTW found from spring to fall, the density of mixed-layer water was equal to or less than that of DTW. Thus the thick mixed layer was accompanied with deepening of isopycnals. The yearly variability of the isopycnal depth coincided with the variability of the mixed-layer thickness. Nearly northerly winds in the Sea of Okhotsk in winter induce the convergence of the Ekman transport toward the northern and western boundaries of the Sea of Okhotsk and, accordingly, force deepening of isopycnals there. In 1997 the winds, which are usually directed offshore, were more parallel to the coast and the integral of the wind stress along the isobath was larger this
We are deeply indebted to Koukichi Suehiro, Kazuaki Kubo, Yasushi Nabae, Koji Iwamoto, and others at Hydrographic and Oceanographic Department, Japan Coast Guard for their cooperation. We are also grateful to the captain and crew of P/V Soya of Japan Coast Guard for their sincere help in the observation. The SSM/I data were provided by the National Snow and Ice Data Center (NSIDC), University of Colorado. This work was sponsored by the Core Research for Evolutional Science and Technology, Japan Science and Technology Corporation, the International Cooperative Research Programme on Global Ocean Observing System, the Grantin-Aid for Scientific Research on Priority Areas (Nos. 08241201 and 09227201), and a fund from Hokkaido Foundation for the Promotion of Scientific and Industrial Technology.
References Alfultis, M.A., Martin, S., 1987. Satellite passive microwave studies of the Sea of Okhotsk ice cover and its relation to
ARTICLE IN PRESS G. Mizuta et al. / Continental Shelf Research 24 (2004) 643–657 oceanic processes, 1978–1982. Journal of Geophysical Research 92, 13013–13028. Aota, M., 1970. Study of the variation of oceanographic condition north-east off Hokkaido in the Sea of Okhotsk II (in Japanese with English abstract). Low Temperature Science A 28, 261–279. Aota, M., 1984. Oceanographic structure of the Soya warm current (in Japanese). Bulletin of the Coastal Oceanography 22, 30–39. Aota, M., Kawamura, T., 1978. Observation of oceanographic condition in the Okhotsk Sea coast of Hokkaido in winter (in Japanese with English abstract). Low Temperature Science A 37, 93–105. Chapman, D.C., Barth, J.A., Beardsley, R.C., 1986. On the continuity of mean flow between the Scotian Shelf and the Middle Atlantic Bight. Journal of Physical Oceanography 16, 758–772. Fukamachi, Y., Mizuta, G., Ohshima, K.I., Melling, H., Fissel, D., Wakatsuchi, M., 2003. Variability of sea-ice draft off Hokkaido in the Sea of Okhotsk revealed by a moored iceprofiling sonar in winter of 1999. Geophysical Research Letters 30 10.1029/2002GL016197. 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 (in Japanese with English abstract). Low Temperature Science 3, 143–157. Gill, A.E., Clarke, A.J., 1974. Wind-induced upwelling, coastal currents, and sea-level changes. Deep-Sea Research 21, 325–345. Gladyshev, S., Martin, S., Riser, S., Figukin, A., 2000. Dense water production on the northern Okhotsk shelves: comparison of ship-based spring–summer observations for 1996 and 1997 with satellite observations. Journal of Geophysical Research 105, 26281–26299. Itoh, M., Ohshima, K.I., 2000. Seasonal variations of water masses and sea level in the southwestern part of the Okhotsk Sea. Journal of Oceanography 56, 643–654. Kimura, N., Wakatsuchi, M., 2000. Relationship between seaice motion and geostrophic wind in the Northern Hemisphere. Geophysical Research Letters 27, 3735–3738. Kitani, K., 1973. An oceanographic study of the Okhotsk Sea: particularly in regard to cold waters. Bulletin of Far Sea Fishery Research Laboratory 9, 45–77. Large, W.G., Pond, S., 1981. Open ocean momentum flux measurements in moderate to strong winds. Journal of Physical Oceanography 11, 324–336. Martin, S., Drucker, R., Yamashita, K., 1998. The production of ice and dense shelf water in the Okhotsk Sea polynyas. Journal of Geophysical Research 103, 27771–27782. Matsuyama, M., Aota, M., Ogasawara, I., Matsuyama, S., 1999. Seasonal Variation of Soya Current (in Japanese with English abstract). Oceanography in Japan 8, 333–338.
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McPhee, M.G., 1990. Small-scale processes. In: Smith, W.O. (Ed.), Polar Oceanography, Part A. Academic Press, New York, pp. 287–334. Mizuta, G., Fukamachi, Y., Ohshima, K.I., Wakatsuchi, M., 2003. Structure and seasonal variability of the East Sakhalin Current. Journal of Physical Oceanography 33, 2430–2445. Ohshima, K.I., Mizuta, G., Itoh, M., Fukamachi, Y., Watanabe, T., Nabae, Y., Suehiro, O., Wakatsuchi, M., 2001. Winter oceanographic conditions in the southwestern part of the Okhotsk Sea and their relation to sea ice. Journal of Oceanography 57, 451–460. Ohshima, K.I., Wakatsuchi, M., Fukamachi, Y., Mizuta, G., 2002. Near-surface circulation and tidal currents of the Okhotsk Sea observed with the satellite-tracked drifters. Journal of Geophysical Research 107 10.1029/ 2001JC001005. Parkinson, C.L., Gratz, A.J., 1983. On the seasonal sea ice cover of the Sea of Okhotsk. Journal of Geophysical Research 88, 2793–2802. R^ed, L.P., O’Brien, J.J., 1983. A coupled ice-ocean model of upwelling in the marginal ice zone. Journal of Geophysical Research 88, 2863–2872. Takizawa, T., 1982. Characteristics of the Soya Warm Current in the Okhotsk Sea. Journal of Oceanographic Society of Japan 38, 281–292. Thorndike, A.S., Colony, R., 1982. Sea ice motion in response to geostrophic winds. Journal of Geophysical Research 87, 5845–5852. Toyota, T., 1998. A study on growth processes of sea ice in the southern region of the Okhotsk Sea, evaluated from heat budget and sea ice sample analysis. Doctoral Dissertation, Hokkaido University, 143pp. Toyota, T., Wakatsuchi, M., 2001. Characteristics of the surface heat budget during the ice growth season in the southern Sea of Okhotsk. Annals of Glaciology 33, 230–236. Toyota, T., Ukita, J., Ohshima, K.I., Wakatsuchi, M., Muramoto, K., 1999. A measurement of sea ice albedo over the southwestern Okhotsk Sea. Journal of Meteorological Society of Japan 77, 117–133. Ukita, J., Kawamura, T., Tanaka, N., Toyota, T., Wakatsuchi, M., 2000. Physical and stable isotopic properties and growth processes of sea ice collected in the Southern Sea of Okhotsk. Journal of Geophysical Research 105, 22083–22093. World Meteorological Organization, 1970. WMO sea-ice nomenclature. Terminology, codes and illustrated glossary (WMO/OMM/BMO 259, TP 145). Secretarait of World Meteorological Organization, Geneva, Switzerland, 147pp. Watanabe, K., 1963. 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). Oceanographical Magazine 14, 117–130.
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Winter mixed layer and its yearly variability under sea ice in the southwestern part of the Sea of Okhotsk Genta Mizutaa,*, Kay I. Ohshimab, Yasushi Fukamachib, Motoyo Itoha,1, Masaaki Wakatsuchib,c a
Graduate School of Environmental Earth Science, Hokkaido University, Sapporo, Japan b Institute of Low Temperature Science, Hokkaido University, Sapporo, Japan c Japan Science and Technology Corporation, Kawaguchi, Japan
Received 9 December 2002; received in revised form 7 January 2004; accepted 23 January 2004
Abstract Hydrographic observations under sea ice were conducted in the southwestern part of the Sea of Okhotsk for 4 years from 1996 to 1999. Every year a cold mixed layer with near-freezing temperature was distributed from the sea surface to a depth of 150–300 m near the shelf break under sea ice. The thicknesses of the mixed layer and sea ice were largest in 1997. While the depth of the mixed layer was considerably deeper than that of dichothermal water, which is identified as a temperature minimum from spring to fall in this region, the density of water in the mixed layer was equal to or less than that of dichothermal water. It is shown that deepening of isopycnals due to the alongshore component of the wind stress is essential for thickening of the mixed layer. In 1997 the nearly northerly winds, which are usually directed offshore, were more parallel to the coast than those in the other years. Thickening of the mixed layer in 1997 is attributed to this wind condition. The air temperature and wind indicate that the sea-ice production rate was low in 1997, whereas the wind direction was favorable for rafting and ridging. Thus the change in wind direction is proposed as an important factor in determining the thickness of both the mixed layer and the sea ice in this region. r 2004 Elsevier Ltd. All rights reserved. Keywords: Sea ice; Water masses; The Sea of Okhotsk (a marginal sea in the Northwestern Pacific); Hydrographic data
1. Introduction The Sea of Okhotsk is a marginal sea in the Northwestern Pacific and a seasonal ice zone, which is connected with the Japan Sea and the *Corresponding author. Graduate School of Environmental Earth Science, Hokkaido University, Kita 10, Nishi 5, Sapporo 060-0810, Japan. E-mail address: [email protected] (G. Mizuta). 1 Present address: Japan Marine Science and Technology Center, Yokosuka, Japan.
North Pacific with narrow straits (Fig. 1). Two major water masses exist in the southwestern part of the Sea of Okhotsk, with their distribution being markedly different with season. The Soya Warm Current Water (SWCW), which is warm and saline water originating from the Japan Sea through the Soya Strait, is dominant in summer. The Soya Warm Current weakens in winter and appears intermittently as a bottom trapped current (Aota, 1984). From November to December East Sakhalin Current Water (ESCW), which is less
0278-4343/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.csr.2004.01.006
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644
60 58
Kam cha tka
NW
The Sea of Okhotsk
54 52
SAK
Amur River
200
50
00 10
Sakhalin
LATITUDE
56
TER
48
2000 3000
SSA
46 44
Soya St.
The Japan Sea
136
140
30
00
North Pacific
Hokkaido
144
148
152
156
160
LONGITUDE Fig. 1. Bathymetry of the Sea of Okhotsk. A rectangle frame indicates our observation area. Dashed lines denote the path for averaging the temperature and geostrophic wind used in Table 2.
saline water affected by Amur River discharge, extends southward along Sakhalin Island. The ESCW forms a less saline layer from the surface to a depth around 50 m accompanied with a strong halocline underneath it (Watanabe, 1963). Fukutomi (1950) pointed out that a strong, shallow halocline due to ESCW tends to reinforce the production of sea ice by suppressing the vertical convection in winter. In fact water with near-freezing temperature, which is thought to be a remnant of winter convection, is found at a depth of 50–100 m from spring to fall in this region. This water is called dichothermal water (DTW). On the other hand, satellite images suggest that sea ice is drifted southward by northerlies and a southward current called the East Sakhalin Current (ESC) near the coast of Sakhalin Island (Parkinson and Gratz, 1983; Kimura and Wakatsuchi, 2000). Watanabe (1963) hypothesized that the thick sea ice found in the southwestern part of the Sea of Okhotsk originates from the northern part. By using heatflux calculation, Alfultis and Martin (1987) and
Martin et al. (1998) showed that a considerable amount of sea ice is produced in the northern part of the Sea of Okhotsk because of severe atmospheric condition there. A recent study based on direct current measurements shows that the ESC is most intense in winter (Mizuta et al., 2003). Surface-drifter measurements show that a part of the ESC reaches the southwestern end of the Sea of Okhotsk in October–January (Ohshima et al., 2002). Thus it is suggested that the ESC and ESCW are important to both the production and advection of sea ice observed in the southwestern part of the Sea of Okhotsk. However, there are no measurements of the current speed under sea ice in this region except in the shallow shelf region (Aota and Kawamura, 1978; Matsuyama et al., 1999). Hydrographic data are quite limited because of the presence of sea ice. Thus it is still not clear what kind of water mass really exists under sea ice in this region. Recently, Ohshima et al. (2001) conducted hydrographic observations in February 1997 in the southwestern part of the Sea of Okhotsk. They found that a mixed layer, in which the temperature is nearly at the freezing point, extends from the surface to a depth of 300 m under sea ice. The lower boundary of the mixed layer is much deeper than that of DTW or the less saline layer associated with ESCW. Because of its low temperature and large vertical extent, water in the mixed layer seems to originate from the northern part of the Sea of Okhotsk. As a possible candidate of the mechanism that maintains the large thickness of the mixed layer, they proposed a downwelling due to the Ekmantransport convergence at the east coast of Sakhalin forced by northerly winds. However, it is not clear whether the Ekman downwelling can account for the thickening of the mixed layer quantitatively. The yearly variability of the mixed-layer properties and their relation to sea ice are also still unknown. In order to answer these questions we analyze the hydrographic data obtained in the southwestern part of the Sea of Okhotsk in every February from 1996 to 1999. The purpose of this study is to investigate the yearly variability of the mixed-layer properties and to clarify the mechanism that maintains the thick mixed layer. Then we
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discuss the relation between the yearly variability of the thick mixed layer and that of sea ice. In the next section, we describe the data used in this study. After describing the ice condition in the observation area in Section 3, we examine the properties of the cold mixed layer and their relation to sea ice in Section 4. The yearly variability of the mixed layer and sea ice are compared with that of the atmospheric condition in Section 5. All these results are summarized in Section 6.
2. Data Hydrographic observation was conducted in early February from 1996 to 1999 by the icebreaker Soya of Japan Coast Guard, under cooperation of the Hydrographic and Oceanographic Department of the Japan Coast Guard and Hokkaido University. The observation period was 1–7 February 1996, 1–10 February 1997, 3–12 February 1998, and 3–12 February 1999. The observation area is indicated by a rectangle frame in Fig. 1. The temperature and salinity were measured by a CTD (SeaBird SBE 19) and XBTs (Tsurumi Seiki) from the surface to a depth of 800 m in maximum. Water samples were collected with Niskin bottles. The CTD salinities were calibrated against water samples. Except for 1996, post-cruise calibration of the CTD sensors was also performed. The dissolved-oxygen content was measured with the Winkler method. Sea-ice condition and thickness and meteorological parameters such as the temperature, relative humidity, wind, shortwave radiation, and fractional cloud cover were also measured on the icebreaker. Detailed results of those measurements are presented by Toyota et al. (1999) and Ukita et al. (2000).
3. Sea-ice condition Fig. 2 shows the ice concentration on 31 January observed by the Defense Meteorological Satellite Program (DMSP) Special Sensor Microwave Imager (SSM/I) in the 4 years. Sea ice was
Fig. 2. Distribution of ice concentration on 31 January observed by the SSM/I in (a) 1996, (b) 1997, (c) 1998, and (d) 1999. Crosses, open circles, solid circles indicate ice concentration 20–50%, 50–80%, and 80–100%, respectively.
distributed from the northern part to the southern part of the Sea of Okhotsk along the western boundary. The observation area was covered with sea ice 3, 7, 7, and 3 days before the start of the observation in the 4 years, respectively. Fig. 3 shows the sea-ice distribution during the observation period. This figure was redrawn from ice charts published by the Hydrographic Department of the Japan Coast Guard and the in-situ visual observations to supplement region without data in the ice chart. Since the ice condition changes day by day, several ice charts were combined so that the ice condition at each observation site reflects that at the observation time. Most of the observation area was covered with sea ice every year except for open waters in the western part near the
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F
C
F Br, Y
Ni
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,P
,G
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LATITUDE
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,C s
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1997
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Feb 11 O
LATITUDE
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C F Gr, P
C
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C Cs, Br
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New
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C New
44 1998
142
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C
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(c)
O
Y
LATITUDE
O
F Y, Gr
1999
143 144 LONGITUDE
145
146
142
(d)
143 144 LONGITUDE
145
146
Fig. 3. Distribution of sea ice in (a) 1996, (b) 1997, (c) 1998, and (d) 1999. Shading indicates ice covered area. The darker shade indicates the more developed ice. The sign ‘F’ denotes ice floe, ‘C’ denotes ice cake, ‘Y’ denotes young ice, ‘P’ denotes pancake ice, ‘Br’ denotes brash ice, and ‘New’ denotes new ice, which consists of grease ice (‘Gr’), and nilas (‘Ni’). The definition of these ices are based on that of the World Meteorological Organization (WMO) (1970). The sign ‘O’ denotes open water. In 1998, the ice distribution near the Soya Strait is for 11 February. Whereas in the other region observation date changes continuously from 4 to 10 February from the western to eastern part, respectively. Solid and open circles denote CTD and XBT stations, respectively.
Soya Strait, in the offshore part northeastward, and near the coast. Several types of sea ice were distributed in the observation area. The iceproduction rate based on the heat flux calculation is about 1 cm day1 in open water in this area (Toyota and Wakatsuchi, 2001; Ohshima et al., 2001). Thus it is most likely that ice floe, which is defined as the sea ice whose diameter is larger than 200 m, and probably ice cake, which is defined as the sea ice whose diameter is between 20 and 200 m, originate from the northern part of the Sea of Okhotsk. In 1997 most of sea ice observed in this area consisted of ice floe, whereas new ice, pancake ice, and ice cake occupied more than half of the area in the other years. In the northernmost part of the observation area the ice thickness in 1997 was more than 1 m, which is the largest among the 4 years. The mean sea-ice thickness
monitored by the video was 18.5, 54.9, 30.1, and 29.0 cm in 1996, 1997, 1998, and 1999, respectively (Toyota and Wakatsuchi, 2001). Thus sea ice was the thickest in 1997.
4. Results 4.1. General features of the temperature and salinity field Fig. 4 shows the distribution of surface temperature. The temperature was nearly at the freezing point in most part of the ice covered area every year (see Fig. 3). Relatively warm water was found every year near the Soya Strait. This water corresponds to SWCW coming from the Japan Sea. The temperature was higher than 1.5 C in
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.5 -1-1.6
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1996
1997
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-1
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.7
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-1
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LATITUDE
-1
.5 6 -1 -1.
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.5
-1
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-1.5
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7
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5
-1.7
44
44 1999
1998
142
(c)
143 144 LONGITUDE
145
146
142
(d)
143 144 LONGITUDE
145
146
Fig. 4. Contours of the surface temperature in (a) 1996, (b) 1997, (c) 1998, and (d) 1999. Shaded region indicates temperature less than 1.5 C. Dashed lines indicate the sections shown in Figs. 5 and 6.
the northeastern area, which was largest in 1998 and absent in 1999, and near the coast in 1997 and 1999. These distributions of water warmer than the freezing point generally coincided with that of open water. Figs. 5 and 6 show the distributions of temperature and salinity at the vertical crosssections along the dashed lines in Fig. 4, respectively. The temperature was vertically uniform near the surface to form a cold mixed layer under the sea ice (Fig. 5). Cold and warm waters were interleaved with each other at the offshore side of the cold mixed layer, for example, around stations 9608, 9710, and 9810. We define the depth, hT, of the mixed layer as the maximum depth at which the difference of the temperature from that at the surface does not exceed 0.2 C. Note that even if there is an intrusion of warm water at the mid depth of the cold mixed layer, the depth of the mixed layer can be deeper than the intrusion depth. We adopted this definition as a measure of
the thickness of the mixed layer unaffected by the intrusion. The mixed-layer thickness attained a maximum at the shelf break every year (dashed line in Fig. 5). The maximum thickness was largest in 1997 with the mixed layer extending to a depth of 300 m or more and reaching the bottom, whereas the maximum thickness was 150–200 m in the other years. The 1997 corresponds to the year when sea ice was the thickest. These depths of the mixed layer were larger than that of DTW, which is found from spring to fall at a depth of 50–100 m, or that of ESCW in late fall, which is found at a depth around 50 m. The salinity was rather uniform in the vertical direction in the thick mixed layer (Fig. 6). The salinity at the deepest part of the mixed layer was 32.8, 32.9, 32.8, and 32.7 in 1996, 1997, 1998, and 1999, respectively. Hence most saline water was found in 1997. At depths greater than 100 m, the isohalines deepened toward the coast. Since the density is almost uniquely determined from the
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648 9601 9603
9606
9607
9608
9609
0
9733
9701 0
9703
9710
9711
0
1
0
-1.5
DEPTH (m)
200
300
1
1
0.5
-1
0 -0.5
1
-1
1
-1.5
0.5
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300
9720
0 -0.5
100
0
0.5
DEPTH (m)
9709
-1
-1.5 2
100
9704
-1 -1.5
2
0.5 0. 5
0
400
400 1.5
50
(a)
100
150
200
0
9825
9824
100
(b)
DISTANCE (km) 0
1997
9810
9811
9822
0
9901
0.5
0
-1.5
2 1
9904 0-0.5
9907
1 -0.5
100
400
200
-0.5
-0.5 0 0
300
1.5
1
1.5
500 100
200
300
DISTANCE (km)
1999 0
(d)
2
1998 0
(c)
-1.5 -1 0 1
400
500
9913
5 0.
DEPTH (m)
0.5
1
300
9909 9910 9911 9912
-1 -0.5
0.5
1.5
DEPTH (m)
-1
100 0
200
300
-1
1.5
3
1
9804 9807 9808 -1.5
200
DISTANCE (km)
-0. 5
0
500
0
1996
500
50
100
150
200
250
DISTANCE (km)
Fig. 5. Contours of the temperature at the vertical cross-sections along the dashed lines in Fig. 4 in (a) 1996, (b) 1997, (c) 1998, and (d) 1999. Shading indicates the temperature less than 1.5 C. Bars at the top of each panel denote ice covered area. Ticks with and without numbers indicate CTD and XBT stations, respectively. Dashed lines indicate the lower boundary of the mixed layer.
salinity for the low temperature range, the inclination of the isohalines indicates a southward geostrophic flow in the upper layer, suggesting that mixed-layer water was advected from the north. Geostrophic velocity referenced to 500 db or the bottom was typically 10 cm s1 to the south on the slope. The dissolved-oxygen content was also uniform in the vertical direction within the mixed layer (not shown). The oxygen content in the mixed layer was slightly less than saturation, suggesting the presence of intense cooling or entrainment of deep water. Other water masses were also identified in these sections. Saline water existed around the Soya Strait near the bottom. Since the temperature was also high there (Fig. 5), this water is SWCW. The SWCW extended largely in 1999 and intruded
below cold mixed-layer water at the shelf break. On the other hand, less saline water existed on the shelf near the surface. Because of its low salinity this water seems to be a remnant of ESCW in late fall, which is defined as the water with salinity less than 32. Less saline water was also observed near the surface in the offshore area where the temperature was higher than the freezing point. The less saline feature was especially clear at the ice edge in 1997. This feature seems to indicate melting of sea ice (Ohshima et al., 2001). Such features were also observed in other years, though they were not so clear. The relations between the potential temperature and salinity obtained at all the CTD stations and regions of water masses defined by Aota (1970) and Takizawa (1982) are displayed in Fig. 7 for
ARTICLE IN PRESS
9607 32.7
0
9733
9701 33 32.4
9703
33 33 .1 33 33.2 .3
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9709
33.4
9710
9711
9720
32.7 32.5
33.1 33.2
200
33.3 33.4
300
33.
5
300
9704
.3 .4 32 32
32.9
32.9
200
DEPTH (m)
9609
32.8
1 .3 2.5 32. 32 3
100
9608
8 32.
32.9 33
9
31.
9606
649
32 .5 32 .6 32.7
9601 9603
DEPTH (m)
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32.7
G. Mizuta et al. / Continental Shelf Research 24 (2004) 643–657
500
1996 0
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9824
32.8
33 .1
9825
32.5
9810
9811
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9822
32. 33
33.8 33.4
9904 9907 33 32.7 32.4
32.8
33
300
.2 .3
33
33
DEPTH (m)
200
33
.6
33.4 33.5
33.7
500
1998 0
(c)
100
200
9913
.1
.5
9909 9910 9911 9912
3 32. 32.5 32.7 33.1
400
400
500
300
33
.4
33
300
9901
100
33
200
0
8
200
DISTANCE (km)
32.6
32.6
.3 33.2
100
9804 9807 9808 32.2
.1 32 32 32.2 .4 32.6
100
(b)
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0
1997 0
33
500
DEPTH (m)
400
33.6
32. 5
400
0
300
(d)
DISTANCE (km)
1999 50
100
150
200
250
DISTANCE (km)
Fig. 6. Similar to Fig. 5 except for the salinity.
each year. Most water in the observation area was more saline than ESCW except for the least saline water found in the observation period. This water corresponds to the less saline water on the shelf in Fig. 6. Warm and saline water observed at the stations closest to the Soya Strait (9601, 9733, 9825, and 9901) is classified as SWCW. Some curves extend almost linearly from the cold and less saline range to the warm and saline range in 1997 and 1999. These curves clearly show that ESCW and SWCW are mixed diapycnally over the shelf. This mixed water was also observed in other years. The curves near the freezing temperature correspond to the cold mixed layer and less saline layer near the surface. The maximum salinity of cold mixed-layer water was 32.9. Although the thickness of the mixed layer was 150–300 m in maximum, the salinity and, accordingly, the
density of mixed-layer water was similar to that of DTW observed at shallower depths from spring to fall. Aota (1970) showed that even more saline water, which is around 33.2 and colder than 1 C, exists in the dichothermal layer in summer. Thus the density of cold mixed-layer water was higher than that of ESCW and equal to or lower than that of DTW. There are spiky features at the portion of curves in the density range less than 26.7. These spiky features indicate that water in and below the mixed layer were mixed isopycnally with relatively warm water, which is the mixture of SWCW and ESCW. The mixing is associated with the interleaving shown in Fig. 5. Cold mixed-layer water lost its original characteristics due to the mixing. The curves in the density range greater than 26.7 asymptote to a single curve. Kitani (1973) showed that cold water with a density of 26.8–27.0sy is formed in the northern part of the Sea of Okhotsk
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650 5
5
4
4
9733
SWCW 26.0
25.5
26.5
25.5
9601
ESCW 2 27.5
1
Potential Temperature
3 Potential Temperature
SWCW
27.0
0
26.0
26.5
ESCW 2 27.5
1
0
-1
DTW
-1
DTW
1996 -2 31.5
32.0
32.5
(a)
33.0 Salinity
33.5
34.0
1997 34.5
(b)
5
32.0
32.5
33.0 Salinity
33.5
4
SWCW 26.0
26.5
3
25.5
9825
ESCW 27.5
1
0
34.5
26.0
26.5
3
2
ESCW 27.5
1
-1
DTW
DTW
1998 -2 31.5
27.0 9901
0
-1
(c)
34.0
SWCW
27.0
Potential Temperature
25.5
Potential Temperature
-2 31.5 5
4
2
27.0
3
32.0
32.5
33.0 Salinity
33.5
34.0
1999 -2 31.5
34.5
(d)
32.0
32.5
33.0 Salinity
33.5
34.0
34.5
Fig. 7. Plots of the potential temperature, y, versus salinity, S, at all the CTD stations in (a) 1996, (b) 1997, (c) 1998, and (d) 1999. Solid and dashed curves indicate the y–S relation in the region where the bottom depth is larger and smaller than 200 m, respectively. The solid curve near the bottom indicates the freezing point. Rectangular regions denote SWCW, ESCW, and DTW after the definition by Aota (1970) and Takizawa (1982). Station numbers are displayed for some curves near the Soya Strait.
in winter. However, cold water in this density range was not observed in our stations. 4.2. Spatial and temporal variability of the cold thick mixed layer The horizontal distribution of the mixed-layer thickness defined in the previous subsection is displayed in Fig. 8. The mixed-layer thickness was
largest in 1997 not only along the section shown in Fig. 5 but also in the whole observation area. The thick mixed layer was distributed along the slope just offshore of the shelf break (dashed line in Fig. 8). The thickness was largest or attained a local maximum every year at almost the same site around (144 00 E, 45 100 N). The ice condition at this site changed from year to year. In 1997, this site was covered with the heaviest ice observed in
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46 10 0
20
20 300 0
0
15
LATITUDE
10
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100 100
0
0
44 1996
142
(a)
1997
143
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143
(b)
LONGITUDE 46
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LATITUDE
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44
50
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45
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0
20
44
142
10
45
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0
50
150 200
100
44
1998
(c)
20 10
50
45 50
LATITUDE
0
50
45
10
1999
143
144
145
146
LONGITUDE
142
143
(d)
144
145
146
LONGITUDE
Fig. 8. Contours of the mixed-layer thickness in (a) 1996, (b) 1997, (c) 1998, and (d) 1999. Dashed lines denote the isobath of 200 m, which roughly corresponds to the shelf break.
this year (Fig. 3). In 1998 this site was located near the ice edge and the ice condition was rather light there. Ice cake and young ice were observed there in 1996 and 1999, respectively. Thus bottom topography controlled the precise location of the thick mixed layer more strongly than sea ice, though the overall distribution of cold water coincided with ice cover. The vertical uniformity of the thick mixed layer weakened in the southeastern part of the observation area because of the intrusion of warm water from the offshore such as shown in Fig. 5 (see also Fig. 3 in Ohshima et al., 2001). Fig. 9 shows the scatter plot of isopycnal depths at CTD stations versus the horizontal distances between the stations and the 200 m isobath. Crosses and circles denote the depths of 26.5 and 26.7sy surfaces, respectively. These densities correspond to those of water with salinity about 32.9 and 33.2 at the freezing temperature, respectively. The 26.5sy surface, which was nearly coincident with or slightly deeper than the lower
boundary of the mixed layer, tended to deepen near the shelf break every year. A similar feature was also found for 26.7sy surface, which was deeper than the lower boundary of the mixed layer. Solid lines in Fig. 9 indicate depths of the isopycnals interpolated with a Gaussian weight, where we set the e-folding scale of the Gaussian to 30 km to smooth out small scale fluctuations. On the deeper stations, the depths of 26.5 and 26.7sy surfaces were not so different among the 4 years with their depths being about 110–140 m and 200– 250 m, respectively, whereas the isopycnal depths near the shelf break varied considerably. The isopycnals in 1997 were the deepest among the 4 years.
5. Discussion In the previous section we have shown that there was a thick mixed layer with near-freezing temperature under sea ice. The thick mixed layer
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0
100
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DEPTH (m)
DEPTH (m)
652
200
300
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1997
1996 500
500 0
50
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50
(b)
DISTANCE (km)
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DISTANCE (km)
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DEPTH (m)
(a)
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200
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1999
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500 0
(c)
50
100
150
200
250
DISTANCE (km)
0
(d)
50
100
150
200
250
DISTANCE (km)
Fig. 9. Scatter plots of depths of isopycnal surfaces at CTD stations versus the distance between the stations and the position of the 200 m isobath in (a) 1996 (b) 1997, (c) 1998, and (d) 1999. Isopycnal depths at the all CTD stations at which the bottom was deeper than 200 m are plotted. Crosses and circles denote 26.5 and 26.7sy surfaces, respectively. Solid lines indicate depths of the isopycnals interpolated with a Gaussian weight. Dotted lines in (a), (b), and (d) indicate the isopycnal depths in 1997.
was associated with deepening of isopycnals near the shelf break and the mixed layer was thickest in 1997 when the sea ice was thickest among the 4 years. In this section, we examine first whether or not the accumulation of mixed-layer water can be explained dynamically by winds. Then we discuss the relation of winds and the air temperature with the ice thickness. 5.1. Thick mixed layer Itoh and Ohshima (2000) compiled climatological data in the southwestern part of the Sea of Okhotsk and showed that depths of isohalines change seasonally (see their Fig. 6). The isohalines
are almost level from April to November, except that they shoal near the coast where SWCW is dominant. From December to February the isohalines deepen from the offshore toward the coast. Thus the deepening of isohalines and isopycnals occurs first and cold mixed-layer water appears later in this area. Itoh and Ohshima (2000) suggested that the deepening is associated with the convergence of the Ekman transport toward the coast by winds. This change in depth of the isopycnals propagates as coastally trapped waves and moves at a much higher speed than the water in the boundary current caused by the waves. So we examine the deepening of isopycnals due to the wave excited by winds.
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correspond to the 200 m isobath, for each year. We defined the sign of the wind stress so that positive values cause deepening of isopycnals at the shelf break. The wind stress was calculated from the wind speed, U10, at every 12 h by using a drag coefficient, CD, by Large and Pond (1981) in neutral stability,
Figs. 10a and b are the vector plots of the monthly-mean wind at 10 m in January calculated from the European Centre for Medium-Range Weather Forecasts (ECMWF) wind data for 1997 and that averaged for the other 3 years, respectively. In both figures northeasterly (northwesterly) winds prevailed in the northern (southern) part. The along-shore component of wind causes the Ekman transport directed toward the coast along the northern and western boundaries of the Sea of Okhotsk. The wind was more parallel to the coast in 1997 than that in the other years, especially near the southern end of Sakhalin, and in the northern part. Thus it is suggested that the Ekman transport and the corresponding deepening of isopycnals were large in 1997. Depths of the isopycnal surfaces at our observation sites are determined from the wind stress integrated along the isobaths from the site to the direction opposite to the propagation direction of the coastally trapped wave (e.g. Gill and Clarke, 1974). Table 1 shows the integration of the monthly-mean wind stress in January along the paths indicated by dashed lines in Fig. 10a, which
( CD ¼
for U10 o11 m s1 for U10 X11 m s1 :
60 -30
KAM
56
54
5 -1
-10
-10
54
-25
-10
-20 -15
NSH
-20
58
58
56
1:2 103 ð0:49 þ 0:065U10 Þ 103
We first neglected the change of the drag coefficient by the presence of sea ice for simplicity. The total integrated wind stress was positive every year due to the positive stress along southern Sakhalin (SSAK), northern Sakhalin (NSAK), and northern shelf (NSH). The integrated wind stress in 1997 was stronger than those in the other years, because the wind direction was more parallel to the coast in this year especially along SSAK and NSH (Fig. 10). We estimate the vertical displacement of the isopycnal surface associated with the wind stress in a two-layer model. For simplicity we assume that
-25
60
653
-25
-5
-5
52
52 NSAK 50
50
48
48
46
46
SSAK
-5
44
44 0
138 140 142 144 146 148 150 152 154 156
(a)
0
138 140 142 144 146 148 150 152 154 156
(b)
Fig. 10. Vector plots of the monthly mean wind at 10 m and contours of the temperature at 2 m in January for (a) 1997 and (b) those averaged for 1996, 1998, and 1999. A dashed line in (a), which is divided into four segments by dots, indicates the integration path used in Table 1.
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Table 1 The monthly mean wind stress in January integrated along the paths indicated by dashed lines in Fig. 10, which are the 200 m isobath in the southern Sakhalin (SSAK), northern Sakhalin (NSAK), northern shelf (NSH), and Kamchatka peninsula (KAM) R x h (m) r1=40 day Dz26.5 (m) h (m) Year t dx ð103 N m1 Þ
96 97 98 99 x, km
SSAK
NSAK
NSH
KAM
Total
10 15 7 5 368
101 73 69 61 924
47 168 88 129 1556
60 66 67 46 979
98 190 96 149 3826
59 90 53 69
94 180 72 97
r1=20 day
r1=N
Ice
52 69 44 52
62 121 61 95
93 137 87 100
At the bottom of the table x denotes the length of the integration path. The vertical displacement, h, of the isopycnal estimated from the stress with a two-layer model is listed in the 7th column. The displacement, Dz26.5, of the isopycnal surface sy=26.5 in Fig. 9 is also listed in the 8th column for comparison. The h calculated for r1=20 day and r1=N are listed in the 9th and 10th columns, respectively. The 11th column represents h calculated from the wind stress considering the change of the drag coefficient on sea ice.
the sea floor is flat. Then the vertical displacement, h, of the interface between the two layers is determined from the response of baroclinic Kelvin waves to the wind. After Gill and Clarke (1974), we have Z 0 x t ðx þ x0 ; t þ x0 =c1 Þ r=c1 x0 hðx; tÞ ¼ e dx0 ; ð1Þ r1 c21 N when the propagation speed, c1, of the baroclinic Kelvin wave is a constant. Here x is the curvilinear coordinate along the coast with its origin taken at the observation site, t the time, tx the alongshore component of the wind stress, and r a (linear) damping coefficient. c1 is expressed as sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi gðr2 r1 ÞH1 H2 ; ð2Þ c1 ¼ r2 ðH1 þ H2 Þ
Ohshima, 2000). Then the monthly mean wind stress in January, which is listed in Table 1, would be appropriate for the estimation of the isopycnal depth at the observation period, i.e. early February. We set r1=40 days, though there are some ambiguities for the value of r. This value corresponds to the damping coefficient for about 500 m depth in Chapman et al. (1986). Then waves coming from the North Pacific, which enter the Sea of Okhotsk turning around the southern end of Kamchatka peninsula, damp before they reach the observation area. With these assumptions, Eq. (1) reduces to a steady response of the twolayer model to the monthly-mean wind stress applied in the Sea of Okhotsk, Z 0 x 0 t ðx Þ r=c1 x0 hð0Þ ¼ e dx0 ; ð3Þ 2 x0 r1 c1
where H1 and H2 are the upper and lower layer thicknesses, respectively, r1 and r2 are the upper and lower layer densities, respectively, and g is the acceleration of gravity. Since the depth of the Sea of Okhotsk is typically 1000 m, except for the southernmost part, we set g=9.8 m s2, r1= 1026 kg m3, r2=1027 kg m3, H1=200 m, and H2=800 m. Then we have c1=1.24 m s1. Hence it takes 36 days for the baroclinic Kelvin wave to propagate from the southern end of Kamchatka peninsula to the observation area. We neglect the fluctuation of the wind whose time scale is shorter than 1 month, since deepening of isopycnals is observed in monthly-mean climatology (Itoh and
where x=x0 corresponds to the southern end of Kamchatka peninsula. The value of h calculated by Eq. (3) and the displacement of the 26.5sy isopycnal surface at the shelf break evaluated from Fig. 9 are listed in Table 1. Here the original position of the isopycnal is assumed to be the average isopycnal depths at all observation sites whose distance from the shelf break is 100 km or more among the 4 years. The simple two-layer model can reproduce deepening of isopycnals and its yearly variability, except that the values are smaller than the observational ones. To check the sensitivity of the model result to the damping coefficient, r, we also calculated h for r1=20 days
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and r1=N (Table 1). Values of h show the similar tendency from year to year for these values of r. We also estimated the change of the wind stress due to sea ice, taking the weighted mean of the stress on open water and sea ice with respect to their areas (R^ed and O’Brien, 1983) and neglecting the internal stress of sea ice for simplicity. We used daily ice concentration obtained by the SSM/ I and set the drag coefficient on sea ice to 2.5 103 after McPhee (1990). Values of h with the larger drag coefficient on sea ice and r1=40 days become closer to the observational ones (Table 1). Although ice concentration was different from year to year, yearly variability of h is similar in the cases with and without the sea ice effect. Therefore, we can conclude that the Ekman transport by northerlies accounts for the deepening of isopycnals and, accordingly, the thickening of the mixed layer in the observation area. We also calculated h for other months using the wind stress in a previous month with the sea-ice effect and r1=40 days. The value of h averaged in the 4 years increases from 34 m in December to 74 m and 105 m in January and February, respectively, and decreases to 72 m in March, 41 m in April, and 15 m in May. Thus in spring the cold mixed-layer water, which probably becomes DTW, spreads offshore at depths around 100 m. The internal Kelvin wave also changes the seasurface height, Z. Since Z={(r2r1)H2}/{r2(H1+ H2)}h B 0.8 103h, the seasonal variability of Z resembles that of the anomalous sea level observed in the Sea of Okhotsk and the North Pacific along the Hokkaido coast (Matsuyama et al., 1999). 5.2. Yearly variability of ice thickness By using satellite and hydrographic data, Gladyshev et al. (2000) pointed out that both ice production and dense water formation associated with brine rejection were less in 1997 than those in 1996. Their analysis showed that relatively high temperature in 1997 in the major polynyas, which are the sites of ice production, weakened the ice production. In fact the monthly-mean ECMWF temperature at 2 m in January 1997 was higher than that in the other years in the Sea of Okhotsk (Fig. 10). As candidates of major sources of sea
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ice in the observation area, we consider three polynyas in the western half of the Sea of Okhotsk (Fig. 1), that is, the Northwest Shelf polynya (NW), Sakhalin Shelf polynya (SAK), and Terpenia Bay polynya (TER). Table 2 shows the monthly-mean temperature and the offshore component of the monthly-mean geostrophic wind in January, where these quantities are averaged along the dashed lines in Fig. 1. Temperatures in 1997 were the highest in the 4 years from 1996 to 1999. The offshore component of the wind also affects the ice production, because it drifts sea ice offshore and enhances new ice production in coastal polynyas (Martin et al., 1998). It is unlikely that ice drifted offshore more intensely in 1997, in which the wind direction was rather parallel to the coast (Fig. 10). The geostrophic wind is a better indicator of ice motion than the 10 m wind, because ice motion is directed to the right of the 10 m wind and roughly parallel to geostrophic wind (Thorndike and Colony, 1982). The offshore geostrophic wind was not strong but rather weak in 1997 compared with that in the other years (the right part of Table 2). Thus, although we observed thick ice in 1997, both the temperature and geostrophic wind indicate that ice production was less in 1997 than that in the other years. In fact the offshore ice extent near the western boundary of the Sea of Okhotsk was smallest in 1997 among the 4 years (Fig. 2). The thickening of sea ice is not only due to the thermodynamic processes but also due to Table 2 The monthly mean temperature, T2, in January from the ECMWF data averaged along the four lines in Fig. 1, which are the representative of the Northwest Shelf polynya (NW), the Sakhalin Shelf polynya (SAK), the Terpenia Bay polynya (TER), and the path of sea ice along the southern part of Sakhalin (SSA) Vg (m s1)
Year T2 ( C) NW 96 97 98 99
SAK
TER
SSA
NW SAK TER SSA
21.3 11.5 11.3 9.4 9.5 13.0 8.4 7.9 6.0 5.2 23.9 19.4 14.3 10.7 7.2 16.8 10.3 9.6 8.3 3.4
0.9 10.3 1.2 9.3 0.5 7.0 0.9 9.4
0.1 2.5 0.9 0.3
The mean geostrophic wind, Vg, normal to the lines is also listed, where positive values indicate the wind directed offshore.
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dynamical processes such as rafting and ridging. Toyota (1998) showed that first-year ice and young ice collected in our observation area in 1997 consisted of multiple layers of columnar structure and granular structure sandwiched each other. This indicates that several ice floes are piled on each other to produce the collected ice. Fukamachi et al. (2003) showed that deformed ice dominates in the total ice mass in our observation area based on data from a moored sonar. Thus rafting and ridging are important for the thickening of sea ice in the observation area. The wind direction in 1997 was more parallel to the coast and more favorable for rafting and ridging of sea ice converged toward the coast than the other years. Note that the offshore geostrophic wind is smallest in this year along SAK and SSA which is the path of sea ice along the southern part of Sakhalin (Table 2). Thus it is suggested this wind direction was important for the thickening of sea ice as well as the mixed layer.
year. A two-layer model in a flat ocean suggests that this wind stress is responsible for the deepening of the isopycnal surface and its yearly variability. Therefore, we can conclude that the thick mixed layer was formed by the Ekman transport by northerlies. Although the air temperature and the offshore component of the geostrophic wind indicate that the production rate of sea ice was low in 1997 compared with that in the other years, relatively thick ice was observed in this year in the observation area. It is most likely that rafting and ridging of sea ice were responsible to this thickening of sea ice. In this year the wind direction, which was responsible for the thickening of the mixed layer, was also favorable for rafting and ridging at the coast. Thus the change of the wind direction is proposed as a factor in determining the thicknesses of both the mixed layer and sea ice in the southwestern part of the Sea of Okhotsk.
Acknowledgements 6. Summary We conducted hydrographic observations in the southwestern part of the Sea of Okhotsk in early February from 1996 to 1999. A thick mixed layer with near-freezing temperature, which extended to a depth of 150–300 m, was observed along the shelf break under sea ice. The thickness of the mixed layer was largest in 1997 when the ice condition was the heaviest among the 4 years. Although the mixed layer extended to a depth deeper than that of DTW found from spring to fall, the density of mixed-layer water was equal to or less than that of DTW. Thus the thick mixed layer was accompanied with deepening of isopycnals. The yearly variability of the isopycnal depth coincided with the variability of the mixed-layer thickness. Nearly northerly winds in the Sea of Okhotsk in winter induce the convergence of the Ekman transport toward the northern and western boundaries of the Sea of Okhotsk and, accordingly, force deepening of isopycnals there. In 1997 the winds, which are usually directed offshore, were more parallel to the coast and the integral of the wind stress along the isobath was larger this
We are deeply indebted to Koukichi Suehiro, Kazuaki Kubo, Yasushi Nabae, Koji Iwamoto, and others at Hydrographic and Oceanographic Department, Japan Coast Guard for their cooperation. We are also grateful to the captain and crew of P/V Soya of Japan Coast Guard for their sincere help in the observation. The SSM/I data were provided by the National Snow and Ice Data Center (NSIDC), University of Colorado. This work was sponsored by the Core Research for Evolutional Science and Technology, Japan Science and Technology Corporation, the International Cooperative Research Programme on Global Ocean Observing System, the Grantin-Aid for Scientific Research on Priority Areas (Nos. 08241201 and 09227201), and a fund from Hokkaido Foundation for the Promotion of Scientific and Industrial Technology.
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