Progress in Oceanography xxx (2014) xxx–xxx
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Progress in Oceanography
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Freshening and dense shelf water reduction in the Okhotsk Sea linked
with sea ice decline
Kay I. Ohshima a,⇑, Takuya Nakanowatari a, Stephen Riser b, Yuri Volkov c, Masaaki Wakatsuchi a
a
Institute of Low Temperature Science, Hokkaido University, Kita-19, Nishi-8, Kitaku, Sapporo 060-0819, Japan
School of Oceanography, University of Washington, Seattle, WA 98195, USA
c
Far Eastern Regional Hydrometeorological Research Institute, 24, Fontannaya St., Vladivostok 690600, Russia
b
a r t i c l e
i n f o
Article history:
Available online xxxx
a b s t r a c t
A recently constructed hydrographic dataset from the Okhotsk Sea reveals a prominent freshening to
depths of 500 m during the past four decades, with the maximum in the northwestern part of the
sea. Averaged over the sea, this freshening corresponds to an input of 0.55 m of freshwater. This leads
to a decrease in density of the intermediate water and deepening of the isopycnals, with the maximum
deepening at 26.8 rh of 60 m averaged over the sea. The intermediate water is significantly warmed
along the pathway of dense shelf water (DSW). A simple box model shows that DSW production has
decreased by 30% during the past four decades. We propose that the freshening and DSW reduction
are caused by the weakening of salt/freshwater redistribution through sea ice decline as well as by the
increase of excess precipitation over evaporation. Since the overturning in the North Pacific originates
from the Okhotsk Sea through the DSW, these changes possibly weaken the shallow overturning of the
North Pacific.
Ó 2014 Elsevier Ltd. All rights reserved.
Introduction
In many regions of the polar and subpolar oceans, significant
freshening has been observed and attributed to an acceleration of
the global hydrological cycle, perhaps linked to a warming climate
(Wong et al., 1999; Curry et al., 2003; Curry and Mauritzen, 2005;
Boyer et al., 2005, 2007; Bindoff et al., 2007). Particularly in the
subpolar North Atlantic, where deep water formation occurs, freshening extends to the bottom, resulting in the freshening of North
Atlantic Deep Water (NADW) (Dickson et al., 2002). There has been
an argument that the freshening and associated decrease in density
of NADW might cause a weakening of the large-scale meridional
overturning circulation (Hansen et al., 2004).
As a result of the great increase in hydrographic observations by
the international Argo Program, some statistical analyses of
salinity trends are possible for the global ocean. A fifty-year trend
analysis at the sea surface has revealed salinity increases in
evaporation-dominated regions and freshening in precipitationdominated regions, consistent with an amplification of the global
hydrological cycle (Hosoda et al., 2009; Durack and Wijffels,
2010; Durack et al., 2012; Helm et al., 2010). Further, Durack and
⇑ Corresponding author. Tel.: +81 11 706 5481; fax: +81 11 706 7362.
E-mail addresses: [email protected] (K.I. Ohshima), nakano@
lowtem.hokudai.ac.jp (T. Nakanowatari), [email protected] (S. Riser),
[email protected] (Y. Volkov), [email protected] (M. Wakatsuchi).
Wijffels (2010) have shown that subsurface salinity changes can
be explained by broad-scale surface warming and the associated
poleward migration of isopycnal outcrops in each ocean basin. In
the subpolar North Pacific, where net precipitation exceeds evaporation, the freshening is significant in the central subpolar region
and broad northeast areas, although not in the western limb area
(Freeland et al., 1997; Whitney and Freeland, 1999; Hosoda et al.,
2009; Ren and Riser, 2010).
Although deep water formation does not generally occur in the
North Pacific, ventilation or overturning into the intermediate layers between 200–800 m occurs from the Sea of Okhotsk (Talley,
1991; Warner et al., 1996) through the dense shelf water (DSW)
formation due to active sea ice production in the northwestern
shelf polynya (Martin et al., 1998; Shcherbina et al., 2003;
Nihashi et al., 2009). Recent observations (Nishioka et al., 2007,
2013) suggest that the ventilated intermediate water from the
Okhotsk Sea is a potential source of the micronutrient iron in the
iron-limited, high-nitrate, low-chlorophyll North Pacific. Over the
past five decades, warming of intermediate water and weakening
overturning have been suggested to occur in northwestern North
Pacific, likely originating from the Okhotsk Sea (Ono et al., 2001;
Nakanowatari et al., 2007; Itoh, 2007). Since the overturning in
the North Pacific originates from brine rejection by sea ice formation in the Okhotsk Sea, sea ice change and the associated salinity
changes in the Okhotsk Sea could be a key component in driving
changes to the overturning of the North Pacific.
http://dx.doi.org/10.1016/j.pocean.2014.04.020
0079-6611/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Ohshima, K.I., et al. Freshening and dense shelf water reduction in the Okhotsk Sea linked with sea ice decline. Prog.
Oceanogr. (2014), http://dx.doi.org/10.1016/j.pocean.2014.04.020
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K.I. Ohshima et al. / Progress in Oceanography xxx (2014) xxx–xxx
Despite the importance of the Okhotsk Sea to North Pacific
overturning and its possible weakening, this sea is one of the most
sparsely sampled in the world ocean: the international Argo Program does not include the Okhotsk Sea, nor have there been very
many quality ship-based surveys in the region over the past
50 years. Thus, in most global analyses the Okhotsk Sea is missing.
As for the salinity change, no examination has been made except a
snapshot analysis (Hill et al., 2003). Although the DSW in the Okhotsk Sea has been discussed based on the climatological dataset by
several studies (Itoh et al., 2003; Gladyshev et al., 2003; Uehara
et al., 2012), the long-term variation of DSW production had not
been examined quantitatively. Very recently, Uehara et al. (this
issue) has examined the decadal variability of the DSW based on
the Russian historical dataset. For this data-void sea, Hokkaido
University and the University of Washington have made an effort
over the past 15 years to accumulate hydrographic data by cooperating with colleagues at the Far Eastern Regional Hydrometeorological Research Institute in the Russian Federation. These efforts
include six international research cruises and the deployment of
24 profiling floats (Ohshima et al., 2010) in the Okhotsk Sea since
1998. Based on these data, together with historical archives, we
create a new dataset for this area and examine the changes in
salinity and the associated DSW production in greater quantitative
detail than has previously been possible.
Data and methods
We created our climatologically-averaged data set covering
three periods: 1930–2009, 1930–1980, and 1990–2009 for the
Okhotsk Sea and the adjacent area of the North Pacific (see Fig. 1
for the study area). For each period, two types of averages, over
depths and over isopycnal surfaces, were constructed based on
the available hydrographic data taken from all the seasons. Salinity, our focus in this work, is relatively less affected by seasonal
Russia
60 N
Kam
c
hatk
a
200
Amur
River
Sea of 1000
Okhotsk
00
10
50 N
Sakhalin
55 N
3000
variability than temperature. For isopycnal averages, we focused
on the density range 26.8–27.4 rh, where winter convection does
not occur in the open ocean, thus having little seasonal variation.
Our focus is to examine the spatial structure of water mass
changes, with a trade-off for temporal resolution.
Most of the temperature and salinity data were taken from the
World Ocean Database 2001 (WOD01) (Conkright et al., 2002) and
from the Japan Oceanographic Data Center (JODC). In addition, we
have included the data collected by R/V Oyashio-Maru of Hokkaido
Central Fisheries Research Institute from 1969 to 1976, those
derived from R/V Khromov by the Japan–Russia–United States joint
study of the Sea of Okhotsk from 1998 to 2007, and from 24 profiling floats deployed in the Sea during this latter period. While
WOD09 is now the most updated version of the World Ocean Database, we have found that newly added Okhotsk Sea data in WOD09
tend to show large scatter (standard deviation) for the salinity
data, compared to that of WOD01 (details in appendix A1). Thus,
to insure the highest quality of data we have decided to use
WOD01 rather than WOD09.
To obtain the standard-depth/standard-density data, the bottle
and profiling float data were linearly interpolated in the vertical
direction. Prior to the construction of gridded data set, we performed a quality control check similar to that by Itoh et al.
(2003) for all the data. Fig. 2 shows the time series of the number
of quality-controlled stations taken in the Okhotsk Sea during
1930–2009. Increase in the quantity of data after 2000 is mainly
due to observations by profiling floats. Fig. 3 shows the positions
of all of the quality-controlled stations for salinity at 200 m depth
during the 1930–1980 and 1990–2009 periods. For the 1990–2009
period, the stations are rather sparse in the northeast part of the
sea, which should be regarded as less reliable area. The qualitycontrolled data contains 2061 stations for the 1990–2009 period,
compared to 2753 stations for 1930–1980 in the case of salinity
at 300 m depth. For the 1930–1980 dataset, we find that the year
1962 is a representative year from averaging of years of all the
observations. For the 1990–2009 dataset, the year 2000 is a representative year from the averaging. Thus, a representative time difference between the two data sets is regarded to be about 40 years.
The calculations were made on a 0.25° latitude/longitude grid. A
similar dataset was separately produced for the Northwestern
Pacific. To conduct a gridded dataset, an objective analytical
method similar to that used by Levitus and Boyer (1994) and
Itoh et al. (2003) was applied. The autocorrelation functions on
each depth and isopycnal bin can be fit to the Gaussian distribution
with an e-folding scale of 75 km. Thus, to calculate a grid value
from the station data, we used the Gaussian distribution as a
weight function with an e-folding scale of 75 km and an influence
radius of 150 km. The number of observations within the influence
radius did not reach the minimum requirement of five for a small
percentage of grid cells. In those cells the influence radius was
increased to 300 km. The influence radius and e-folding scale in
the North Pacific were set to 300 km and 150 km.
00
30
0
00
1
Bussol’ Strait
Results
Freshening
00
10
3000
Hokkaido
140 E
145 E
Pacific
150 E
155 E
Fig. 1. Bottom topography (contours of 200 m by dotted lines, and 1000 m and
3000 m by solid lines) of the Okhotsk Sea from the ETOPO5 dataset. The arrows
indicate the cyclonic gyre inferred from the observations. The solid line from the
northwestern shelf through the Bussol’ Strait denotes the section shown in Fig. 6.
The amount of freshening is calculated for the area encompassed by the dashed
lines in ‘Discussion’.
We first examine salinity changes based on the depth-averaged
dataset. Fig. 4 shows the vertical profile of salinity differences
averaged over the Okhotsk Sea between the 1930–1980 and
1990–2009 periods in two ways. One is by examining the
spatially-averaged salinity difference between the climatology of
the two periods (solid lines in Fig. 4a). The other is based purely
from the raw data: the average in anomalies of all the station
data from the climatology of the whole period (1980–2009) is
Please cite this article in press as: Ohshima, K.I., et al. Freshening and dense shelf water reduction in the Okhotsk Sea linked with sea ice decline. Prog.
Oceanogr. (2014), http://dx.doi.org/10.1016/j.pocean.2014.04.020
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K.I. Ohshima et al. / Progress in Oceanography xxx (2014) xxx–xxx
300
Number of stations
250
200
150
100
50
0
1930
1940
1950
1960
1970
1980
1990
2000
2010
Year
Fig. 2. Time series of the number of quality-controlled stations in the Okhotsk Sea for salinity at 300 m depth. The lines with arrow-head at both ends indicate the periods for
which the climatology map was made.
(a)
(b)
Fig. 3. Positions of all of the quality-controlled stations for salinity at 200 m depth during the (a) 1930–1980 and (b) 1990–2009 periods.
(a)
(b)
Fig. 4. (a) Vertical profiles of difference in salinity (solid lines) and temperature (dotted lines) averaged over the Okhotsk Sea between the 1930–1980 and 1990–2009 periods
(i.e., values from 1990–2009 minus values from 1930–1980) based on the climatology maps. The error (lateral) bars show the standard deviation of the difference for all the
grid points in the Okhotsk Sea. (b) Vertical profiles of difference in salinity based on the average in anomalies of all the station data from the climatology of the whole period
(1980–2009) for the two periods. The 99% confidence intervals (indicated by error bars) are calculated based on a Student t distribution with one degree of freedom per one
year.
Please cite this article in press as: Ohshima, K.I., et al. Freshening and dense shelf water reduction in the Okhotsk Sea linked with sea ice decline. Prog.
Oceanogr. (2014), http://dx.doi.org/10.1016/j.pocean.2014.04.020
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K.I. Ohshima et al. / Progress in Oceanography xxx (2014) xxx–xxx
calculated for the two periods and then the difference of these
averages of the two periods is shown with the 99% confidence
interval (Fig. 4b). In both figures, a prominent freshening can be
found to a depth of 500 m, with salinity decrease of 0.05 PSU
averaged in the upper 500 m. From our dataset, the salinity
decrease between the two periods averaged in the upper 500 m
layer over the Northwestern Pacific north of 49°N and west of
165°E is only 0.01 PSU. This is consistent with previous studies
(Ren and Riser, 2010; Hosoda et al., 2009), which showed that significant freshening occurs in northeast to central North Pacific
down to 600 m but not in the northwestern area of the North
Pacific.
Our depth-averaged dataset also shows a significant warming in
the Okhotsk Sea (dotted lines in Fig. 4a). For all the waters of the
Okhotsk Sea, the mean temperature increase is 0.154 °C, corresponding to an increase of the total heat content of 8.0 1020 J.
With the assumption of a time difference of 40 years, this corresponds to a heat storage rate (time derivative of heat content per
unit area) of 0.42 W m2, which is about two and three times larger
than the heat storage rate of the global ocean and the North Pacific,
respectively (Levitus et al., 2005; Bindoff et al., 2007).
Fig. 5a shows the salinity difference between the two periods
averaged over the 50–500 m layer. In the top 50 m, where effects
of seasonal variation are relatively large, the salinity difference
shows a strong patchiness, and thus this layer is excluded in
Fig. 5a. The figure suggests that salinity has decreased over most
areas of the Okhotsk Sea. An area of salinity increase exists only
in the eastern part of the Okhotsk, where the data density and thus
reliability is relatively low (indicated by dotted shading; see also
Fig. 3). The salinity decrease is particularly large in the western
part of the Okhotsk Sea. On the other hand, a significant salinity
change does not appear to occur in the northwestern North Pacific
on average, with salinity decreasing and increasing in patches,
where effects of mesoscale eddies cannot be removed.
Fig. 5b shows the integrated salinity decrease in the upper
500 m layer, represented by the corresponding fresh water thickness Fw, defined as Fw = DS/Sa L, where DS is salinity decrease
averaged over the layer L, the upper 500 m layer or the whole
water column for grid points with water depth <500 m, and Sa
(=33.2) is the averaged salinity of sea water in the layer. For depths
shallower than 50 m the salinity value is extrapolated from that of
50 m depth. Clear freshening can be seen in the western part of the
Okhotsk Sea, particularly around the center of the cyclonic gyre,
where the residence time is relatively large. The freshening volume
over the entire Okhotsk Sea is estimated to be 763 km3, which is
equivalent to a net fresh water input of 0.55 m over the entire
basin.
(a)
Fig. 6 shows the salinity from climatology and the salinity
change between the two periods along the vertical section from
the northwestern shelf to the Bussol’ Strait, as denoted in Fig. 1.
It is found that the freshening is particularly large in the northwestern region of the Sea, to depths of about 500 m, with the maximum change between 50–150 m. The salinity decrease of about
0.05 PSU over this time interval in the Okhotsk Sea is comparable
to the salinity decrease of the North Atlantic or Labrador Sea,
although the freshening integrated over the total column is considerably smaller in the Okhotsk Sea than in the North Atlantic or Labrador Sea, where freshening occurred down to much greater
depths (Curry et al., 2003; Yashayaev, 2007).
Changes in intermediate water
We examine changes in the intermediate water, based on the
isopycnal-averaged dataset. Since the freshening extends to the
intermediate levels (about 500 m), as seen in Figs. 4 and 6b, the
layer of Okhotsk Sea Intermediate Water (OSIW), which is the
source water of North Pacific Intermediate Water (Talley, 1991;
Shcherbina et al., 2003), is affected. One remarkable change is
the significant deepening of the isopycnals at the intermediate levels (Fig. 7) due to decreases in salinity and the resulting density.
Although our dataset indicates that temperature in the intermediate depths (200–800 m) of the Okhotsk Sea has increased by about
0.2 °C for the past four decades (Fig. 4a), the density is mostly
determined by the salinity in the cold water range. The maximum
deepening has occurred at 26.8 rh, with the spatial structure of the
deepening (Fig. 7) resembling the freshening (Fig. 5b). The amount
of the deepening averaged over the Okhotsk Sea reaches about
60 m, considerably larger than in the adjacent North Pacific.
We have estimated the volume of water in each density range
inside the Okhotsk Sea for the two periods (Table 1), where 26.8
rh water is defined as the water between the 26.75 and 26.85 isopycnals. The volume of heavier intermediate water has decreased
with the maximum decrease at 27.0 rh, by 7400 km3 (11%),
whereas the volume of lighter intermediate water has increased
with the maximum increase at 26.8 rh, by 14,000 km3 (16%).
We examine the change in properties of the intermediate water.
We show the potential temperature distribution on the 26.9 isopycnal surface for each period (Fig. 8a and b), since the feature of
DSW and its difference between the two periods are most prominent on this surface. It is found that the coldest water exists on
the northwestern shelf and extends southward along the east
Sakhalin shelf, suggesting that the intermediate water is ventilated
through the dense shelf water (DSW) from the northwestern shelf
in both periods. It is found that the temperature is lower in the
(b)
Fig. 5. Horizontal distributions of (a) salinity difference averaged over 50–500 m layer and (b) fresh water thickness (m) corresponding to the salinity change integrated in
the upper 500 m layer between the 1930–1980 and 1990–2009 periods. The areas with fewer than 30 observations within a half-width of the window for either period are
indicated by dotted shading.
Please cite this article in press as: Ohshima, K.I., et al. Freshening and dense shelf water reduction in the Okhotsk Sea linked with sea ice decline. Prog.
Oceanogr. (2014), http://dx.doi.org/10.1016/j.pocean.2014.04.020
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K.I. Ohshima et al. / Progress in Oceanography xxx (2014) xxx–xxx
(a)
(b)
Fig. 6. Vertical section of (a) salinity from climatology for 1930–2009 and (b) difference in salinity between the 1930–1980 and 1990–2009 periods, along the section from
the northwestern shelf through the Bussol’ Strait, denoted in Fig. 1.
Decreasing ratio of dense shelf water production
Although a decrease in DSW production was also suggested in
Nakanowatari et al. (2007), a quantitative discussion as to the
degree that DSW production is reduced has not yet been made.
Here we attempt to estimate the decreasing ratio of DSW production by utilizing the datasets from the two periods. Fig. 9 shows the
relationship between potential temperature and salinity for OSIW
from the two periods, averaged over the region where the water
depth exceeds 1000 m in the Okhotsk Sea, and for Western Subarctic Water (WSAW) averaged over the region north of 50°N and
west of 160°E in the North Pacific. Since the average properties
of WSAW have not significantly changed between the two periods,
only the averages for the whole period of 1930–2009 are shown.
The figure demonstrates that the properties of OSIW have significantly changed. The averaged potential temperature (h) and salinity (S) of the OSIW for each isopycnal during the two periods are
also listed in Table 1, which shows that on isopycnal surfaces the
OSIW has warmed (by 0.3–0.4 °C) and become more saline (by
0.03 PSU) over the four decades.
Since the OSIW is approximately formed by the mixing of DSW
and inflowing WSAW, the change of OSIW is likely attributed to the
change in the mixing ratio between the DSW and WSAW, owing to
the reduction of DSW. To investigate this idea, we have estimated
the change of DSW production rate in density bins of 0.1 rh from a
simple box model, assuming that the OSIW is formed by the isopycnal mixing between the DSW and WSAW. We also assume that
the WSAW is unchanged, with the properties of WSAW calculated
as averages north of 48°N and west of 160°E in the North Pacific
(Table 1). We further assume that the formation of OSIW is nearly
steady in the annual mean during each period with the constant
mixing ratio. Under these assumptions, the inflowing heat content,
Fig. 7. Horizontal distribution of depth change (m) on the isopycnal 26.8 rh
between the 1930–1980 and 1990–2009 periods. The areas with fewer than 30
observations within a half-width of the window for either period are indicated by
dotted shading.
period 1930–1980 compared to the later period. The difference of
the two periods (Fig. 8c) is particularly large in the formation area
of DSW and its pathway along the cyclonic gyre, the southward
East Sakhalin Current and the subsequent eastward flow (the
arrows in Fig. 1; Ohshima et al., 2004). These spatial features of
Fig. 8c clearly suggest that the change (warming) of OSIW is caused
by the decrease of cold DSW production.
Table 1
Changes in Okhotsk Sea Intermediate Water (OSIW).a
rh
Volume change (103 km3)
hO1 (°C)
hO2 (°C)
hp (°C)
SO1
SO2
Sp
Change of DSW prod. (%)
26.8
26.9
27.0
27.1
14.0 (+16%)
2.4 (3%)
7.4 (11%)
2.2 (4%)
0.86
1.15
1.44
1.75
1.19
1.54
1.83
2.07
2.57
3.09
3.35
3.37
33.442
33.589
33.737
33.888
33.467
33.621
33.771
33.919
33.560
33.747
33.913
34.051
72
71
68
74
a
The symbols h and S denotes potential temperature and salinity. The subscripts O1, O2, and P indicate OSIW for 1930–1980, OSIW for 1990–2009 and WSAW for 1930–
2009, respectively.
Please cite this article in press as: Ohshima, K.I., et al. Freshening and dense shelf water reduction in the Okhotsk Sea linked with sea ice decline. Prog.
Oceanogr. (2014), http://dx.doi.org/10.1016/j.pocean.2014.04.020
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K.I. Ohshima et al. / Progress in Oceanography xxx (2014) xxx–xxx
5
mixing by the tidal currents in the Kuril Straits. At the present
stage of our understanding, however, it is still difficult to assess
the effect of the diapycnal mixing on the water mass formation.
The above scenario will need to be modified somewhat in the
future by including such effects.
26.
5
27.
0
Discussion
27.
Potential Temperature ( )
Fig. 8. Horizontal distributions (a) potential temperature (°C) on the 26.9 rh surface for 1930–1980, (b) same as (a) except for 1990–2009, (c) difference between (a) and (b).
The areas with fewer than 30 observations within a half-width of the window are indicated by dotted shading.
Salinity (psu)
Fig. 9. Potential temperature (°C) versus salinity (psu) for the Okhotsk Sea
Intermediate Water (OSIW) in 1930–1980 (green square) and 1990–2009 (orange
triangle), and Western Subarctic Water (WSAW: red solid circle). The temperature
of DSW is assumed to be the freezing point of 1.8 °C (blue open circle). (For
interpretation of the references to color in this figure legend, the reader is referred
to the web version of this article.)
Vdhd + Vphp, should be balanced by the outflowing heat content
(Vd + Vp) ho, where Vd and Vp are the annual inflow volumes of
DSW and WSAW, respectively, and ho, hd, and hp are potential temperatures of OSIW, DSW and WSAW, respectively.
With these definitions the volume flux of DSW for 1930–1980,
Vd1, is given by
V d1 ¼ V p ðho1 hp Þ=ðhd ho1 Þ
ð1Þ
and that for 1990–2009, Vd2, is given by
V d2 ¼ V p ðho2 hp Þ=ðhd ho2 Þ
ð2Þ
where hd is assumed to be the freezing point of sea water,
1.80 °C; hp and Vp are assumed to be constant throughout the
analyzed period; the subscripts 1 and 2 indicate the period of
1930–1980 and 1990–2009, respectively. Then the change ratio r
of DSW production between the two periods, Vd2/Vd1, is calculated
to be
r ¼ V d2 =V d1 ¼ ðho2 hp Þðhd ho1 Þ=ðhd ho2 Þðho1 hp Þ
ð3Þ
The results of this calculation from our data set are listed in
Table 1, showing that the change ratio of DSW production is about
70%, with the minimum at 27.0 rh. This implies that DSW production is reduced by about 30% over the four decades between 1960
and 2000. Small differences in the reduction rate for different density layers might be partly due to the smoothing effect of diapycnal
What is the cause of the inferred freshening of the Okhotsk Sea?
From an examination of long-term variations in Amur River discharge, a major source of fresh water to the Sea, an increasing
trend in fresh water discharge cannot be identified (Tachibana
et al., 2008; Dai et al., 2009). Using precipitation data from the Global Precipitation Climatology Project (GPCP: Adler et al., 2003) and
the evaporation data from the Objective Analyzed Air-Sea Fluxes
dataset (OAFlux: Yu and Weller, 2007), an increasing trend in the
excess of precipitation over evaporation (P–E) can be found in
the Okhotsk Sea, with the value of the trend averaged over the
Okhotsk Sea being 29 mm per year over the 30 years. Calculating
the cumulative fresh water flux due to this P–E increase by neglecting the water exchange with the Pacific yields an increase of about
0.43 m for 30 years, which is less than the observed equivalent
freshening of 0.55 m. It should be noted that the prominent freshening is confined to the northwestern part of the Okhotsk Sea
(Figs. 5 and 6b) and does not occur in the northwestern North Pacific. If the precipitation excess evaporation were the main cause, it
is likely that significant freshening would occur over a wider area
that includes regions outside of the Okhotsk Sea. Thus, we suggest
that the increase in P-E can likely explain only a portion of the
observed freshening. We note that in spite of great advances in
the estimation and compilation of surface fluxes in recent year
(e.g., Trenberth, 2011), P–E data are still somewhat problematic
and sometimes unreliable.
As an alternative, we explore another possible cause of the
observed freshening, a decrease in sea ice production. The freshwater and salt budgets of Okhotsk Sea are governed by a large amount
of sea ice formation in the north, the subsequent southward transport of this ice by the prevailing northerly wind, and its melting in
the south; together these processes provide a salt flux in the north
and fresh water flux in the south, as schematically shown in the
upper panel of Fig. 10. This important role of sea ice in the redistribution of salt and freshwater in the Okhotsk Sea has been demonstrated by Nihashi et al. (2012) using the satellite sea ice
information and heat flux estimates and by Watanabe et al.
(2004) using an ice-ocean coupled model. We propose that a
decrease in sea ice production in recent decades weakens this system, resulting in the freshening in the north (lower panel of
Please cite this article in press as: Ohshima, K.I., et al. Freshening and dense shelf water reduction in the Okhotsk Sea linked with sea ice decline. Prog.
Oceanogr. (2014), http://dx.doi.org/10.1016/j.pocean.2014.04.020
K.I. Ohshima et al. / Progress in Oceanography xxx (2014) xxx–xxx
Fig. 10. (Upper panel) schematics of the role of sea ice in the redistribution of salt/
freshwater and the overturning of the Okhotsk-North Pacific system. (Lower panel)
a proposed mechanism of the freshening and subsequent weakening of North
Pacific overturning.
Fig. 10). While we expect that in the southern portion of the Okhotsk Sea salinity would likely increase in the upper layer, such a
signal is unlikely to be observed due to the outflow to the Pacific
and the inflow of the Soya Warm Current Water.
Based on moored observations of sea ice drift and thickness,
Fukamachi et al. (2009) made an estimate of the annual southward
ice transport from the north of the Okhotsk as 3.1–7.3 1011 m3.
Simizu et al. (this issue) has further refined this estimate, using a
more precise evaluation of wind and ocean ice drifts and has
shown the southward transport of sea ice to be 3.0 ± 0.9 1011
m3 at 53°N over 2003–2010. The corresponding freshwater transport Tf can be calculated as
T f ¼ ðS0 Si Þ=S0 qi =qw T i
ð4Þ
where S0 (=33.2) is sea-water salinity, Si (=6.0) is sea-ice salinity,
qw (=1000 kg m3) is water density, qi (=920 kg m3) is sea-ice
density, and Ti is volume transport of sea-ice. From eq.(4), the
annual sea-ice transport of 3.0 ± 0.9 1011 m3 results in an annual
freshwater transport of 2.3 ± 0.7 1011 m3. This value is comparable to the annual Amur River discharge of 3.1 1011 m3 (Dai
et al., 2009). Kashiwase et al. (this issue) has shown that sea ice
production in Okhotsk Sea coastal polynyas has decreased by
11.4% for the past 34 years, due to a significant warming trend of
air temperature northwest of the Okhotsk Sea in late fall. Based
on these estimates, we assume that the annual southward transport of sea ice (3.0 ± 0.9 1011 m3 at present) has linearly
decreased by 10% over the past 40 years. Then the associated
cumulative freshening is calculated to be 5.1 ± 1.5 1011 m3,
with the use of (4). According to our dataset, the amount of
freshening during the four decades is calculated to be
3.3 1011 m3 in the northwestern part of the Okhotsk Sea (the
portion encompassed by the dashed lines in Fig. 1). Hence the
cumulative freshening by the decrease in ice transport exceeds
the observed freshening.
In reality, water exchange with the North Pacific would relax
this freshening. The residence time of the Okhotsk Sea water has
been estimated for the intermediate water to be somewhere
between 2 and 10 years (Yamamoto et al., 2001; Gladyshev et al.,
2003; Itoh et al., 2003). Additionally, the inflow from the North
7
Pacific originating in the Bering Sea would affect the salinity
change in the Okhotsk Sea, as suggested by Uehara et al. (this
issue) for the decadal variability using the Russian historical
dataset. It should be pointed out that the prominent freshening
has occurred in the active ice formation area of the northwestern
part of the Sea (Figs. 5 and 6b) and the center of the cyclonic gyre
where the residence time is relatively large. This geographic
distribution of the freshening, along with the estimated value of
the cumulative freshening, suggests that the freshening is caused
by the weakening of salt/freshwater redistribution through sea
ice decline.
Once the upper-intermediate layer has freshened in the active
sea ice formation area, the ambient source water for the DSW
should become lighter, Fig. A1 causing a decrease in production
and decreased density of DSW, even if the sea ice production is
the same. This implies that DSW is affected indirectly by sea ice
reduction through freshening of the source water with a multiyear time scale, in addition to a direct decrease of brine rejection
water to DSW by sea ice reduction. Further, the weakening of the
salt/freshwater redistribution by sea ice decline possibly leads to
the recent suggested decrease in overturning of the North Pacific
(Nakanowatari et al., 2007), as schematically shown in Fig. 10. In
conjunction with a recent freshening in high latitude oceans, previous studies have mainly examined changes in the enhanced global hydrological cycle due to changes in net evaporation and
precipitation. This study demonstrates that sea ice decline can be
an important contributor to freshening in ice-covered regions of
the world ocean.
Acknowledgments
The profiling floats were prepared at the University of Washington by Dana Swift, Rick Rupan, and Dale Ripley. We thank Kyoko
Kitagawa and Motoyo Itoh for support in drawing the figures. This
work is supported by CREST, Japanese Science and Technology Corporation and Grants-in-aid 20221001 and 22221001 for scientific
research from the Ministry of Education, Science, Sports, and Culture of Japan.
Appendix A
We here describe the data quality of the World Ocean Database
in the Okhotsk Sea. We compare the data quality of newly added
data in WOD09 (abbreviated as WOD09-01 hereinafter) with that
of WOD01. For WOD01, the total number of data station with both
temperature and salinity is 8638, among which 80% are from Japan
and US. On the other hand, for WOD09-01, the total number of data
station with both temperature and salinity is 7604, among which
94% are from Russia and USSR. To check the data quality of
WOD09-01 compared to that of WOD01, we have calculated the
standard deviation of salinity for the center area (49°–54°N,
146°–150°E) of the Okhotsk Sea, where salinity is relatively spatially uniform. Fig. A1a shows the standard deviation as a function
of depth for the two datasets (solid lines: WOD01, dotted lines:
WOD09-01). The standard deviation of WOD09-01 is nearly twice
as that of WOD01 for depths of 75–1000 m which we focus on.
Fig. A1b shows the standard deviation as a function of potential
density. Even isopycnally, WOD09-01 shows larger (by about 50%
in average) standard deviation for 26.2–26.6 rh. As for temperature
data, there are no significant differences in the standard deviation
between WOD01 and WOD09-01. We have judged that the large
scatter (standard deviation) of WOD09-01 salinity data arises from
relatively low quality of data and not from the real one. For discussion of slight change of water property, we consider that WOD0901 salinity data do not have enough quality.
Please cite this article in press as: Ohshima, K.I., et al. Freshening and dense shelf water reduction in the Okhotsk Sea linked with sea ice decline. Prog.
Oceanogr. (2014), http://dx.doi.org/10.1016/j.pocean.2014.04.020
8
K.I. Ohshima et al. / Progress in Oceanography xxx (2014) xxx–xxx
(a)
(b)
Fig. A1. The standard deviation of salinity as a function of (a) depth and (b) potential density over the center area (49°–54°N, 146°–150°E) of the Okhotsk Sea, where salinity
is relatively uniform spatially, for the two datasets (solid lines: WOD01, dotted lines: WOD09-01) during 1930–2009.
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