On the basis of all available data, it is found that intermediate water temperature on the 26.8–27.4σθ isopycnals in the northwestern North Pacific has significantly increased during the past 50 years. The largest warming area exists in the western part of the Sea of Okhotsk with a 0.68°C/50-yr temperature increase observed at 27.0σθ. The warming in the Pacific is found over the Oyashio and Subarctic Current regions, where the Okhotsk water extends along the subarctic gyre. This suggests that the warming originates from the Sea of Okhotsk. The warming trend is also accompanied by the significant decreasing trend of dissolved oxygen content, suggesting the weakening of overturning in the northwestern North Pacific. We propose that these trends of the water mass property are caused by a decrease in dense shelf water production in the northwestern shelf of the Sea of Okhotsk, which is a sensitive area to the current global warming.
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 It is known that North Pacific Intermediate Water (NPIW), characterized by a salinity minimum at 26.8σθ, is a major water mass at the intermediate level of the North Pacific [e.g., Reid, 1965]. Several studies have suggested that the ventilation source of intermediate water in the North Pacific, including NPIW, is the Sea of Okhotsk [e.g., Talley, 1991; Warner et al., 1996]. Over the northwestern shelf in the Sea of Okhotsk, a large amount of sea ice is produced due to severe winds from northeastern Eurasia in winter. The sea ice production leads to production of cold, oxygen-rich dense shelf water (DSW) with densities of up to 27.0σθ [Shcherbina et al., 2003]. The DSW is transported southward into the intermediate layer of the deep Okhotsk basin in the southern Okhotsk Sea, and mixed with intermediate water coming from the North Pacific. This mixing forms the coldest, freshest and oxygen-richest water in the North Pacific in the density range of 26.8–27.4σθ [Talley, 1991], which is called Okhotsk Sea Mode Water [Yasuda, 1997] or Okhotsk Sea Intermediate Water (OSIW) [Itoh et al., 2003]. The signal of OSIW extends downward to 27.4σθ owing to diapycnal mixing caused by strong tidal currents around the Kuril Straits [Wong et al., 1998].
 The OSIW outflows to the North Pacific through the Kuril Straits, mainly Bussol' Strait [Talley, 1991], and then mixes with East Kamchatka Current Water, which flows southwestward along the northern Kuril Islands, forming the Oyashio water. The Oyashio water extends to the intermediate layer, flowing southwestward along the Kuril Island chain as the western boundary current of the subarctic gyre. The Oyashio water reaches the confluence of the subtropical and subarctic gyres, and then part of the Oyashio water flows northeastward as the Subarctic Current (SAC), bounding the subarctic gyre on the south.
 Recently, warming of intermediate water in the Sea of Okhotsk [Hill et al., 2003] (also M. Itoh, Warming of intermediate water in the Sea of Okhotsk since the 1950s, submitted to Journal of Oceanography, 2007) (hereinafter referred to as Itoh, submitted manuscript, 2007) and decreasing trends of dissolved oxygen content in intermediate water in the Oyashio [Ono et al., 2001] and western subarctic [Andreev and Watanabe, 2002; Emerson et al., 2004] regions have been reported. However, the spatial extent and origin of these trends have not been clarified. In this paper, we examine the origin and spatial extent of the warming and oxygen decreasing trends of the intermediate water in these regions, by constructing gridded data set on isopycnal surfaces on the basis of all available data.
2. Data and Methods
 Temperature, salinity and dissolved oxygen data were mainly taken from the World Ocean Database (WOD01) [Conkright et al., 2002]. In addition, we used observational data obtained by the Japan-Russia-United States international joint study of the Sea of Okhotsk from 1998 to 2004 and data archived by the Japan Oceanographic Data Center. We also used profiling float data obtained by the international Argo program from 2000 to 2004. The data from the recent international project in the Sea of Okhotsk and the Argo project enable us to discuss linear trend of intermediate water mass property until very recently. In this study, we focused on the density range 26.8–27.4σθ, on which the influence of deep water is small and to which winter convection does not directly reach in the open North Pacific [Reid, 1965]. These isopycnal surfaces correspond to the intermediate layer influenced by ventilation from the Sea of Okhotsk [Talley, 1991; Warner et al., 1996]. Data at discrete depths, mostly from bottle samples, were linearly interpolated to 1 m intervals, and then the values of potential temperature and dissolved oxygen were selected at 0.1σθ intervals. We applied a quality control, resulting in a data set of ∼63,000 stations at 27.0σθ in the northwestern North Pacific and the Sea of Okhotsk (35°–60°N, 135°E–140°W). For dissolved oxygen, the number of stations at 27.0σθ was about 30% of that for temperature.
 Since the seasonal variation is negligible in the intermediate depths, annual mean climatologies of potential temperature and dissolved oxygen were calculated on a 0.25° latitude/longitude grid with a method similar to that by Levitus and Boyer . Since water mass properties in the Okhotsk Sea are distinctly different from those in the northwestern North Pacific, we produced their climatologies separately. For constructing the gridded data set, we used weighted averaging with a Gaussian window. We chose 150 km as a half-width of the window and 75 km as an e-folding scale to resolve regional features in the Sea of Okhotsk and the boundary current and fronts in the North Pacific. If the number of observations within the window of the center of a grid box was less than 5, that grid box was regarded as a no-data box. The derived climatologies on each isopycnal surface are quite similar to those of the North Pacific Hydrobase [Macdonald et al., 2001].
 A gridded data set of potential temperature anomalies on isopycnal surfaces was then prepared for the period 1955–2004, and one of dissolved oxygen for the period 1960–2004. First, we calculated anomalies of observed values as differences of the observed values from the produced climatologies. All the calculated anomalies were gridded by using simple averaging in a yearly 2.5° × 2.5° grid box, taking account of the trade-off between spatial and temporal resolution. Neither spatial interpolation nor spatial smoothing was applied to the anomaly field.
Figure 1 shows linear trend maps of intermediate water temperature on selected isopycnal surfaces for the last 50 years. Significant warming trends are observed in the northwestern North Pacific and the Sea of Okhotsk on all isopycnal surfaces. The warming trend in these regions is most prominent at density 27.0σθ, and the largest warming area exists in the western part (47.5°–55°N, 145°–147.5°E) of the Sea of Okhotsk with an average of 0.68°C/50-yr (Figure 1b). At 27.2σθ, the magnitude of the linear trend is smaller than that at 27.0σθ, though the spatial pattern is similar. On the other hand, the spatial pattern of the linear trend at 26.8σθ is different from those at 27.0 and 27.2σθ; warming occurs in the western subarctic gyre, while cooling occurs over the region from the Kuroshio Extension to the south of the subpolar front (Figure 1a). The cooling trend is consistent with the fact that the significant cooling occurred down to the depth of 400 m, which roughly corresponds to the 26.8σθ surface, in the early 1980's [Deser et al., 1996].
 The warming trend at 27.0σθ seems to extend along the pathway of the OSIW. Climatology of the acceleration potential at 27.0σθ (Figure 2) shows that the western subarctic gyre, which consists of the Oyashio and Subarctic Current (SAC), extends to the intermediate depth of 27.0σθ. A significant warming trend is observed in the Oyashio and SAC regions, but not in the East Kamchatka Current region, i.e., upstream of the Sea of Okhotsk. Since the intermediate water masses in the Oyashio and SAC regions are largely affected by the OSIW [Yasuda, 1997], these results indicate that the warming trend in the northwestern North Pacific may be caused by advection of warmed OSIW.
Figure 3a shows the time series of temperature anomalies at 27.0σθ for the Sea of Okhotsk, Oyashio and SAC regions (Figure 2). A positive linear trend is the most significant feature in all three regions. Table 1 summarizes estimates of the linear trend of potential temperature from 26.8 to 27.4σθ for each region. The warming trend is most prominent at 26.9–27.0 σθ in the above three regions. At 27.0 σθ, the temperature has increased by 0.62 ± 0.18°C (significant at 99% confidence interval) in the Sea of Okhotsk during the past 50 years from 1955 through 2004, which is roughly consistent with the result reported by Itoh (submitted manuscript, 2007). The magnitude of the warming trend in the other two regions is about half of that.
Table 1. Linear Trends of Potential Temperature (°C/50-yr) and Dissolved Oxygen (ml/l/45-yr) Anomaly at Density 26.8–27.4 σθ in the Sea of Okhotsk, Oyashio, and SAC regionsa
Bold numbers indicate that the trend is significant at the 99% confidence level. The statistical significance is based on a Student t distribution.
 We next examine the linear trend of dissolved oxygen content. For all three regions, significant negative trends are found (Figure 3b). The decrease of dissolved oxygen is most prominent at 26.9–27.0σθ, where the warming trend is most prominent (Table 1). At 27.0σθ, the linear trend in the Sea of Okhotsk is −0.58 ± 0.34 ml/l (significant at 95% confidence interval) for the past 45 years. The Oyashio and SAC regions have the value less than that for the Sea of Okhotsk. It is noted that the oxygen trend difference between the Sea of Okhotsk and Oyashio regions is relatively small when compared to the temperature trend difference between the two regions. Table 1 shows that the trends in temperature and dissolved oxygen are significant up to 27.4σθ for the Sea of Okhotsk and up to 27.2–27.3σθ for the Oyashio and SAC regions. Taking account of the effect of temperature variation on oxygen solubility, we examine trends of apparent oxygen utilization (AOU) of intermediate waters. The trend of AOU is approximately inversely proportional to that of dissolved oxygen, and both trends are equally statistically significant.
 It is shown that warming and oxygen-decreasing trends in the intermediate water are most prominent in the Sea of Okhotsk. Moreover, these trends appear to extend to the northwestern North Pacific along the pathway of the water mass originating from the Sea of Okhotsk. These facts suggest that trends in the northwestern North Pacific are due to preceding changes of water-mass properties in the Sea of Okhotsk. Intermediate water in the Sea of Okhotsk retains its cold and oxygen-rich properties by mixing with dense shelf water (DSW) associated with sea ice production in the coastal polynya of the northwestern shelf. The largest warming trend occurs in the western Sea of Okhotsk (Figure 1b), to which DSW is transported from the northwestern shelf [Fukamachi et al., 2004]. Therefore, we suppose that the main cause of the warming and oxygen-decreasing trends is the weakening of DSW production.
 Although reliable estimation of DSW production is not yet available, there is some indirect evidence for a decrease trend in DSW production. Figure 4 shows the time series of surface air temperature anomaly in the cold season averaged over northeastern Eurasia, which is upwind of the Sea of Okhotsk; this air temperature can be an index of sea ice extent. This air temperature has increased considerably during the past 50 years (2.0 ± 1.4°C/50-yr, significant at 99% confidence level). Sea ice extent in the Sea of Okhotsk derived from satellite measurements, which is highly correlated with this air temperature (r = −0.61, significant at 95% confidence level), has decreased (−9.2%/27-yr) (Figure 4). Although satellite measurements have only been available since the 1970's, visual observations at Hokkaido coast, located on the southern boundary of sea ice extent in the Sea of Okhotsk, show the decreasing trend of sea ice season length during the past 100 years [Aota, 1999]. These trends of air temperature and sea ice season suggest that sea ice extent, accordingly sea ice production, have likely decreased during the past 50 years. During the current global warming, the surface air temperature anomaly in autumn and winter is particularly large over northeastern Eurasia [Serreze et al., 2000]. The DSW production area of the northwestern shelf in the Sea of Okhotsk is located where the winter monsoon from northeastern Eurasia directly transports cold air masses. Therefore, intermediate water in the Sea of Okhotsk which is ventilated through DSW may be sensitive to the global warming.
 Several studies showed that most of the North Pacific has freshened in the upper layer recently [Wong et al., 1999; Boyer et al., 2005], which may be also related to the global warming. Also for the Sea of Okhotsk, Hill et al.  reported a possibility of freshening in the upper layer. The freshening in the upper ocean would strengthen the stratification and thus weaken the ventilation or DSW production.
 Recent studies suggest that OSIW has a significant role in material circulation of the intermediate layer in the North Pacific. Hansell et al.  indicated that dissolved organic carbons in NPIW originate from the Sea of Okhotsk. Nakatsuka et al.  showed that large amounts of dissolved and particulate organic carbons are exported from the highly productive northwestern shelf into the intermediate layer in the Sea of Okhotsk through the outflow of DSW. Moreover, recent observational data show that in the northwestern North Pacific, iron, which is an essential micronutrient for phytoplankton, may come from the intermediate water of the Sea of Okhotsk [Nishioka, 2004]. The co-occurrence of warming and decrease in dissolved oxygen concentration in the northwestern North Pacific, originating from the Sea of Okhotsk, implies that overturning in the northwestern North Pacific has weakened in the sense of material cycle. Therefore, such a trend has a possibility of substantial impacts on the material cycle and biological productivity in the North Pacific.
 The Argo float data used in this study were collected and made freely available by the International Argo Project and the national programs that contribute to it. Part of the hydrographic data in the Okhotsk Sea were obtained under the cooperation with Far Eastern Regional Hydrometeorological Research Institute, Scripps Institute of Oceanography, and S. C. Riser of University of Washington. We wish to thank M. Itoh and K. Ono for data processing and anonymous reviewers for their constructive comments. Some figures are produced with the GrADS package.