Changes in water properties around North Pacific intermediate water between the 1980s, 1990s and 2000s

ARTICLE IN PRESS Deep-Sea Research II 57 (2010) 1177–1187

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Changes in water properties around North Pacific intermediate water between the 1980s, 1990s and 2000s Shinya Kouketsu a,, Masao Fukasawa a, Daisuke Sasano b, Yuichiro Kumamoto a, Takeshi Kawano a, Hiroshi Uchida a, Toshimasa Doi a a b

Institute of Observational Research for Global Change, Japan Agency for Marine-Earth Science and Technology, 2-15, Natsushima-cho, Yokosuka 237-0061, Japan Meteorological Research Institute, 1-1 Nagamine, Tsukuba 305-0052, Japan

a r t i c l e in fo

abstract

Article history: Received 8 December 2009 Accepted 16 December 2009 Available online 11 January 2010

We detected temperature (salinity) decreases on the neutral density surfaces above the salinity minimum and increases below throughout the western North Pacific subtropical gyre (253 N2403 N, west of 1703 W) by comparing World Ocean Circulation Experiment (WOCE) hydrographic sections from the 1980s, 1990s, and 2000s. The temperature (salinity) changes around the salinity minimum were associated with an apparent oxygen utilization (AOU) increase, and could not be explained by an increase of the subpolar input. These property changes were due to a density decrease of the salinity minimum. Furthermore, similar changes in temperature (salinity) were detected along 1653 E as a 12year trend from 1996 to 2007. This suggests that the changes evident in the comparison of the WOCE sections are the representative of changes at the intermediate depths in the last decade. This pattern was not detected in the eastern subtropical gyre (253 N2403 N, east of 1703 W) and along 473 N, where the AOU increase due to ventilation reduction has been reported. Along 473 N, changes in temperature, salinity and AOU from 1999 to 2007 were opposite to those from 1985 to 1999. & 2010 Elsevier Ltd. All rights reserved.

Keywords: NPIW WHP Decadal change Apparent oxygen utilization

1. Introduction North Pacific Intermediate Water (NPIW) is widely observed as a salinity minimum at depths of 3002800 m in the North Pacific subtropical gyre (the region with salinities o 34:2 in Fig. 1) (Sverdrup et al., 1942; Reid, 1965). The waters responsible for origins of the low salinity of NPIW originate from the Sea of Okhotsk and the Gulf of Alaska in the subarctic gyre (e.g., Reid, 1965; Talley, 1991; You et al., 2000). The circulation of NPIW play an important role in transporting the low-salinity and nutrientrich subarctic water to the intermediate depths of the subtropical gyre. This nutrient-rich water can diffuse to the surface in the subtropical gyre, and may be important for the biological productivity there (Sarmiento et al., 2004). The circulation associated with NPIW has been identified as a medium for transferring water that has absorbed CO2 at the sea surface to the mid-depths of the subtropical gyre (e.g., Tsunogai et al., 1993; Ono et al., 2003). Furthermore, the formation and distribution of NPIW might influence the current structures in the North Pacific, because NPIW contains Oyashio water with low potential vorticity (Tatebe and Yasuda, 2004).

 Corresponding author.

E-mail address: [email protected] (S. Kouketsu). 0967-0645/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr2.2009.12.007

NPIW is presumed to form mainly in the Mixed Water Region, which is the region between the Kuroshio Extension and the Subarctic Front east of Japan (e.g., Hasunuma, 1978; Talley, 1993). Oyashio water has low temperature, low salinity and low potential vorticity, and is one of the source waters of NPIW, and it is transported to the intermediate depths of this region (e.g., Yasuda et al., 1996; Shimizu et al., 2001). This water is then modified by mixing with warm and saline Kuroshio water transported by the Kuroshio. Through mixing, the new NPIW is formed by about 1503 E (Masujima et al., 2003). NPIW formed in the Mixed Water Region is then distributed along the circulation in the subtropical gyre (e.g., You et al., 2003). Long-term changes where NPIW is formed and modified might be reflected in variability of NPIW properties in the subtropical gyre. Wong et al. (1999) compared salinities observed during the World Ocean Circulation Experiment (WOCE) in the 1980s with the historical data from the 1960s and reported that the core of NPIW had freshened by 0.017 around 243 N in the North Pacific. Nakano et al. (2005) found inter-annual variability in the crosssectional area of NPIW core in a section along 1373 E and explained that the variability was mainly caused by the east–west shift of the NPIW distribution, which extends as a tongue from east to west. Furthermore, long-term freshening by 0:0015 y1 above the salinity minimum was detected along the same section; this freshening was caused by deepening of the isopycnal surfaces, mainly due to warming (Nakano et al., 2007). Kouketsu et al.

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(2007) reported that the pattern of salinity and temperature decreases above the salinity minimum and increases below it were detected in the southwest subtropical region from a comparison of WOCE hydrographic sections and later revisits on

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neutral density ðgn Þ surfaces, and suggested that the changes might be caused by warming of NPIW. However, they showed only changes in potential temperature and salinity in the western subtropical gyre, and the distribution of the NPIW changes was not described in detail. In the present study, we show changes in salinity and temperature since the 1980s by comparing observations along revisited WOCE sections with the original WOCE observations throughout the subtropical gyre and the subarctic gyre with updated sections along 1803 and 473 N. From the updated sections, a pattern in temperature changes on neutral density surfaces due to the temperature increase, which was reported in Kouketsu et al. (2007), was also detected north of 303 N. In addition, we show changes in apparent oxygen utilization (AOU), which were not shown in Kouketsu et al. (2007). We report that the AOU changes were consistent with the result that the temperature decreases on neutral density surfaces above the salinity minimum (Kouketsu et al., 2007) were not due to increase of subpolar input. Furthermore, using long-term observational data, we suggest that the changes widely detected in the western subtropical gyre are representative of changes during the 1990s and 2000s clearly.

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2. Data and methods

20°N

80

240

0

0° AOU

-1

Fig. 1. Distribution of salinity and apparent oxygen utilization ðm mol kg Þ on gn ¼ 26:9 kg m-3 ðsy C 26:8 kg m-3 Þ, where the salinity minimum is frequently observed. This figure was produced from the World Ocean Atlas 2005 (WOA05) (Antonov et al., 2006; Garcia et al., 2006): (a) Salinity and (b) AOU.

We used the data from the hydrographic sections collected under the WOCE Hydrographic Programme (WHP) in the 1980s and 1990s and the revisited sections from the 2000s (Fig. 2, Table 1). Most data are available from the Climate Variability and Predictability/Carbon Hydrographic Data Office (CCHDO; http:// whpo.ucsd.edu/), and we used additional data from revisits to sections P1, P3 and P14, which we conducted. These data sets

Table 1 The WHP and revisited sections used in this study. Section

Location

Years

Chief scientists

Research vessels

P1 P1C P1E P1W P1 P2C P2E P2W P2 P3

473 N

1985 1999 1999 1999 2007 1994 1993 1994 2004 1985

L. D. Talley (SIOa) M. Fukasawa (TUb) R. G. Perkin (IOSc) T. Watanabe (JFAd) T. Kawano (JAMSTEC) M. Fukasawa (TU) T. Bando (MSAe) Y. Iwanaga (MSA) J. H. Swift (SIO) J. H. Swift (SIO) D. H. Roemmich (SIO) T. Kawano (JAMSTEC) A. Murata (JAMSTEC) S. Watanabe(JAMSTEC) M. Hole (WHOIf) T. Kawano (JAMSTEC) G. I. Roden (UWg) T. Kawano (JAMSTEC) J. L. Bullister (PMELh,SIO) L. D. Talley (SIO) C. L. Sabine (PMEL) R. A. Feely (PMEL) D. Masgrave (UAi) M. Fukasawa (TU)

Thomas G. Thompson (USA) Mirai (Japan) John P. Tully (Canada) Kaiyo maru (Japan) Mirai (Japan) Bosei maru (Japan) Shoyo maru (Japan) Shoyo maru (Japan) Melville (USA) Thomas G. Thompson (USA)

303 N

243 N

P3

P10 P10 P14 P14 P16 P16 P16 P17 P17 a

2005

1503 E 3

180

1503 W

1353 W

SIO: Scripps Research Institute. TU: Tokai University. IOS: Institute of Ocean Sciences. d JFA: Japan Fisheries Agency. e Japanese Maritime Safety Agency. f WHOI: Woods Hole Oceanographic Institution. g UW: University of Washington. h PMEL: Pacific Marine Environmental Laboratory, NOAA. i UA: University of Alaska. b c

1993 2005 1993 2007 1991 1991 2006 1993 2001

Mirai (Japan)

Thomas G. Thompson (USA) Mirai (Japan) Thomas G. Thompson (USA) Mirai (Japan) Discoverer (USA) Thomas Washington (USA) Thomas G. Thompson (USA) Thomas G. Thompson (USA) Mirai (Japan)

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were carefully processed and checked for accuracy. In these WHP sections, the accuracies of temperature and salinity measurements are 0:001 3 C and 0.002, respectively. The accuracy of oxygen 1 in the deep layer. We measurements is about 1 mmol kg neglected differences less than 0:003 3 C ( C 0:0003 3 C y1 for almost the sections) in comparing temperature values. However, compatibility is a problem when comparing salinity and oxygen values between different sections. Johnson et al. (2001) estimated the standard deviation for differences in measurements at crossover points of the WOCE sections to be about 2%. For salinity, the standard sea water (SSW) batch correction for WOCE expeditions led to a significant reduction in the salinity variance at crossovers of WHP sections in the Pacific. In order to avoid the apparent density differences in deep layers that result from bias in salinity measurements, we applied the latest batch correction (Kawano et al., 2006) to salinity data, except for those data from the western part of P2 in 1994, for which the batch number has not yet been provided. For oxygen, we neglected differences less 1 1 than 3 mmol kg ( C 0:3 m mol kg ; y1 for almost the sections), which is about 2% of oxygen value in the intermediate layers from gn ¼ 26:6 ðsy C 26:5 kg m3 Þ togn ¼ 27:4 kg m3 ðsy C 27:3 kg m3 Þ. The accuracy of the measurements in the WHP sections was high enough to allow detection of the changes on neutral density surfaces in the intermediate layers in the North Pacific during recent decades (shown in Section 3). To investigate the variability of water property changes, we used data from long-term conductivity, temperature, and depth (CTD) profiler observational sections along 1653 E, which has been maintained by the Japan Meteorological Agency (JMA)

120°E

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180°

150°W

120°W

60°N P1 JMA

40°N

P17

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0° Fig. 2. Map showing the locations of the hydrographic observation sections used in this study. Blue and red lines denote the WHP and revisit sections, respectively. Green line indicates long-term observational sections that has been maintained by the Japan Meteorological Agency (JMA).

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Fig. 3. All CTD stations along 1653 E used in this study.

1179

(Figs. 2 and 3). The CTD observations were carried out at 13 intervals from 1996 to 2007. We compared the water properties on the neutral density surfaces in all hydrographic sections to avoid the effects from the heaving of isopycnal depths caused by short-term variability such as mesoscale eddies. We used the method of Jackett and McDougall (1997) to calculate neutral density surfaces in this study. It is noted that differences between density values in the neutral density and potential density are about 0:1 kg m3 in most of the areas in the North Pacific.

3. Results 3.1. Property changes along WHP sections We normalized the potential temperature changes on neutral density surfaces to annual rates of change so that the amplitudes of the changes along each section are comparable (Figs. 4 and 5). Along 303 N and 243 N, the potential temperature changes on neutral density surfaces followed patterns that were almost the same (Fig. 4B and C). West of 1703 W, potential temperature (salinity) decreased above the salinity minimum (at gn ¼ 26:9 kg m3 ) and increased below, as reported by Kouketsu et al. (2007). The amplitude of the changes around the salinity minimum along 303 N ð 4 0:010 3 C y1 Þ was larger than that along 243 N. Since the annual rates of change corresponded to temperature (salinity) differences of 0:1 3 C (0.01) over about 10 years, they were large enough to detect with data from the WHP sections. East of 1703 W, we detected a pattern similar to that around the salinity minimum west of 1703 W, but the boundary between positive and negative changes at around gn ¼ 26:5 kg m3 ðsy C 26:4 kg m3 Þ. The amplitudes of the changes were more than 0:050 3 C y1 in the layers with gn ¼ 26:0 kg m3 ðsy C 25:9 kg m3 Þ and more than 0:010 3 C y1 in the layers around gn ¼ 26:7 kg m3 ðsy C 26:6 kg m3 Þ. These changes were detected around the shallow salinity minimum, which are frequently observed east of 1653 W (Reid, 1973; Talley, 1993). Changes in the layers for gn o 26:0 kg m3 were corresponding to changes in the properties of eastern subtropical mode water defined by Hautala and Roemmich (1998), and the recent invasion of subarctic water near the west coast of America (e.g., Freeland et al., 2003). Along 473 N (section P1), temperature minimum in the layers with sy ¼ 26:2226:6 kg m3 and maximum in the layers with sy ¼ 26:7227:1 kg m3 , are frequently observed (e.g., Uda, 1963; Ueno and Yasuda, 2000), although no salinity minimum is observed. From 1985 to 2007, potential temperature increased around 1703 E and 1653 W in the layers from gn ¼ 26:7 to gn ¼ 27:2 kg m3 , where the local maximum of temperature are observed (Fig. 4A). In the deep layers (from gn ¼ 27:3 to gn ¼ 27:8 kg m3 ), potential temperature increased throughout the section. Along 1503 E (section P10), the pattern of decreasing temperatures above the salinity minimum and increasing temperatures below was also detected between 203 N an 283 N, a pattern also reported by Kouketsu et al., 2007 (Fig. 5A). Furthermore, we also detected a similar pattern between 243 N and 423 N along 1803 (P14) (Fig. 5B) as one of the new findings in this study. The temperature increase below the salinity minimum was over 0:010 3 C y1 and the decrease was less than 0:050 3 C y1 . However, the pattern around the salinity minimum was not clear along 1503 W (P16), and temperature increases of more than 0:010 3 C y1 were detected north of 203 N in the layers with gn 4 26:0 kg m3 (Fig. 5C). Temperature increases of more than 0:010 3 C y1 also appeared along 1353 W south of 403 N (Fig. 5D). These changes along 1353 W were the same as those detected in the eastern part of the sections along 243 N and 303 N.Though these

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25.0

0.04

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34.6

26.0

33.8

4.2

26.5

3

34.2

0.02 0.01 0.00 −0.01

27.0 34.2

34.2

27.5

−0.02 −0.03

28.0

−0.04

P3 Fig. 4. Potential temperature changes ð C y-1 Þ on neutral density surfaces for (a) P1 : 473 N (2007 values minus 1985 values), (b) P2 : 303 N (2004–1993, 1994), and (c) P3 : 243 N (2005–1985). Contours denote the salinities for revisit observations, with a contour interval of 0.2. (a) and (b) are adopted from Kouketsu et al. (2007). White and black regions denote areas with changes less than 0:0003 3 C y-1 and with no value (due to outcrop, topography, and no observation station), respectively. Green curves denote the late winter mixed-layer density calculated from sea-surface temperatures and salinities in WOA05 (Locarnini et al., 2006; Antonov et al., 2006). 3

positive temperature changes reached as far as 423 N, there was a different pattern north of 423 N. In the layers with gn 426:0 kg m3 , the patterns of changes in potential temperature (salinity) on neutral density surfaces were different among three regions. The pattern that the positive and negative changes were separated by the salinity minimum was observed in the western subtropical gyre (253 N2403 N, west of 1703 W). In the eastern subtropical gyre (253 N2403 N, east of 1703 W), potential temperature increased around gn ¼ 26:7 kg m3 ðsy C 26:6 kg m3 Þ and decreased above gn ¼ 26:0 kg m3 ðsy C 25:9 kg m3 Þ. The potential temperature increases were detected in the layer where the local maximum of temperature are observed (gn ¼ 26:6 and gn ¼ 27:2 kg m3 ) around 1703 E and 1653 W in the subarctic gyre (along 473 N). In each region, we compare the changes in potential temperature (salinity) with those in apparent oxygen utilization (AOU) below, which were not shown in Kouketsu et al. (2007). Although AOU is calculated as the difference between the measured oxygen concentration and the oxygen saturation concentration estimated following Garcia and Gordon (1992), saturation concentration changes due to the temperature changes were small enough to neglect for most of the sections.

In the western subtropical gyre, AOU increased both above and below the NPIW salinity minimum (Figs. 6BC and 7AB), which separates the positive and negative changes in potential temperature and salinity (Figs. 4BC and 5AB). This shows that the temperature (salinity) decreases on neutral density surfaces above the salinity minimum cannot be explained simply by an increase in the subpolar input, since the increase in AOU implies that the subpolar input to the subtropical gyre decreased (see. Fig. 1B). Because the potential temperature and salinity relationships corresponding to the changes detected west of 1703 W around the salinity minimum appear to be shifting in the direction of the temperature increase (Fig. 8), these property changes can be revealed by a density decrease of the salinity minimum due to the temperature increase of the NPIW core. This temperature increase of NPIW core appear as the typical property change pattern on the neutral density surfaces around the salinity minimum in the vertical sections (Figs. 4 and 5) , while the salinity decrease around the salinity minimum was also detected in the temperature and salinity relationships (Fig. 8). The pattern of change in AOU along 303 N east of 1703 W (Fig. 6B) is similar to the pattern observed for changes in potential temperature (Fig. 4B). These temperature and AOU increase

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Fig. 5. Same as Fig. 4, but for (a) P10:1503 E (2005–1994), (b) P14:1803 (2007–1993), (c) P16:1503 W (2006–1991), and (d) P17:1353 W (2001–1993).

(decrease) in the layers with gn 426:5 kg m3 ðgn o 26:5 kg m3 Þ east of 1703 W were due to decrease (increase) in advection of water from the subarctic region along the neutral density surfaces. Along 1503 W, AOU also increased in the layers around gn ¼ 26:7 kg m3 (Fig. 7C), which were lighter density layers than where AOU increase were detected along 1503 E and 1803 (gn ¼ 26:9 kg m3 around the salinity minimum in Fig. 7A and B). Mecking et al. (2008) studied about the AOU increases on gn ¼ 26:7 kg m3 ðsy ¼ 26:6 kg m3 Þ along 1503 W and 303 N in detail, and have been reported and suggested the AOU increases were caused by reduction of ventilation. The potential temperature changes might be affected by the ventilation changes west of 1703 W along 243 N(Fig. 6C), because the horizontal pattern of AOU changes around gn ¼ 26:7 kg m3 was also similar to that of temperature changes. However, AOU increased in the nearsurface layers ðgn o25:5 kg m3 Þ. In these layers, changes in biological production are also important to AOU changes. In the subtropical gyre, the pattern of changes in temperature (salinity) and AOU east of 1703 W was different from that west of 1703 W. East of 1703 W, changes in the Alaskan gyre could affect the properties of water around the NPIW salinity minimum because of the estimated transport of 0.2 Sv of Gulf of Alaska intermediate

water (GAIW) to the layers containing NPIW in the eastern subtropical gyre (You et al., 2003). In the subarctic gyre (along 473 N), AOU changes (Fig. 6A) showed a pattern different from more southern sections at 243 N and 303 N. The AOU increase associated with the temperature increase was detected around gn ¼ 26:7 kg m3 ðsy C26:6 kg m3 Þ around 1703 E, and this AOU increase was not detected east of 1653 W from 1985 to 2007. Throughout this section, an AOU increase around gn ¼ 26:7 kg m3 was detected from 1985 to 1999 (Emerson et al., 2004). In this study, we detected the opposite changes in temperature (salinity) as well as in AOU after 1999 (Fig. 9).

3.2. Temperature variability along long-term sections In the previous subsection, we showed that similar water property changes around NPIW were observed widely. However, there was no clear time scale for the changes. To clarify the variability around the NPIW, we used long-term observation sections along 1653 E (see Fig. 2), which is located at the center of

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Fig. 6. AOU changes ðmmol kg y-1 Þ on neutral density surfaces for (a) P1 : 473 N (2007–1985), (b) P2 : 303 N (2004–1993, 1994), and (c) P3 : 243 N (2005–1985). White and -1 Black regions denote areas with changes less than 0:3 m mol kg y-1 and with no value (due to outcrop, topography, and no observation station), respectively. Contours denote the salinity distribution for the revisit observations; contour interval is 0.2.

the region showing the pattern of potential temperature decreases above the salinity minimum and increases below. The 12-year trend on neutral density surfaces at each station along 1653 E was estimated from the long-term observations (Fig. 10). The potential temperature decreased above the salinity minimum and increased below in the area between 243 N and 403 N. The amplitudes of these changes were up to 0:010 3 C y1 and the confidence levels for the trend were over 0.8. Since this pattern is similar to that in the changes between WHP sections, this widely observed pattern might be the representative of change during the 1990s in the North Pacific subtropical gyre. To show the spatial differences in the property changes around the salinity minimum, we calculated the potential temperature changes averaged on the neutral density surfaces for three regions along 1653 E : 253 N2303 N, 303 N2353 N, and 353 N2403 N (Fig. 11). Although there was large short-term variability in the northern and southern regions on both gn ¼ 26:7 kg m3 ðsy C 26:6 kg m3 Þ and gn ¼ 27:2 kg m3 ðsy C 27:1 kg m3 Þ, which might have been caused by the Kuroshio Extension and subarctic front shifts or by mesoscale eddies, the potential temperature decrease above the salinity minimum and the increase below were detected in the central region ð303 N2353 NÞ (Fig. 11A and C). The changes

explained by the density decrease of the salinity minimum before 2004 were revealed in the potential temperature-salinity relationships (Fig. 12). The temperature (salinity) decrease at the salinity minimum isopycnal of gn ¼ 26:9 kg m3 after 2004 showed the freshening of the NPIW (Fig. 11B). The freshening appeared both above and below the salinity minimum after 2004 (Fig. 12), and the clear temperature (salinity) decrease at gn ¼ 27:2 kg m3 (Fig. 11C) might have been caused by this freshening. Furthermore, no clear lags among the latitudes were detected in water property changes, although a slight potential temperature maximum during 2002–2004 in the northern region ð353 N2403 NÞ did not appear in the regions south of 353 N (Fig. 11).

4. Summary and discussion We compared data from WHP sections and revisits to those sections, and detected a temperature (salinity) decrease on the neutral density surface above the salinity minimum and an increase below throughout the western North Pacific subtropical gyre (253 N2403 N, west of 1703 W), although these changes were detected only in the southwestern region of the subtropical gyre

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Neutral density (kg m−3)

25.5

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28.0

−2.0

P16

P17

3

3

Fig. 7. Same as Fig. 6, but for (a) P10 : 150 E (2005–1994), (b) P14 : 180 (2007–1993), (c) P16 : 150 W (2006–1991), and (d) P17 : 1353 W (2001–1993).

Salinity

6

34.0

26.2

10 26.4

8

34.2

26.6

26.8

Potential temperature (°C)

Potential temperature (°C)

10

34.1

8

6

34.0 10

26.6

26.8

34.1

34.2

26.2

26.4

8

6

27

26.6

26.8

27

4

4 24° N

34.2

26.2

26.4

27

4

34.1

Salinity

Potential temperature (°C)

Salinity 34.0

3

30° N

180°

Fig. 8. Potential temperature-salinity relationships for WOCE (dashed curve) and the revisit (solid curve) sections averaged on neutral density surfaces from 1503 E to 1703 W for along (a) 243 N, (b) 303 N and (c) from 253 N to 403 N along 1803 . Solid and dashed curves represent the confidence levels of 0.8 and 0.7, respectively. Gray contours denote potential density.

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150°E 160°E 170°E 180° 170°W 160°W 150°W 140°W 130°W

150°E 160°E 170°E 180° 170°W 160°W 150°W 140°W 130°W

0.04

25.5

0.03

26.0

0.02 0.01

26.5

0.00

33.4

27.0

−0.01

33.8

−0.02

34.2

27.5

−0.03 34.6

28.0

−0.04

25.0 Neutral density (kg m−3)

Neutral density (kg m−3)

25.0

2.0

25.5

1.5

26.0

1.0 0.5

26.5

0.0

27.0

−0.5

27.5 28.0

−1.0

34.2

−1.5 34.6

−2.0

1999–1985

1999–1985

150°E 160°E 170°E 180° 170°W 160°W 150°W 140°W 130°W

150°E 160°E 170°E 180° 170°W 160°W 150°W 140°W 130°W

0.03 0.02

33

26.0

0.01

26.5

0.00

33.4

27.0

−0.01

33.8

−0.02

34.2

27.5

−0.03 34.6

28.0

−0.04

2.0

25.5

33

25.5

25.0 Neutral density (kg m−3)

0.04 33

Neutral density (kg m−3)

25.0

0.5

26.5

33.4

27.0 27.5 28.0

0.0 −0.5

33.8

−1.0

34.2

−1.5 34.6

−2.0

2007–1999 -1

1.5 1.0

33

26.0

2007–1999 -1

-1

3

Fig. 9. Changes in potential temperature ð C y Þ and AOU ðm mol kg y Þ on neutral density surfaces along P1 : 47 N for (a, b) 1999–1985 and (c, d) 2007–1999. Contours denote the salinity distribution; contour interval is 0.2. 3

0° 25.0

10°N 20°N 30°N 40°N 50°N 0.04

25.5

0.03

26.0

0.02 0.01

26.5 27.0 27.5

0.00 −0.01 −0.02 −0.03

28.0

−0.04

Fig. 10. Potential temperature trend ð3 C y-1 Þ over 12 years (1996 to 2007) on neutral density surfaces along 1653 E. Gray curve is the 34.2 contour for salinity. Solid and dashed curves represent the confidence levels of 0.8 and 0.7, respectively. Green curves denote the late winter mixed-layer density calculated from sea-surface temperatures and salinities in WOA05 (Locarnini et al., 2006; Antonov et al., 2006). Gray regions denote areas with no value (due to outcrop, topography, and no observation station), respectively.

in the previous studies (Kouketsu et al., 2007; Nakano et al., 2007). AOU increased in the intermediate layers where the typical temperature (salinity) change pattern was detected around the salinity minimum. Since the AOU increase, which was not shown in Kouketsu et al. (2007), is presumed to be due to a decrease in the subpolar input, under the assumption that changes in the oxygen utilization rate from biological activity are negligible, the temperature (salinity) decrease on the neutral density surface above the salinity minimum cannot be explained by an increase in the subpolar input. From changes in potential temperature and salinity relationships, the pattern of the temperature (salinity) decrease on the neutral density surface above the salinity minimum and increase below was caused by temperature increase of NPIW core. Furthermore, this pattern of the tempera-

ture (salinity) changes detected in the vertical sections was also detected along 1653 E as the general 12-year trend from 1996 to 2007. This suggests that the changes evident in the comparisons of the WHP sections are the representative of changes in the intermediate depths in the subtropical gyre in the last decade. In this study, we could not conclude whether the property changes were caused by advection or modification changes on neutral density surface without a ventilation reduction or by a ventilation reduction, since we focused on the property changes of water masses apart from the ventilation regions. The temperature increase of NPIW, which was called ‘‘warming of NPIW’’ by Kouketsu et al. (2007), might not be caused by direct warming from the atmosphere, since the isopycnal surfaces for the deep layers in the North Pacific, with gn Z26:7 kg m3 , do not outcrop widely. The density decrease of NPIW may be strongly related to the changes in the formation and modification processes east of Japan, since the property changes observed in this study were detected in the western part of the salinity minimum distribution, where NPIW is influenced by Oyashio water and not by Gulf of Alaska intermediate water (GAIW) (You et al., 2000). The recently reported trends in property changes of Oyashio water (Ono et al., 2001; Osafune and Yasuda, 2006; Takatani et al., 2007), which is one of the source waters of NPIW, are consistent with the changes that were found in this study. For example, south east of Hokkaido, the temperature of Oyashio water which is considered one of the source waters of NPIW, decreased on sy ¼ 26:4 kg m3 and increased on sy ¼ 27:0 kg m3 in the 1990s (see Fig. 3D and A in Osafune and Yasuda, 2006). The temperature, salinity, and AOU differences observed in our study might have originated from these variations in Oyashio water, and then spread widely throughout most of the subtropical gyre. This might also explain why the amplitude of the property changes in the northern regions was larger than that in the southern regions (Fig. 11). Takatani et al. (2007) suggested that changes in Oyashio water east of Japan can be mainly explained by changes in the mixing ratio between Okhotsk Sea water and western subarctic

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7.8

7.4

7.2

Potential Temperature

Potential Temperature

Potential Temperature

6.2 7.6

6.0

5.8

4.6

4.4

5.6

7.0 1996 1998 2000 2002 2004 2006 Year

1996 1998 2000 2002 2004 2006 Year

n = 26.7 kg m−3

n = 26.9 kg m−3

1996 1998 2000 2002 2004 2006 Year n = 27.2 kg m−3

Fig. 11. Time series of averaged potential temperature on the neutral density surfaces: (a) gn ¼ 26:7 kg m-3 , (b) gn ¼ 26:9 kg m-3 , and (c) gn ¼ 27:2 kg m-3 . The solid, dotted, dash-dotted, and dashed lines denote averages over the regions from 253 N2403 N, 253 N2303 N, 303 N2353 N, and 353 N2403 N, respectively.

Salinity 34.0

34.2

26.2

10 Potential temperature (°C)

34.1

1996−1998 1999−2001 2002−2004 2005−2007

26.4

8 26.6

6

26.8

27

4 Fig. 12. Potential temperature and salinity relationships averaged over 253 N2403 N for 3 years along 1653 E. Gray contours denote potential density.

gyre water. Osafune and Yasuda (2006) suggested the Oyashio water property changes are caused by changes in vertical mixing around the Kuril Straits induced by the diurnal tide whose amplitude modulated with the 18.6-year nodal cycle. These changes in themodification processes of NPIW might cause the property changes of NPIW. However, the effect of property changes in Kuroshio water, which is the other source waters for NPIW, and transport remains unclear, although the transport of the Kuroshio increased from the 1990s to the 2000s (e.g., Kouketsu et al., 2009). In addition, the property changes of Okhotsk Sea Water, which is the main source water for the low-salinity feature of NPIW (Talley, 1991; Warner et al., 1996), might also cause the changes revealed in this study, since Nakanowatari et al. (2007) reported that the temperature and AOU significant increase in the intermediate layers in the Okhotsk Sea in 50-year historical data. Along 1653 E, the property changes on isopycnals above and below the salinity minimum did not show clear time lags among the three latitudinal regions. This might be evidence of the short circulation pathway to the southern subtropical region at the

depths of the salinity minimum, which is suggested by Nishina and Fukasawa (1998) based on detailed analysis of the WHP-P2 section, and also suggested by Nakano et al. (2007) in a comparison between the freshening trend in section P2 and along 1373 E. However, lags among isopycnals was not clear, though the AOU increase was detected above and below the salinity minimum. The small decrease in AOU above the salinity minimum (around gn ¼ 26:7 kg m3 ) north of 353 N along 1803 (Fig. 7B) might be due to the vertical phase lag of signals from the NPIW formation region or due to the transport shear changes between water above and below the salinity minimum. Furthermore, changes around the salinity minimum might be affected by longer-period changes. Nakanowatari et al. (2007) reported the temperature increases of about 0:220:3 3 C in 50 years was detected along 1653 E below the salinity minimum. Since these temperature increases was smaller than the one of about 0:1 3 C in 10 years (Fig. 11C) in this study, the temperature increases in this study may be revealed by the decadal changes superimposed on the long-term trend. Wong et al. (1999) reported the salinity decreases around the salinity minimum from 1960s to 1985 along 243 N. Although the freshening around the salinity minimum continued in 1990s (Fig. 8A), this study suggested that temperature also increased after 1985. Above the salinity minimum, Deser et al. (1996) reported that low-temperature anomaly on isolevels propagated from the surface around 353 N to the depth of about250 m around 283 N (along with sy ¼ 25:0226:0 kg m3 ) from 1977 to 1991 in the central region ð1703 21453 WÞ of the North Pacific. In this study, temperature (salinity) decreases on gn ¼ 25:0226:0 kg m3 in the central region along 243 N and 303 N were detected (Fig. 4). Since the temperature decrease on neutral density surfaces appear the temperature increase in the temperature-salinity relationships (similar to the changes above salinity minimum in Fig. 8), the temperature decrease on neutral density surfaces might be due to the hightemperature anomaly on isolevels propagating from the sea surface in the northern region after 1992. Furthermore, the hightemperature anomaly at the sea surface might be consistent with the transition from the positive to the negative phases of Pacific Decadal Oscillation index, of which positive (negative) value means the basin-scale negative (positive) anomalies were observed in sea surface temperatures (Mantua et al., 1997). The changes typically observed around the salinity minimum were not clearly detected east of 1703 W, and the strong AOU

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increase around gn ¼ 26:7 kg m3 reported in previous studies (e.g., Emerson et al., 2001; Mecking et al., 2006) was shown again along with the increases in temperature and salinity. The AOU changes along 1503 W were similar to the AOU increases observed over the period between the WHP-P16 section in 1991 to the cruise carried out in 1997 (Emerson et al., 2001). These AOU increases along 1503 W were caused by a ventilation reduction as reported in previous studies (Mecking et al., 2006, 2008). This increase along 1503 W from 1991 to 1997 was due to a reduction in the ventilation as determined by the AOU data and the chlorofluorocarbon-12 ages considering oxygen utilization rate changes (Mecking et al., 2006). Furthermore, along 473 N, AOU around gn ¼ 26:7 kg m3 increased from 1985 to 1999 and decreased from 1999 to 2007. The changes along 473 N are consistent with the changes revealed by long-term observations at the eastern and western edges of the subarctic region. Ono et al. 1 (2001) reported positive trend of 1:28 mmol kg y1 and 18.5year cycle sinusoidal variability with the amplitude of 1 in AOU time series on sy ¼ 26:7 kg m3 near 22 mmol kg Hokkaido from 1975 to 2000, and the negative trend of 1 0:68 m mol kg y1 was detected in oxygen time series on sy ¼ 26:7 kg m3 at the Station Papa from 1956 to 2007 (Whitney et al., 2007). Although the value of the AOU increase from 1985 to 2007 along 473 N (Fig. 6A) was similar to these reported long-term trends, the large changes in Fig. 9 might be corresponding to the 18-year cycle changes. This pattern of changes in AOU (Fig. 9) is similar to that revealed by an ocean general circulation model that considers the biological processes (Deutsch et al., 2006). However, the changes around gn ¼ 26:7 kg m3 in the western subarctic gyre might not directly transfer to the eastern subarctic gyre, since relationships between the changes in potential temperature and AOU were different between in the region of 1703 E21703 W and in the region of 1503 W21703 W. Both potential temperature and AOU increased (decreased) from 1985 to 1999 (from 1999 to 2007) in the region of 1703 E21703 W, although potential temperature decreased (increased) but AOU increased (decreased) from 1985 to 1999 (from 1999 to 2007) in the region of 1503 W21703 W (Fig. 9). Since the local maximum of potential temperature observed in the region of 1703 E21703 W is maintained by advection of NPIW from east of Japan (Ueno and Yasuda, 2000, 2003), the potential temperature and AOU increases (decreases) from 1985 to 1999 (from 1999 to 2007) there might be due to changes in the advection of NPIW. Along 1503 W, AOUincrease around gn ¼ 26:7 kg m3 from 253 N to 503 N, although there was no clear pattern in salinity (temperature) changes (Figs. 5C and 7C), as Mecking et al. (2008) have been also pointed out. Although these changes were similar to the change at 1503 W along 473 N from 1985 to 1999 (Fig. 9AB), it remains unclear whether these changes were transferred from the north to the subtropic region along 1503 W, because there was the horizontal boundary for temperature changes at 1503 W along 473 N (Fig. 9AC). However, since our results are based on the temporary occupation of only two or three hydrographic sections, more detailed analysis is needed to clarify the propagation of the signals in the subarctic region, although Masuda et al. (2006) suggested that changes in the local maximum of potential temperature there is strongly affected by changes in the Kuroshio Extension region by using their 4D-VAR assimilation results.

Acknowledgments We thank the investigators, captains, and crews of many organizations that conducted the WHP observations and the revisits. The long-term observation data along 1653 E were provided by the JMA. We are grateful to everyone who has been maintaining

the observations for a long time. Discussions with M. Ishii, H. Ueno and T. Kobayashi were very useful. Detailed comments from the anonymous reviewers helped to improve the manuscript. We extend our appreciation to late Dr. Nobuo Suginohara for his encouragement to study of repeat hydrography. References Antonov, J.I., Locarnini, R.A., Boyer, T.P., Mishonov, A.V., Garcia, H.E., 2006. World Ocean Atlas 2005, vol. 2, Salinity. U.S. Government Printing Office, Washington, DC. Deser, C., Alexander, T.M., S, T.M., 1996. Upper-ocean thermal variations in the North Pacific during 1970–1991. J. Climate 9, 1840–1855. Deutsch, C., Emerson, S., Thompson, L., 2006. Physical-biological interactions in North Pacific oxygen variability. J. Geophys. Res. 111, doi:10.1029/ 2005JC003179. Emerson, S., Mecking, S., Abell, J., 2001. The biological pump in the subtropical North Pacific Ocean: Nutrient sources, Redfield ratios, and recent changes. Global Biogeochem. Cycles 15 (3), 535–554. 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