Subsurface Water Masses in the Central North Pacific Transition Region: The Repeat Section along the 180 Meridian

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1 Journal of Oceanography, Vol. 59, pp. 435 to 444, 2003 Subsurface Water Masses in the Central North Pacific Transition Region: The Repeat Section along the 180 Meridian TOSHIO SUGA*, KAZUNORI MOTOKI and KIMIO HANAWA Department of Geophysics, Graduate School of Science, Tohoku University, Aoba-ku, Sendai , Japan (Received 7 June 2002; in revised form 14 October 2002; accepted 14 October 2002) A repeat hydrographic section has been maintained over two decades along the 180 meridian across the subarctic-subtropical transition region. The section is naturally divided into at least three distinct zones. In the Subarctic Zone north of 46 N, the permanent halocline dominates the density stratification, supporting a subsurface temperature minimum (STM). The Subarctic Frontal Zone (SFZ) between N is the region where the subarctic halocline outcrops. To the south is the Subtropical Zone, where the permanent thermocline dominates the density stratification, containing a pycnostad of North Pacific Central Mode Water (CMW). The STM water colder than 4 C in the Subarctic Zone is originated in the winter mixed layer of the Bering Sea. The temporal variation of its core temperature lags months behind the variations of both the winter sea surface temperature (SST) and the summer STM temperature in the Bering Sea, suggesting that the thermal anomalies imposed on the STM water by wintertime air-sea interaction in the Bering Sea spread over the western subarctic gyre, reaching the 180 meridian within a year or so. The CMW in this section originates in the winter mixed layer near the northern edge of the Subtropical Zone between 160 E and 180. The CMW properties changed abruptly from 1988 to 1989; its temperature and salinity increased and its potential density decreased. It is argued that these changes were caused by the climate regime shift in 1988/1989 characterized by weakening of the Aleutian Low and the westerlies and increase in the SST in the subarctic-subtropical transition region. Keywords: Subsurface temperature minimum, North Pacific Central Mode Water, dichothermal water, western subarctic gyre, climate regime shift. 1. Introduction The subarctic-subtropical transition region of the central North Pacific is one of the regions that show the largest interannual to interdecadal signals of winter sea surface temperature (SST) in the open ocean (e.g., Tanimoto et al., 1993; Deser et al., 1996; Nakamura et al., 1997). The large winter SST variability is associated with the variability in the atmospheric circulation above and thus presumably results from vigorous atmosphereocean interaction. Winter SST anomalies represent anomalies in the winter mixed layer temperature, which can spread into the subsurface layer through advective and diffusive processes in the ocean. It has been shown that some of the decadal SST signals in the subarctic-subtropi- * Corresponding author. suga@pol.geophys.tohoku.ac.jp Copyright The Oceanographic Society of Japan. cal transition region are subducted into the subtropical thermocline (Deser et al., 1996; Schneider et al., 1999), which is apparently consonant with ventilated thermocline theory (Luyten et al., 1983) and likely involves changes in thermocline water masses. It is not clear, however, what types of water mass changes are associated with the oceanic response to the winter SST variations. The subarctic-subtropical transition region is characterized by a series of property fronts. The fronts divide the region into several zones of distinct water mass structures, from the purely subarctic water mass to the purely subtropical water mass (e.g., Roden, 1970; Anma et al., 1990; Zhang and Hanawa, 1993; Yuan and Talley, 1996). A permanent halocline dominates the upper density stratification in the subarctic region, where a subsurface temperature minimum (STM) frequently occurs. On the other hand, a permanent thermocline dominates the density stratification in the subtropical region. The subsurface oceanic response to the SST variability likely differs de- 435

2 pending on these water mass structures. The purposes of the present study are to identify a characteristic subsurface water mass in each zone of the central subarctic-subtropical transition region and to examine its source area and temporal variability. The rest of this paper is organized as follows: Section 2 describes the data used in the present study, which include the repeat hydrographic section along the 180 meridian and an isopycnally averaged hydrographic climatology. Section 3 describes major features of the mean section, focusing on two characteristic water masses corresponding to the subarctic STM and the subtropical pycnostad, respectively. Section 4 examines interannual to decadal changes in these water masses. Section 5 gives a discussion and summary. 2. Data and Processing The repeat hydrographic section along the 180 meridian across the subarctic-subtropical transition region has been maintained over two decades since 1979 (Anma et al., 1990). The observation has been conducted every June by the T/V Oshoro Maru belonging to Faculty of Fisheries, Hokkaido University. The temperature and salinity profile data for were used in the present study (Fig. 1). The typical section consists of stations with spacing of 0.5 or less from 37 N to N. The profile data were obtained by the serial observation until 1983 and by the conductivity-temperature-depth (CTD) sensor after that. We used the publicly available standard depth data at 0, 10, 20, 30, 50, 75, 100, 150, 200, 250, 300 m and each 100 m below. Each profile of temperature and salinity was interpolated onto a vertical grid with 10 dbar interval using a shape-preserving local spline (Akima, 1970). The 10 dbar gridded profile data were used to calculate several derived properties at 10 dbar intervals. The resulting profiles of the observed and derived properties are further interpolated linearly onto a series of isopycnal surfaces with a 0.05σ θ interval. Potential vorticity (Q) was calculated by Q f σ = θ ρ z, where σ θ is the fixed increment of 0.1σ θ, z is the variable thickness between the isopycnal surfaces immediately above and below the given surface for which Q is calculated, f is the Coriolis parameter and ρ is the in-situ density. Geostrophic velocities were estimated relative to 1000 dbar for each station pair using the 10 dbar-gridded profiles. The isopycnally averaged climatology of the North Pacific, called North Pacific HydroBase (NPHB), was Fig. 1. Station positions along the 180 meridian from 1979 to Circles denote stations with STM water colder than 4 C. The stations with coldest STM water in each year are indicated by open circles. Crosses denote the station with STM water warmer than 4 C. Dots denote the stations without substantial STM features. originally produced by Macdonald et al. (2001) from the observed data of World Ocean Atlas 1994 (NODC, 1994) and the CTD data of several pre-woce and WOCE sections. Suga et al. (2003) has added more WOCE CTD data, which were available as of January 2001, and other CTD data collected by National Oceanic and Atmospheric Administration/Pacific Marine Environmental Laboratory (NOAA/PMEL) (Johnson and McPhaden, 1999) and recalculated the climatology. The present study utilizes this updated version of NPHB to relate the water masses in the repeat section to the basin-wide features. The NPHB climatology includes 1 gridded annual mean properties on a series of isopycnal surfaces. The averaging and smoothing were performed isopycnally and with the smallest possible smoothing scale, typically 1 to 3 in latitude or longitude. As a result, the NPHB preserves more realistic water mass structures than previous climatological maps, especially near the western boundary region and the frontal regions (Macdonald et al., 2001). Geostrophic flow fields on the isopycnal surfaces were represented with the pressure anomaly streamfunction introduced by Zhang and Hogg (1992). The pressure anomaly streamfunction, ψ σ, on a certain isopycnal of σ, is defined as 436 T. Suga et al.

3 Fig. 3. Mean sections of (a) potential temperature ( C), (b) salinity (psu), and (c) potential vorticity ( m 1 s 1 ). Dashed contours in (a) are drawn with a 0.1 C interval. Fig. 2. Mean sections of (a) potential temperature ( C), (b) salinity (psu), (c) potential density (σ θ ) and (d) geostrophic velocities (10 2 ms 1 ). Dashed contours in (a) are drawn with a 0.1 C interval. p ψσ = δdp + δp, p0 where p and δ are the pressure and the specific volume anomaly on the surface of σ, p 0 is the reference pressure and p is the pressure anomaly on the surface of σ. The pressure anomaly is defined as p = p p, where p is the lateral mean pressure on the surface of σ. We adopted the pressure anomaly streamfunction instead of the acceleration potential because the inherent errors in the latter quantity on a surface of constant potential density are reduced substantially in the former (Zhang and Hogg, 1992). The winter mixed layer climatology was produced by Suga et al. (2003) primarily based on the same hydrographic dataset as described above but only for the late-winter two months, February and March. The mixed layer depth was defined at each station as the depth at which potential density was 0.125σ θ greater than the sea surface density; the individual mixed-layer properties were then averaged and smoothed typically over 1 to 3 in latitude or longitude to create a 1 gridded dataset. As for producing the contour maps presented in this paper, a 2 block mean was applied to reduce the grid scale variability, followed by the final smoothing with the mapping algorithm of Smith and Wessel (1990), which fits a surface with continuous second derivatives and minimal curvature to the gridded data. This last procedure is the same as that used by Lozier et al. (1995) and Macdonald et al. (2001). 3. Mean Sections and Characteristic Water Masses The mean sections along the 180 meridian averaged over the period from 1979 to 2000 are presented in Figs. 2 and 3 to show overall features of the section. The north- Water Masses in the North Pacific Transition Region 437

4 Fig. 4. NPHB climatological map of (a) potential temperature (color) and pressure anomaly streamfunction relative to 2000 dbar (white contours) at 26.6 σ θ, and (b) potential vorticity (color) and pressure anomaly streamfunction relative to 2000 dbar (white contours) at 26.4 σθ. Thick white curves indicate the winter outcrops of these isopycnals. Typical station positions of the 180 repeat section are indicated by circles. Pressure anomaly streamfunction is contoured with an interval of 1 m2s 2. ernmost zone north of about 46 N is characterized by the sharp permanent halocline (Fig. 2(b)). The halocline corresponds to the sharp pycnocline; the salinity stratification maintains the subsurface density stratification in this zone (Figs. 2(b), 2(c) and 3(b)). We call this zone the Subarctic Zone in the present study. A remarkable feature of the Subarctic Zone is the STM at about 150 m (Fig. 2(a)). Its core has temperature of nearly 3 C and potential density centered at σθ and is located at N (Figs. 2(a) and 3(a)). The STM water is a ubiquitous feature of the North Pacific subarctic region and is often called dichothermal water (e.g., Uda, 1963; Ueno and Yasuda, 2000; Miura et al., 2002). The STM lies within the sharp halocline (Figs. 2(b) and 3(b)). The southern boundary of the Subarctic Zone is approximately marked by the nearly vertical isotherm of 4 C at 100 m and below (Dodimead et al., 1963; Favorite et al., 1976), which is sometimes called the Subarctic Front (e.g., Anma et al., 1990; Ueno and Yasuda, 2000). On the other hand, Belkin et al. (2002) defined the southern boundary of the Subarctic Zone based on its subsurface thermohaline features including the prominent STM. They called this boundary the Polar Front. 438 T. Suga et al. The STM in the Subarctic Zone is believed to be a remnant of the wintertime mixed layer (e.g., Uda, 1963; Ueno and Yasuda, 2000; Miura et al., 2002). It is further suggested that the STM water in the Subarctic Zone of the 180 section is originated in a remote area because the corresponding density surfaces, σθ, hardly outcrop locally even in late winter (Suga et al., 2003). The climatological potential temperature, θ, and geostrophic flow pattern on the 26.6σθ surface based on NPHB are presented in Fig. 4(a). The map indicates that this surface outcrops only in and just outside the Bering Sea, except in the Sea of Okhotsk. Miura et al. (2002) actually showed that the STM water as dense as 26.6σθ is formed in the winter mixed layer in the Bering Sea based on the analysis of extensive historical hydrographic data. A cold tongue extends from the outcrop area along the cyclonic western subarctic gyre and its eastern end marked by the isotherm of 3.5 C reaches the 180 section at N. It is thus suggested that the source of the STM water in the Subarctic Zone of the 180 section is the winter mixed layer in or just outside the Bering Sea, while the water is substantially warmed along the advective path, presumably through mixing.

5 Fig. 5. θ-s plots for the section in Circles denote the profiles with the STM water colder than 4 C. The profile with the coldest STM water in the section is plotted with open circles. Crosses denote a profile with a STM warmer than 4 C. Dots denote profiles without substantial STM features. The zone immediately south of the Subarctic Zone is characterized as the region where the subarctic permanent halocline outcrops (Fig. 2(b)). Yuan and Talley (1996) defined the Subarctic Frontal Zone (SFZ) as the zone between the outcrops of the 33.0 psu and 33.8 psu isohalines that mark the top and the bottom of the subarctic halocline, respectively. According to their definition, SFZ in the present mean section is located at N. Since the salinity stratification still dominates the density stratification in SFZ, patches of STM are often observed there. The STM water in SFZ, however, is much warmer than that in the Subarctic Zone. An example of the θ-s plots from an individual section is presented in Fig. 5. The θ-s profiles are naturally divided into three groups: the first one with the STM colder than 4 C (large dots), the second one with the STM warmer than 4 C (crosses), and the third one with no clear STM but with the intermediate salinity minimum (small dots). The colder STM associated with the first group is typical in the Subarctic Zone. The warmer STM associated with the second group is observed in SFZ. The station positions in Fig. 1 are plotted with the symbols representing the types of temperature minimum. The southern extent of the colder STM is about N, which is consistent with the position of the Polar Front at 180 detected by Belkin et al. (2002). Our further analysis is focused on Fig. 6. Time series of (a) the cold core θ of the STM water in the 180 section in summer, (b) the cold core θ of the STM water in the Bering Sea in summer and (c) winter SST averaged over the western Bering Sea: N, E as indicated with the box in Fig. 7(b). this colder STM water in the Subarctic Zone. The subsurface salinity stratification almost disappears and the permanent thermocline is developed south of SFZ (Figs. 2(a) and (c)). We call this southernmost zone south of about 42 N the Subtropical Zone in the present study. A remarkable feature of the subsurface stratification in the Subtropical Zone is the pycnostad or the Q minimum centered at 26.4σ θ (Figs. 2(c) and 3(c)). The Q minimum water is known as North Pacific Central Mode Water (CMW), which is formed in the deep winter mixed layer and subducted into the permanent pycnocline (Nakamura, 1996; Suga et al., 1997). The CMW represented by the low Q region extends along the anticyclonic flow of the subtropical gyre (Fig. 4(b)). The Subtropical Zone of the 180 section is situated in the eastward flow. The outcrop area of this isopycnal is some km, or 6 10 in longitude, upstream of the section, which corresponds with the plausible formation area of the CMW at this density proposed by Suga et al. (2003). The CMW is the other water mass on which our analysis is focused in the following section. Water Masses in the North Pacific Transition Region 439

6 4. Interannual/Decadal Variations of STM Water and CMW 4.1 STM water The cold core temperature is defined as the temperature of the coldest vertical temperature minimum for each section as a representative parameter of the STM water in the Subarctic Zone for each year. It is plotted in Fig. 6(a). The core temperature varies over several years, ranging from 2.4 C to 3.0 C. Since the sections in 1979, 1983 and 1987 were limited in spatial coverage for the Subarctic Zone (Fig. 1), the core temperature for those years is not plotted to avoid possible inadequate sampling of the cold core. While the time scale of the core temperature variation is thus rather obscure for the 1980s, it is quite clear for the 1990s. The core temperature decreased from 1991 to 1995, increased from 1995 to 1998, and then decreased sharply from 1998 to The peak-to-peak period for the 1990s is 7 years. As discussed in the previous section, the plausible source region of the STM water in the Subarctic Zone is the Bering Sea. The National Research Institute of Far Seas Fisheries/Fisheries Research Agency (NRIFSF/FRA) has been conducting hydrographic survey in the Bering Sea every summer (June July) since 1991 (Tomowo Watanabe, personal communication). The survey mainly consists of the two sections across the deep basin of the Bering Sea: one along the 180 meridian and the other along the northeast-to-southwest line approximately from 57 N/179 W to 53 N/170 E. Miura (2000) presented the time series of the cold core temperature in each of the two sections. We chose the coldest temperature in each year from his time series and the data are plotted in Fig. 6(b). The cold core temperature in the Bering Sea is considerably colder than that in the Subarctic Zone along the 180 meridian, ranging from 1.0 C to 2.3 C during Nevertheless, its temporal variation is fairly similar to that of the latter with a time lag of one year; the minimum in 1994 and the maximum in 1997 occurred a year ahead of the corresponding minimum and maximum in the Subarctic Zone along the 180 meridian. This correspondence supports the connection between the STM water in the Subarctic Zone of the 180 section and that in the Bering Sea. In order to further examine the relation between the temporal change in the STM water in the Subarctic Zone at 180 and changes occurring in the Bering Sea, we calculated correlation coefficients between the cold core temperature shown in Fig. 6(a) and the 2 2 wintertime SSTs obtained by averaging monthly mean analyzed SSTs for February and March. The analyzed SSTs were produced by the Oceanographical Division and the Office of Marine Prediction of the Japan Meteorological Agency (JMA) based on in-situ SST data compiled by the JMA. Fig. 7. Correlation coefficients between SST in one winter and the cold core θ of the STM water in the 180 section (a) in the following summer with the time lag of 4 months and (b) in the summer a year later with the time lag of 16 months. Crosses indicate grid points, which have significant correlation with the confidence level greater than 95%. Box indicates the area for which the SST time series is shown in Fig. 6(c). The cold core temperature of the STM water in summer at 180 is significantly correlated with the winter SST in the western Bering Sea with the time lag of 16 months (Fig. 7(b)). The area of the significant correlation in the western Bering Sea corresponds with the wintertime outcrop of the 26.6σ θ surface (Fig. 4(a)) and thus with the probable source region of the STM water of the Subarctic Zone at 180. Figure 6(c) shows a time series of the winter SST averaged over the part of the Bering Sea at N and E where the correlation is higher than 0.4, as indicated by the box in Fig. 7(b). The temporal change in the winter SST in this region is fairly similar to that in the cold core temperature in the Bering Sea in the following summer. The correspondence among these time series (Fig. 6) can plausibly be interpreted as follows: a change in the winter SST or mixed-layer temperature in the western Bering Sea remains as a change in the subsurface temperature of the Bering Sea in the following summer and also propagates along the western subarctic 440 T. Suga et al.

7 gyre to the Subarctic Zone at 180 by the next summer. Figure 7(b) also shows significant correlation between the cold core temperature in summer at 180 and the winter SST in the fairly wide area at about N, 160 E 180 with the time lag of 16 months. Since the wintertime sea surface density in this area is not as large as the STM core density, it is unlikely that the area should correspond to the source region of the STM cold core. Instead, the significant correlation suggests that the interannual variability of the STM cold core temperature may also be related to the vertical mixing along the advective path. In order to examine this idea, we calculated the correlation between the winter SST and the STM cold core temperature in summer at 180 with a time lag of 4 months (Fig. 7(a)). If the vertical mixing has a major effect on the interannual variability of STM, the correlation in just upstream of the 180 meridian around 45 N should be even higher with this shorter time lag. However, the correlation is considerably lower and is not significant in this area (Fig. 7(a)). Since this low correlation is not consistent with the idea that the vertical mixing is important, we have chosen not to examine it further in this paper and to leave it for future study. Meanwhile, Fig. 7(a) shows significant correlation between the STM cold core temperature at 180 and the winter SST in the southwestern part of the Bering Sea with a time lag of 4 months. However, this area of significant correlation is slightly east of the wintertime outcrop of the STM core isopycnal, 26.6σ θ, and the lag time of 4 months is too short for the water to be carried from the Bering Sea around the western subarctic gyre to the 180 section, as estimated in Section 5. It is thus unlikely that the area should be directly related to the interannual variability of the cold core temperature at CMW While the low Q core of CMW in the mean section is about m 1 s 1 (Fig. 3(c)), the core layer of CMW in an individual section is more clearly defined as a layer with the Q lower than m 1 s 1 between 26.0σ θ and 26.5σ θ (not shown). We first extracted all the isopycnal levels at each station within the CMW core layer defined above. The sectional mean properties of the CMW were then calculated for each year. The time series of the sectional averaged properties are presented in Figs. 8(a) (c). The most remarkable feature in the time series of θ of the CMW is an abrupt increase by 1 C or more from 1988 to The θ during each of the decades before and after this change was fairly steady centered at 8.5 C for the former decade and 10 C for the latter. Since the density change is dominated by temperature in the Subtropical Zone, the shift in the θ is accompanied by the abrupt decrease of potential density of the CMW by 0.2σ θ Fig. 8. Time series of the (a) potential temperature, (b) salinity and (c) potential density of the CMW in the 180 section and (d) the winter SST averaged over its plausible formation region: N, 160 E 180 as indicated with the box in Fig. 10. or more. The change in the potential density is even more clearly regarded as a shift between the two decades with fairly steady core density of the CMW. Even though the time series of salinity of the CMW is somewhat more dominated by year-to-year variation, a rather abrupt increase from 1988 to 1989 or 1990 is still discernible. In order to confirm that the shift in the sectional mean CMW properties is not an artifact due to averaging procedure, all the θ-s pairs within the CMW core layer are plotted along with their sectional means in Fig. 9. It is obvious that the CMW during is systematically warmer, more saline and lighter than that during It is thus concluded that the CMW in the 180 section abruptly became warmer, more saline and lighter from 1988 to The correlation coefficients between the sectional mean θ of the CMW and the 2 2 gridded winter SST Water Masses in the North Pacific Transition Region 441

8 Fig. 9. θ-s plots of CMW. Individual θ-s pairs of CMW are plotted in red for and in blue for An averaged θ-s pair for each year is plotted with a open circle for and with an closed circle for described above were calculated (Fig. 10). Significant correlation coefficients, as high as 0.6, appear in the plausible formation region of the CMW in the 180 section, that is, the region between 160 E and 180 along about 40 N. The time series of the winter SST averaged over this region at N and 160E 180 are presented in Fig. 8(d). The wintertime SST in the CMW formation region increased by 1.5 C or more from 1988 to 1989, which corresponds very well to the shift in θ of the CMW found in the 180 section. It is thus inferred that the shift in the CMW properties in the 180 section from 1988 to 1989 directly reflects a shift in the winter mixed layer properties of its formation region. Fig. 10. Correlation coefficients between SST in one winter and the CMW θ in the 180 section in the following summer with the time lag of 4 months. Crosses indicate grid points, which have significant correlation with the confidence level greater than 95%. Box indicates the area for which the SST time series is shown in Fig. 8(d). 5. Discussion The correlation among the winter SST in the Bering Sea, the core temperature of the STM water in the Bering Sea and that in the Subarctic Zone at the 180 meridian suggests that the thermal anomalies produced in the winter mixed layer in the Bering Sea have a widespread impact on the subsurface temperature in the western subarctic gyre. While we cannot directly show the propagation of the anomalies from the Bering Sea to the 180 section, the time lag of 16 months between the time series of the winter SST in the Bering Sea and those of the cold core temperature in the 180 section supports the connection of the anomalies in the two remote areas. Suppose that the propagation of the anomaly is mainly by advection. The distance along the western subarctic gyre from the exit of the Bering Sea to the Subarctic Zone of the 180 section is some km (Fig. 4(a)). This implies an advection speed of km/month or ms 1. The geostrophic speed on the 26.6σ θ along the western subarctic gyre estimated from the pressure anomaly streamfunction relative to 2000 dbar (Fig. 4(a)) is ms 1. The two estimates of the speeds correspond reasonably with each other. The formation area of CMW has been sought by several authors. Suga et al. (1997) inferred that CMW is formed near the northern edge of the subtropical gyre between 175 E and 160 W based on the climatology of the winter mixed layer evaluated by the temperature profile data. Nakamura (1996) produced other climatological maps of the mixed layer depth and SST, which suggest the CMW formation area spanning the similar longitudinal range to that indicated by Suga et al. (1997). On the other hand, Mecking and Warner (2001) suggested that the formation area is shifted westward because of the presence of CMW at 165 E during some years (Kawabe and Taira, 1998). More recently, Suga et al. (2003) compared the winter mixed layer properties including its temperature, salinity and density with the CMW properties and identified its probable formation area between 160 E and 170 W near the northern edge of the subtropical gyre, which is consistent with the suggestion by Mecking and Warner. The present analysis demonstrates a fairly good correlation between the temperature of CMW in the 180 section and the winter SST in the probable formation area 442 T. Suga et al.

9 identified by Suga et al. (2003). In other words, the present result confirms the location of the CMW formation area from the viewpoint of its temporal variability. The CMW in the 180 section exhibits a dramatic shift in its properties from 1988 to This shift can be qualitatively explained as a response of the winter mixed layer properties to the climate regime shift in 1988/ 1989 (e.g., Hare and Mantua, 2000; Yasunaka and Hanawa, 2001, 2002). The regime shift was associated with weakening of the Aleutian Low and the westerlies, and warming of the winter SST in the subarctic-subtropical transition region, which is consistent with the warming of CMW. The weaker westerlies further imply the weaker southward Ekman advection of cold and fresh water from the subarctic region, which is consistent with both warming and salinification of CMW. Yasuda and Hanawa (1997) reported the cooling of CMW associated with the climate regime shift in mid-1970s based on the quantitative analysis of changes in both water properties and forcing terms including the surface heat flux and the heat divergence in the E kman layer. Further quantitative examination of the shift in the CMW properties reported in the present paper and its relation to the climate regime shift will be addressed in future study. In summary, the repeat hydrographic section along the 180 meridian across the subarctic-subtropical transition region captures the distinct water masses: the STM water originating in the Bering Sea and the CMW originating between 160 E and 180 near the northern edge of the subtropical gyre. The STM water exhibited temporal variation in its temperature with a peak-to-peak period of 7 years during 1990s, which corresponds well with the winter SST and the summer subsurface temperature in the Bering Sea, lagged by months. It is thus inferred that the thermal anomalies imposed on the winter mixed layer in the Bering Sea propagate as subsurface anomalies along the western subarctic gyre and reach the 180 section within a year or so. The CMW properties shifted abruptly from 1988 to 1989; they became warmer by about 1 C, more saline and lighter by 0.2σ θ or more. The shift is qualitatively explained as a response to the climate regime shift in 1988/1989. Acknowledgements We are grateful to the crew of T/V Oshoro Maru and the scientists at Graduate School of Fisheries, Hokkaido University for their efforts to maintain the repeat section for a long time. We thank members of the Physical Oceanography Group, Tohoku University for helpful comments and useful discussion. Dr. M. J. McPhaden kindly provided the CTD data collected by NOAA/PMEL. Many useful comments from F. M. Bingham and an anonymous reviewer are greatly appreciated. This study was made as part of Subarctic Gyre Experiment (SAGE), which was financially supported by the former Science and Technology Agency and the present Ministry of Education, Culture, Sports, Science and Technology. The authors (TS and KH) were also supported by the Japan Society for Promotion of Science (Grant-in-Aid for Scientific Research (B), No ). References Akima, H. (1970): A new method of interpolation and smooth curve fitting based on local procedures. J. Assoc. Comput. Mech., 17, Anma, G., K. Masuda, G. Kobayashi, H. Yamaguchi, T. Meguro, S. Sasaki and K. Ohtani (1990): Oceanographic structures and changes around the Transition Domain along 180 longitude during June Bull. Fac. Fish. Hokkaido Univ., 41, (in Japanese with English abstract). Belkin, I., R. Krishfield and S. Honjo (2002): Decadal variability of the North Pacific Polar Front: Subsurface warming versus surface cooling. Geophys. Res. Lett., 29(9), art. no Deser, C. M., A. Alexander and M. S. 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