Decadal Variability of Subsurface Temperature in the Central North Pacific

Transcription

1 Journal of Oceanography, Vol. 59, pp. 945 to 955, 2003 Short Contribution Decadal Variability of Subsurface Temperature in the Central North Pacific SATOSHI SUGIMOTO*, TAKASHI YOSHIDA and TADASHI ANDO Climate and Marine Department, Japan Meteorological Agency, Chiyoda-ku, Tokyo , Japan (Received 11 July 2002; in revised form 11 July 2003; accepted 18 July 2003) Decadal variability of subsurface temperature in the North Pacific has been investigated. Two dominant regions were found; the central subarctic region (CSa) and the north-eastern subtropical region (NESt). In CSa, cooling (warming) of wintertime subsurface temperature corresponds to the large (small) temperature gradient and southward (northward) shift of subsurface temperature front, associated with the increase (decrease) of positive wind stress curl and the southward (northward) shift of curl τ zero line with 2 years delay. It is suggested that the relocation of subtropicalsubarctic boundary plays an important role. In NESt, importance of heat flux through the sea surface and heat divergence in the Ekman layer is also discussed. Keywords: Decadal variability, subsurface temperature, North Pacific, wind stress curl, heat flux. 1. Introduction In recent years, much attention has been paid to the remarkable variability on a decadal time scale in the North Pacific. It is well known that wintertime sea surface temperature (SST) in the central North Pacific has decreased significantly since the mid 1970s, accompanied by an intensification of the Aleutian low (e.g., Nitta and Yamada, 1989; Trenberth, 1990). Previous studies have shown that the decrease of SST corresponds to the change of heat flux through the sea surface and heat divergence in the Ekman layer, which is caused by the intensification of the Aleutian low and the associated westerlies (Cayan, 1992a, b; Yasuda and Hanawa, 1997). SST in the tropical Pacific increased almost simultaneously with the decrease of SST in the central North Pacific, suggesting that the variability in the central North Pacific was strongly associated with that in the tropical Pacific through the Pacific-North American (PNA) pattern, which is related to the intensification of the Aleutian low (Trenberth and Hurrell, 1994; Deser and Blackmon, 1995; Tanimoto et al., 1997). The variability of SST and atmospheric fields for a longer period was examined by Kachi and Nitta (1997), who discovered two dominant decadal modes and pointed out that the first mode changed its sign in the late 1970s with the intensification of the PNA pattern, and the second mode changed sign around Recently, it was pointed out that the fluctuation in the late 1980s was * Corresponding author. s_sugimoto@met.kishou.go.jp Copyright The Oceanographic Society of Japan. related to the Arctic Oscillation (Yasunaka and Hanawa, 2001). The variability of subsurface temperature on a decadal time scale has also been discussed in several studies. Watanabe and Mizuno (1994) found cooling of subsurface temperature in the central North Pacific and a southward shift of the subtropical-subarctic boundary by comparison of m averaged subsurface temperature between the period and , suggesting the intensification of the subtropical-subarctic front. It has also been pointed out that the heat divergence in the Ekman layer plays an important part in the change of subsurface temperature in the central North Pacific (Yasuda and Hanawa, 1997). Zhang and Levitus (1997) revealed a significant zonal and meridional propagation of subsurface temperature anomaly around the North Pacific, suggesting a decadal-scale cycle of the anomaly circulating clockwise around the subtropical gyre. Zhang et al. (1999) discovered two distinct modes of subsurface temperature in the Pacific, one being interannual and the other having a decadal time scale. Most of these previous studies on decadal fluctuation of subsurface temperature were focused on that in the mid 1970s. Few studies have been done on the variability of subsurface temperature and relationship to the atmospheric field in the late 1980s. The purpose of this study was to analyze subsurface temperature in the North Pacific for the period and examine decadal time scale variability in the midhigh latitudes. The cause of the fluctuation of subsurface 945

2 temperature is also investigated in relation to the atmospheric field. Such analyses over both the mid 1970s and the late 1980s may improve our understanding of decadal fluctuation of subsurface temperature and its relation to the atmospheric field. The data and the objective analysis method are outlined, and the results of a spectrum analysis and EOF analysis, which are first applied to the data to determine the area where subsurface temperature variability on a decadal time scale is dominant, are described in the next section. Section 3 discusses wintertime subsurface temperature variability on a decadal scale, focusing on the difference between the mid 1970s and the late 1980s. Section 4 describes the variability of the wintertime wind stress field, heat flux through the sea surface and heat divergence in the Ekman layer, and the relationship to the decadal variability of subsurface temperature is discussed. 2. Data and Method of Analysis The historical temperature profiles, viz., the World Ocean Database 1998 (Levitus et al., 1998), data set of Japan National Research Institute of Far Seas Fisheries (JNRIFSF) for , TRANSPAC XBT data co-operated on by Japan Meteorological Agency (JMA) and US National Oceanic and Atmospheric Administration (1997 ), and ship/buoy data collected operationally by JMA after 1986, are used for an objective analysis of subsurface temperature. The Comprehensive Ocean-Atmosphere Data Set (Woodruff et al., 1987) was also used for an SST analysis. The JNRIFSF data are profiles at standard depths from the surface to 250 m, mainly observed west of the date line. In the region of N, 140 E 180, they account for 15% of the total number of data between 1966 and From the historical data above, the monthly 2 2 (latitude longitude) data in the Pacific at the standard levels were prepared by the authors, applying the optimum interpolation method (OI). We adopted the climatology of World Ocean Atlas 1994 (Levitus and Boyer, 1994) as the first guess of OI. The spatial and temporal decorrelation scales, standard deviations of the first guess error and of the observation error were calculated from the TOPEX/POSEIDON altimetry data and the historical in situ data (Kuragano and Kamachi, 2000). The monthly 2 2 data include erroneous grid point values which are introduced by erroneous observation data or uneven distribution of data, so that any grid point value which has departure greater than 4 times standard deviation from the surrounding average value is replaced by the average. In this study, to examine the variability of subsurface temperature including the mid 1970s and the late 1980s, subsurface temperature was objectively analyzed at the 15th day of every month for Hereafter, we treat this as monthly subsurface temperature. We computed a climatological monthly mean for 40 years at each grid point. The monthly anomaly of subsurface temperature is defined as the departure from the climatological monthly mean. First, we examined the reliability of the monthly subsurface temperature from the analysis error provided by OI. Analysis errors in 1960s are significantly greater than those after That is due to uneven distribution of the data in 1960s, when most of the data were fixed point observations. The errors can be reduced by spatial or temporal averaging; accordingly, we used the data for 40 years when using spatially or temporally averaged data, and the data from 1971 to 2000 for the EOF analysis. The spectrum analysis and EOF analysis were applied to subsurface temperature and its anomaly field to first mode second mode Fig. 1. First (left) and second (right) EOF modes of wintertime (Jan. Mar.) m subsurface temperature anomalies and their time coefficients. The anomalies were smoothed with a 4-year running mean filter prior to the EOF analysis. Thick rectangles denote the north-eastern subtropical region (left) and the central subarctic region (right) in the North Pacific, respectively, defined in this study. 946 S. Sugimoto et al.

3 determine the area where subsurface temperature fluctuation on a decadal time scale is dominant. The Maximum Entropy Method, which is recognized as a valuable technique, especially for obtaining a very accurate power spectrum with short data samples, was adopted for the spectrum analysis. Spectra peaks longer than 10 years were found in N, W at a depth of about 150 m. Therefore, m averaged temperature anomaly, where the power spectrum was large for a period longer than 10 years, was used for the EOF analysis. The ocean condition is most influenced by the atmosphere from January to March, and we choose wintertime (Jan. Mar.) to examine decadal variability and to detect the relationship between the atmosphere and the ocean. We are interested in decadal variability of subsurface temperature in the mid-high latitudes, so that a 4-year running mean filter is applied prior to the EOF analysis and the area of the EOF analysis is confined to north of 20 N. The first mode of the EOF, which accounts for about 35% of the total variance, has a strong signal south of 40 N from 170 to 140 W, and the second mode (about 17%) north of 40 N from 180 to 150 W, respectively (Fig. 1). Our first two EOFs have different patterns from those obtained by Zhang et al. (1999). The strongest signal of our first mode is found between W south of 40 N (Fig. 1), though that reported in Zhang et al. (figure 5b in their study) is found near the date line along 40 N. Our second mode is apparently different from their second one (figure 5c in theirs). The strongest signal of our second mode is found west of 150 W between N, which is rather similar to their first mode. It seems as if the first mode in Zhang et al. (1999) is separated into the first two modes found in the present study. Our third mode (not shown) closely resembles their second mode. The difference of the period in EOF analysis, in the present study and in theirs, produces the difference of EOF modes. EOF modes in the present study include both fluctuations in the mid 1970s and late 1980s on a decadal scale. The use of only subsurface temperature may be an advantage of the present study because the analysis errors in the 1960s are significantly greater than those after CEOF modes for m heat content were reported by Tourre et al. (1999). Their first mode is associated with ENSO, in view of the analysis region (30 S 60 N). Their second mode has maximum interdecadal variability from Japan to 150 W within the subarctic frontal zone, similar to our second mode. Based on the first two EOFs, we define two regions where subsurface temperature variability on a decadal time scale is dominant, namely, the north-eastern subtropical region in the North Pacific (30 40 N, W; hereafter NESt) and the central subarctic region (40 50 N, W; hereafter CSa). 3. Decadal Scale Variability of Subsurface Temperature Decadal variability of subsurface temperature in CSa and NESt was examined using areal averages of wintertime m subsurface temperature anomaly (Fig. 2). In CSa, wintertime m temperature anomaly is positive from 1963 to 1976, changes its sign to negative around 1976/77, to positive around 1990, and to negative around On the other hand, in NESt it is positive from 1962 to 1977, changes its sign to negative around 1977/78, and to positive around These sign reversals in CSa and NESt correspond to the phase reversals of bidecadal and pentadecadal variations of the wintertime North Pacific Index obtained by Minobe (1999). The method for quantitatively detecting the climate jump reported by Yamamoto et al. (1986) was applied to the time series of the wintertime m temperature anomaly for both regions. Climate jump is detected when the difference between time averages of two periods longer than 10 years is greater than the sum of the 95% confidence limits. The difference between and in CSa is greater than the sum of the confidence limits, and this means that the definition of a climate jump is satisfied. It is not satisfied for the periods and in CSa because the latter period is shorter than 10 years, but the difference of time averages is greater than the sum of the confidence limits. The definition is also satisfied for the periods and in NESt. In NESt, a warming may be found in the late 1980s, but it is too small (warming of 0.15 C as compared with the sum of the confidence limits of 0.24 C) to be recognized as a climate jump. CSa : 40-50N, W NESt : 30-40N, W Fig. 2. Time series of areal averaged wintertime (Jan. Mar.) m subsurface temperature anomalies ( C) for CSa (40 50 N, W, top) and NESt (30 40 N, W, bottom). Solid lines denote time averages and dashed lines 95% confidence limits. Decadal Variability of Subsurface Temperature 947

4 (average of )- (average of ) (average of )- (average of ) (average of )- (average of ) (average of )- (average of ) normal( ) Fig. 3. Difference of wintertime (Jan. Mar.) m subsurface temperature anomalies ( C) between the periods. Contour interval is 0.2 C. Dashed contours denote negative differences. Areas with confidence greater than 95% are shaded. In order to detect the spatial pattern of decadal variability of subsurface temperature, we computed the time averages of wintertime m temperature anomalies for three periods and the differences of the time averages. Three periods , , are selected, considering the difference of signs between NESt and CSa in Wintertime m temperature for is almost 1 C lower than that for in N, 170 E 165 W over the western part of CSa, and almost 0.5 C lower than over NESt (Fig. 3). By contrast, wintertime m temperature for is almost 1 C higher than that for in N, 175 E 160 W. The changes are greatest along the temperature front at the subtropical-subarctic boundary. Figure 4 shows the difference of wintertime temperature anomalies between the periods along the W longitudinal band. The subsurface temperature for is lower than that for and is higher than between 40 N and 50 N, respectively. A significant change at the subtropical-subarctic boundary, where a strong meridional temperature gradient exists, was found from the surface to 700 m along the isotherms between 40 N and 45 N. On the other hand, the change Fig. 4. Difference of wintertime (Jan. Mar.) subsurface temperature anomalies ( C) between the periods (top, middle) and the normal in February (bottom) along W longitudinal band. Contour interval is 0.2 C (top, middle) and 1 C (bottom). Dashed contours denote negative differences. Areas with confidence greater than 95% are shaded. between 30 N and 40 N around 1977 is confined from the surface to 250 m. 4. Relationship to the Atmospheric Field The amount of heat flux through the sea surface and heat divergence in the Ekman layer changed with the intensification of the westerlies due to the strong Aleutian low (Cayan, 1992a, b; Yasuda and Hanawa, 1997). It has also been pointed out that the intensification of the wind stress field in the North Pacific caused the subtropical gyre to strengthen (Qiu and Joyce, 1992; Yasuda and Hanawa, 1997), and that the anomalous southward intrusion of the Oyashio is related to the wind stress field in the North Pacific (Sekine, 1988). In order to detect the cause of the decadal variability of the subsurface temperature, its relationship to the wind stress and heat flux 948 S. Sugimoto et al.

5 (average of )- (average of ) (average of )- (average of ) Fig. 6. As Fig. 5 but their difference (N/m 2 ) between the periods. Fig. 5. Wind stress fields (N/m 2 ) in winter (Dec. Feb.) during (top), (middle) and (bottom). has been investigated, using the monthly mean wind stress and heat flux data from NCEP (National Centers for Environmental Prediction)/NCAR (National Center for Atmospheric Research) reanalysis (Kalnay et al., 1996). The wintertime wind stress and wintertime heat flux are defined as averages from December to February, when wind stress is intensified and the atmospheric-ocean interactions are strongest in the North Pacific. 4.1 Wind stress field It is well known that the regime shifts occurred in the mid 1970s and the late 1980s. Yasunaka and Hanawa (2002) specified the year of the shifts as 1976/77 and 1988/89. Because the atmospheric fields such as wind stress change significantly when a regime shift occurs, the oceanic field can be changed after a regime shift. In order to examine the relationship to the wind stress field on a decadal time scale, averages of wind stress for three periods , , were calculated. As mentioned later in this section, the decadal fluctuation of subsurface temperature in CSa occurs after that of wind stress curl with 2 years delay. Accordingly, subsurface temperatures in CSa for three periods , and are used to discuss the relationship to the wind stress field. The averaged wind stress field for each period and the differences between the periods are shown in Figs. 5 and 6. The wind stress along 40 N for is stronger than that for The intensification of the westerlies is remarkable east of the date line. The difference of the wind stress between and represents the intensification of the counterclockwise wind stress centered at 50 N, 165 W and of the westerlies near N, 170 E 140 W. The intensification of the wind stress is accompanied by the intensified Aleutian low after the mid 1970s (Hanawa et al., 1996). It is pointed out that the intensification of the subtropical gyre and a southward shift of the subtropical-subarctic boundary in the North Pacific are caused by the intensification of the wind stress (Watanabe and Mizuno, 1994). On the other hand, the difference of the wind stress between and represents southward anomalous wind stress near Decadal Variability of Subsurface Temperature 949

6 (average of )- (average of ) (average of )- (average of ) Fig. 8. As Fig. 7 but their difference between the periods. Contour interval is N/m 3. Areas with difference exceeding N/m 3 are shaded darkly and areas exceeding N/m 3 shaded lightly. Fig. 7. As Fig. 5 but wind stress curl (curl τ). Contour interval is N/m 3. Areas with wind stress curl exceeding N/m 3 are shaded darkly and areas exceeding N/m 3 shaded lightly. 140 W and westward anomalous wind stress near 30 N, W, that is, the counterclockwise wintertime wind stress in the mid-high latitude had weakened. Figures 7 and 8 show the averaged wind stress curl (curl τ) in winter for each period and their differences. The areas with strong positive wind stress curl exceeding N/m 3 in the subarctic region and strong negative wind stress curl exceeding N/m 3 in the subtropical region are widely distributed during The corresponding areas of the other two periods ( and ) are smaller than those for , especially near 45 N, 150 W. Wind stress curl differences show that the counterclockwise/clockwise wind stress in the subarctic/subtropical region strengthened after 1977, and weakened after Another difference is detected in the latitude of curl τ zero. The latitude where curl τ is equal to zero shifted southward between 170 E and 150 W for in comparison with those for the other two periods. The southward (northward) migration of the curl τ zero line corresponds to the positive (negative) anomaly of curl τ between N (Fig. 8). We now go on to consider subsurface temperature fluctuation associated with these two points, that is, the magnitude change of wind stress curl and the latitude change of curl τ zero line. Miller et al. (1998) suggested that the gyres strengthened after the early mid 1970s, associated with the increase of wind stress curl in the subarctic and subtropical regions from the model velocity field. Yasuda and Hanawa (1997) also pointed out the intensification of the subtropical gyre after the mid 1970s from the Sverdrup transport and geostrophic transport. Accordingly, we can expect strengthening (weakening) of subsurface temperature gradient at the subtropical-subarctic boundary resulting from the intensification (weakening) of the gyres after the mid 1970s (the late 1980s). We can also expect a southward (northward) shift of the temperature front at the subtropical-subarctic boundary associated with the southward (northward) shift of curl τ zero line (as described by Seager et al., 2001) on the assumption of Sverdrup balance. 950 S. Sugimoto et al.

7 (a) (b) Fig. 10. (a) Time-series of the latitude where wintertime (Dec. Feb.) wind stress curl (curl τ) = 0, averaged from 180 to 150 W. Thick line denotes 4-year running mean. (b) Timeseries of areal averaged wintertime (Jan. Mar.) m subsurface temperature anomaly ( C) for CSa. Thick line denotes 4-year running mean. Fig. 9. Subsurface temperatures in February during (top), (middle) and (bottom) along W longitudinal band. Contour interval is 1 C. Areas with horizontal gradient of subsurface temperature greater than C/m are shaded lightly and areas greater than C/m darkly. Arrows denote the latitude where wintertime wind stress curl is equal to zero during the corresponding periods. Figure 9 shows subsurface temperature and its horizontal gradient along a W longitudinal band with the latitude where wintertime curl τ is zero. First, we look at the area where horizontal gradient of subsurface temperature is greater than C/m (shaded lightly in the figure). The areas are found both in the subtropical region (30 35 N) and in the subarctic region (40 45 N) for each period. In comparison with the area for , the shaded area for in the subtropical region is larger, suggesting that the subtropical gyre strengthened after the mid 1970s. The shaded area for is clearly smaller than that for in both the subtropical and subarctic regions, suggesting that the gyres weakened after the late 1980s. Second, we look at the position of the subtropical-subarctic boundary. The 9 13 C contours in the upper 100 m, where horizontal gradient of subsurface temperature is greater than around in CSa, shift southward about degree in latitude between 1979 and 1990 relatively to the other periods, suggesting that the temperature front near N shifts southward (northward) accompanied with the southward (northward) shift of the latitude where curl τ is zero. On the other hand, the shift of C contours in the subtropical region is smaller than 0.5 degree. The next question is whether or not the temperature front of subtropical-subarctic boundary shifts simultaneously with the shift of curl τ zero line on a decadal scale. If relocation of the gyres is brought about by the change of wind stress curl, it may take years to adjust the gyres with the baroclinic response, and thus the change of subsurface temperature at the temperature front. We represent time-series of the latitude where wintertime wind stress curl is zero, and m subsurface temperature anomaly in CSa (Fig. 10). The latitude is an average between 180 and 150 W where the decadal change of wintertime m temperature anomaly was large around 1976/77 and A 4-year running mean filter is applied in order to remove shorter interannual variations. The latitude where curl τ is zero is near 40 N from the mid 1960s to the mid 1970s, near 36 N from the late 1970s to the mid 1980s, and near 38 N after the late 1980s. The tendency of southward (northward) shifts of the latitude corresponds to the cooling (warming) of wintertime m temperature anomaly in CSa. A time-lag correlation coefficient between the 4-year running mean of the latitude and that of m temperature gives the maximum value 0.59 in case the change of m temperature is 2 years delayed, suggesting the baroclinic response of the ocean to the change of wind stress field on a decadal Decadal Variability of Subsurface Temperature 951

8 NESt : 30-40N, W CSa : 40-50N, W Fig. 11. Time series of wintertime (Dec. Feb.) heat flux through the sea surface ( ) and wintertime heat divergence in the Ekman layer ( ) ( W/m 2, top), and the sum of them (bottom) for NESt and CSa. Solid lines denote time averages and dashed lines 95% confidence limits in the bottom panel. scale. The tendency of southward/northward shifts and lag correlation are almost same for the annual mean wind stress curl. 4.2 Heat flux Previous studies have pointed out that the latent and sensible heat flux anomalies are significantly correlated to the tendency of SST anomaly (Cayan, 1992b), and that heat divergence in the Ekman layer plays a dominant role in the long-term variability of the wintertime SST tendency (Yasuda and Hanawa, 1997). Consider the equation below for estimating the wintertime heat flux near the sea surface, Qtotal = Q+ C w k τ T f surface where the first term of the right-hand side represents heat flux through the sea surface, and the second term represents heat divergence in the Ekman layer. C w (=4180 J/kgK) is the specific heat of sea water, τ is the wind stress from the atmosphere to the ocean, f is the Coriolis parameter, T surface is temperature in the Ekman layer, assuming that the temperature is identical to SST, as supposed by Yasuda and Hanawa (1997). In addition to the terms above, the importance of entrainment heat flux in mixing has been pointed out (Miller et al., 1994; Schneider et al., 1999). On the other hand, Nakamura et al. (1997) suggest that the southward ocean mass transport in the Ekman layer across the subarctic front maintains the SST variability. In this study we try to explain the decadal variability of subsurface temperature in NESt and CSa from the estimation of heat flux through the surface and heat divergence in the Ekman layer. Although the m temperature in NESt changed its sign in the mid 1970s and did not change in the late 1980s, we are also interested in heat flux change before and after the late 1980s, and therefore averaged heat flux has been calculated for each three period. Figure 11 shows the time-series of wintertime (Dec. Feb.) heat flux anomaly through the sea surface and of the wintertime heat divergence in the Ekman layer, and their sum. The anomaly from the average of the whole period is used to easily compare the amount of the variation. Interannual variation of heat divergence in the Ekman layer is greater than that of heat flux through the surface in NESt. The increase of heat flux loss in NESt after 1977 is estimated to be 35 W/m 2 ; 7 W/m 2 through the sea surface and 28 W/m 2 in the Ekman layer. Heat flux loss decreased after 1989; 22 W/m 2 in the Ekman layer as decrease and 10 W/m 2 through the sea surface as increase. The sum of these values is a decrease of 12 W/m 2, which is relatively small compared to the change before and after 1976/77. It is consistent with the decadal subsurface temperature fluctuation in the mid 1970s (a large cooling as a climate jump) and in the late 1980s (a small warming) in NESt. Differences of three periods suggest that the heat divergence in the Ekman layer is dominant for the decadal subsurface temperature fluctuation in NESt. In CSa, the increase of the sum of heat loss after 1977 is estimated as 34 W/m 2, suggesting that there was some contribution to the decadal variation over CSa in the upper layer, but there was little between and Figure 12 shows the difference of wintertime heat flux through the surface, heat divergence in the Ekman layer and their sum between and , and In the left figures, the increase of heat 952 S. Sugimoto et al.

9 (average of )-(average of ) (average of )-(average of ) Fig. 12. Difference of wintertime (Dec. Feb.) heat flux through the sea surface (top), wintertime heat divergence in the Ekman layer (middle) and sum of them (bottom) between and (left figures), between and (right figures). Areas with difference exceeding +20 W/m 2 are shaded darkly and areas exceeding 20 W/m 2 shaded lightly in the top and middle figures; exceeding +40 W/m 2 darkly and exceeding 40 W/m 2 lightly in the bottom figure. Thick rectangles in the bottom figure denote NESt and CSa. loss more than 20 W/m 2 by heat flux through the surface is found from east of Japan to the date line between 30 N and 40 N. The increase of heat loss by heat divergence in the Ekman layer is found between 35 N and 45 N. The differences between and correspond to those reported by Yasuda and Hanawa (1997) between and in terms of both pattern and amount. The increase of the sum of heat loss is found between 35 N and 40 N over NESt where the increase of heat divergence in the Ekman layer is large. This suggests that the cooling of subsurface temperature from the surface to 250 m in NESt after 1977 is associated with the increase of heat loss. The differences between and are different from those between and While negative anomalous heat flux of both through the sea surface and in the Ekman layer spread zonally along about 35 N over NESt between and , a negative difference is found through the surface and positive/negative west/east of 150 W in the Ekman layer in NESt between and Conclusion and Discussion In this study, decadal variability of subsurface tem- perature in the North Pacific was investigated using objectively analyzed subsurface temperature from 1961 to Temporal fluctuation of subsurface temperature was examined from wintertime m subsurface temperature anomalies for the regions N, W (CSa) and N, W (NESt). Wintertime m temperature anomaly changed its sign from positive to negative around 1976/77 and from negative to positive around 1990 in CSa. The anomaly in NESt changed its sign from positive to negative around 1977/ 78. Previous studies have suggested that the ocean interior changed after the mid 1970s, that is, the gyres intensified and the main thermocline deepened, which is a baroclinic response to the wind stress curl field in the North Pacific (Deser et al., 1999; Venzke et al., 2000). Seager et al. (2001) showed that SST cooling in the latitude of the Kuroshio-Oyashio Extension from the early 1980s to 1989 is associated with a southward displacement of the latitude of the confluence at the subtropicalsubarctic boundary in consistent with the southward shift of the curl τ zero line. The present study has shown that the horizontal gradient of subsurface temperature became large and the subtropical-subarctic boundary shifted Decadal Variability of Subsurface Temperature 953

10 southward after 1979 in CSa, consistent with the shift of the curl τ zero line. These results are consistent with those of Seager et al. (2001): the intensification of gyres (large temperature gradient) and the southward shift of the Kuroshio-Oyashio Extension (subtropical-subarctic boundary) around Furthermore, a relatively small gradient of subsurface temperature and a northward shift of temperature front were found after 1991 in the present study, consistent with the change of wind stress curl field in the late 1980s. This is suggestive of the importance of the subtropical-subarctic boundary shift for the subsurface temperature change in CSa on a decadal scale, considering the large meridional temperature gradient and the magnitude of the shift ( degree in latitude) of the temperature front. The lag between the latitude of the curl τ zero line and subsurface temperature for CSa is 2 years after the shift of the curl τ zero line, which suggests a baroclinic response of the ocean to the wind stress curl (Deser et al., 1999; Venzke et al., 2000). There is uncertainty about the lag period, because the correlation calculated for lags of 1 year and 3 years is almost the same as that for 2 years. Westward propagation of temperature anomaly at a depth of 400 m in the subtropical gyre (not shown), which is similar to the propagation of sea level anomalies in ocean model with an observed wind stress forcing obtained by Venzke et al. (2000), is consistent with the slow baroclinic response. The importance of Ekman transport for the cooling in the central North Pacific in the mid 1970s has already been pointed out in previous studies (Yasuda and Hanawa, 1997; Seager et al., 2001). In the present study, we showed that the heat divergence in the Ekman layer is dominant relative to the heat flux through the surface for the decadal subsurface temperature change in NESt from the analysis between This result is consistent with the previous studies. In addition, temperature anomaly spreading south-southwestward in the eastern section of the subtropical gyre, with downward propagation, has been found by Deser et al. (1996) and Tourre et al. (1999, 2001). The advected anomaly can contribute to the decadal variability in NESt. Other terms, such as entrainment, may be important for the temperature fluctuation (Schneider et al., 1999). More investigations are needed to give us a better understanding of the subsurface temperature variability. Another interesting thing is a negative anomalous heat flux between and west of the date line along 40 N. The negative anomalous heat flux could cause negative anomalous subsurface temperature, while the m temperature difference is positive in the corresponding region (Fig. 3). The increase of advection of warm water by the Kuroshio after the mid 1970s has been pointed out by Yasuda and Hanawa (1997), but it is inconsistent in this case for the late 1980s; a positive anomalous wind stress curl in the subtropical region (Fig. 8) could cause weakening of subtropical gyre, and a subsequent negative anomalous subsurface temperature in the Kuroshio-Oyashio Extension. It is suggested that a displacement of the latitude of the subtropical-subarctic boundary plays a dominant role for decadal subsurface temperature fluctuation in this region, as described by Seager et al. (2001). Our results indicate the importance of an almost simultaneous response in the Ekman layer in NESt and a relocation of the gyres with some delay as a response of the ocean to the wind stress curl field, supporting the theory proposed in Seager et al. (2001). Subsurface temperature in both regions of NESt and CSa may act independently, as has already been pointed out for decadal SST variability by Nakamura et al. (1997). However, we did not consider the decadal fluctuation quantitatively in the present study. Quantitative analyses would shed more light on a decadal-scale fluctuation. In recent years, many studies have been done to clarify a feedback mechanism of the fluctuation of decadal subsurface temperature to the atmosphere. Venzke et al. (2000) reported the delayed eastward spreading along the Kuroshio Extension as the result of baroclinic response and proposed a negative feedback mechanism, following Latif and Barnett (1994, 1996). On the other hand, Seager et al. (2001) obtained a positive feedback in the same region from a hindcast performed with an atmosphereocean coupled model forced with the observed surface winds. An influence from the atmosphere to the ocean on a decadal scale was examined in this study, but the mechanisms that generate atmosphere/ocean decadal variability are not clear. Further studies are needed to understand the decadal variability by comprehensive analyses of atmospheric and oceanic data. Acknowledgements The authors sincerely thank the National Research Institute of Far Seas Fisheries for providing their historical data. Thanks are extended to Dr. Tsurane Kuragano and Mr. Ichiro Ishikawa of the Meteorological Research Institute for their help in analyzing subsurface temperature and preparing NCEP reanalysis data, respectively. We are also grateful to Dr. Yoshiteru Kitamura of JMA and Dr. Ikuo Kaneko of the Nagasaki Marine Observatory for providing useful comments and constructive suggestions on an early draft of this paper. Two anonymous reviewers provided comments that greatly improved the manuscript. Graphics were produced using GrADS, developed by COLA/IGES. This study was performed as part of the Subarctic Gyre Experiment (SAGE), financially supported by the Ministry of Education, Culture, Sports, Science and Technology. 954 S. Sugimoto et al.

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