Long-Term Variability of North Pacific Subtropical Mode Water in Response to Spin-Up of the Subtropical Gyre

Transcription

1 Journal of Oceanography, Vol. 59, pp. 279 to 290, 2003 Long-Term Variability of North Pacific Subtropical Mode Water in Response to Spin-Up of the Subtropical Gyre TAMAKI YASUDA* and YOSHITERU KITAMURA Meteorological Research Institute, Japan Meteorological Agency, Nagamine, Tsukuba, Ibaraki , Japan (Received 17 May 2002; in revised form 24 October 2002; accepted 24 October 2002) The Meteorological Research Institute s ocean general circulation model (MRI- OGCM) has been used to investigate the temperature variability of the North Pacific Subtropical Mode Water (NPSTMW) over a time series longer than 5 years via the spin-up of the subtropical gyre. Besides an interannual variation, the wintertime sea surface temperature in the area where the NPSTMW is formed, and the temperature of the NPSTMW itself, both change remarkably in a >5-year time scale. An analysis of heat budgets showed that the long-term changes in NPSTMW temperature are due mainly to a leading advection of heat by the Kuroshio Extension and compensating surface heat flux. As a result of a dynamical adjustment to the wind stress fields, the transports of the Kuroshio and the Kuroshio Extension increased in the mid 1970s with a lag of 3 years after the wind stress curl in the central North Pacific. The increased heat advection by the Kuroshio Extension induces a warming in the mixed layer in the NPSTMW formation area, followed by a warming of the NPSTMW itself. Both these warming actions increase the heat release to the atmosphere. These results imply that the surface heat flux over the Kuroshio Extension area varies in response to the change in the ocean circulation through the spin-up of the subtropical gyre. Keywords: North Pacific Subtropical Mode Water, ocean general circulation model, gyre spin-up. 1. Introduction South of the Kuroshio Extension, a large amount of the heat is released in winter, leading to the formation of the deep mixed layer with temperature ranging from 15 to 19 C. This water, which has vertically uniform properties, is subducted into the thermocline layer as a thermostad. As a result of the advection by the Kuroshio recirculation, thermostad of C distributes widely in the northwestern part of the subtropical gyre. The water of the thermostad is called the North Pacific Subtropical Mode Water (NPSTMW; Masuzawa, 1969, 1972; Hanawa and Talley, 2001). The properties of the NPSTMW have a significant interannual variability (2 4 years) which is closely related to the local change in the East Asian wintertime monsoon (Suga and Hanawa, 1995; Yasuda and Hanawa, 1999; Taneda et al., 2000). When the wintertime monsoon is stronger, the ocean releases a larger amount of heat to * Corresponding author. tyasuda@mri-jma.go.jp Present address: Climate and Marine Department, Japan Meteorological Agency, Ote-machi, Chiyoda-ku, Tokyo , Japan. Copyright The Oceanographic Society of Japan. the atmosphere, leading to the formation of the deeper mixed layer and the thicker and colder NPSTMW. Recently, it has also been reported that the decadal variability of the NPSTMW is related to the change in the gyre circulation. Yasuda and Hanawa (1997) examined the change in the thermal structure of the NPSTMW that occurred in the mid 1970s and pointed out that the change in the Kuroshio transport possibly causes the decadal variability of the mode water properties. Hanawa and Kamada (2001) compared the time series among the NPSTMW temperature, climate indices such as the North Pacific Index, and the oceanic transport derived from hydrographic repeat section data. They showed that the change in the Kuroshio transport is related to a change in the strength of the Aleutian Low with a time lag of 5 years, and they concluded that the subtropical gyre dynamically adjusts to the large-scale wind stress fields for about 5 years, as pointed out by Deser et al. (1999). In addition, since the temperature of the NPSTMW varies with a lag of 2 years from the Kuroshio transport, they suggested that the temperature of the NPSTMW is affected by the Kuroshio heat advection with a 2-year lag. Yasuda et al. (2000) estimated the change in the heat budget of the mixed layer based on the observed data and 279

2 pointed out the importance of the horizontal advection terms for the shift of the sea surface temperature (SST) in the Kuroshio Extension region observed in the late 1980s. Xie et al. (2000) published a numerical investigation of the decadal variability of the North Pacific Central Mode Water (NPCMW) which is characterized by a thermostad of 9 13 C and distributes over the area of W, N. They concluded that increase of the eastward Kuroshio Extension, in addition to the deepening of the winter mixed layer, contributed to the eastward movement of the NPCMW formation area, and thus to the resulting change in the subsurface temperature in the central North Pacific. The Kuroshio Extension region is a key region where feedback from ocean to atmosphere could occur (Latif and Barnett, 1994, 1996). In fact, the climatological and long-term variability of the surface heat flux is significant in the Kuroshio Extension region, i.e., the formation area of the NPSTMW (Yasuda and Hanawa, 1997). In order to understand the decadal climate change it is important to elucidate the long-term variability of the NPSTMW and its causes. In this study we have investigated the variability of the NPSTMW using an ocean general circulation model (OGCM). The observational data series is not long enough to discuss the long-term changes in the thermal structure of the NPSTMW and their relationship with causal factors. We can examine the heat budget of the NPSTMW by analyzing the model results spanning from 1960 to In particular, the mechanism of its long-term variability, i.e., the role of the subtropical gyre circulation suggested by Yasuda and Hanawa (1997) and Hanawa and Kamada (2001) has been evaluated. The paper is organized as follows. Section 2 explains the model and boundary conditions used in this study. Climatological features of the NPSTMW simulated in our model are described in Section 3. Section 4 presents the long-term variability of the NPSTMW temperature. Mechanisms of the long-term variability in the mode water are discussed by analyzing the heat budget in the NPSTMW formation area in Section 5. Section 6 summarizes our results. 2. Model and Boundary Conditions The OGCM used in this study is a z-coordinate primitive equation model developed at the Meteorological Research Institute (MRI-OGCM; Yamanaka et al., 1998). The model domain extends from 15 S to 60 N in the North Pacific with realistic topography, and the horizontal resolution is 2 in longitude and 1 in latitude. The model has 30 vertical levels with 2 m to 50 m resolutions in the upper 500 m. The model includes the isopycnal diffusion scheme of Gent and McWilliams (1990) where the isopycnal and diapycnal diffusivities are m 2 s 1 and m 2 s 1, respectively. The horizontal viscosity is m 2 s 1, while the vertical viscosity and diffusivity are determined by the level-2 turbulent closure scheme of Mellor and Yamada (1982). Within 5 of the model s southern boundary, temperature and salinity are restored to the climatological monthly values of the World Ocean Atlas 1994 (WOA94; Levitus and Boyer, 1994; Levitus et al., 1994). The model was initialized with the WOA94 January condition and integrated for 40 years to spin up the gyrescale circulation. The spin-up was driven by the climatological annual cycle of wind stress and buoyancy forcing. The wind stress data are the monthly mean climatology values of da Silva et al. (1994). On the surface heat flux, short and long wave radiative heat fluxes are taken from the monthly mean climatology of da Silva et al. (1994). The latent and sensible heat fluxes are calculated using the bulk formula of Large and Pond (1982) from monthly mean data of 10-m air temperature, specific humidity, wind speed, and sea level pressure of da Silva et al. (1994) with the simulated SST. The surface freshwater flux is parameterized by relaxing the sur- Fig. 1. (a) Simulated and (b) WOA94 climatological temperature sections at 150 E averaged from May to December. Contour interval is 2 C. Dashed line indicates MLD in March. Shading shows vertical temperature gradient. 280 T. Yasuda and Y. Kitamura

3 face salinity to the monthly mean climatology of WOA94 with a time scale of 2 days. Using final climatological fields generated by the 40year spin-up as initial conditions, the interannual simulation from January 1960 through December 1993 is performed using the historical monthly data of wind stress, radiative fluxes, and the atmospheric factors published by da Silva et al. (1994). Because of the lack of a reliable historical dataset for the surface freshwater flux, salinity at the uppermost level is restored to the monthly climatology of sea surface salinity in WOA Simulated NPSTMW Climatology In this section we describe the features of the NPSTMW simulated by the OGCM. Figure 1(a) shows the latitude-depth section along 150 E of the temperature averaged from May to December. Since the NPSTMW is characterized by a thermostad, a vertical temperature gradient is a good indicator to detect the NPSTMW (Suga et al., 1997). In Fig. 1(a) there is an area where the vertical temperature gradient is minimum at mainly 28 N. The water in this area can be regarded as the NPSTMW. In this paper, simulated NPSTMW is defined as the water of C that has a vertical temperature gradient less than 1 C/100 m, whereas the ob- served NPSTMW is defined as the water with a vertical temperature gradient less than 1.5 C/100 m (Hanawa and Suga, 1995; Suga et al., 1997; Hanawa and Talley, 2001). In addition, following to Hanawa and Kamada (2001), we define the temperature of the NPSTMW as the temperature averaged from May to December at the depth where the vertical temperature gradient is vertically minimum in m depth ranges, and hereafter we call this depth the core depth. For example, temperature of the NPSTMW at 150 E, 30 N is about 19 C (Fig. 1(a)). The temperature section along 150 E based on the observational data (WOA94) is shown in Fig. 1(b). The distribution of the model NPSTMW is similar to the observational data, although the model water is warmer. In Figs. 1(a) and (b), a dashed line denotes the mixed layer depth (MLD) in March. MLD is defined as the depth where temperature equals to SST 0.5 C in this study. Compared with observation, the model mixed layer front at which the mixed layer deepens sharply northward exists further south. The model mixed layer is deeper in the area north of the front. This seems to indicate that a larger amount of the mode water is formed in the model than in reality. Figure 2(a) shows the horizontal maps of the model SST and MLD in March in the northwestern part of the subtropical gyre. The mixed layer front extends from Fig. 2. (a) Simulated climatological SST (contour) and MLD (shade) in March. Contour interval is 1 C. (b) As in (a) but for the WOA94 data. (c) Simulated climatological temperature and vertical temperature gradient at the core depth averaged from May to December. Contour interval is 0.5 C. Thick line denotes the mixed layer front (150 m contour of MLD) in March. (d) As in (c) but for the WOA94 observed data. Long-Term Variability of North Pacific Subtropical Mode Water 281

4 140 E, 27 N to 175 E, 33 N, and the mixed layer deeper than 200 m is located between the front and the Kuroshio Extension at approximately 35 N. Moreover, water of C distributes in the deep mixed layer. Thus, it can be considered that the simulated NPSTMW is formed in winter between the mixed layer front and the Kuroshio Extension, and we can regard the area E, N as the formation area of the model NPSTMW. Even if we were to choose an area deviating a few degrees from that area as the formation region, conclusions based on the heat budget analysis below do not change as far as this study is concerned. Figure 2(b) shows the observed SST and MLD in March, and reveals that the mixed layer front extents from 140 E to 170 E approximately at 32 N. The observed NPSTMW formation area (Fig. 2(b)), which is considered to be the deep mixed layer region north of the mixed layer front, is narrower than the model formation area (Fig. 2(a)). This difference in the formation areas as between the model and the observations (WOA94) can mainly be attributed to the broad model Kuroshio. As seen in Fig. 1, the meridional extent of the model Kuroshio is apparently wider than that of WOA94 (32 38 N). Since the model Kuroshio flows more broadly than the observed one, the subsurface temperature is warmer. In the Kuroshio Extension area, a large amount of heat is released because of the warm water advected from lower latitude and the wintertime cold air, which results in the deepening of the mixed layer. In this model, as a result of SST forced by the observed atmospheric variables and the warmer water in the subsurface layer, the deeper mixed layer develops more during winter than is observed. The underestimate of observed MLD with the large spatial and temporal smoothing in WOA94 partly causes MLD given in WOA94 to be shallower than the model. Using hydrographic data smoothed by the scale that was smaller than the WOA94, Suga and Hanawa (1990) and Suga et al. (1997) calculated MLD in the midlatitude North Pacific and revealed a mixed layer deeper than 200 m in the northwestern part of the subtropical gyre. NPSTMW is subducted below the seasonal thermocline and spreads extensively in the northwestern part of the subtropical gyre. Figure 2(c) shows that the temperature and its vertical gradient at the core depth averaged from May to December. According to the definition that NPSTMW is the water with a vertical temperature gradient less than 1 C/100 m, NPSTMW distributes in the northwestern part of the subtropical gyre, i.e., E, N. The mode water exists south of the wintertime mixed layer front. This indicates that the NPSTMW formed north of the mixed layer front is advected by the geostrophic flows. More than half of the volume of the NPSTMW remains in the north of the front, i.e., in the formation area, even after May, and the temperature in the March mixed layer is lower than that of the NPSTMW in this area. This part of the NPSTMW is re-entrained into the deep mixed layer in the next winter. We can see that MLD in March reached the depth of the NPSTMW at 150 E in Fig. 1(a). Hanawa and Yoritaka (2001) showed that a part of the volumes of the observed NPSTMW remains in the formation area in the late fall. Figure 2(d) shows the observed temperature and its vertical gradient at the core depth. As described above, the observed NPSTMW is defined as the water mass with a vertical temperature gradient less than 1.5 C/100 m. The observed NPSTMW is distributed in the area E, N, which is almost same as the distribution area of the model NPSTMW (Fig. 2(c)). 4. Long-Term Variability of the NPSTMW Figures 3(a) and (b) show the time series of SST in March averaged in the formation area of the NPSTMW ( E, N), both in the model and as observed (da Silva et al., 1994), respectively. The variability of the simulated NPSTMW temperature agrees well with that found by observation. Remarkable temperature variations for the time scale longer than 5 years are seen, in addition to the interannual (2 4 year) variability. Figures 4(a) and (b) show the longitude-time sections of SST in March and the NPSTMW temperature. Fig. 3. Time series of (a) simulated and (b) observed SST anomalies in March averaged in the NPSTMW formation area ( E, N), and (c) simulated temperature anomalies at the core depth averaged in the NPSTMW distribution area ( E, N). Dashed line indicates 5-year running mean values. 282 T. Yasuda and Y. Kitamura

5 Temperature is plotted only in the area with the mixed layer deeper than 150 m. The model temperature between 140 E and 170 E varies in the time scale longer than 5 years (Fig. 4(a)). Although the model temperature is warmer than the observation as mentioned in Section 3, the contours especially in E indicates similar temporal changes in the simulated and observed temperature (Fig. 4(b)). The deep mixed layer region extended further east from the late 1970s to the early 1980s. This occurred simultaneously with and is related to the increase of the transport of the Kuroshio Extension through the spin-up of the subtropical gyre, as discussed in the next section. The long-term change in SST in March is directly related to that in the NPSTMW temperature. Temperature change in the NPSTMW at the core depth averaged in E, N is shown in Fig. 3(c). In , the NPSTMW became colder. The NPSTMW temperature rose from 1976 to 1980 and then decreased in the early 1980s, to increase again after The longitude-time section of the NPSTMW temperature in Fig. 4(c) shows a similar pattern to that in the wintertime SST (Fig. 4(a)). These long-term variations are in good agreement with the analysis of the observed data by Yasuda and Hanawa (1997) and Hanawa and Kamada (2001). Figure 5 shows the latitude-time sections of the NPSTMW temperature. The temperature was colder and the NPSTMW distributed further north in During this period, the Kuroshio Extension was located further north than the climatological position, and its transport was small (not shown here). We can also see the NPSTMW warming from 1978 along both 160 E and 170 E. 5. Heat Budget Analysis; Intensified Advection via the Gyre Spin-Up In the preceding section we demonstrated that the NPSTMW has a remarkable temporal variability longer than 5 years. This long-term variability has no significant correlation with the surface wind speed and surface heat flux over the formation area. In this section, in order to examine any influence of the ocean circulation on the Fig. 4. Longitude-time plots of (a) simulated and (b) observed SST in March (shade). Simulated SST is drawn only in the area with MLD greater than 125m. (c) As in (a) but for the simulated temperature at the core depth in the area with vertical temperature gradient less than 1 C/100 m. Contours indicate 5-year running mean values and contour interval is 0.5 C. Long-Term Variability of North Pacific Subtropical Mode Water 283

6 Fig. 5. Latitude-time plots of simulated temperature at the core depth in the area with vertical temperature gradient less than 1 C/100 m (shade). Contours indicate 5-year running mean values and contour interval is 0.5 C. NPSTMW long-term variability, as suggested by Yasuda and Hanawa (1997) and Hanawa and Kamada (2001) based on their analysis of the observed data, we analyze the heat budget of the surface layer in the formation area. Figures 6(a) and (b) present the annual mean temporal change rate and the major heat budget terms averaged in the NPSTMW formation area. We can see that the temperature variations are largely due to the horizontal advection and the net surface heat flux terms. Anomalies of the horizontal advection term were negative in the first half of the 1970s, but there was a steep rise in the mid 1970s, and the anomalies turned positive in The surface net heat flux had positive anomalies in the early 1970s, followed by a gradual decline in the late 1970s. The heat flux anomalies continued to be negative from 1978 to The horizontal advection term can be divided into the advection terms due to the geostrophic flow and to the surface Ekman flow, as shown in Fig. 6(c). This figure clearly illustrates that the change in the geostrophic component dominates in the horizontal advection term. Hence, long-term changes in the NPSTMW temperature are mainly determined by the geostrophic advection and the surface net heat flux terms. Figure 7 shows the horizontal maps of anomalies of these two terms yearly from 1977 to The anomaly fields of the annual mean horizontal velocity at 150 m are superimposed. Both the geostrophic advection and surface heat flux terms display large anomalies in 140 E 180, N, and this area corresponds to the region Fig. 6. Time series of the annual mean heat budget in the upper 200 m in the formation area ( E, N) smoothed with a 5-year running mean filter. Anomalies of (a) temporal change rate, terms of (b) horizontal advection (closed circle), vertical advection (cross), and net surface heat flux (open circle), and (c) horizontal advections by geostrophic (closed circle) and Ekman (open circle) flows. with large anomalies in the horizontal velocity. Correlation coefficients of the zonal velocity and of the zonal temperature gradient with the geostrophic advection term are 0.75 and 0.32, respectively. Thus, it is clear that the change in the geostrophic advection term is due to that in the zonal velocity. This result is consistent with that reported by Latif and Barnett (1994, 1996) who showed that the change in the Kuroshio transport results in the SST change in the northwestern part of the subtropical gyre in their scenario of the interdecadal variability of the North Pacific. Figure 8(a) shows the correlation of the annual mean Kuroshio transport across 138 E with annual mean wind stress curl with a time lag of 3 years. The transports of the Kuroshio and the Kuroshio Extension vary simultaneously. The wind stress curl around 180, 30 N has a significant correlation with the Kuroshio transport. We can recognize the 3-year lag between the Kuroshio transport and the wind stress curl over the central North Pacific in Fig. 8(b). Figure 9 shows the longitude-time plots of the temperature anomalies at 620 m along 30 N. It takes about 3 years for the anomalies to propagate westward from 180 to 138 E. Compared with the phase speed of 3.2 cm/s of the first baroclinic Rossby wave at 30 N, estimated using the linear theory with model density stratification, the propagation speed estimated in Fig. 9 is 50% 284 T. Yasuda and Y. Kitamura

7 Fig. 7. Anomalies of (a) heat advection term by the geostrophic flow vertically averaged in the upper 200 m, and (b) surface heat flux anomalies, smoothed with a 5-year running mean filter. Contour interval is 0.2 C/year. Light (dark) shading indicates values greater than 0.2 C/year (less than 0.2 C/year). Anomalies of the horizontal velocity vector at 150 m are superimposed. faster. Such a discrepancy between the observed and the theoretical phase speeds of the Rossby wave has often been reported (e.g., Chelton and Schlax, 1996). Killworth et al. (1997) explained that the linear theory is partly modified by the change in the potential vorticity gradient due to the presence of the baroclinic flows. Although the Kuroshio transport decreased in the mid 1980s, as seen in Fig. 8(b), we cannot recognize such a reduction of the horizontal heat advection term in Fig. 6. This discrepancy results from the change in the spatial distribution of the anomalous horizontal advection term in the Kuroshio Extension region (Fig. 7(a)). In the mid Long-Term Variability of North Pacific Subtropical Mode Water 285

8 Fig. 8. (a) Correlation coefficient of the annual mean wind stress curl with the annual mean Kuroshio transport with time lead of 3 years. Contour interval is 0.3 and areas with coefficient greater than 0.6 (less than 0.6) are lightly (darkly) shaded. (b) Time series of the wind stress curl averaged over 170 E 170 W, N (open circle), and the Kuroshio transport (closed circle), smoothed with a 5-year running mean filter. 1980s the area of the maximum anomalies of the heat advection term shifted southward to 30 N, i.e., the NPSTMW formation region. The horizontal heat advection in the formation area, therefore, does not decrease in spite of a reduction of the Kuroshio transport. In the late 1980s, by contrast, the area of the maximum heat advection anomalies shifted northward to 37 N, far from the formation region, corresponding to the increase of the Kuroshio transport. The spatial position of the NPSTMW formation region does not change temporally. In the 1980s, therefore, the change in the horizontal advection term in the formation region (Fig. 6(b)) did not directly reflect that in the Kuroshio transport. It is concluded that the change in the geostrophic advection is a result of the dynamical adjustment of the gyre circulation to the large-scale wind change by baroclinic Rossby waves (Miller et al., 1998; Deser et al., 1999; Lysne and Deser, 2002). Moreover, the increased transport of the Kuroshio Extension due to the gyre spin-up has an advection impact on the NPSTMW temperature suggested by Yasuda and Hanawa (1997) and Hanawa and Kamada (2001). Figure 10(a) presents the time series of the net surface heat flux anomalies and their components over the Fig. 9. Longitude-time plots of temperature anomalies at 620 m along 30 N (shade). Contours indicate 5-year running mean values and contour interval is 0.05 C. Fig. 10. (a) Time series of the anomalies of model net surface heat flux (solid), sum of the short and long wave radiative heat fluxes (dashed) and sum of the model latent and sensible heat fluxes (dotted), over the formation area ( E, N) smoothed with a 5-year running mean filter. (b) As in (a) but for (Ta Ts )<Vw> (solid), (<Ta> <Ts>)Vw (dashed), and (Ta Ts )Vw (dotted). Ta, Ts and Vw denoted observed surface air temperature, model sea surface temperature and observed surface wind speed, respectively. Marks < > and mean time-mean and anomaly, respectively. 286 T. Yasuda and Y. Kitamura

9 Fig. 11. Anomalies of (a) model sea surface temperature (Ts) and (b) observed surface air temperature (Ta). Contour interval is 0.1 C. Light (dark) shading indicates Ta Ts greater than 0.1 C (less than 0.1 C). NPSTMW formation area. The temporal change in the surface net heat flux is mainly due to the change in the latent and sensible heat fluxes from 1970 until It should be noted that the negative anomalies of the net surface heat flux are explained by the radiative heat fluxes since In our model the latent and sensible heat fluxes are calculated by the bulk formula using the observed atmospheric parameters and model SST, whereas we use data of da Silva et al. (1994) as the radiative heat fluxes, as mentioned in Section 2. Latent and sensible heat fluxes can be approximately determined by the product of the difference between the observed surface air temperature (Ta) and model SST (Ts), i.e., Ta Ts, and the observed surface wind speed (Vw). In order to show which component (either Ta Ts or Vw) is important for the change in the latent and sensible heat fluxes, Ta Ts and Vw are divided into time-mean (denoted by < >) and anomalies (denoted by ), and the products of those components Long-Term Variability of North Pacific Subtropical Mode Water 287

10 [(Ta Ts )<Vw>, (<Ta> <Ts>)Vw, and (Ta Ts )Vw ] are computed separately. These products are shown in Fig. 10(b). Change in the latent and sensible heat fluxes (Fig. 10(a)) corresponds well to that of Ta Ts rather than Vw (Fig. 10(b)). If the change in the model SST (Ts) dominates in the change in Ta Ts, it is considered that the ocean interior influences the net surface heat flux through Ts. Figure 11 shows the anomalies of Ts, Ta and Ta Ts from 1977 to The patterns of Ta Ts anomalies are consistent with those of Ts rather than Ta. This means that the change in Ts mainly determines that in Ta Ts. Therefore, it is concluded that the negative anomalies of the latent and sensible heat fluxes in the period are responsible for SST rise that results from the increased heat advection by the Kuroshio Extension. In summary, the change in the ocean interior, i.e., the increase of the geostrophic advection due to the spinup of the subtropical gyre in the late 1970s, causes the increase of SST in the formation area of the NPSTMW, which leads to the increase of the surface heat release to the atmosphere. 6. Summary and Discussion We have simulated the NPSTMW from 1960 to 1993 using the MRI-OGCM. The basic climatological features and the temporal variability of the NPSTMW are reproduced. SST in the NPSTMW formation area and the NPSTMW temperature change remarkably in a time scale longer than 5 years, in addition to the interannual time scale (2 4 years). A heat budget analysis shows that the long-term (longer than 5 years) change in the NPSTMW temperature is mainly controlled by the sum of the heat balance between the heat advection by the Kuroshio Extension geostrophic flow and the surface heat flux. The transports of the Kuroshio and the Kuroshio Extension increase in association with the change in the wind stress curl in the central North Pacific with a lag of 3 years. The increase of the heat advection by the Kuroshio Extension into the formation area of the NPSTMW results in the warming of the NPSTMW. Warming in the Kuroshio Extension region leads to an increase of the surface heat release to the atmosphere. The NPSTMW temperature has long-term variability that is different from the change in SST in the central North Pacific, which is one of the centers of action of the decadal variability in the North Pacific (Nakamura et al., 1997). Our results present a mechanism of the thermal change in the NPSTMW formation area different from that in the central North Pacific region. We have demonstrated that the warming of the NPSTMW after the mid 1970s and the resulting increases of the heat release were caused by the heat advection in the Kuroshio Extension region through the gyre spin-up, as suggested by Yasuda and Hanawa (1997) and Hanawa and Kamada (2001). This implies that the NPSTMW is a good indicator of the North Pacific climate variability, because the water is rich in information concerning the variability, not only of the air-sea interaction but also of the ocean circulation related to the change in the large scale wind field. In the Kuroshio Extension region, the climatological and long-term variability of the surface heat flux is significantly larger than other areas in the North Pacific. Time series of the observed surface heat flux shows that wintertime heat release increased gradually after the mid 1970s in the northwestern part of the subtropical gyre, in contrast to the rapid rise in the central North Pacific (Yasuda and Hanawa, 1997). Yasuda and Hanawa (1997) suggested that the gradual increase of the heat release in the Kuroshio Extension region results from the increased advection of the warm water by the gradually increased Kuroshio transport. Our results support their suggestion by detailed analysis of time series of the NPSTMW temperature, the Kuroshio transport, and the surface heat flux. It is not clear whether SST change influences the atmospheric circulation through the surface heat flux. Using the coupled atmosphere ocean general circulation model, Latif and Barnett (1996) concluded that the Kuroshio Extension region is a key region where negative feedback from ocean to atmosphere occurs though the surface heat flux. Our heat budget analysis in the Kuroshio Extension region does not contradict the delayed negative feedback. Recently, SST change in the subarctic front region and the meridional shift of the boundary between the subtropical and subpolar gyres have been focused on (Miller and Schneider, 2000; Seager et al., 2001; Schneider et al., 2002). That is, the negative heat advection in the subarctic front region caused by the southward shifts of the Aleutian Low cools the SST in that area. Schneider et al. (2002) showed that there is no negative feedback loop in the midlatitude North Pacific in their coupled climate model. In our study, a quantitative comparison between the two (subarctic front and Kuroshio Extension) regions shows that the temperature and heat flux variations are much larger in the subarctic front region than those in the Kuroshio Extension region (not shown here). This gives the impression that the positive feedback is dominant, as suggested by Schneider et al. (2002). Since the atmosphere does not necessarily respond linearly to the surface heat flux change, however, we cannot discuss the feedback in the two regions directly in terms of the amplitude of the surface heat flux change in this study. Thus, we still do not know whether the SST change in the gyre boundary between the subtropical and subpolar gyres leads to the atmospheric feedback. Furthermore, since the Kuroshio and the Oyashio meet at approximately 40 N and their Extensions extend 288 T. Yasuda and Y. Kitamura

11 because of the coarse resolution, even in the state-of-theart climate models, the surface temperature changes related to the Kuroshio Extension and the subarctic front are not well reproduced. Therefore, it is not clear which of the subtropical or subpolar gyres is more crucial in inducing the feedback in the area of the Kuroshio and Oyashio Extensions. Our simulation shows that long-term change in the surface temperature in the Kuroshio Extension region is dynamically distinct from that in the subarctic front region, where SST change results from the shift of the boundary between the subpolar and subtropical gyres, as examined by Seager et al. (2001). In the Kuroshio Extension region, on the other hand, SST change is mainly due to that in the heat advection by the Kuroshio Extension due to the spin-up/down of the subtropical gyre. In addition, Peng and Whitaker (1999) showed that the response to SST anomalies in the atmosphere general circulation model is sensitive to the representation of the storm track and the relationship between the positions of the storm track and SST anomalies. Further studies are required to reach conclusions on the possibility of feedback associated with the midlatitude SST anomalies. The model SST is well reproduced due to the bulk formula with observed atmospheric parameters. Although the model SST is forced toward the observed surface air temperature through the latent and sensible heat fluxes in this method (Seager et al., 1995), the surface heat flux term is not a leading factor in our long-term heat budget analysis. However, the surface heat flux calculated in the simulation does not necessarily agree with the observed surface heat flux given by da Silva et al. (1994). The difference in the annual mean heat fluxes between the model and the observation is about 20W/m 2, and long-term variability of the model heat flux is smaller than that found by observation. Although these differences result from both errors in the model s physics and the observed data set, the main reason is the weak and broad Kuroshio due to the coarse resolution of the model. Furthermore, in addition to the large-scale change in the ocean circulation such as the Kuroshio Extension, the temporal and spatial variations of the Kuroshio recirculation system possibly change the NPSTMW distribution though geostrophic advection. Qiu (2000) showed that the spatial structure of the Kuroshio recirculation varied significantly in the 1990s. In order to minimize the errors related the model physics and to investigate the influence of the Kuroshio recirculation system on the NPSTMW distribution, we intend to simulate the NPSTMW structure using a high resolution OGCM. 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