3.1. Changes in the Kuroshio and the Kuroshio Extension
 Figure 1shows stream functions of barotropic volume transport (BVT) and Sverdrup volume transport (SVT) for the present-day climate over the North Pacific north of 10°N, and for the future climate scenario. The BVT was directly calculated from the ocean velocity field:
where λ and ϕ are longitude and latitude, λe is longitude of the eastern boundary, v is meridional velocity, re is Earth's radius and D is bottom depth. The SVT was estimated from wind data based on the Sverdrup relation:
Where β is the meridional derivative of the Coriolis parameter, ρ0 is the mean density of the ocean, and is the wind stress curl.
Figure 1. (a and b) Barotropic and (c and d) Sverdrup stream functions of the present-day climate (contour) and their SRES A1B-induced changes (color shading) for (top) MIROC3.2-Hires and (bottom) EM LCM. These are calculated fromequations (1) and (2), respectively. Here and in subsequent figures as defined in the text the present-day climate means the average of variables from 20C3M scenario between 1901 and 2000 , and the future climate means the average of variables from SRES A1B between 2051 and 2100.
Download figure to PowerPoint
 In the present-day climate, spatial patterns of BVT and SVT for the interior ocean are similar. The western boundary current is successfully reproduced in the BVT of both MIROC3.2-Hires and EM LCM. In MIROC3.2-Hires, this western boundary current is more intense than that in EM LCM, and the KE appears to meander. Such characteristics of pattern and amplitude in MIROC3.2-Hires are in agreement with other high-resolution ocean general circulation models (OGCMs) [e.g.,You, 2005]. Application of scenario A1B created distinct changes in the SVT. In both MIROC3.2-Hires and EM LCM, the southern (northern) part of the North Pacific subtropical gyre weakens (strengthens) and the southern part of the North Pacific subarctic gyre weakens. This dipole type volume transport change indicates that the entire North Pacific subtropical gyre is shifted poleward in the future climate. The BVT also shows a similar change, which means that the poleward shift of the North Pacific subtropical gyre is mainly attributed to changes in wind stress as inSakamoto et al. . The change in BVT, however, is smaller than that in SVT, and the northward shift of BVT is not substantial both in high- and low-resolutions.
 In EM LCM the anti-cyclonic recirculation gyre is observed only south of Japan, whereas in MIROC3.2-Hires it is divided into two: one south and another east of Japan. By applying scenario A1B, in MIROC3.2-Hires the BVT of the recirculation gyre east of Japan is increased by up to about 16 Sv (16%), indicating that the KE jet is intensified. The recirculation gyre south of Japan is shifted poleward, which leads to a partial intensification of the Kuroshio between Taiwan and the southern coast of Japan. Although in EM LCM the southern recirculation gyre is also shifted poleward, the Kuroshio between Taiwan and the southern coast of Japan appears to weaken slightly. This difference seems to be due to the intensity and size of the respective southern recirculation gyres. That is, the southern recirculation gyre of MIROC3.2-Hires is located in a relatively narrow area just south of Japan, whereas that of EM LCM extends farther northeastward toward the center of the North Pacific. Therefore, even for the similar change in the interior subtropical gyre the response of the Kuroshio, the western boundary current, can be resolution dependent.
 Changes in the KOE region are examined in Figure 2in terms of the zonally averaged zonal velocity averaged from the surface to 100 m depth, the upper ocean temperature and its meridional gradient between 145°E and 160°E. In the present-day climate, two zonal velocity peaks, which are indicative of the KE and OE jets, are distinctly observed in MIROC3.2-Hires along with the local maxima of meridional temperature gradient that represent the subsurface KE front at about 37°N and the surface OE front at about 40°N. It is in good agreement withNonaka et al. . However, in EM LCM these two fronts merge into one broad surface front, which is a typical feature of low-resolution climate models, as discussed by earlier studies [Hurlburt et al., 1996; Yamanaka et al., 1998].
Figure 2. (top) Sections of zonal velocity averaged between 145°E and 160°E for the present-day climate (black lines) and the future climate (red lines). Potential temperature and its meridional gradient (shading) averaged between 145°E and 160°E over 0–500 m depth for the (middle) present-day climate and (bottom) future climate. Red isothermal lines indicate 15°C, and green isothermal lines 10°C.
Download figure to PowerPoint
 With the application of scenario A1B, in MIROC3.2-Hires the subsurface thermal front is significantly intensified, and its overlying zonal velocity peak, which is indicative of the KE jet, almost doubles in intensity from 0.26 m/s to 0.48 m/s. The OE front also intensifies while strengthening the jet from 0.09 m/s to 0.16 m/s. The intensification of the fronts results in clearer separation of the fronts under a global warming scenario. On the contrary, in EM LCM the scenario A1B does not have substantial influence on the KOE front. The thermal front intensifies slightly and shifts northward, but the change is not significant.
 To investigate if this intensification is a misinterpretation due to internal variability we display the time series of the KE jet from MIROC3.2-Hires for the 20th and 21st centuries inFigure 3. To find quantify the strength of the KE jet, we first zonal velocity averaged from the surface to 100 m depth and between 145°E and 160°E. We then defined the maximum in the meridional range between 30°N and 40°N as the strength of the KE jet. The KE jet reveals a slightly declining trend during the 20th century. Although there is strong short term variability, during the 21st century the long term inclining trend is significantly surpassing the declining trend during the 20th century, confirming that the intensification is not due to decadal or centennial scale internal variability. The standard deviations of the maximum zonal velocity for the 20th and 21st centuries are 0.07 m/s and 0.13 m/s, respectively. As discussed before, the KE jet is intensified by 0.2 m/s. This amount is well beyond the standard deviation and we could conduce that the intensification is not due to internal variability. For EM LCM similar to the KE jet we define the KOE jet between 30°N and 50°N. Note that we adopted a wider meridional range considering the meridional extent of the KOE jet in low resolution models. The KOE jet intensifies by about 0.01 m/s by the end of the 21st century, which is beyond one standard deviation, 0.004 m/s, for both the 20th and 21st centuries. However, the intensified amount is much smaller than that of MIROC3.2-Hires.
Figure 3. Time-series of the maximum of zonal velocity averaged between the surface and 100 m depth and between 145°E and 160°E, determined in the meridional range (MIROC3.2-Hires) between 30°N and 40°N and (EM LCM) between 30°N and 50°N and (EM LCM), for the entire (left) 20th and (right) 21st centuries. Red lines indicate linear trends of the maximum zonal velocity for the respective periods. It should be noted that the upper and lower panels have different vertical scales.
Download figure to PowerPoint
 One notable result from MIROC3.2-Hires is the enhanced internal variability of the KE jet during the 21st century. The intensified KE and OE fronts have a larger amount of potential energy available for baroclinic instability and the formation of transient eddies. The enhanced shear across the KE jet also could enhance eddy activities to generate the greater internal variability, but detail investigation on this subject is outside the scope of this paper.
3.2. Dynamic Atmosphere-to-Ocean Process
 Figure 4 shows ω500hPa, SLP, and wind stress curl over the North Pacific outside the equatorial region for the present-day climate, and their changes under the future climate scenario. Positive (negative) values ofω500hPaindicate descending (ascending) air motion. In the present-day climate, ascending motion is dominant over the northwestern North Pacific off Japan, whereas descending motion is dominant over the southeastern North Pacific outside the tropics. One of the general features depicted in the future climate is the weakening of these vertical motions of air across the whole North Pacific (Figures 4a and 4d), which is interpreted as the weakening and poleward expansion of the Hadley circulation and the poleward displacement of the Ferrel circulation [Yin, 2005; Vecchi and Soden, 2007; Lu et al., 2007]. Over the northwestern Pacific including the Kuroshio and KOE region, the poleward shift of the subtropical subsidence region weakens the ascending motion. In the regions where these ascending motions are weakened, the SLP increases (Figures 4b and 4e).
Figure 4. Vertical velocity (descent positive) at (a, d) 500 hPa, (b, e) sea level pressure, and (c, f) wind stress curl over the North Pacific for the present-day climate (contour lines), and their SRES A1B-induced anomalies (color shading) for (left) MIROC3.2-Hires and (right) EM LCM.
Download figure to PowerPoint
 The increase of SLP over the northwestern Pacific, in turn, induces a negative wind stress curl anomaly along the zero wind stress curl line of the present-day climate via the geostrophic balance both in MIROC3.2-Hires and EM LCM, pushing the zero wind stress curl line poleward (Figures 4c and 4f). Consequently, the negative wind stress curl field driving the North Pacific subtropical gyre is shifted poleward to produce the dipole pattern in SVT (Figures 1c and 1d). Within the interior of the subtropical gyre the change in BVT is comparable to that in SVT except the poleward shift, and we can conclude that the atmospheric change induces ocean change over the interior of the northern Pacific subtropical gyre.
 Both MIROC3.2-Hires and EM LCM reveal that over the KOE region the upward vertical velocity at 500 hPa weakens, the SLP increases, and the negative wind stress curl anomaly occurs. This change generates an Ekman convergence anomaly to elevate the sea surface height. An anti-cyclonic current anomaly is formed in the KOE region to enhance the circulation gyre east of Japan in MIROC3.2-Hires, and the northern part of subtropical gyre in EM LCM. This dynamic atmosphere-to-ocean process indicates a link between changes in the large-scale atmosphere circulation and the KOE.