Interannual-to-decadal variability in the Oyashio Current

Interannual-to-decadal variability in the Oyashio Current and its influence on
temperature in the subarctic frontal zone: An eddy-resolving OGCM simulation
Masami Nonaka
Frontier Research Center for Global Change, JAMSTEC, Yokohama, JAPAN
Hisashi Nakamura
Frontier Research Center for Global Change, JAMSTEC, Yokohama, JAPAN
also Graduate school of Science, the University of Tokyo, Tokyo, JAPAN
Youichi Tanimoto
Frontier Research Center for Global Change, JAMSTEC, Yokohama, JAPAN
also Faculty of Environmental Earth Science, Hokkaido University, Sapporo, JAPAN
Takashi Kagimoto
Frontier Research Center for Global Change, JAMSTEC, Yokohama, JAPAN
Hideharu Sasaki
Earth Simulator Center, JAMSTEC, Yokohama, JAPAN
submitted to the Journal of Climate
October 18, 2007
revised on April 14, 2008
Corresponding author address:
Masami Nonaka
Frontier Research Center for Global Change,
Japan Agency for Marine-Earth Science and Technology (JAMSTEC),
3173-25 Showa-machi, Kanazawa-ku, Yokohama, 236-0001 Japan
[email protected]
-1-
ABSTRACT
Output of an eddy-resolving OGCM simulation is used to investigate mechanisms for
interannual-to-decadal variability in the Oyashio Current and its influence on the subarctic
frontal zone in the western North Pacific. Lag correlation analysis reveals that positive
anomalies both in basin-scale wind-stress curl and in local Ekman pumping can intensify
the southward Oyashio Current almost simultaneously via barotropic and baroclinic Rossby
wave propagations, respectively. The Oyashio Current strength can also be influenced by
anomalous Ekman pumping exerted in the western portion of the basin through the
baroclinic wave propagation with the lag of three years, which appears to arise from a
periodicity in the wind field. The intensification of the Oyashio Current is accompanied by
negative anomalies both in the sea surface temperature and height off the Hokkaido Island
of Japan and followed by their eastward development along the southern branch of the
Oyashio Extension and associated subarctic frontal zone in association with a southward
displacement of their axes. These changes are associated with cool sea surface temperature
anomalies and low potential-vorticity anomalies at the thermocline level in the frontal zone.
The surface cooling thus induced in the frontal zone by those oceanic processes
accompanies anomalous downward surface heat fluxes, indicative of ocean-to-atmosphere
feedback forcing associated with the Oyashio Current variations.
-2-
1. Introduction
For the North Pacific decadal variability, the importance of remote influence from the
tropics (Nitta and Yamada 1989; Trenberth and Hurrell 1994; Newman et al. 2003; Deser et
al. 2004, among others) and that of the midlatitude atmospheric stochastic forcing
(Hasselman 1976; Frankignoul 1985) have been stressed. A recent analytical investigation
by Qiu et al. (2007), however, emphasized the potential importance of ocean-to-atmosphere
feedback in the Kuroshio Oyashio Extension (KOE) region in generating decadal-scale sea
surface temperature (SST) variability, as has been suggested by some previous studies
(Latif and Barnett 1994; Barnett et al. 1999; Pierce et al. 2001; Schneider et al. 2002,
among others). In fact, Nakamura et al. (1997) and Nakamura and Yamagata (1999) pointed
out that the KOE region is the primary center of action of North Pacific decadal variability,
and decadal-scale SST anomalies (SSTAs), especially in its northern portion, exhibits no
significant simultaneous correlation with the tropical SSTAs. Further, it has been shown
that oceanic processes can induce interannual-to-decadal SSTAs in the KOE region on the
basis of ocean general circulation model (OGCM) experiments (Xie et al. 2000; Seager et
al. 2001; Yasuda and Kitamura 2003; Nonaka et al. 2006; Kwon and Deser 2007), a linear
model experiment (Schneider and Miller 2001), and observational data analyses (Qiu 2000,
2002, 2003; Tomita et al. 2002; Kelly and Dong 2004), implying a possibility of an
ocean-to-atmosphere feedback in the region as mentioned below.
In most of those previous studies, the KOE region has been treated as a single frontal
zone owing to rather coarse horizontal resolutions of their models or datasets.
High-resolution observational data, however, unambiguously shows that there are at least
-3-
two prominent oceanic frontal zones in the KOE region: the Kuroshio Extension (KE)
frontal zone (KEFZ) along the KE Current (e.g., Mizuno and White 1983), and the
subarctic frontal zone (SAFZ), or polar frontal zone (Belkin et al. 2002), associated with
the Oyashio Extension and the North Pacific Current (e.g., Yuan and Talley 1996). These
two frontal zones have distinct vertical structures (Nonaka et al. 2006): SSTAs in SAFZ
tend to be substantially stronger than in KEFZ (Nakamura and Kazmin 2003; Nonaka et al.
2006). Tanimoto et al. (2003) showed that decadal-scale warm (cold) SSTAs in SAFZ
locally induce anomalous upward (downward) surface heat flux. Given SSTAs are induced
by oceanic processes, including advective effect by those currents (Kelly and Dong 2004;
Schneider and Cornuelle 2005), this suggests the existence of an ocean-to-atmosphere
feedback in the region. In spite of their potential importance as thermal forcing onto the
overlying atmosphere, how SSTAs are generated in SAFZ has not been clarified yet. The
aforementioned previous studies have suggested that incoming baroclinic Rossby waves are
particularly important for the formation of SSTAs in the KOE region. Nonaka et al. (2006)
have shown, however, that decadal variations of the SAFZ and KEFZ reproduced in an
eddy-resolving OGCM are not necessarily coherent temporally. Their results suggest
potential importance of processes other than the Rossby waves, especially for the SAFZ
variability, including variability of the Oyashio Current, as has been suggested by Sekine
(1988a).
The Oyashio Current is a western boundary current of the western subarctic gyre in the
North Pacific, transporting cold water from that gyre and the Sea of Okhotsk into SAFZ
(Yasuda 2003, for a recent review) with its vertical extent more than 1000 m (Uehara et al.
-4-
2004). The Oyashio Current, especially its along-shore branch, is known to exhibit large
interannual variability (Sekine 1988a). Owing to the limited availability of direct
measurements of the current velocity, interannual and decadal variations in the current has
been detected mainly by examining the corresponding changes in the southern-most
latitude of the Oyashio near-shore branch defined from 100-m depth temperature
distributions off northern Japan (Ogawa 1988; Yasuda 2003, and references therein). While
the importance of barotropic Rossby wave propagation has been pointed out for seasonal
and interannual variations in the Oyashio Current (Sekine 1988b; Hanawa 1995; Isoguchi et
al. 1997; Kono and Kawasaki 1997), it has been recently shown that baroclinic Rossby
wave propagation is also important for interannual and longer term variability of the current
(Qiu 2002; Tatebe and Yasuda 2005). Recently, Ito et al. (2004) examined interannual
variations in the Oyashio Current after 1993 by combining in situ measurements and
satellite altimeter measurements. More recently, Isoguchi and Kawamura (2006) attempted
to estimate interannual variability in the Oyashio Current based on tide gauge data,
although it tends to emphasize the barotropic component.
As mentioned above, the limited availability of observational data limits our
understanding of the mechanisms for interannual and decadal variability in the Oyashio
Current and SAFZ. Nevertheless, recent development of an eddy-resolving OGCM that can
realistically represent the frontal structures in the KOE region (Nonaka et al. 2006) has
enabled us to investigate their long-term variability in detail. In this study we investigate
how long-term variability of the Oyashio Current is induced by wind-stress anomalies and
how the current variations induce SSTAs in SAFZ, taking advantage of an output of the
-5-
eddy-resolving OGCM hindcast simulation for 1950-2003.
This paper is organized as follows. Section 2 introduces the OGCM and datasets.
Section 3 describes the mean state of the Oyashio Current and its variability simulated in
the OGCM. Influence of wind stress variability on the Oyashio Current is investigated in
Section 4. Section 5 shows influences of the Oyashio Current variations on SAFZ. Section
6 provides a summary and discussions.
2. Model and datasets
The OGCM we use in this study is based on the Modular Ocean Model (MOM3)
(Pacanowski and Griffies 2000), developed at the Geophysical Fluid Dynamics
Laboratory/National Ocean and Atmospheric Administration (GFDL/NOAA), but the code
has been substantially modified for attaining its efficient performance on the vector-parallel
hardware system of the Japan’s Earth Simulator (Ohfuchi et al. 2007). Our ocean model for
the Earth Simulator (OFES; Masumoto et al. 2004) covers a near-global domain extending
from 75°S to 75°N, except for the Arctic Ocean, with horizontal resolution of 0.1°. The
model has 54 vertical levels, with resolutions from 5 m at the surface to 330 m near the
bottom. The model topography is based on the 1/30° bathymetry dataset (kindly provided
by GFDL/NOAA) with the maximum depth of 6,065 m. Although the lack of sea-ice
processes leads to somewhat unrealistic features in stratification within the North Pacific
subpolar gyre, the overall circulation system is realistically reproduced in OFES.
The model solves the primitive equation system in spherical coordinates under the
Boussinesq and hydrostatic approximations. The KPP boundary layer mixing scheme
-6-
(Large et al. 1994) is adopted for vertical mixing. For horizontal mixing of momentum and
tracers, we adopt a scale-selective damping with a bi-harmonic operator, to suppress
grid-scale computational noises (Smith et al. 2000). The background horizontal
bi-harmonic viscosity and diffusivity are -27x109 m4s-1 and -9x109 m4s-1, respectively.
These values are the same as those used in Maltrud and McClean (2005) and Smith et al.
(2000) for 0.1°-resolution global and Atlantic OGCMs, respectively.
In the model, the surface heat flux and evaporation rate are calculated with the bulk
formula developed by Rosati and Miyakoda (1988), based on the model-simulated SST and
atmospheric variables from the National Centers for Environmental Prediction/National
Center for Atmospheric Research (NCEP/NCAR) reanalysis (Kalnay et al. 1996). The fresh
water flux is evaluated from the evaporation rate and daily precipitation rate taken from the
reanalysis data, under the constraint for sea surface salinity to be restored to its monthly
climatology with timescale of 6 days, to include the contribution from river run-off. The
climatology is based on World Ocean Atlas 1998 (WOA98; Boyer et al. 1998a, 1998b,
1998c). Within 3° from the model’s artificial boundaries placed at 75°N and 75°S,
temperature and salinity are restored to their local monthly climatologies (WOA98) with
time scale that is 1 day at the boundaries and increasing to infinity into the interior. See
Masumoto et al. (2004) and Sasaki et al. (2008) for details of the model setup.
The model was first integrated for 50 years from the climatological annual-mean fields
of temperature and salinity (WOA98) without motion under the climatological
monthly-mean atmospheric forcing. Following this 50-year spin-up, we then conducted a
54-year hindcast integration with daily-mean atmospheric fields of the NCEP/NCAR
-7-
reanalysis data from 1950 to 2003. This hindcast simulation successfully captures
variability with intraseasonal-to-decadal time scales (Sasaki et al. 2008), and it has been
used for an investigation of decadal variability in SAFZ and KEFZ (Nonaka et al. 2006). In
the following analyses, model monthly climatology and anomalies are defined as the mean
for this 54-year period and deviations from it, respectively.
For our analytical convenience, the full (0.1°) resolution for the model output has been
reduced to 0.5° resolution by picking up the data at every five grid-points both in the zonal
and meridional directions, except in our evaluation of the potential vorticity (PV) field.
Though reduced, the resolution is still adequate for resolving the Oyashio Current and fine
structures in SAFZ as shown below. Variability in frontal structures simulated in SST and
sea surface height (SSH) fields discussed below has been confirmed to have the same
characteristics between the reduced and full resolution datasets.
In addition to the output of the hindcast integration of OFES, we also use the Japan
Meteorological Agency (JMA) SST data set. The data have been compiled for the western
North Pacific (100°-180°E, 0°-60°N) with 1° horizontal resolutions with subjective analysis
of in situ and satellite observations for every 10-day period from 1950 to 19991.
3. Simulated fields in the Oyashio and SAF regions
a. Annual mean fields
Before describing the simulated variability in the Oyashio Current region, we plot
annual mean fields of sea surface current, height and temperature to see general structures
1
The JMA-SST data include satellite data since 1998, yielding a discontinuity in the data
quality in that year, but it affects little our analyses because we focus primarily on earlier
years in this study.
-8-
of the North Pacific subarctic gyre in OFES (Fig. 1). The annual mean sea surface current
field (Fig. 1a) shows that OFES well reproduces the major components of the subarctic
gyre: the Alaskan Stream, the East Kamchatka Current, the Oyashio Current, the North
Pacific Current. The Alaskan gyre is evident in the surface current field, while the western
subarctic gyre is more apparent in the mean SSH field2. In addition, a cyclonic gyre is
simulated in the Bering Sea in either of these fields. It should be noted here that in OFES,
the Kuroshio Current tends to overshoot as far north as ~40°N along the east coast of Japan,
while its large part returns to about 36°N before merging itself into the KE Current. An
unrealistic overshoot of the Kuroshio Current gives rise to a warm bias in SSTs along the
east coast of Japan as shown below. Except these discrepancies around Japan, however,
annual mean SSTs are well represented in OFES.
b. Seasonal variability
In the JMA data (Fig. 2b), strong SST gradient associated with SAFZ extends eastward
off Japan around 40°N, in which the Oyashio front with the particularly tight SST gradient
is embedded with its axis tilted northeastward. In OFES (Fig. 2a), the overshoot Kuroshio
warms off the east coast of Japan, leading to slight relaxation of SST gradient in the
Oyashio front and SAFZ, especially in their southern portions. Nevertheless, the simulated
SST field represents fairly strong horizontal SST gradient in the regions of the Oyashio
front and SAFZ, especially in their northern portions, and the overall SST distribution over
2
In the simulated SSH field the western Subarctic Gyre is recognized as a closed
circulation, but this feature is less apparent in the surface velocity field due to a
contribution from surface Ekman flow embedded. In contrast, the simulated Alaskan Gyre
appears to be closed in the surface velocity field, though not apparently closed in the SSH
field. These discrepancies in gyre configuration between the Western Subarctic and Alaskan
Gyres can also be found in the observational SSH field prepared by N. Maximenko and P.
Niiler at http://apdrc.soest.hawaii.edu/projects/DOT/
-9-
the western North Pacific is well reproduced in OFES. Compared to JMA-SST (Fig. 3b),
the seasonal SST variability in the Oyashio frontal region is well captured in OFES, despite
a warm bias maximized in winter. In contrast to SST, 100-m depth temperature in the same
area (Fig. 3b, thin curve) minimizes in March and maximizes in November. Seasonal
variations in horizontal temperature advection (not shown) suggest that the November peak
is largely due to the seasonal variations in the Oyashio Current shown below.
The seasonal variability in surface currents is characterized by the strongest southward
Oyashio Current off the Kuril Islands and the Hokkaido Island in winter and its weakest
southward penetration in summer (Fig. 4), consistent with previous observational studies
(e.g., Ito et al. 2004, Yasuda 2003 and references therein). Seasonal surface current
variability is also apparent within the Sea of Okhotsk. The southward East Sakhalin Current
strengthens in winter, while an anticyclonic gyre in the Kuril basin (around 44°-47°N,
144°-150°E) develops in summer to autumn, both consistent with observational studies
(Mizuta et al. 2003, Wakatsuchi and Martin 1991). In contrast, seasonal variations in the
Kuroshio and its extension currents are substantially weaker (Kagimoto and Yamagata
1997; Sakamoto and Yamagata 1996).
For the following analyses of the variability in the Oyashio Current and its influence on
temperature field, we define an index of the strength of the Oyashio Current (the Oyashio
Current index, Fig. 5) as the southward velocity for a given level averaged over a
rectangular domain off the Hokkaido Island [40.1°-45.1°N, 143.1°-151.1°E] as indicated in
the upper-left panel of Fig. 43. The area mean was taken in order to exclude the influence of
3
Although the rectangular domain includes southeastern portion of the Sea of Okhotsk, its
- 10 -
meso-scale eddies that are ubiquitous in the Oyashio Current region. Additionally, in the
following analyses, we regard the vertical average of the index values as a measure of the
barotropic component of the current, whereas the deviation of the index value at the 100-m
depth from the vertical mean is regarded as a measure of its baroclinic component. In this
study we focus on the Oyashio Current at the 100-m depth, because temperatures at that
depth have been used to define the southernmost latitude of the Oyashio near-shore branch.
In their climatological mean seasonal march (Fig. 5, bottom panel), the wintertime
intensification of the Oyashio Current at the 100-m depth is largely contributed to by the
barotropic component, while the baroclinic component shows smaller seasonal variability.
Owing to the wintertime intensification of the Aleutian Low and associated positive
wind-stress curl over the North Pacific Ocean, this wintertime intensification in the
barotropic component is consistent with the Sverdrup balance. The same mechanism is also
operative in interannual variations of the current, as shown in the following analyses
(section 4a).
c. Interannual variability
Figure 3a compares simulated interannual variability in winter (January-March) SST
within the Oyashio frontal region with its counterpart in the JMA-SST data. The simulated
area-mean SST anomaly shows its negative peaks in the middle of the 1960s, 1970s, and
1980s, and its positive peaks in the early 1960s and early1970s and in the late 1970s and
late 1980s. Though somewhat weaker, these peaks are also found in the JMA-SST,
consistent with their fairly high correlation (coefficient of r = 0.66). These time series both
exhibit a spectrum peak at the period of ~7 years, while another spectrum peak at the period
contribution was excluded in evaluating the area mean.
- 11 -
of ~20 years is apparent only in the simulated SST. In Fig. 3a, the simulated area-mean
100-m depth temperature is found to be highly correlated with the surface index.
As shown in the top panel of Fig. 5, interannual variations in the baroclinic and
barotropic components of the Oyashio Current intensity are not well correlated mutually
(r=0.26). For the period of 1993-2000, the baroclinic transport of the Oyashio Current
simulated in OFES (Fig. 5a) is in good agreement with its observational counterpart
obtained by Qiu (2002, his Fig. 21c) from satellite altimeter data. In addition, a comparison
with the temperature anomalies (Fig. 3a) suggests that SAFZ tends to be cooler when the
Oyashio Current is intensified (r = 0.36 for annual-mean SSTAs, r = 0.47 for winter
SSTAs), as will be further discussed in section 5. Good correspondence between the
observed and simulated SSTA time series in Fig. 3a thus suggests that the Oyashio Current
variations are well captured in OFES.
d. Decadal variability
On decadal timescales, the observed and simulated SST and simulated 100-m depth
temperature in SAFZ were all relatively high around 1970 and in the late 1980s, and in
between a pronounced cool period was found in the mid-1980s (Fig. 3a). These temperature
variations are concurrent with long-term variations in the southward Oyashio intrusion, as
have been already detected in the 100-m depth temperature measurements along the east
coast of Japan (Ogawa 1988; Sekine 1988b; Hanawa 1995; Yasuda 2003, his Fig. 2a). In
good agreement with its enhanced southward penetration and northward retreat, the
simulated Oyashio Current strengthened in the mid-1980s and weakened around 1970,
respectively (Fig. 5, top panel). In fact, 5-year mean surface current fields in Fig. 6 confirm
that the southward Oyashio Current and its eastward extension along SAFZ are both
- 12 -
stronger in the cool period (1984-88) than in the warm period (1968-72). The
aforementioned concomitance between the Oyashio Current and SSTAs in SAFZ on
interannual and decadal time scales suggests significant influence of the current on the SST,
calling for the investigation of the mechanisms for changes in the Oyashio Current. At the
same time it is also suggested that SSTAs in the upstream of the Oyashio Current may
possibly influence SAFZ through advection (c.f., section 5). Indeed, despite the pronounced
enhancement of the Oyashio Current in the late 1990s (Fig. 5), SSTA was not strongly
negative in SAFZ (Fig. 3), which is likely due to warm SSTA simulated in the upstream
region (not shown).
4. Influence of wind variability on the Oyashio Current
To investigate how the changes in the Oyashio Current are induced in OFES, in this
section we examine lead-lag correlations of the current strength with wind-stress curl and
Ekman pumping fields. Our investigation was carried out separately for the barotropic and
baroclinic components of the current, as we expect that different mechanisms may be
operative in their variability. In the following correlation analysis, we estimate the number
of degree of freedom (DOF) for the barotropic component (100-m depth) Oyashio Current
index to be 50 (25), based on its auto-correlation with one-year (two-year) lag of only 0.135
(0.194)4. With the number of DOF of 50 (25), the 95%, 90% and 80% confidence levels for
correlation are 0.27 (0.38), 0.23 (0.32) and 0.18 (0.26), respectively.
a. Barotropic component
Figure 7 indicates that the barotropic component has significant simultaneous
4
The auto-correlation with one-year lag for the 100-m depth Oyashio Current is 0.368.
- 13 -
correlation (panel b) with the wind-stress curl almost over the whole North Pacific basin
(shaded in the left column), whereas the correlation almost diminishes in either the previous
or following year (panels a and c). The corresponding simultaneous correlation maps for
sea level pressure (SLP) and 500-hPa height anomalies show that the basin-wide cyclonic
wind-stress curl anomalies are associated with the enhancement and southward expansion
of the surface Aleutian Low (Fig. 8, top and bottom panels) and a PNA-like circulation
anomaly pattern aloft (not shown), respectively. Lag regression maps for surface current
vectors (Fig. 7, right panels) also indicate that the southwestward surface Oyashio Current
tends to enhance along the Kuril Islands and Hokkaido Island only simultaneously with the
intensification of its barotropic component, consistent with very fast propagation of
barotropic Rossby waves. Driven by wind-stress curl anomalies, barotropic Rossby waves
can induce changes in the Oyashio Current, consistent with the linear vorticity balance as
suggested by previous studies (Sekine 1988b; Hanawa 1995; Isoguchi et al. 1997).
b. Comparison with Sverdrup transport
The above results suggest that the vertically integrated southward transport by the
Oyashio Current and its interannual variability can be explained by the Sverdrup transport
and its variability, respectively, estimated from the wind-stress curl fields imposed on
OFES as forcing. In Fig. 9, we plot time series of the meridional transport integrated
vertically from the bottom to the sea surface and zonally from the western boundary to
151°E (solid line) and the Sverdrup transport based on wind-stress curl over the whole
width of the basin (dashed line) as well. Consistent with the above results, their
simultaneous correlation is high (r = 0.76) and significant, but the estimated Sverdrup
transport and its variations are substantially larger than those of the meridional transport of
- 14 -
the Oyashio Current in OFES. Specifically, the mean transport is approximately six times
larger (24.4 Sv vs. 4.3 Sv), and the standard deviation is approximately three times larger
(26.2 Sv vs. 8.5 Sv). We have also estimated the Sverdrup transport based on the same
wind-stress curl field as above but only from the western boundary to 170°E (dash-dotted
line in Fig. 9). The corresponding mean (10.1 Sv) and its standard deviation (8.8 Sv) are
better correspondent to their counterpart in the simulated meridional transport of the
Oyashio Current with respective to their magnitudes, although the temporal correlation is
slightly lower (r = 0.67) between the estimated and simulated transport values. Kono and
Kawasaki (1997), who made qualitatively the same comparison as above based on their
observations for several years, argued that the Emperor Seamounts located around 170°E
block barotropic Rossby waves propagating from their east, and therefore wind-stress curl
only to their west can effectively force the Oyashio Current and its variability (see also
Sekine 1989; Ito et al. 2004). Indeed, a longitude-time section of simulated daily SSH
anomalies that can well depict propagating barotropic waves (not shown) reveals that
westward propagating signals are sometimes halted or weakened around 170°E, in a
manner consistent with the above argument. Though beyond the scope of this study,
however, more detailed investigations are necessary to understand specifically how the
Emperor Seamounts influence the propagation of barotropic Rossby waves and thereby the
meridional transport of the Oyashio Current.
c. 100-m depth Oyashio Current including baroclinic component
Unlike its vertically averaged strength, the Oyashio Current in the surface layer can be
influenced also by propagation of baroclinic Rossby waves (Qiu 2002; Tatebe and Yasuda
2005). As exemplified at the 100-m depth (Fig. 5, top panel), the JFM-mean Oyashio
- 15 -
Current strength in the surface layer varies interannually in a manner different from that of
the vertical mean current intensity. A map of simultaneous correlation in Fig. 10d indicates
that the enhancement of the southward Oyashio Current at the 100-m depth tends to occur
concomitantly with anomalous upward Ekman pumping in the region around [45°N,
160°E]. Consistent with linear theory, this relationship strongly suggests that the southward
Oyashio Current off Japan can be driven by wind-induced upwelling just to the east.
The wintertime 100-m depth Oyashio Current index is also correlated significantly
with the Ekman pumping associated with northeasterly wind anomalies within the region
[42°-45°N, 160°-170°E], if the pumping leads the Current index by three years (Fig. 10a).
That is almost the same region in which the simultaneous correlation is significant. With
their slow propagation speed, it takes nearly three years for baroclinic Rossby waves to
propagate from 170°E to the western boundary at these latitudes until they influence the
Oyashio Current. A correlation analysis shows that anomalous Ekman pumping in the
region off the Hokkaido Island [43-47N, 155-165E], which has significant simultaneous
correlation with the Oyashio Current index at 100-m depth, has weak but significant
(>95%) positive correlation with 3-year-lead anomalous Ekman pumping around 43N,
160-170E. In the 2-year-lead field, there is also positive correlation but with smaller spatial
extent, and in the 1-year-lead field, weak but negative correlations are found. These results
suggest that in the 1-year and 2-year lead fields, Ekman pumping is unlikely to effectively
enhance the Oyashio, and thus only the 3-year lead forcing is selected due to particular
periodicity in the wind field. The similar three-year periodicity is also suggested for the
barotropic Oyashio transport by Hanawa (1995).
- 16 -
Correlation maps between the Ekman pumping and the baroclinic component of the
100-m depth Oyashio Current (dashed line in the top panel of Fig. 5) show the similar
results with slightly weaker correlations (not shown). Figure 8b indicates that the
strengthening of the 100-m depth Oyashio Current tends to accompany anomalous
near-local Ekman pumping associated with the intensification of the Aleutian Low on its
western flank, but no significant correlation is found over the North Pacific Ocean in the
three-year-lead SLP field (not shown).
A correlation analysis reveals that the anomalous surface Oyashio Current concomitant
with the barotropic component of the anomalous transport is confined to the coastal region
just of the Kuril Islands and northern Japan (Fig. 7e). It also reveals that anomalous surface
current signal with the baroclinic component of the transport exhibits even stronger
meridional confinement (Fig. 10h), reflecting the meridionally confined forcing (Fig. 10d).
No significant correlation is found, however, along the eastward extension of the Oyashio
Current in SAFZ, where meso-scale eddy activity is particularly high. In fact, a scatter plot
shown in Fig. 11a between the 100-m depth Oyashio Current index and the axial latitude of
the eastward Oyashio Extension Current averaged for 150°-160°E indicates weak negative
correlation between them, which is embedded in bimodality in the axial position between
its northern (~44°N) and southern (~41°N) branches. The bimodality is apparent only when
the 100-m depth southward Oyashio Current is relatively weak (with its index < 3 cm s-1),
the eastward velocity maximizes exclusively along the southern branch when the southward
current intensifies (with its index > 3 cm s-1). The bimodality is consistent with dual frontal
structure that is sometimes observed in satellite-measured SST (Nakamura and Kazmin
- 17 -
2003, their Fig. 5). As long as manifested as the primary current axis of the Oyashio
Extension, the latitude of the southern branch exhibits significant negative correlation (r =
-0.37) with the 100-m depth Oyashio index (Fig. 11a), showing the tendency for the
Oyashio Extension to be displaced southward as the southward Oyashio Current
strengthens. The corresponding scatter plot in Fig. 11b shows that the maximum velocity of
the Extension current tends to be weaker along the northern branch than along the southern
branch, as evident in decadal differences in the current field as shown in Fig. 6. When
manifested as the primary axis, the latitude of the southern branch exhibits no such clear
tendency with the current velocity along it (r = -0.24).
5. Influence of Oyashio Current variability on SAFZ
In the preceding sections, we have investigated how variations in the Oyashio Current
are caused by wind forcing. In this section how those variations induce changes in SAFZ is
examined for spring (March-May) with lag correlation analysis. Qualitatively the same
results are obtained for winter (not shown), although the correlation is slightly lower
probably due to the stronger influences of atmospheric noise. On the basis of discussion
near the end of the preceding section, we focus on the southern branch of the Oyashio
Extension in SAFZ, along which signals associated with variations in the Oyashio Current
strength tends to emerge.
Lag correlation maps of SST with the 100-m depth Oyashio Current index (Fig. 12)
clearly indicate that the intensification of the southward Oyashio Current tends to
accompany surface cooling off northern Japan and in SAFZ, as discussed in the previous
- 18 -
studies (Sekine 1988a; Hanawa 1995). A close inspection reveals that the cool SST
anomalies in SAFZ tend to maximize two or three years after the peak intensification of the
Oyashio Current, but the cooling signal starts emerging even one-year earlier than the peak.
The cool anomalies mature just off the Hokkaido Island first and then to the east along the
Oyashio front/SAFZ, which is evident in the mid- to late 1980s in the OFES simulation and
hinted also in the JMA-SST data (Fig. 13).
It should be pointed out that the SST anomalies along SAFZ (Fig. 12) can be generated
not only by thermal advection by the Oyashio Extension current but also by the meridional
migration of SAFZ. Figure 14a shows longitude-lag diagram of SSTAs along the
time-varying SAFZ axis regressed linearly with lags on the 100-m depth Oyashio Current
index, while the anomalous axial latitude of SAFZ has been regressed on that index for
each longitude in Fig. 14b. After generated by the enhancement of the Oyashio Current, the
maximum negative SSTA associated with the largest southward displacement of the local
frontal axis tends to shift eastward with time. The figure indicates that both the advective
effect and the axial migration of SAFZ are likely contributors to the eastward developing
SSTAs.
Consistently with the aforementioned signal of meridional migration of SAFZ (Fig.
14b), similar downstream development of negative SSH anomalies tends to occur along
SAFZ following the intensification of the Oyashio Current (Fig. 15, right panels). This
suggests that the SSTA evolution is coherent with subsurface temperature changes. Indeed,
there are anomalies in a subsurface potential vorticity (PV) field developing along SAFZ
following the intensification of the Oyashio Current (Fig. 16). The left-top panel of Fig. 16
- 19 -
shows the climatological-mean PV field on an isopycnal surface of σθ=27.0 kg m-3 in the
lower portion of the thermocline. Low-PV water is spilling out of its pool in the Sea of
Okhotsk5 southward along the Kuril Islands and the east coast of the Hokkaido Island as
observed (Yasuda 1997, and references therein), forming fairly tight meridional PV
gradient across SAFZ with higher PV to the south. A lag correlation analysis reveals that
one year before the peak of the Oyashio intensification, low-PV anomalies start emerging
in the western portion of SAFZ off Hokkaido, followed by their downstream development
along SAFZ (right panels). The whole sequence suggests that the PV anomalies are caused
probably by the enhanced advection of low-PV water southward along the coast and then
develop eastward along SAFZ by the Oyashio Extension Current. The low-PV anomalies in
SAFZ correspond to its southward displacement (c.f., left-top panel of Fig. 16), indicating
that negative anomalies in SSH (Figs. 14b and 15) and a portion of cool SSTA (Fig. 12) can
be interpreted as being associated with the southward displacement of SAFZ. At the
σθ=27.0 kg m-3 isopycnal surface, zonal velocity averaged over [40°-42°N, 150°-170°E] is
2.8 cm s-1 or ~10° in longitude a year, to which the eastward propagation/extension speed
of those anomalies along SAFZ is found comparable or just slightly less.
Generated by the oceanic processes as discussed above, the SSTAs in SAFZ can exert
feedback forcing onto the overlying atmosphere via anomalous surface heat fluxes. Indeed,
another set of lag correlation maps (Fig. 17) indicates that the intensification of the Oyashio
5
Recent studies (Nakamura et al. 2004, Itoh et al. 2003) have suggested that tidal mixing
around the Kuril Islands may be of crucial importance for the formation of the low-PV
water. With the lack of this mixing, OFES simulates the PV minimum on that isopycnal
surface that appears to spill out of a region just off the Sakhalin Island into the Kuril Basin.
It is suggested that, unlike in reality, the low-PV water appears to form around the Sakhalin
Island in the model without any contribution from the tidal mixing.
- 20 -
Current tends to yield negative (downward) anomalies in turbulent surface heat fluxes off
the Hokkaido Island and later along SAFZ, collocated with the cool SSTAs. The collocation
means that the anomalous heat flux acts to damp the SSTAs, exerting thermal forcing on
the overlying atmosphere, as observed by Tanimoto et al. (2003).
6. Summary and discussions
In the present study, mechanisms for the low-frequency variability of the Oyashio
Current and its influence on SAFZ have been investigated, by using a hindcast experiment
with an eddy-resolving OGCM (OFES) that can reproduce those two features in the
western North Pacific reasonably well with respect to their seasonal and interannual
variations. Our lag correlation analysis has indicated that basin-scale wind forcing induces
variations in the barotropic component of the Oyashio Current almost instantaneously. In
addition, wind forcing in the western portion of the basin can impact the Oyashio Current
strength through baroclinic Rossby wave propagation within three years. The strong
correlation found between the basin-scale wind-stress curl and the vertically-integrated
Oyashio transport suggests that overall temporal variability in the transport can be
understood by the time-varying Sverdrup transport, although the transport estimated for the
whole basin width leads to an overestimation, as has been pointed by observational studies
(Kono and Kawasaki 1997; Ito et al. 2004). This discrepancy may be caused by the
disturbing effect of the Emperor Seamounts on barotropic Rossby wave propagation. Note
that OFES cannot resolve the 18.6-year variations in tidal mixing around the Kuril Islands
(Yasuda et al. 2006), which may also contribute to changes in the Oyashio Current.
Our lag correlation analysis suggests that the intensification of the southward Oyashio
- 21 -
Current generates SSTAs off the Hokkaido Island probably through the enhanced cold
advection. It also suggests that the subsequent downstream development of cool SSTAs
along the southern branch of SAFZ can be attributed to both the advection by the eastward
Oyashio Extension Current and the southward migration of the SAFZ axis. As indicated by
SSH anomalies along the front, the frontal cooling is not limited to the surface. Indeed, in
the lower portion of the thermocline (surface of σθ=27.0 kg m-3), the enhanced transport of
low-PV water originated from the Sea of Okhotsk by the intensified Oyashio Current yields
low-PV anomalies off the Hokkaido Island. Their eastward development probably by the
Oyashio Extension Current leads to southward displacement of SAFZ and associated
cooling. It is those oceanic processes of thermal advection and meridional frontal
displacement that induce the cool SSTAs in SAFZ following the intensification of the
Oyashio Current. Consistently in SAFZ, anomalous surface heat flux is distinctively
downward, acting to damp the cool SSTAs with their potential to exert thermal feedback
forcing onto the atmosphere.
Longitude-time sections of OFES-simulated SSH anomalies for the North Pacific basin
(Fig. 18) reveal latitudinal dependence of wave propagation characteristics. At 38°N to the
south of the mean axial position of SAFZ, coherent westward propagating signals are
evident across the whole width of the basin. Likewise, basin-wide signature of westward
propagation is evident also at the mean latitude of KEFZ (36°N, not shown). Their
propagation is fairly consistent with the theoretical values of baroclinic Rossby wave
propagation speeds for the respective latitudes (Nonaka et al. 2006, their Fig. 12). At 40°N,
the corresponding westward propagating signals are still apparent, but they become less
- 22 -
coherent as they approach to the western boundary (west of ~150°E). At 42°N, within
SAFZ, coherent westward propagating signals are apparent only in the eastern through
central portions of the basin. In the western portion of the basin (west of ~170°E), however,
they tend to be disturbed, and even some eastward propagating signals are hinted, for
example, around 1970 and the end of the 1980s, as suggested by Nonaka et al. (2006). The
same feature is also noticeable at 44°N, on the northern flank of SAFZ. These eastward
propagating signals, which cannot be regarded as Rossby waves excited in the central or
eastern portion of the basin, seem to be consistent with the eastward developing signals
found in SAFZ following the enhancement or weakening of the Oyashio Current (Figs. 15
and 16). It is noteworthy that dominant time scales of the SSH fluctuations plotted in Fig.
18 also exhibit substantial latitudinal dependence, with longer time scales with latitude in
both the westward and eastward propagating signals. The longer time scales of the
fluctuations and the stronger influence of the Oyashio Current in SAFZ than in KEFZ can
be two of the factors that give rise to unsynchronized decadal variations between the two
adjacent frontal zones, as pointed out by Nonaka et al. (2006). Furthermore, the tendency
for baroclinic Rossby waves to be disturbed or dissipated in SAFZ until they reach the
western boundary acts to prevent the wind forcing to the east from effectively influencing
the Oyashio Current. In fact, virtually no significant correlation is found in our analysis
between the Oyashio Current intensity and Ekman pumping in the central and eastern
portions of the basin with leading time longer than three years (not shown). Unlike in
KEFZ, where Rossby waves can reach the western boundary several years after their
excitation, predictability of oceanic variations in SAFZ is thus likely to be limited.
- 23 -
Very recently, air-sea interactions associated with oceanic frontal zones are gaining
increasing attention (Nakamura et al. 2004; Xie 2004; Minobe et al. 2008). It has been
confirmed with archived shipboard (Tanimoto et al. 2003; Tokinaga et al. 2005), satellite
(Nonaka and Xie 2003; Xie 2004; Chelton et al. 2004, among others), and in situ (Tokinaga
et al. 2006) observational data that SSTAs in midlatitude oceanic frontal zones can
influence the overlying atmospheric boundary layer by modulating surface heat fluxes. It is
also suggested that oceanic fronts act to anchor the atmospheric storm tracks (Nakamura
and Sampe 2002; Nakamura and Shimpo 2004; Inatsu and Hoskins 2004; Sampe 2006) by
maintaining the atmospheric surface baroclinicity through cross-frontal differential heat
supply (Nakamura et al. 2008, submitted to Geophys. Res. Lett.). The present study
suggests the potential for the Oyashio Current variations to induce SSTAs in SAFZ.
Similarly, it is suggested that variations in KEFZ are governed by oceanic processes,
including unknown nonlinear dynamics, under the influence of westward propagating
Rossby waves (e.g., Taguchi et al. 2005; 2007). How these anomalies in the frontal zones
can exert feedback forcing to large-scale atmospheric circulation is under investigation with
very-high resolution atmospheric GCM (AFES; Ohfuchi et al. 2004, 2007) and
ocean-atmosphere coupled GCM (CFES; Komori et al. 2008).
Acknowledgments. The OFES simulations were conducted on the Earth Simulator under the
support of JAMSTEC. We thank the members of the OFES group, including Drs. Y.
Masumoto, H. Sakuma and T. Yamagata, for their efforts and support in the model
development and the anonymous referees for their sound criticism and constructive
- 24 -
comments. This study is supported in part by a research project of the Agriculture, Forestry
and Fisheries Research Council of Japan and by Grand-In-Aid for Scientific Research
defrayed by the Ministry of Education, Culture, Sports, Science and Technology of Japan
(17340137 and 18204044).
- 25 -
References
Barnett, T. P., D. W. Pierce, R. Saravanan, N. Schneider, D. Dommenget, and M. Latif,
1999: Origins of the midlatitude Pacific decadal variability. Geophys. Res. Lett., 26,
1543-1546.
Belkin, I., R. Krishfield, nd S. Honjo, 2002: Decadal variability of the North Pacific Polar
Front: Subsurface warming versus surface cooling. Geophys. Res. Lett., 29,
doi:10.1029/2001GL013806.
Boyer, T. P., S. Levitus, J. I. Antonov, M. E. Conkright, T. O’Brien, and C. Stephens, 1998a:
World Ocean Atlas 1998 Vol. 4: Salinity of the Atlantic Ocean, NOAA Atlas NESDIS
30. US Government Printing Office, Washington, D.C.
Boyer, T. P., S. Levitus, J. I. Antonov, M. E. Conkright, T. O’Brien, and C. Stephens,
1998b: World Ocean Atlas 1998 Vol. 5: Salinity of the Pacific Ocean, NOAA Atlas
NESDIS 31. U.S. Government Printing Office, Washington, D.C.
Boyer, T. P., S. Levitus, J. I. Antonov, M. E. Conkright, T. O’Brien, C. Stephens, and B.
Trotsenko, 1998c: World Ocean Atlas 1998 Vol. 6: Salinity of the Indian Ocean, NOAA
Atlas NESDIS 30. U.S. Government Printing Office, Washington, D.C.
Chelton, D. B., M. G. Schlax, M. H. Freilich, and R. F. Milliff, 2004: Satellite radar
measurements reveal short-scale features in the wind stress field over the world ocean.
Science, 303, 978-983.
Deser, C., S. Phillips, and J. W. Hurrell, 2004: Pacific interdecadal climate variability:
Linkages between the Tropics and North Pacific during boreal winter since 1900. J.
Climate, 17, 3109-3124.
Frankignoul, C., 1985: Sea surface temperature anomalies, planetary waves, and air-sea
feedback in the middle latitudes. Rev. Geophys., 23, 357-390.
Hanawa, K., 1995: Southward penetration of the Oyashio water system and the wintertime
condition of midlatitude westerlies over the North Pacific. Bull. Hokkaido Natl. Fish.
Res. Inst., 59, 103-119.
Hasselmann, K., 1967: Stochastic climate models. Part I. Theory. Tellus, 28, 473-485.
Inatsu, M., and B. J. Hoskins, 2004: The zonal asymmetry of the Southern Hemisphere
winter storm track. J. Climate, 17, 4882-4892.
Isoguchi, O., and H. Kawamura, 2006: Seasonal to interannual variations of the western
boundary current of the subarctic North Pacific by a combination of the altimeter and
tide gauge sea levels. J. Geophys. Res., 111, C04013, doi:10.1029/2005JC003080.
- 26 -
Isoguchi, O., H. Kawamura, and T. Kono, 1997: A study on wind-driven circulation in the
subarctic North Pacific using TOPEX/POSEIDON altimeter data. J. Geophys. Res.,
102, 12457-12468.
Ito, S., K. Uehara, T. Miyao, H. Miyake, I. Yasuda, T. Watanabe, and Y. Shimizu, 2004:
Characteristics of SSH anomaly based on TOPEX/Poseidon altimetry and in situ
measured velocity and transport of Oyashio on OICE. J. Oceanogr., 60, 425-437.
Itoh, M., K. I. Ohshima, and M. Wakatsuchi, 2003: Distribution and formation of Okhotsk
Sea Intermediate Water: An analysis of isopycnal climatological data. J. Geophys. Res.,
108, 3258, doi:10.1029/2002JC001590.
Kagimoto, T., and T. Yamagata, 1997 : Seasonal Transport Variations of the Kuroshio : An
OGCM Simulation. J. Phys. Oceanogr., 27, 403-418.
Kalnay, E., et al., 1996: The NCEP/NCAR 40-year reanalysis project, Bull. Amer. Meteor.
Soc., 77, 437-471.
Kelly, K. A., and S. Dong, 2004: The relation of western boundary current heat transport
and storage to midlatitude ocean-atmosphere interaction. In Earth Climate: The
Ocean-Atmosphere Interaction, C. Wang, S.-P. Xie, and J.A. Carton (eds.), Geophys.
Monogr., 147, AGU, Washington D.C., 347-363.
Komori, N., A. Kuwano-Yoshida, T. Enomoto, H. Sasaki, and W. Ohfuchi, 2008:
High-resolution simulation of the global coupled atmospheric-ocean system:
Description and preliminary outcomes of CFES (CGCM for the Earth Simulator), in
High Resolution Numerical Modelling of the Atmosphere and Ocean, W. Ohfuchi and
K. Hamilton (eds.), chapter 14, pp. 241-260, Springer, New York.
Kono, T., and Y. Kawasaki, 1997: Results of CTD and mooring observations southeast of
Hokkaido 1. Annual velocity and transport variations in the Oyashio. Bull. Hokkaido
Natl. Fish. Res. Inst., 61, 65-81.
Kushnir, Y., W. A. Robinson, I. Blade, N. M. J. Hall, S. Peng, and R. Sutton, 2002:
Atmospheric GCM response to extratropical SST anomalies: Systhesis and evaluation.
J. Climate, 15, 2233-2256.
Kwon, Y.O., and C. Deser, 2007: North Pacific Decadal Variability in the Community
Climate System Model Version 2. J. Climate, 20, 2416–2433.
Large, W. G., J. C. McWilliams, and S.C. Doney, 1994: Oceanic vertical mixing: A review
and a model with a nonlocal boundary layer parameterization, Rev. Geophys., 32,
363-403.
- 27 -
Latif, M., and T. P. Barnett, 1994: Causes of decadal climate variability over the North
Pacific and North America. Science, 266, 634-637.
Maltrud, M.E., McClean, J.L., 2005: An eddy resolving global 1/10° ocean simulation.
Ocean Modelling, 8, 31-54.
Masumoto, Y., H. Sasaki, T. Kagimoto, N. Komori, A. Ishida, Y. Sasai, T. Miyama, T.
Motoi, H. Mitsudera, K. Takahashi, H. Sakuma, and T. Yamagata, 2004: A Fifty-Year
Eddy-Resolving Simulation of the World Ocean -Preliminary Outcomes of OFES
(OGCM for the Earth Simulator)-, J. Earth Simulator, 1, 31-52.
Minobe, S., A. Kuwano-Yoshida, N. Komori, S.-P. Xie, and R. J. Small, 2008: Influence of
the Gulf Stream on the troposphere. Nature, 452, 206-209, doi:10.1038/nature06690.
Mizuno, K., and W. B. White, 1983: Annual and interannual variability in the Kuroshio
Current System. J. Phys. Oceanogr., 13, 1847-1867.
Mizuta, G., Y. Fukamachi, K. I. Ohshima, and M. Wakatsuchi, 2003: Structure and seasonal
variability of the East Sakhalin Current. J. Phys. Oceanogr., 33, 2430-2445.
Nakamura, H., G. Lin, and T. Yamagata, 1997: Decadal climate variability in the North
Pacific during the recent decades. Bull. Amer. Meteor. Soc., 78, 2215-2225.
Nakamura, H., and T. Yamagata, 1999: Recent decadal SSST variability in the northwestern
Pacific and associated atmospheric anomalies. In Beyond El Niño: Decadal and
Interdecadal Climate Variability, A. Navarra, Ed., Springer, 49-62.
Nakamura, H., and T. Sampe, 2002: Trapping of synoptic-scale disturbances into the
North-Pacific subtropical jet core in midwinter. Geophys. Res. Lett., 29,
doi:10.1029/2002GL015535.
Nakamura, H., and A. S. Kazmin, 2003: Decadal changes in the North Pacific oceanic
frontal zones as revealed in ship and satellite observations. J. Geophys. Res., 108,
3078-3094.
Nakamura, H., and A. Shimpo, 2004: Seasonal variations in the Southern Hemisphere
storm tracks and jet streams as revealed in a reanalysis dataset. J. Climate, 17,
1828-1844.
Nakamura, H., T. Sampe, Y. Tanimoto, and A. Shimpo, 2004: Observed associations among
storm tracks, jet streams and midlatitude oceanic fronts. In Earth Climate: The
Ocean-Atmosphere Interaction, C. Wang, S.-P. Xie, and J.A. Carton (eds.), Geophys.
Monogr., 147, AGU, Washington D.C., 329-345.
Nakamura, T., T. Toyoda, Y. Ishikawa, and T. Awaji, 2004: Tidal mixing in the Kuril Straits
- 28 -
and its impact on ventilation in the North Pacific. J. Oceanogr., 60, 411-423.
Newman, M., G. P. Compo, and M. A. Alexander, 2003: ENSO-forced variability of the
Pacific Decadal Oscillation. J. Climate, 16, 3853-3857.
Nitta, T., and S. Yamada, 1989: Recent warming of tropical sea surface temperature and its
relationship to the Northern Hemisphere circulation. J. Meteor. Soc. Jpn., 67, 375-382.
Nonaka, M., and S.-P. Xie, 2003: Covariations of sea surface temperature and wind over the
Kuroshio and its extension: Evidence for ocean-to-atmosphere feedback. J. Climate, 16,
1404-1413.
Nonaka, M., H. Nakamura, Y. Tanimoto, T. Kagimoto, and H. Sasaki, 2006: Decadal
variability in the Kuroshio-Oyashio Extension simulated in an eddy-resolving OGCM.
J. Climate, 19, 1970–1989.
Ogawa, Y., 1988: Variations in latitude at the southern limit of the first Oyashio intrusion.
Bull. Tohoku Reg. Fish. Res. Lab., 51, 1-10 (in Japanese with English abstract).
Ohfuchi, W., H. Sasaki, Y. Masumoto, and H. Nakamura, 2007: “Virtual” atmospheric and
oceanic circulations in the Earth Simulator. Bull. Amer. Meteor. Soc., 88, 861-867.
Ohfuchi, W., S. Shingu, H. Nakamura, M. K. Yoshioka, T. Enomoto, K. Takaya, S. Yamane,
T. Nishimura, X. Peng, H. Fuchigami, M. Yamada, Y. Kurihara, and K. Ninomiya,
2004: 10-km mesh meso-scale resolving simulations of the global atmosphere on the
Earth Simulator - Preliminary outcomes of AFES (AGCM for the Earth Simulator), J.
Earth Simulator, 1, 8-34.
Pacanowski R. C., and S. M. Griffies, 2000: MOM 3.0 Manual, Geophysical Fluid
Dynamics Laboratory/National Oceanic and Atmospheric Administration, 680pp.
Pierce, D. W., T. P. Barnett, N. Schneider, R. Saravanan, D. Dommenget, and M. Latif,
2001: The role of ocean dynamics in producing decadal climate variability in the North
Pacific. Clim. Dyn. 18, 51-70.
Qiu, B., 2000: Interannual variability of the Kuroshio Extension system and its impact on
the wintertime SST field. J. Phys. Oceanogr., 30, 1486-1502.
Qiu, B., 2002: Large-scale variability in the midlatitude subtropical and subpolar North
Pacific Ocean: Observations and causes. J. Phys. Oceanogr., 32, 353-375.
Qiu, B., 2003: Kuroshio Extension variability and forcing for the Pacific decadal
oscillations: Responses and potential feedback. J. Phys. Oceanogr., 33, 2465-2482.
Qiu, B., N. Schneider, and S. Chen, 2007: Coupled decadal variability in the North Pacific:
An observationally constrained idealized model. J. Climate, 20, 3602-3620.
- 29 -
Rosati, A., and K. Miyakoda, 1988: A general circulation model for upper ocean circulation.
J. Phys. Oceanogr., 18, 1601-1626.
Sakamoto, T., and T. Yamagata, 1996 : Seasonal Transport Variations of the Wind-Driven
Ocean Circulation in a Two-Layer Planetary Geostrophic Model with a Continental
Slope. J. Mar. Res., 54, 261-284.
Sampe, T., 2006: Importance of Midlatitude Oceanic Frontal Zones for the General
Circulation of the Extratropical Troposphere. Ph.D. dissertation, University of Tokyo.
Sasaki, H., M. Nonaka, Y. Masumoto, Y. Sasai, H. Uehara, and H. Sakuma, 2007: An
eddy-resolving hindcast simulation of the quasi-global ocean from 1950 to 2003 on the
Earth Simulator, in High resolution numerical modelling of the atmosphere and ocean,
edited by K. Hamilton and W. Ohfuchi, chapter 10, pp. 157-185, Springer, New York.
Schneider, N., and A. J. Miller, 2001: Predicting western North Pacific ocean climate. J.
Climate, 14, 3997-4002.
Schneider, N., and B.D. Cornuelle, 2005: The forcing of the Pacific Decadal Oscillation. J.
Climate, 18, 4355-4373.
Schneider, N., A. J. Miller, and D. W. Pierce, 2002: Anatomy of North Pacific decadal
variability. J. Climate, 15, 586-605.
Seager R., Y. Kushnir, N. H. Naik, M. A. Cane, and J. Miller, 2001: Wind-driven shifts in
the latitude of the Kuroshio-Oyashio extension and generation of SST anomalies on
decadal timescales. J. Climate, 14, 4149-4165.
Sekine, Y., 1988a: Anomalous southward intrusion of the Oyashio east of Japan, 1,
Influence of the interannual and seasonal variations in the wind stress over the North
Pacific. J. Geophys. Res., 93, 2247-2277.
Sekine, Y., 1988b: A numerical experiment on the anomalous south-ward intrusion of the
Oyashio east of Japan, J. Oceanogr. Soc. Jpn., 44, 60-67.
Sekine, Y., 1989: On the volume transport of the Oyashio and subarctic circulation in the
North Pacific. Umi-to-Sora, 65, 85-94 (in Japanese with English abstract).
Smith, R. D., M. E. Maltrud, F. O. Bryan, and M. W. Hecht, 2000: Numerical simulation of
the North Atlantic Ocean at 1/10°., J. Phys. Oceanogr., 30, 1532-1561.
Taguchi, B., S.-P. Xie, H. Mitsudera, and A. Kubokawa, 2005: Response of the Kuroshio
Extension to Rossby waves associated with the 1970s climate regime shift in a
high-resolution ocean model. J. Climate, 18, 2979-2995.
Taguchi, B., S.-P. Xie, N. Schneider, M. Nonaka, H. Sasaki, and Y. Sasai, 2007: Decadal
- 30 -
variability of the Kuroshio Extension: Observations and an eddy-resolving model
hindcast. J. Climate, 20, 2357-2377.
Tanimoto, Y., H. Nakamura, T. Kagimoto, and S. Yamane, 2003: An active role of
extratropical sea surface temperature anomalies in determining anomalous turbulent
heat flux. J. Geophys. Res., 108 (C10), 3304, doi:10.1029/2002JC001750.
Tatebe, H., and I. Yasuda, 2005: Interdecadal variations of the coastal Oyashio from the
1970s to the early 1990s. Geophys. Res. Lett., 32, L10613, doi:10.1029/2005
GL022605.
Tokinaga, H., Y. Tanimoto, and S.-P. Xie, 2005: SST-induced surface wind variations over
the Brazil-Malvinas confluence: Satellite and in situ observations. J. Climate, 18,
3470-3482.
Tokinaga, H., Y. Tanimoto, M. Nonaka, B. Taguchi, T. Fukamachi, S.-P. Xie, H. Nakamura,
T. Watanabe, and I. Yasuda, 2006: Atmospheric sounding over the winter Kuroshio
Extension: Effect of surface stability on atmospheric boundary layer structure. Geophys.
Res. Lett., 33, L04703, doi:10.1029/2005GL025102.
Trenberth, K. E., and J. W. Hurrell, 1994: Decadal atmosphere-ocean variations in the
Pacific. Clim. Dyn., 9, 303-319.
Uehara, K., S. Ito, H. Miyake, I. Yasuda, Y. Shimizu, and T. Watanabe, 2004: Absolute
volume transport of the Oyashio referred to moored current meter data crossing the
OICE. J. Oceanogr., 60, 397-409.
Wakatsuchi, M., and S. Martin, 1991: Water circulation in the Kuril Basin of the Okhotsk
Sea and its relation to eddy formation. J. Oceanogr. Soc. Jpn., 47, 152-168.
Xie, S.-P., 2004: Satellite observations of cool ocean-atmosphere interaction. Bull. Amer.
Meteor. Soc., 85, 195-208.
Xie, S.-P., T. Kunutani, A. Kubokawa, M. Nonaka, and S. Hosoda, 2000: Intedecadal
thermocline variability in the North Pacific for 1958-1997: A GCM simulation. J. Phys.
Oceanogr., 30, 2798-2813.
Yasuda, I., 1997: The origin of the North Pacific Intermediate Water. J. Geophys. Res., 102,
893-909.
Yasuda, I., 2003: Hydrographic structure and variability in the Kuroshio-Oyashio transition
area. J. Oceanogr., 59, 389-402.
Yasuda, I., S. Osafune, and H. Tatebe, 2006: Possible explanation linking 18.6-year period
nodal tidal cycle with bi-decadal variations of ocean and climate in the North Pacific.
- 31 -
Geophys. Res. Lett., 33, L08606, doi:10.1029/2005GL025237.
Yasuda, T., and Y. Kitamura, 2003: Long-term variability of North Pacific subtropical mode
water in response to spin-up of the subtropical gyre. J. Oceanogr., 59, 279-290.
Yuan, X., and L. D. Talley, 1996: The subarctic frontal zone in the North Pacific:
Characteristics of frontal structure from climatological data and synoptic surveys. J.
Geophys. Res., 101, 16491-16508.
- 32 -
Figure 1. Annual climatologies (1950-2003) of (a) surface current vectors and speeds
(shaded as indicated to the right), (b) sea surface height (SSH; every 5 cm), and (c) sea
surface temperature (SST; every 1°C) in the OFES hindcast integration. The scaling for the
vectors is indicated at the upper-right corner with unit of cm s-1. In (a), thick vectors
represent currents with mean speeds greater than 30 cm s-1. In (a), some currents and
geographies are labeled (AS for the Alaskan Stream, EKC the East Kamchatska Current,
OC the Oyashio Current, NPC the North Pacific Current, KP Kamchatska Peninsula, HI
Hokkaido Island, and KIs Kuril Islands).
Figure 2. Climatological mean (1950-1999) wintertime (Jan.-Mar.) SST based on (a) the
OFES hindcast integration and (b) JMA-SST. Contour intervals are 1°C. The rectangular
domain indicated in (a) is used for defining the temperature indices shown in Fig. 3.
Figure 3. Time series of temperature (°C) averaged over the rectangular domain [40°-42°N,
147.5°-152.5°E] in Fig. 2a, based on OFES-simulated SST (heavy solid; left axis) and
JMA-SST (heavy dashed; left axis), in addition to OFES-simulated 100-m depth
temperature based on the OFES simulation (thin solid; right axis). (a) Annual averages, and
(b) monthly climatologies (1950-1999).
Figure 4. Climatological-mean (1950-2003) surface current vectors simulated in OFES for
(top-left) Jan.-Mar., (top-right) Apr.-Jun., (lower-left) Jul.-Sep., and (lower-right) Oct.-Dec.
The scaling for the vectors (unit of cm s-1) is indicated below the lower-right panel. Thick
vectors indicate current speeds greater than 50 cm s-1. In each panel, shading denotes
surface meridional velocity as indicated below the lower-left panel. The rectangular domain
in the upper-left panel is used for defining the current velocity indices shown in Fig. 5.
Figure 5. Time indices of OFES-simulated meridional velocity (cm s-1 ; southward negative)
averaged over the rectangular domain [40.1°N-45.1°N, 143.1°E-151.1°E] shown in Fig. 4.
(Top) Wintertime (Jan.-Mar.) averages. (Bottom) Monthly climatologies (1950-2003). In
each panel, the velocity at 100-m depth and its vertical mean are plotted with solid and
dotted lines, respectively, and the difference between the 100-m velocity and the vertical
mean (the former minus the latter) is superimposed with a dashed line.
- 33 -
Figure 6. OFES-simulated surface current vectors and their magnitudes (shading; as
indicated to the right of the top panel) averaged separately over the two five-winter periods
(Jan.-Mar.) of (top) 1984-88 and (bottom) 1968-72. The scaling for the vectors (unit: cm
s-1) is indicated between the panels.
Figure 7. (a, d) One-year lead, (b, e) simultaneous, and (c, f) one-year lagged wintertime
(Jan.-Mar.) (d, e, f) surface current and (a, b, c) wind-stress vectors, regressed linearly on
the standardized wintertime vertical mean Oyashio Current index, based on the OFES
simulation. The scaling vectors for the wind stress (unit: dyn cm-2) and currents (unit: cm
s-1) are indicated at the upper-right corners of panels a and d, respectively. The regressed
current vectors are plotted only where the corresponding correlation is significant at the
95% confidence level (|r|>0.27) for both their zonal and meridional components, whereas
for the regressed wind-stress black vectors are plotted where the corresponding correlation
is significant at the 95% confidence level for either their zonal or meridional component. In
the left panels, the corresponding local correlation for the wind-stress curl is superimposed
with shading as indicated below panel c.
Figure 8. Maps of wintertime (Jan.-Mar.) SLP anomaly (contoured for every 1 hPa; dashed
for negative values) regressed linearly on standardized indices of the wintertime Oyashio
Current velocity (top) for its barotropic component and (middle) at 100-m depth based on
the OFES simulation. The corresponding correlation is superimposed with shading as
indicated to the right of each panel. The SLP field is based on the NCEP-NCAR reanalysis,
and its wintertime climatology (1950-2003) is shown in the bottom panel (contoured for
every 4 hPa; heavy lines for 1000 and 1020 hPa).
Figure 9. Time series of wintertime (Jan.-Mar.) meridional transport of the OFESsimulated Oyashio Current integrated from the western boundary to 151°E (solid line;
northward is positive; left axis), and those of two estimates of the Sverdrup transport
(southward is positive), one integrated from 145°E to the eastern boundary (dashed line;
right axis) and the other between 145°E and 170°E (dash-dotted line; left axis). All values
are averaged between 40°N and 45°N. Unit for the transport is Sv (=1.0x106 m3 s-1).
- 34 -
Figure 10. Anomalous vectors of wintertime (Jan.-Mar.) (e-h) OFES-simulated surface
ocean current and (a-d) surface wind stress regressed linearly on the standardized
wintertime 100-m depth Oyashio Current index. The surface current and wind fields lead
the index by three, two, one and zero (simultaneous) year(s) as indicated (from top to
bottom). The scaling vectors for the wind stress (unit: dyn cm-2) and currents (unit: cm s-1)
are indicated at the upper-right corners of the top panels of the corresponding column. The
regressed vectors for the surface current are plotted only where the corresponding
correlation is significant at the 90% confidence level (|r|>0.32) for both their zonal and
meridional components, whereas for the regressed wind stress black vectors are plotted
only where the corresponding correlation is significant at the 90% confidence level for
either their zonal or meridional component. The corresponding correlation coefficient of
local Ekman pumping velocity with the Oyashio Current index is plotted with shading in
the left panels as indicated below panel d.
Figure 11. Scatter plots for the axial latitude of the Oyashio Extension Current (ordinate)
versus (a) the 100-m depth Oyashio Current index and (b) the maximum eastward current
velocity (abscissa), based on OFES-simulated seasonal mean fields for every winter
(Jan.-Mar.). The axis is defined as the latitude of the maximum surface eastward velocity
averaged for 150-160°E with SST lower than 10°C, in order to prevent the KE jet from
being selected as the Oyashio Extension axis. Correlation coefficient between the
corresponding variables within the southern branch of SAFZ (to the south of 43°N) is
indicated in each panel.
Figure 12. Maps of OFES-simulated springtime (Mar.-May) SSTA (°C) regressed linearly
on the wintertime 100-m depth Oyashio Current (shading as indicated below the bottom
panels). In the left column, the SSTAs lead the index by three, two and one year(s), whereas
in the right column they lags the index by zero (simultaneous), one, two and three years, as
indicated. The corresponding correlation of r = ±0.32 is plotted with dashed lines to show
the significance of the anomalies at the 90% confidence level. Springtime SST climatology
(1950-2003) is superimposed with solid contours for every 1°C (thickened for every 5°C).
Figure 13. Longitude-time section of SSTA (°C; color shading as indicated below the panel)
at 42°N based on the JMA-SST. The plot is drawn for the whole width of the North Pacific
- 35 -
basin for easy comparison with Fig. 18.
Figure 14. Longitude-lag diagrams for springtime (Mar.-May) (left) SSTAs (°C) along and
(right) the axial latitude (degree) of the time-varying southern branch of SAFZ regressed
linearly on the wintertime 100-m depth Oyashio Current index in the OFES simulation.
Positive (negative) lags mean SSTAs and latitude anomalies lag (lead) the index. In each
panel, the corresponding correlation is superimposed with shading as indicated below the
right panel. The southern branch of SAFZ is defined as the latitude of the maximum surface
eastward velocity with SST lower than 10°C to the south of a straight line connecting
[141°E, 43.1°N] and [175°E, 44.1°N]..
Figure 15. Maps of OFES-simulated springtime (Mar.-May) SSH anomalies (color shading
as indicated below the bottom panels; unit: cm) regressed linearly on the wintertime 100-m
depth Oyashio Current index in the OFES simulation. In the left column, the SSH
anomalies lead the index by three, two and one year(s), whereas in the right column they
lag the index by zero (simultaneous), one, two and three year(s), as indicated. The
corresponding correlation of r = ±0.32 is plotted with dashed lines to show the significance
of the anomalies at the 90% confidence level. Springtime SSH climatology (1950-2003) is
superimposed with solid contours for every 5 m (thickened for every 25 m).
Figure 16. (top-left) Map of springtime (Mar.-May) climatology (1950-2003) of
OFES-simulated absolute PV (10-12 cm-1 s-1) on σθ=27.0 kg m-3 isopycnal surface (shading
as indicated below the panel). Other panels show maps of OFES-simulated springtime PV
anomalies (shading as indicated below the bottom panels) regressed linearly on the
wintertime 100-m depth Oyashio Current index in the OFES simulation. In the left column,
the anomalies lead the index by three, two and one year(s), whereas in the right column
they lag the index by zero (simultaneous), one, two and three year(s) as indicated. The
corresponding correlation is superimposed with contours only for the coefficients of 0.32
(solid) and – 0.32 (dashed), to show the significance of the anomalies at the 90%
confidence level.
Figure 17. Maps of OFES-simulated springtime (Mar.-May) anomalies in upward sea
surface heat flux (shading as indicated below the bottom panels; unit: W m-2) regressed
- 36 -
linearly on the wintertime 100-m depth Oyashio Current index in the OFES simulation. In
the left column, the heat flux anomalies lead the index by three, two and one year(s),
whereas in the right column they lag the index by zero (simultaneous), one, two and three
year(s), as indicated. The corresponding correlation coefficients are superimposed with
contours (dashed for negative) only for ±0.26 (thin) and ±0.32 (thick), to show the
significance of the anomalies at the 80% and 90% confidence levels.
Figure 18. Longitude-time sections of SSH anomalies (cm; color shading as indicated
below the lower panels) at (upper-left) 38°N, (upper-right) 40°N, (lower-left) 42°N, and
(lower-right) 44°N. 13-month running mean is applied to remove annual signal.
- 37 -
Figure 1. Annual climatologies (1950-2003) of (a) surface current vectors and speeds
(shaded as indicated to the right), (b) sea surface height (SSH; every 5 cm), and (c) sea
surface temperature (SST; every 1°C) in the OFES hindcast integration. The scaling for
the vectors is indicated at the upper-right corner with unit of cm s-1. In (a), thick vectors
represent currents with mean speeds greater than 30 cm s-1. In (a), some currents and
geographies are labeled (AS for the Alaskan Stream, EKC the East Kamchatska
Current, OC the Oyashio Current, NPC the North Pacific Current, KP Kamchatska
Peninsula, HI Hokkaido Island, and KIs Kuril Islands).
- 34 -
Figure 2. Climatological mean (1950-1999) wintertime (Jan.-Mar.) SST based on (a) the
OFES hindcast integration and (b) JMA-SST. Contour intervals are 1°C. The
rectangular domain indicated in (a) is used for defining the temperature indices shown
in Fig. 3.
- 35 -
Figure 3. Time series of temperature (°C) averaged over the rectangular domain
[40°-42°N, 147.5°-152.5°E] in Fig. 2a, based on OFES-simulated SST (heavy solid; left
axis) and JMA-SST (heavy dash-dotted; left axis), in addition to OFES-simulated 100-m
depth temperature based on the OFES simulation (thin solid; right axis). (a) Annual
averages, and (b) monthly climatologies (1950-1999).
- 36 -
Figure 4. Climatological-mean (1950-2003) surface current vectors simulated in OFES
for (top-left) Jan.-Mar., (top-right) Apr.-Jun., (lower-left) Jul.-Sep., and (lower-right)
Oct.-Dec. The scaling for the vectors (unit of cm s-1) is indicated below the lower-right
panel. Thick vectors indicate current speeds greater than 50 cm s-1. In each panel,
shading denotes surface meridional velocity as indicated below the lower-left panel. The
rectangular domain in the upper-left panel is used for defining the current velocity
indices shown in Fig. 5.
- 37 -
Figure 5. Time indices of OFES-simulated meridional velocity (cm s-1; southward
negative) averaged over the rectangular domain [40.1°N-45.1°N, 143.1°E-151.1°E]
shown in Fig. 4. (Top) Wintertime (Jan.-Mar.) averages. (Bottom) Monthly climatologies
(1950-2003). In each panel, the velocity at 100-m depth and its vertical mean are plotted
with solid and dotted lines, respectively, and the difference between the 100-m velocity
and the vertical mean (the former minus the latter) is superimposed with a dashed line.
- 38 -
Figure 6. OFES-simulated surface current vectors and their magnitudes (shading; as
indicated to the right of the top panel) averaged separately over the two five-winter
periods (Jan.-Mar.) of (top) 1984-88 and (bottom) 1968-72. The scaling for the vectors
(unit: cm s-1) is indicated between the panels.
- 39 -
Figure 7. (a, d) One-year lead, (b, e) simultaneous, and (c, f) one-year lagged wintertime
(Jan.-Mar.) (d, e, f) surface current and (a, b, c) wind-stress vectors, regressed linearly
on the standardized wintertime vertical mean Oyashio Current index, based on the
OFES simulation. The scaling vectors for the wind stress (unit: dyn cm-2) and currents
(unit: cm s-1) are indicated at the upper-right corners of panels a and d, respectively. The
regressed current vectors are plotted only where the corresponding correlation is
significant at the 95% confidence level (|r|>0.27) for both their zonal and meridional
components, whereas for the regressed wind-stress black vectors are plotted where the
corresponding correlation is significant at the 95% confidence level for either their zonal
or meridional component. In the left panels, the corresponding local correlation for the
wind-stress curl is superimposed with shading as indicated below panel c.
- 40 -
Figure 8. Maps of wintertime (Jan.-Mar.) SLP anomaly (contoured for every 1 hPa;
dashed for negative values) regressed linearly on standardized indices of the wintertime
Oyashio Current velocity (top) for its barotropic component and (middle) at 100-m depth
based on the OFES simulation. The corresponding correlation is superimposed with
shading as indicated to the right of each panel. The SLP field is based on the
NCEP-NCAR reanalysis, and its wintertime climatology (1950-2003) is shown in the
bottom panel (contoured for every 4 hPa; heavy lines for 1000 and 1020 hPa).
- 41 -
Figure 9. Time series of wintertime (Jan.-Mar.) meridional transport of the OFESsimulated Oyashio Current integrated from the western boundary to 151°E (solid line;
northward is positive; left axis), and those of two estimates of the Sverdrup transport
(southward is positive), one integrated from 145°E to the eastern boundary (dashed line;
right axis) and the other between 145°E and 170°E (dash-dotted line; left axis). All
values are averaged between 40°N and 45°N. Unit for the transport is Sv (=1.0x106 m3
s-1).
- 42 -
Figure 10. Anomalous vectors of wintertime (Jan.-Mar.) (e-h) OFES-simulated surface
ocean current and (a-d) surface wind stress regressed linearly on the standardized
wintertime 100-m depth Oyashio Current index. The surface current and wind fields
lead the index by three, two, one and zero (simultaneous) year(s) as indicated (from top
to bottom). The scaling vectors for the wind stress (unit: dyn cm-2) and currents (unit:
cm s-1) are indicated at the upper-right corners of the top panels of the corresponding
column. The regressed vectors for the surface current are plotted only where the
corresponding correlation is significant at the 90% confidence level (|r|>0.32) for both
their zonal and meridional components, whereas for the regressed wind stress black
vectors are plotted only where the corresponding correlation is significant at the 90%
confidence level for either their zonal or meridional component. The corresponding
correlation coefficient of local Ekman pumping velocity with the Oyashio Current index
is plotted with shading in the left panels as indicated below panel d.
- 43 -
Figure 11. Scatter plots for the axial latitude of the Oyashio Extension Current
(ordinate) versus (a) the 100-m depth Oyashio Current index and (b) the maximum
eastward current velocity (abscissa), based on OFES-simulated seasonal mean fields for
every winter (Jan.-Mar.). The axis is defined as the latitude of the maximum surface
eastward velocity averaged for 150-160°E with SST lower than 10°C, in order to prevent
the KE jet from being selected as the Oyashio Extension axis. Correlation coefficient
between the corresponding variables within the southern branch of SAFZ (to the south
of 43°N) is indicated in each panel.
- 44 -
Figure 12. Maps of OFES-simulated springtime (Mar.-May) SSTA (°C) regressed
linearly on the wintertime 100-m depth Oyashio Current (shading as indicated below
the bottom panels). In the left column, the SSTAs lead the index by three, two and one
year(s), whereas in the right column they lags the index by zero (simultaneous), one,
two and three years, as indicated. The corresponding correlation of r = ±0.32 is plotted
with dashed lines to show the significance of the anomalies at the 90% confidence level.
Springtime SST climatology (1950-2003) is superimposed with solid contours for every
1°C (thickened for every 5°C).
- 45 -
Figure 13. Longitude-time section of SSTA (°C; color shading as indicated below the
panel) at 42°N based on the JMA-SST. The plot is drawn for the whole width of the
North Pacific basin for easy comparison with Fig. 18.
- 46 -
Figure 14. Longitude-lag diagrams for springtime (Mar.-May) (left) SSTAs (°C) along
and (right) the axial latitude (degree) of the time-varying southern branch of SAFZ
regressed linearly on the wintertime 100-m depth Oyashio Current index in the OFES
simulation. Positive (negative) lags mean SSTAs and latitude anomalies lag (lead) the
index. In each panel, the corresponding correlation is superimposed with shading as
indicated below the right panel. The southern branch of SAFZ is defined as the latitude
of the maximum surface eastward velocity with SST lower than 10°C to the south of a
straight line connecting [141°E, 43.1°N] and [175°E, 44.1°N]..
- 47 -
Figure 15. Maps of OFES-simulated springtime (Mar.-May) SSH anomalies (color
shading as indicated below the bottom panels; unit: cm) regressed linearly on the
wintertime 100-m depth Oyashio Current index in the OFES simulation. In the left
column, the SSH anomalies lead the index by three, two and one year(s), whereas in the
right column they lag the index by zero (simultaneous), one, two and three year(s), as
indicated. The corresponding correlation of r = ±0.32 is plotted with dashed lines to
show the significance of the anomalies at the 90% confidence level. Springtime SSH
climatology (1950-2003) is superimposed with solid contours for every 5 m (thickened
for every 25 m).
- 48 -
Figure 16. (top-left) Map of springtime (Mar.-May) climatology (1950-2003) of
OFES-simulated absolute PV (10-12 cm-1 s-1) on σθ=27.0 kg m-3 isopycnal surface
(shading as indicated below the panel). Other panels show maps of OFES-simulated
springtime PV anomalies (shading as indicated below the bottom panels) regressed
linearly on the wintertime 100-m depth Oyashio Current index in the OFES simulation.
In the left column, the anomalies lead the index by three, two and one year(s), whereas
in the right column they lag the index by zero (simultaneous), one, two and three year(s)
as indicated. The corresponding correlation is superimposed with contours only for the
coefficients of 0.32 (solid) and –0.32 (dashed), to show the significance of the anomalies
at the 90% confidence level.
- 49 -
Figure 17. Maps of OFES-simulated springtime (Mar.-May) anomalies in upward sea
surface heat flux (shading as indicated below the bottom panels; unit: W m-2) regressed
linearly on the wintertime 100-m depth Oyashio Current index in the OFES simulation.
In the left column, the heat flux anomalies lead the index by three, two and one year(s),
whereas in the right column they lag the index by zero (simultaneous), one, two and
three year(s), as indicated. The corresponding correlation coefficients are superimposed
with contours (dashed for negative) only for ±0.26 (thin) and ±0.32 (thick), to show the
significance of the anomalies at the 80% and 90% confidence levels.
- 50 -
Figure 18. Longitude-time sections of SSH anomalies (cm; color shading as indicated
below the lower panels) at (upper-left) 38°N, (upper-right) 40°N, (lower-left) 42°N, and
(lower-right) 44°N. 13-month running mean is applied to remove annual signal.
- 51 -