INTERNATIONAL JOURNAL OF CLIMATOLOGY
Int. J. Climatol. (2014)
Published online in Wiley Online Library(wileyonlinelibrary.com) DOI: 10.1002/joc.4230

Short Communication
Tree-ring reconstructed temperature index for coastal
northern Japan: implications for western
North Pacific variability
Rosanne D’Arrigo,a* Rob Wilson,a,b Greg Wiles,a,c Kevin Anchukaitis,a,d Olga Solomina,a,e Nicole
Davi,a,f Clara Deserg and Ekaterina DolgovaeaTree-Ring Laboratory, Lamont-Doherty Earth Observatory, Palisades, NY, USA
Department of Geography and Geosciences, University of Saint Andrews, UKc Department of Geology, The College of Wooster, OH, USAd
Department of Geology and Geophysics, Woods Hole Oceanographic Institute, MA, USAe
Institute of Geography, Russian Academy of Sciences, Moscow, Russia
f Department of Environmental Science, William Patterson University, NJ, USAg
Climate and Global Dynamics, NCAR, Boulder, CO, USAb

ABSTRACT: While paleoclimatic studies have extended our understanding of North Pacific climate variability, these havebeen almost exclusively based on proxies from western North America. We present a tree-ring reconstruction of June to
September coastal air temperatures for Nemuro, northeastern Japan for the past four centuries. It explains 36% of the variancein instrumental temperatures and correlates significantly with indices of the atmosphere–ocean circulation. Spectral analyses
reveal robust bidecadal peaks that appear associated with regional modes of western North Pacific variability. At decadal
time scales, Nemuro temperatures appear to be influenced by the confluence of the Kuroshio and Oyashio currents, a primarycentre of action driving Pacific Decadal Variability. Regime shifts (e.g. 1976) are weakly expressed relative to western North
America. These aspects of western North Pacific climate are regionally distinct relative to those elsewhere in the basin, withgreater complexity than can be attributed to the Pacific Decadal Oscillation (PDO) alone.
KEY WORDS

western North Pacific; dendrochronology; Japan; tree rings; temperature reconstruction

Received 8 August 2014; Revised 3 November 2014; Accepted 6 November 2014

1. Introduction
North Pacific climate variability influences marine andterrestrial ecosystems and human populations around theNorth Pacific rim (Mantua et al., 1997). Atmosphere–
ocean phenomena linked to the Pacific Decadal Oscillation (PDO), the dominant mode of sea surface temperature(SST) variability in the North Pacific Ocean, have beenshown to be a strong control on temperature and precipitation over adjacent land areas (Mantua et al., 1997). However, there is much greater complexity inherent in North
Pacific climate variability on interannual to decadal andlonger time scales than only the PDO (Deser et al., 2004;
Schneider and Cornuelle, 2005). We use annual tree-ringrecords to reconstruct temperatures for northeastern Japan,
a region highly sensitive to western North Pacific climate
variability. In so doing, we show that long-term North
Pacific climate patterns and variability are substantially* Correspondence to: R. D’Arrigo, Tree-Ring Laboratory, LamontDoherty Earth Observatory, Palisades 10964, New York, USA.
E-mail: rdd@ldeo.columbia.edu© 2014 Royal Meteorological Society

more complex than can be gleaned from any single indicator such as the PDO, particularly with respect to thewestern side of the basin (Bond et al., 2003).
Fluctuations in North Pacific climate have been observedin the instrumental record on interannual to multidecadal
time scales. A key example is the Pacific regime shift in
1976, which was associated with an abrupt transition froma negative to positive PDO phase, with related impacts
on marine and land ecosystems (Mantua et al., 1997).
This shift was linked to widespread warming along thecoast of western North America, with a deepened andeastward-shifted Aleutian Low. To a lesser extent, it was
associated with cooling in the western North Pacific. Earlier shifts occurred in ∼1924 (from negative to positive
phase), and ∼1946 (from positive to negative) (Mantua
et al., 1997; D’Arrigo et al., 2005). The latter event was
linked to SST and surface air warming centred strongly
over Japan during the colder months (Minobe, 1997, 1999;
Deser et al. 2004). Other regime shifts have been proposedfor 1988/1989 and the late 1990s (Bond et al. 2003). Such
Pacific multidecadal mechanisms, their origins and theirpotential causes (e.g. tropical forcing – Deser et al., 2004;

ROSANNE D’ARRIGO et al.

D’Arrigo et al., 2005; Wang et al., 2012a) are not wellunderstood due to the limited length of instrumental data.
Minobe (1999) theorized that these shifts result from interaction between bidecadal and pentadecadal modes of circulation, but this theory appears to break down when tested
over the last millennium (Wilson et al., 2007). It is perhaps
more likely that the apparent regime shifts are due to therandom superposition of multiple independent processes
and may thus not represent a single physical phenomenon(Newman, 2007; Wang et al., 2012b).
In contrast to western North America, relatively few
studies have investigated the climate of the western North
Pacific sector and its distinct regional characteristics. The
Subarctic Front, Okhotsk High, Kuroshio–Oyashio Extension (KOE) and East Asian monsoon are distinct physicalfeatures of this region’s circulation, climate and geography. The Subarctic Front east of Japan (defined as SST
averaged over 34∘ –48∘ N, 150∘ –180∘ E) features the mostintense temperature gradient in the Pacific basin, extending zonally to 42∘ N, where cool water from the Oyashio
current (OC) joins warm water in the Kuroshio Extension (Nakamura et al., 1997). An above-average Okhotsk
High [sea level pressure (SLP), averaged over 50∘ –60∘ N,
140∘ –160∘ W] is linked with cool summers in northern
Japan and vicinity (Ogi et al., 2004). There is also increasing evidence that Eurasian climate plays a critical role insuch variability (Frauenfeld and Davis, 2002; Nakamura
et al., 2002), including the effects of the Siberian High(SH) pressure cell. These observations illustrate some of
the unique and complex features of the climate of the western North Pacific sector.
Longer time series, gleaned from precisely dated paleoclimatic records, can aid considerably in understanding
the basic mechanisms that drive climate variability acrossthe North Pacific, particularly on time scales beyond thoseresolvable in the instrumental record. For western North
America, tree rings from the mountain ranges along the
Gulf of Alaska (GOA) and other locations were used togenerate reconstructions of North Pacific climate linked
to the PDO (e.g. D’Arrigo et al., 2001, 2005; Gedalof
and Smith, 2001; Wiles et al., 2014a). Building on theseand other prior studies, Wilson et al. (2007) describeda 1300-year tree-ring reconstruction of GOA January
to September coastal air temperatures, identifying statistically significant spectral peaks in North Pacific climate, and outlining evidence for multiple regime shifts
over time.
On the western side of the North Pacific basin, however, tree-ring research has been much more limited, but
this situation has recently improved. This work includes
individual-site tree-ring investigations in northern coastal
Japan [Hokkaido (D’Arrigo et al., 1997; Davi et al.,2001)], the Russian Far East [Kurile Islands (Jacoby et al.,2004), Kamchatka (Solomina et al., 2007; Sano et al.,2010) and Sakhalin (Wiles et al., 2014b)], Japan and
Korea (Ohyama et al., 2013). Cook et al. (2012) reconstructed temperatures for temperate East Asia, focusing
mainly on continental climate. D’Arrigo and Wilson(2006) reconstructed the Asian expression of the spring© 2014 Royal Meteorological Society

PDO, which identified significant regime shifts similar to
those in the instrumental period, although distinct featuresreflecting local and regional response to volcanic eventsand the El Niño-Southern Oscillation (ENSO) were noted.
Other reconstructions of the PDO have been developed
from Asia based on historical rainfall records (Shen et al.,2006), and corals from the South China Sea (Deng et al.,2013).

2. Data and methods
We use a network of tree-ring chronologies (Table 1,
Figure 1) from the Russian Far East and Japan to reconstruct northern Japanese coastal air temperatures over the
past four centuries. We target this region because it isparticularly sensitive to northwestern Pacific maritime climate, and is potentially impacted by the Subarctic Front,
KOE and other aspects of northwestern Pacific circulation
with features distinct from those observed in either theeastern northern Pacific or interior Asia.
Our tree-ring reconstruction of coastal summer surface
air temperatures is based on the meteorological station
record at Nemuro, northeastern Japan [Figure 1, 43.33∘ N,
145.58∘ E, elevation 39 m, Global Historical Climate Network (GHCN); Peterson and Vose, 1997]. This station issituated on a narrow peninsula that extends eastward into
the North Pacific Ocean from the island of Hokkaido. We
selected this station for analysis because it is considerably
longer than any other records in the region (beginning in
1880), allowing for longer intervals for model calibrationand verification (Cook and Kairiukstis, 1990). Importantly,
it is located at the northwestern edge of the region of peakcorrelation of land-based summer surface air temperatures
and SSTs with the PDO and related indices, thus providing
important information on western North Pacific climate
and atmosphere–ocean circulation (Figures 1 and 2). It isin this region that the OC to the north meets the Kuroshio
current (KC) from the south, representing the KOE and
Arctic Front (Nakamura et al., 1997).
Nemuro is also in close proximity to several of our individual tree-ring sites developed over the past few decades,
and these records respond positively to summer temperatures during the growing season (Figure 1, Table 1).
Figure 2(a) shows the spatial correlation pattern of Nemurowith summer land/sea temperatures for the region. All
tree-ring sites used in our reconstruction are situated within
the region of positive correlation. Likewise, they all sit
within an area of broad, negative correlation between
the PDO and summer temperature over northern Japan(including Nemuro) and the southern Russian Far East
(Figure 2(b)). This association is weaker over Japan during the winter to spring seasons (February to May), when
the area of strongest correlation appears to shift eastward(Figure 2(c)), and when the positive PDO/temperature
relationship is much stronger over western America.
A network of six tree-ring chronologies was used asa set of candidate predictors in principal components
regression (PCR) modelling (Cook and Kairiukstis, 1990)
Int. J. Climatol. (2014)

WESTERN NORTH PACIFIC CLIMATE OVER FOUR CENTURIES

Table 1. Site information for six tree-ring chronologies used to reconstruct Nemuro, Japan June to September temperatures. Also
shown are their correlations (all significant at or above the 0.05 level) with Nemuro instrumental temperatures based on the
1900–1993 calibration period, and beta coefficients. All are ring-width records except for Asahikawa, which is maximum latewood
density (MXD).
Site location

Country

Chamga, Sakhalin
Kunashir, Kuriles
Asahikawa, Hokkaido (MXD)
Saroma, Hokkaido
Tokachi, Hokkaido
Omebetsu, Hokkaido

RussiaRussia
Japan
Japan
Japan
Japan

LengthLatitudeLongitude1620–2004 51∘ 01′ 48′′ N 143∘ 35′ 24′′ E
1610–2000 43∘ 52′ 47′′ N 145∘ 36′ 24′′ E
142∘ 33′ E
1640–1997 43∘ 46′ 12′′ N
142∘ 20′ E
1700–199343∘ 48′ N
143∘ 12′ E
1520–199743∘ 30′ N′∘
145∘ 28′ E
1580–1998
43 13 N

0.37
0.49
0.23
0.23
0.22
0.49

Beta
Tree species
coefficient
0.27
0.15
0.05
0.09
0.19
0.21

Larix spp.
Quercus crispula
Picea glehnii
Quercus dentata
Picea spp.
Picea spp.

NEM vs HADCRUT JJAS

(a)

o

54 N

Correlation

o

o

64 N
o
56 N
o
48 N
o
40 N

CHAMGA

51 N

SAKHALIN

(b)

S

LE

I
UR

K
ASAHIKAWA

SAROMA

TOKACHIHOKKAIDO

o

42 N

138oE

141oE

KUNASHIR

o

144 E

o

147 E

o

150 E

Figure 1. Map showing locations of the tree-ring sites used in thisreconstruction and the Nemuro (*) meteorological station. The islandsof Hokkaido, Sakhalin and the Kuriles are also labelled.

to reconstruct northern Japanese temperatures over the
past several centuries. These chronologies are based onwood collections from living trees that mainly show sensitivity to temperature. The sites are from coastal and nearcoastal regions impacted significantly by North Pacificclimate. These data include chronologies developed by theTree-Ring Laboratory-Lamont-Doherty Earth Observatory (TRL-LDEO) and collaborators, and several donatedby contributors to the NOAA Paleoclimatology International Tree-Ring Data Bank (ITRDB) and other sources.
We include only those records that expressed significant
agreement with the Nemuro record and which were situated within the ‘zone of coherence’ identified between
the PDO and HADCRU data shown in Figure 2(b). The
records used herein were mainly from maritime sites
situated closest to the Nemuro station (Table 1, Figure 1).
Five series are based on ring width; one, for Hokkaido, ismaximum latewood density or MXD (Davi et al., 2001).
Four are of coniferous species of spruce and larch, andtwo of oak (Table 1). Several were utilized in prior local© 2014 Royal Meteorological Society

(c)

175oE

o

o

150 oE

175oE

o

o

o

o

160 W 135 W
PDO vs HADCRUT JJAS

o

64 N
o
56 N
o
48 N
o
40 N

32oN
125 o
E

OMEBETSU

150 E

o

o

o

KC

o

32 N
125 o
E

48 N

45 N

OC

160 W 135 W
PDO vs HADCRUT FMAM

64oN
o
56 N
o
48 N
o
40 N

32oN
o
o
125 o
E 150 E 175 E−0.6 −0.4 −0.2
0160 W 135 W
0.2
0.4
0.6

Figure 2. (a) Spatial correlation maps for 1900–1993 comparing the
Nemuro, Japan meteorological station temperature data (*) and gridded
HADCRUT land/sea temperatures (Morice et al., 2012) for the June to
September season. Large black dots denote the six tree-ring sites (seeTable 1, Figure 1) used in the reconstruction. (b) As in (a), but between
the PDO index (1900–2012) and the HADCRUT data for the June to
September season. (c) As in (b), but for the February to May season.
Kuroshio–Oyashio Currents/Extension also shown by arrows. Nemuro
is near the confluence of these currents.

to regional studies (e.g. D’Arrigo et al., 1997, 2005;
Davi et al., 2001; Jacoby et al., 2004; D’Arrigo andWilson, 2006; Cook et al., 2012; Wiles et al., 2014b),
whereas others have not been previously published. Data
were detrended using standard methods (e.g. negative
exponential or straight line/regression functions, to remove
non-climatic biological age-related trends), with theInt. J. Climatol. (2014)

ROSANNE D’ARRIGO et al.

Table 2. Correlations between Nemuro, Japan instrumental and reconstructed June to September temperatures and indices of the
PDO, West Pacific pattern (http://www.cpc.ncep.noaa.gov/data/teledoc/wp.shtml), Arctic Front, Okhotsk High, KOE and Enoshima,
Japan SST. First difference comparisons in parentheses. The first difference in this context refers to the value of a time series attime t minus the series at time t-1 (Cook and Kairiukstis, 1990). Arctic Front: SST averaged over 34∘ –48∘ N, 150∘ –180∘ E; Okhotsk
High Index averaged over 50∘ –60∘ N, 140∘ –160∘ E; KOE Index of SST averaged over 35∘ –45∘ N, 140∘ –180∘ E, Enoshima, Japan
SST averaged over 38∘ –39∘ N, 141∘ –142∘ E. Although the JJAS season was used for all of these comparisons, correlations can behigher for individual months and datasets. For the Okhotsk High, for example, values are much higher for June alone: −0.44 for thereconstruction.
Nemuro data

IndicesPDO

W. PacificIndex

Arctic Front
Index

Okhotsk High
Index

Kuroshio
Extension
Index

Enoshima
Index

Tree-ring reconstruction−0.40 (−0.49)
0.59 (0.54)
0.33 (0.40)−0.26 (−0.36)
0.49 (0.57)
0.16 (0.23)
0.0001 (0.0000) 0.000 (0.0002) 0.0002 (0.0000) 0.0018 (0.0000) 0.0000 (0.0000) 0.0813 (0.0091)
n 94 (93)n 44 (43)n 124 (123)n 144 (143)n 104 (100)n 124 (123)Instrumental temperatures −0.52 (−0.63)
0.37 (0.52)
0.55 (0.57)−0.27 (−0.42)
0.67 (0.71)
0.47 (0.48),
0.000, (0.0000) 0.0000 (0.0000) 0.0000 (0.0000) 0.0017 (0.0000) 0.0000 (0.0000) 0.0000 (0.0000)
n 114 (113)
n 64 (63)n 134 (133)n 134 (133)n 123 (119)n 134 (133)caveat that a certain amount of secular variability will
be removed related to mean sample length (Cook andKairiukstis, 1990). We therefore focus herein on the annual
to multidecadal variability in Pacific climate gleaned fromthese tree-ring records.
Climate analyses were performed using the HADISST(Rayner et al., 2003), HADCRUT land/sea temperatures(Morice et al., 2012), HADSST3 (Kennedy et al., 2011)
and HADISLP (Allan and Ansell, 2006) data sets. The
strongest overall correlations were found for the June to
September (JJAS) summer season based on iterative PCRmodel trials. This result differs from that of D’Arrigo
and Wilson (2006), who reconstructed the PDO basedon a much larger data set (including not only some of
the chronologies used herein, but also tree-ring data frommainland north Asia), with March–April–May as theiroptimal target season. Correlations with Nemuro summer
temperatures were positive in sign for all six chronologies. Correlations were strongest for the Kunashir andOmebetsu, Japan chronologies, which are situated closest to Nemuro, within a region where temperatures arewell correlated with the PDO and related indices (Tables 1
and 2, Figures 1 and 2, Jacoby et al., 2004). The significance of interannual correlations was evaluated using
the method described by Dawdy and Matalas (1964)
to account for temporal autocorrelation. For correlation
between smoothed series, we used the simulation method
described by Ebisuzaki (1997) to estimate significance.
A nested procedure (Cook et al. 2002), in which thenumber of chronologies declines back in time, was used
to optimize the length of the reconstruction. Iterative
nests begin in 1560, 1590, 1640, 1650, 1700 and 1710.
The final reconstruction was developed by splicing thesenested series together after their variance and mean had
been adjusted to that of the best-replicated 1710–1993
nest. This was done in order to avoid any changes invariance in the overall reconstruction due to weakening of
the modelled signal back/forward in time. For each model© 2014 Royal Meteorological Society

nest, we used 1900–1993 as the calibration period and
1880–1899 as the period of independent verification.
Figure 3(a) shows the observed and estimated Juneto September Nemuro temperatures for the 1900–1993
calibration period for the best replicated recent period or
nest, based on all six chronologies. The level of temperature variance (adjusted r2 ) explained by the regression
model over the common 1900–1993 calibration period
for the most replicated nest is 36%. The calibration
period root mean square error (RMSE, Cook and Kairiukstis, 1990) was utilized to derive error estimates for thereconstruction back through time. The full temperaturereconstruction based on spliced model nests spans from1640 to 1993 (Figure 3(b)). The strength of the models
declines back in time as chronologies leave the data
matrix (Figure 3(c)), with the ar2 values ranging as low
as 14% for the early 1640 model nest (note that earlier
nests were truncated although they retained some model
skill), based on only three chronologies, indicating that
additional long tree-ring records are needed to improvemodel reliability prior to this time. The nested regression
models all passed the rigorous coefficient of efficiency(CE) test (Figure 3(d), indicating predictive skill – Cook
and Kairiukstis, 1990), and model residuals show no
significant autocorrelation (Figure 3(c)).
3. Results and discussion
The Nemuro temperature reconstruction reveals annual tomultidecadal variability in western North Pacific climate
over the past four centuries (Figure 3). Unlike other reconstructions for western North America, where the PDO
signal is typically much stronger (Wilson et al., 2007;
Figure 3), the Nemuro reconstruction does not show clear
evidence for the three 20th century major regime shifts
identified in the cold season and annual PDO (Mantua
et al., 1997; Sano et al., 2010). This is particularly true forthe two most recent transitions (1946 and 1976), and theInt. J. Climatol. (2014)

WESTERN NORTH PACIFIC CLIMATE OVER FOUR CENTURIES

Figure 3. (a) Nemuro, Japan observed (red in online) and reconstructed (green in online) JJAS temperatures based on western North Pacific tree rings
for most replicated nest and full calibration period from 1900–1993. (b) full Nemuro temperature reconstruction over 1640–1993 with 2-sigma error
bars. Smoothed line is a 28-year spline function; (c) Calibration ar2 and Durbin-Watson (DW) statistic results for the 1900–1993 period for each
nest; (d) Verification (1880–1899) r2 and CE values for each nest. Calibration and verification r2 values refer to the levels of variance explained by
the tree-ring data in regression analysis (Cook and Kairiukstis, 1990).

summer instrumental data from Nemuro likewise fails to
indicate abrupt transitions at these times. These observations are largely explained by the fact that the trees respondprimarily to summer conditions, whereas the PDO is moststrongly expressed in the colder months, and in the eastern
North Pacific sector. This point is illustrated in Figure 4,
which shows spatial anomaly maps comparing the difference in temperature for the 10 years before and after 1976.
The ‘classic’ PDO spatial pattern is clear, particularly for
the FMAM season, but it is only weakly expressed for the
Nemuro region by summer. An intervention analysis (notshown) identifies a significant shift in the Nemuro FMAM
time-series at 1988, although not in 1976. This shift may berelated to the Arctic Oscillation, which underwent a substantial change in 1988/1989 coinciding with strong warming over northern Japan (Thompson and Wallace, 1998).
There is no evidence of a significant change in temperature
for the Nemuro region for the reconstructed JJAS season
in 1988, nor for the mid-1940s shift, believed to be thestrongest in the instrumental record over Japan (Nakamura
et al., 1997), at least in the colder months.
© 2014 Royal Meteorological Society

To further investigate the large-scale climatic signal in
the Nemuro data, we compare the Nemuro data to six
indices of North Pacific atmosphere–ocean circulation(Table 2): (1) the PDO index (Mantua et al., 1997), (2)the West Pacific index (indicator of intensity of the East
Asian jet stream – Barnston and Livezey, 1987), (3) the
North Pacific Subarctic Frontal Zone index (a regionof sharp gradients in SST near the Oyashio–Kuroshio
boundary, 34∘ –48∘ N, 150∘ –180∘ E), (4) the Okhotsk Highindex (a semi-permanent SLP system over the Okhotsk
Sea in warmer months; 50∘ –60∘ N, 140∘ –160∘ E), (5) the
Kuroshio Extension SST index (35∘ –45∘ N, 140-180∘ E)
and (6) the Enoshima index of SST, considered a proxyof Oyashio southward penetration along the Japanese
coast (Minobe, 1997). Nemuro temperatures are mostsignificantly correlated with these indices during thewarm season months, when they are linked to conditions
over northern Japan.
The Nemuro reconstruction provides an opportunity toanalyse long-term variability, including spectral properties, in western North Pacific climate. Multi-taper method
Int. J. Climatol. (2014)

ROSANNE D’ARRIGO et al.
HADCRUT JJAS

(a)

o

64 N
o

56 N
48oN
o

40 N
32oN
125 o
E

150 oE

175oE

o

160 W

o

135 W

HADCRUT FMAM

(b)

o

64 N
56oN
o

48 N
o

40 N
32oN
125 o
E

o
150 oE
175oE
160 W−1 −0.8−0.6−0.4−0.2 0 0.2 0.4 0.6 0.8 1

o

135 W

Figure 4. Spatial difference plot (∘ C) for the 10 years on either side of1976 (1966 to 1975 vs 1977 to 1986) using the HADCRUT temperature
data set for the JJAS and FMAM seasons.

tidal mixing induced by anomalous circulation around the
Kurile Straits, with resulting SST and SLP patterns similar to the PDO that can significantly impact Pacific climate (Osafune and Yasuda, 2006; Yasuda et al., 2006;
Hasumi et al., 2008; Tanaka et al., 2012). KOE variability impacts extratropical storm tracks and the surface windstress curl across the North Pacific basin with clearly
defined decadal modulations, exerting major influences
on western North Pacific climate and ecosystems (Qiu
et al., 2014). A KOE SST index based on oxygen isotopes
from California speleothems identified a 22-year periodicity that was attributed to possible solar influences on North
Pacific climate (McCabe-Glynn et al., 2013). SSTs fromnear Enoshima, off the coast of Japan (38.4∘ N, 141.5∘ E)
are a useful proxy for the OC that runs southward along theJapanese coast (Figure 2, Table 2), and feature bidecadal topentadecadal variability (Minobe, 1997, 1999) which arealso observed in the PDO (Mantua et al., 1997; Minobe,
1997; Yasuda et al., 2006) and related reconstructions forwestern North America (Wilson et al., 2007). We can further examine the frequency-dependent spatial structure of
the relationship between reconstructed Nemuro temperatures and Pacific SSTs. Figure 6(a) shows the correlation
between our reconstruction and SSTs (coloured shading)as well as the PDO-SST correlation (contours). As in
Figure 2, it is clear these two correlation patterns are spatially distinct. A decadal timescales (reconstruction and
SST field smoothed with 11-year lowpass Butterworth filter), the correlation between reconstructed Nemuro is morestrongly zonal, and the ‘horseshoe’ shape of the PDO mode
disappears. The structure of the zonal correlation patternoccurs along the boundary of the KOE.

4. Conclusions

Figure 5. MTM spectral analysis results for Nemuro temperature reconstruction over its length from 1640–1993, showing robust (99% level)
peaks centered at ∼28 years, 18 and 4 years. Levels of significance shown(null, 90, 95 and 99%).

(MTM) spectral analysis (Thomson, 1982; Mann and Lees,
1996) performed on the reconstruction over 1640–1997(Figure 5) reveals a robust (above 99% level) peak centred at ∼28 years, with another peak, also significant above
the 99% level, at ∼18 years. There is a ∼4-year peakthat is within the ENSO bandwidth. While the causes ofthese modes are not well known, winter SST anomalies
for the KOE do show a spectral power at 20–30 years(Pierce et al., 2001). Dominant peaks in the Community
Climate System Model (CCSM) KOE SST index havealso been described at 15–20 years (Kwon and Deser,2007). KOE SST anomalies were shown to occur withina period of 18.6 years, with warmer SSTs linked to strong© 2014 Royal Meteorological Society

We have described a new temperature reconstruction fora climatically sensitive, remote region of the western
North Pacific. Results indicate that coastal land areas ofnortheast Asia feature distinct information relative to theeastern side of the basin. This overall finding is illustrated by the significant agreement between the Nemuro
reconstruction and large-scale indices (SST, KOE, Arctic Front and Okhotsk High) of regional scale circulation(Table 2). Specifically, it is noteworthy that the correlation structure in Figure 6 between Nemuro temperatures and SST changes sign in the general region of the
Oyashio–Kuroshio frontal boundary and currents, possibly indicating that Nemuro temperatures might be anindicator of frontal position and atmospheric circulation
related to this primary centre of action (Nonaka et al.,2006). We note that our reconstruction is based on summer temperatures and appears to express a combinationof local scale variability and large-scale dynamics. Twentieth century regime shifts, as seen more clearly in thenortheast Pacific instrumental PDO and PDO reconstructions (e.g. D’Arrigo et al., 2001; Gedalof and Smith,2001), tend to be more muted in summer, and in the
far western north Pacific. There is some high-frequency
Int. J. Climatol. (2014)

WESTERN NORTH PACIFIC CLIMATE OVER FOUR CENTURIES

(a)

−0.3

60oN

0.3
0.3
0.3
0.3
0.3
0.3
0.3

−0.3
−0.3

45oN

0.3

−0.6

o

−0.3
−0.3
−0.3
−0.3
−0.3
−0.3
−0.3
−0.3
−0.3

−0.3
−0.3
−0.3
−0.3

−0.3

30 N

−0.3

o

0.3

15 N

0.3

0.3

0.3
0.3 0 3

o

0

0.3

0.3
0.3

o

50 E

o

100 E

150 E

100oE

150oE
−0.2

160 W
0.2
0.4

o

o

o

160 W

110 W

o

110 W

03

o

60 W

60oN

(b)
o

45 N
30oN
o

15 N
0o

0.2 m s–1

50oE

−0.4

0

o

o

60 W

Figure 6. (a) Correlation (colour shading) between Nemuro (NEM) temperature reconstruction and JJAS HadISST (1870–1993; Rayner et al. 2003).
Pointwise field correlation values larger than ±0.19 are significant at the p < 0.10 level. Overlying line contours show the correlation (1900–2012)
between the JJAS PDO index and SSTs, and demonstrates that there are differences in the spatial correlation structure between PDO/SST and theNEM reconstruction/SST. The contour interval is 0.1; (b) 11-year low pass correlations between reconstruction and JJAS HadISST (Rayner et al.
2003), showing considerable decadal regional zonal structure likely linked to position of Oyashio/Kuroshio frontal and current dynamics, rather
than the PDO. Pointwise field correlation values larger than ±0.42 are significant at the p < 0.10 level following the method described by Ebisuzaki(1997). Also shown are JJAS North Pacific (north of 20∘ N) mean surface current velocities from Lumpkin and Johnson (2013). Only surface currents
with zonal and meridional components greater than 0.1 m s−1 are plotted.

coherence with the PDO in our summer record, but with
distinct decadal differences relative to those expressedover western North America, indicating spatial complexity. The robust quasi-bidecadal peaks at Nemuro appear
to correspond more to regional features of western North
Pacific coupled atmosphere–ocean processes, and specifically to Kuril and KOE-related SST anomalies in the western North Pacific. Effects of intense mixing can include
anomalous SSTs and sea fog in the Okhotsk Sea in summer (Tokinaga and Xie, 2009), which could impact forest
growth via cooler temperatures and decreased incoming
solar radiation. Understanding of air–sea interactions in
the northwest Pacific Ocean and greater basin are still not
well understood. In particular one question is whether or
not low-frequency variability associated with the ENSO
system in the tropics is the cause of decadal to longer-term
variations at higher latitudes (e.g. Allan, 2000; Deseret al., 2004). An alternative explanation might be related
to air–sea exchanges on the local to regional level, butthere is currently too little data for these intrinsic modes
of interaction to be explored. Additional data and comparisons of proxies from around the North Pacific rim, including spatial reconstructions, will aid understanding of thelong-term processes, forcing mechanisms, and spatiotemporal extent of complex climate phenomena on time scales
not currently resolvable by the instrumental record.

Acknowledgements
We thank the National Science Foundation Paleoclimatology Program, Grant AGS-1159430 and AGS-1202218.
© 2014 Royal Meteorological Society

We also gratefully acknowledge the ITRDB and othercontributors of tree-ring data, including M. Sano of Kyoto
University and K. Yasue of Shinshu University, Japan.
These data are either readily available on the ITRDB orfrom the authors, or will be submitted to the ITRDB prior
to publication of this manuscript. Lamont-Doherty Earth
Observatory Contribution No. 0000.

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