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The Pacific Decadal Oscillation, Revisited
MATTHEW NEWMAN,a,b MICHAEL A. ALEXANDER,b TOBY R. AULT,c KIM M. COBB,d
CLARA DESER,e EMANUELE DI LORENZO,d NATHAN J. MANTUA,f ARTHUR J. MILLER,g SHOSHIROMINOBE,h HISASHI NAKAMURA,i NIKLAS SCHNEIDER,j DANIEL J. VIMONT,k ADAM S. PHILLIPS,eJAMES D. SCOTT,a,b AND CATHERINE A. SMITHa,ba

CIRES, University of Colorado Boulder, Boulder, Colorado
NOAA/Earth Systems Research Laboratory, Boulder, ColoradocDepartment of Earth and Atmospheric Sciences, Cornell University, Ithaca, New YorkdSchool of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgiae
Climate and Global Dynamics, National Center for Atmospheric Research, Boulder, Coloradof
NOAA/Southwest Fisheries Science Center, Santa Cruz, CaliforniagScripps Institution of Oceanography, La Jolla, CaliforniahGraduate School of Science, Hokkaido University, Sapporo, Japani
Research Center for Advanced Science and Technology, University of Tokyo, Tokyo, and Application
Laboratory, JAMSTEC, Yokohama, Japanj
Department of Oceanography and International Pacific Research Center, University of Hawai‘i at Manoa, Honolulu, HawaiikDepartment of Atmospheric and Oceanic Sciences, and Nelson Institute Center for Climatic Research,
University of Wisconsin–Madison, Madison, Wisconsinb(Manuscript received 23 July 2015, in final form 19 February 2016)
ABSTRACTThe Pacific decadal oscillation (PDO), the dominant year-round pattern of monthly North Pacific sea surface
temperature (SST) variability, is an important target of ongoing research within the meteorological and climate
dynamics communities and is central to the work of many geologists, ecologists, natural resource managers, andsocial scientists. Research over the last 15 years has led to an emerging consensus: the PDO is not a single
phenomenon, but is instead the result of a combination of different physical processes, including both remotetropical forcing and local North Pacific atmosphere–ocean interactions, which operate on different time scales
to drive similar PDO-like SST anomaly patterns. How these processes combine to generate the observed PDO
evolution, including apparent regime shifts, is shown using simple autoregressive models of increasing spatialcomplexity. Simulations of recent climate in coupled GCMs are able to capture many aspects of the PDO, but doso based on a balance of processes often more independent of the tropics than is observed. Finally, it is suggested
that the assessment of PDO-related regional climate impacts, reconstruction of PDO-related variability into thepast with proxy records, and diagnosis of Pacific variability within coupled GCMs should all account for theeffects of these different processes, which only partly represent the direct forcing of the atmosphere by North
Pacific Ocean SSTs.

1. Introduction
Since its identification in the late 1990s as the dominant year-round pattern of monthly North Pacific sea
surface temperature (SST) variability, the Pacific decadal

Denotes Open Access content.

Corresponding author address: Matthew Newman, NOAA/ESRL, 325 Broadway, R/PSD1, Boulder, CO 80305.
E-mail: matt.newman@noaa.gov
DOI: 10.1175/JCLI-D-15-0508.1
Ó 2016 American Meteorological Society

oscillation (PDO) has been connected both to other partsof the climate system and to impacts on natural resources
and marine and terrestrial ecosystems. Subsequent research, however, has found that the PDO is not a single
physical mode of climate variability but instead largely
represents the combination of three groups of processes:
1) changes in ocean surface heat fluxes and Ekman (winddriven) transport related to the Aleutian low, due to both
local unpredictable weather noise and to remote forcing
from interannual to decadal tropical variability (largely El
Niño) via the ‘‘atmospheric bridge’’; 2) ocean memory, or
processes determining oceanic thermal inertia including

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JOURNAL OF CLIMATE‘‘reemergence,’’ which act to integrate this forcing andthus generate added PDO variability on interannualand decadal time scales; and 3) decadal changes in the
Kuroshio–Oyashio system, forced by winds over the
North Pacific driving westward-propagating oceanic
Rossby waves, manifested as SST anomalies along thesubarctic front at about 408N in the western Pacific
Ocean. Thus, the PDO represents the effects of different
processes operating on different time scales, and its apparent impacts elsewhere only partly represent direct
forcing of the atmosphere by the North Pacific Ocean.
What has often been characterized as PDO impacts in theliterature may, in fact, reflect correlations with processes
that drive simultaneous variations in both the PDO andthe impact variables. Consequently, care should be taken
when positing the PDO as a forcing of nonoceanic responses without a convincing argument for the physicalforcing mechanism.
This paper synthesizes this current view of the PDO anddiscusses its implications for climate diagnosis, including
PDO climate impacts and predictability (both oceanographic and atmospheric); potential decadal-regimelike behavior; PDO simulations in climate models; theinterpretation of multicentennial PDO reconstructions;and its relationship to another widely used index, theinterdecadal Pacific oscillation (IPO). We conclude
with a few suggested best practices for future PDObased diagnoses and forecasts.

2. What is the PDO?
The PDO was first introduced by Mantua et al.
(1997) as the leading empirical orthogonal function(EOF) of North Pacific (208–708N) SST monthly averaged anomalies, or SSTAs, defined as departures fromthe climatological annual cycle after removing the
global mean SSTs. Figure 1a shows the PDO pattern,
calculated by regressing SST anomalies on the associated principal component (PC) time series, obtained
from the HadISST dataset (Rayner et al. 2003), for theyears 1901–2014. (Unless otherwise noted, this SSTA
dataset and period are used for all calculations in thispaper.) Over this period, the PDO is fairly similar
across four SST datasets, all using different methods tofill in missing grids, with relatively minor differences
both in time series (Fig. 1b) and pattern. The latter
point can be illustrated using the Taylor diagram(Taylor 2001) in Fig. 2, which shows that the rootmean-square (RMS) differences between the HadISST
PDO pattern and PDO patterns determined from theother SST datasets are relatively small. The PDO isalso reasonably robust to sampling; for example, continually repeating the EOF analysis upon randomly

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chosen (with replacement) 50-yr draws from theHadISST dataset yields patterns that are all highly similar to the PDO EOF (as shown by the black dots in
Fig. 2). Dataset dependencies are more pronounced earlyin the observational record, especially prior to about 1920
(Fig. 1b, bottom) or when only a few decades are used
to define the climatology and leading EOF (Wenet al. 2014).
Initial research in the 1990s suggested that the PDO
might represent a distinct physical ‘‘mode’’ of North
Pacific variability. First, even using monthly anomalies, the PDO time series has a slowly varying component, with episodic changes of sign, hence ‘‘decadaloscillation.’’1 Second, low simultaneous correlation
between time series of the PDO and the easternequatorial Pacific SST anomalies resulted in the PDO
and the tropics appearing to be only weakly coupled.
Additionally, some early modeling work (Latif andBarnett 1994, 1996) raised the possibility that the
PDO might correspond to a physical mode, oscillating
on decadal time scales, of coupled atmosphere–oceaninteraction within the North Pacific.
As the leading North Pacific EOF, the PDO is, byconstruction, the single pattern that best encapsulates
the variability of monthly SSTAs within the domain
where it is defined. However, the PDO is also associated with variability outside the North Pacific, and indeed the observed regression pattern in Fig. 1a shows astrong connection between the North Pacific and thetropics, despite relatively low correlation values within
the narrow cold tongue located in the eastern equatorial Pacific between 28N and 28S (not shown; see Deseret al. 2004). In particular, positive SSTAs in the eastern
tropical Pacific accompany negative SSTAs in thecentral and western North Pacific and positive SSTAs
in the eastern North Pacific (Fig. 1a). Some PDO details depend on the domain used for the EOF calculation: as this domain is expanded southward (Fig. 2,orange symbols), the tropical portion of the pattern
becomes relatively more pronounced, even along theequator, with the North Pacific anomaly shiftingslightly eastward to become more symmetric with its
South Pacific counterpart. The leading SST EOF of theentire Pacific basin resembles a global ENSO-related
pattern (e.g., Deser and Blackmon 1995).
While the PDO exists throughout the year, it undergoes some seasonal evolution both in its amplitude

1
Note that in meteorological parlance, ‘‘oscillation’’ was first
related to spatial seesaw patterns in the Atlantic and Pacific byWalker and Bliss (1932), but has more recently been occasionallyconfused with temporal oscillations.

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FIG. 1. The PDO over the historical record (1901–2014). (a) Regression of global monthly SST (shading; interval is 0.058C) and DJF SLP(contours; interval is 1 hPa) anomalies onto the PDO time series from the HadISST dataset. Note that a positive PDO is associated withnegative central North Pacific SSTA. (b) PDO index time series determined from the SST datasets, Centennial Observation-Based Estimates(COBE; Ishii et al. 2005), ERSST.v3b (Smith et al. 2008), HadISST (Rayner et al. 2003), and Kaplan (Kaplan et al. 1998). Positive (negative)
values are drawn in red (blue). The thick black line in each panel shows the smoothed (6-yr lowpass; Zhang et al. 1997) time series. The lastseries in (b) shows the departure of each time series from the mean of all four time series. (c) Seasonal cycle of (3-month running mean) PDO
index autocorrelation. Contour (shading) interval is 0.2 (0.1). Only values that are 95% significant are shaded. The month ordinate indicates
the time of the PDO base month, and the lag indicates how far ahead or behind the PDO is; for example, the value plotted at (5, MAY)
represents the correlation between the May value of the PDO and the subsequent October value of the PDO.

and structure. The PDO’s amplitude is greatest from
November through June, with weak maxima both in
midwinter and late spring and a pronounced latesummer–early fall minimum (H. Wang et al. 2012).
However, repeating the PDO regression separately
by month shows the SSTA maximum shifting from thenortheastern to northwestern Pacific between thecold and warm seasons (not shown). In fact, the largest
PDO-related SSTAs in the northwestern Pacific occur inearly fall despite the overall PDO minimum then. In theobserved PDO autocorrelation structure (Fig. 1c), forlong lags persistence is increased (decreased) in spring(autumn) (i.e., the tilted ridges or troughs at long leads orlags in Fig. 1c).

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a. Fluctuations in the Aleutian low (large-scale
stochastic forcing)
Many aspects of climate can be represented by a slow
dynamical system integrating fast forcing approximatedas random, or stochastic, noise (Hasselmann 1976). In a
simple stochastic model of midlatitude SST variability(Frankignoul and Hasselmann 1977), the ocean at agiven location is treated as a motionless (well-) mixed
layer in which surface heat fluxes both force and damp
SSTAs. The forcing F is represented by fluxes associated with weather variations, which relative to oceanic
time scales have approximately no memory and thesame variance at all time scales (effectively, white
noise). The resulting SSTAs are damped by a linear
negative air–sea feedback, representing loss (gain) ofheat with the atmosphere from anomalously warm
(cold) waters. This can be expressed as a first-order
autoregressive or AR1 model,
FIG. 2. Taylor diagram (Taylor 2001) comparing the reference
PDO (HadISST) pattern (Fig. 1a, black circle) with variations due
to sampling, observational dataset, and geographical domain; andto PDOs determined from CGCMs run with historical radiative
forcing. In this diagram, the distance of a point from the origin is
the pattern standard deviation (8C), and the distance from thereference point [at (0.26, 0)] is the root-mean-square error(RMSE) between the pattern and the reference pattern, indicatedby the dashed semicircles spaced at an interval of 0.18C. The
pattern correlation, decreasing in a counterclockwise azimuthal
direction, is mathematically related to these two quantities. The
analysis is taken only over the North Pacific PDO domain (208–708N). Black dots show the PDO estimates based on the 50-yr
Monte Carlo subsamples; triangles show PDO results determinedfrom the ERSST.v3b (blue), COBE (green), and Kaplan (magenta) observed datasets; orange symbols show the SSTA structure (within the North Pacific PDO region) associated with theleading SSTA EOF, where the southern border of the Pacificdomain is instead 08 (square), 208S (diamond), and 708S (circle).
Also shown are the CMIP3 (cyan squares), CMIP5 (red hexagons), and CESM-LE (yellow hexagons) historical simulation
PDOs. EOF spatial patterns were interpolated onto the 28 3 28
grid used for the reference pattern. As a result of differences inlandmasks, metrics for the Taylor diagram were calculated overocean points that were in common between each model and theHadISST data.

3. Processes driving the PDO
Statistical modes may represent physical modes, butthere need not be a one-to-one correspondence between them. A body of research exists showing how
different physical processes, including random atmospheric forcing, teleconnections from the tropical
Pacific, and ocean Rossby waves/shifts in the basinwide ocean gyre circulation, contribute to PDO variability on a variety of time scales and regions in the
North Pacific Ocean.

SSTA(n) 5 rSSTA(n 2 1) 1 F ,(1)where r represents the expected fraction of the SSTA
retained between times n 2 1 and n, determined by the
feedback of air–sea heat fluxes and SST and by thethermal inertia of the upper ocean in direct contact withthe atmosphere. Then, the SSTAs exhibit a red noise
spectrum whose magnitude increases with the inverse
square of frequency, flattening out at periods that arelong compared to the damping time scale.
The simple view of SST variability as the result of noise
integration can be applied basin wide. White noise forcing associated with large-scale weather patterns generates much of the observed SST variability over the entire
North Pacific Ocean (Frankignoul and Reynolds 1983),
where interannual variability in the surface fluxes and
SSTs are closely linked to dominant atmospheric circulation patterns (Cayan 1992; Iwasaka and Wallace 1995).
For example, in an atmospheric general circulation model(AGCM) coupled to a mixed layer ocean model with nocurrents (and hence no ENSO variability or ocean dynamics), the dominant sea level pressure (SLP) pattern is
associated with fluctuations in the strength of the Aleutian low pressure system primarily resulting from internal
atmospheric dynamics (Pierce 2001; Alexander 2010),
including large-scale dominant atmospheric teleconnection patterns such as the Pacific–North American (PNA)
pattern. For periods with a stronger Aleutian low, enhanced wind speeds and reduced air temperature andhumidity along approximately 358N cool the underlying
ocean via surface sensible and latent heat fluxes, whilenorthward advection of warm moist air heats the ocean
near North America (cf. Fig. 1a); the opposite flux
anomalies occur when the low is weaker than average.

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FIG. 3. Illustration of how both local and remote atmospheric forcing can drive PDO variability. (a) One-season lead correlation
between November–January (NDJ) NPI and global SSTAs during FMA. (b) Seasonal cycle of cross correlation between the NPI and the
PDO index (both filtered with 3-month running mean). PDO leads NPI for positive lags; NPI leads PDO for negative lags. In (a) and (b),the NPI index sign has been flipped so that positive refers to a deepening of the Aleutian low, which also will correspond to positive PDO.
(c) One-season lag correlation between the NDJ value of the ENSO index (the leading PC of the tropical Pacific SSTA) and global SSTAs
during FMA. (d) Seasonal cycle of cross correlation between the ENSO and PDO indices (both filtered with 3-month running mean).
PDO leads ENSO for positive lags; ENSO leads PDO for negative lags. All panels are determined from 1901–2014 data; shading interval is0.1. For (b) and (d), only values that are 95% significant are shaded, and the contour line interval is 0.2. The month ordinate indicates thetime of the PDO index base month, and the lag indicates how far ahead or behind the second variable is; for example, the value plotted at(5, MAY) represents the correlation between the May value of the PDO and the subsequent October value of the other variable.

This simulated flux-driven SSTA pattern in the North
Pacific closely resembles the observed PDO. Anomalous
Ekman transports tend to amplify the flux-driven pattern(Miller et al. 1994a; Alexander and Scott 2008).
These physical processes result in observed correlations in which atmospheric variations generally lead
SST variations (e.g., Davis 1976; Deser and Timlin
1997). For example, Fig. 3a shows the PDO-like patternthat results from springtime SST correlation with priorwintertime Aleutian low variability [here measured by
the North Pacific index (NPI); Trenberth and Hurrell
1994]. Throughout the cold season, Aleutian low anomalies typically lead the PDO, even over consecutive years(Fig. 3b). Note that the NPI and PDO are also simultaneously correlated, which has been misinterpreted asindicating that the PDO must force an Aleutian low
response. As Frankignoul (1999) noted, even without
oceanic feedback onto the atmosphere, simultaneous

extratropical atmosphere–SST correlation can occur
as a result of rapid weather forcing of the more slowly
evolving ocean; so, to diagnose interactions between the
North Pacific Ocean and the Aleutian low requires
determining both positive and negative PDO–NPI lag
correlations.

b. Teleconnections from the tropics
1) THE ATMOSPHERIC BRIDGE
We now generalize the ‘‘PDO as integrator’’ paradigm in (1), allowing for other sources of forcing F
beyond noise due to intrinsic midlatitude atmosphericvariability. ENSO tropical Pacific SSTAs induce tropical precipitation shifts forcing global atmosphericteleconnections (e.g., Trenberth et al. 1998; Liu andAlexander 2007), altering near-surface air temperature, humidity, wind, and clouds far from the equatorial

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Pacific. The resulting variations in the surface heat,
momentum, and freshwater fluxes cause changes in
SST and ocean currents. Thus, during ENSO events an
atmospheric bridge extends from the equatorial Pacificto other ocean basins including the North Pacific (e.g.,
Alexander 1990, 1992; Lau and Nath 1994, 1996, 2001;
Alexander et al. 2002). When El Niño events peakduring boreal winter, the Aleutian low deepens and thechanges in the surface heat fluxes, wind-driven mixing,
and Ekman transport in the upper ocean all act tocreate a positive PDO SSTA pattern (Alexander et al.
2002; Alexander and Scott 2008; see also Strong and
Magnusdottir 2009).
The atmospheric bridge is seen when correlating awintertime ‘‘ENSO index’’ (here, defined as the timeseries of the leading tropical SSTA EOF determinedwithin the 188S–188N domain) with subsequent global
springtime SSTAs (Fig. 3c). This correlation pattern isstable throughout the observational record, with someamplitude changes but relatively little modification inpattern (not shown). In general, ENSO leads the PDO
throughout the year (Fig. 3d), but the bridge acts differently in summer and fall compared to winter, modifying SSTAs in the western Pacific primarily throughchanges in cloudiness (Alexander et al. 2004).

2) OCEANIC COASTALLY TRAPPED WAVESThe equatorial thermocline variability associated with
ENSO excites Kelvin and other coastally trapped ocean
waves that propagate poleward along the eastern Pacificboundary in both hemispheres, generating substantial
sea level and SST anomalies (Enfield and Allen 1980;
Chelton and Davis 1982; Clarke and Van Gorder 1994).
However, these waves impact the ocean only within
about 50 km of shore poleward of 158N (Gill 1982).

3) TROPICAL DECADAL VARIABILITY
Tropical Pacific decadal variations will also be communicated to the North Pacific via the atmosphericbridge, driving about 1/ 4–1/ 2 of the PDO-related variability
based on GCM model experiments (Alexander et al.
2002; Alexander 2010). Zhang et al. (1997) employed
several techniques to separate observed interannual andinterdecadal (.6 yr) ENSO variability. The SSTA pattern based on their low-pass-filtered data is similar to theunfiltered ENSO pattern, except meridionally broader inthe eastern equatorial Pacific and with higher amplitudes
both in the central equatorial Pacific and in the extratropics; its North Pacific component resembles the PDO.
Other statistical methods have found similar structuresand have indicated that some PDO decadal variability is
associated with low-frequency anomalies in the tropical
Pacific (e.g., Nakamura et al. 1997; Power et al. 1999;

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Mestas-Nuñez and Enfield 1999; Barlow et al. 2001; Seager
et al. 2004; Deser et al. 2004; Vimont 2005; Alexander et al.
2008; Chen and Wallace 2015).
How much tropical Pacific decadal variability is due tocoupled processes operating on fundamentally decadal
time scales, including possible linkages to the North
Pacific [see section 3c(4)], and how much is the residualof weather noise-driven ENSO dynamics (e.g., Newman
et al. 2011; Wittenberg et al. 2014) remains to be determined. Because the decadal ENSO pattern is similar
to the most persistent portions of the seasonally evolving
ENSO pattern, it may be partly the residual of a nonuniformly evolving interannual phenomenon (Vimont
2005) and/or the asymmetry between warm and cold
phases of ENSO (Rodgers et al. 2004).

c. Midlatitude ocean dynamics and coupledvariability
1) REEMERGENCE
We might expect that, because of the thermal capacityof seawater and typical mixed layer depths, the memory
time scale of the upper ocean is on the order of months.
However, because of seasonal variations in the oceanic
mixed layer depth, the decorrelation time scale of midlatitude SSTAs in successive cold seasons is generally
greater than a year. Figures 4a–c illustrate this process,showing the correlation of February–April (FMA) PDO
values with ECMWF Ocean Reanalysis System, version 4(ORAS4; Balmaseda et al. 2013), ocean temperatures at
depth and increasing lag for three regions of the North
Pacific. Similar results are obtained using the SODA
dataset (Carton and Giese 2008; not shown) and inmodeling studies (e.g., Alexander et al. 2002; H. Wanget al. 2012). Persistent temperature anomalies forming at
the surface mix downward throughout the deep wintermixed layer. Then, when the mixed layer abruptly shallows in spring, thermal anomalies at depth can remain
under the summer seasonal thermocline, insulated fromsurface fluxes that damp anomalies in the thin mixed
layer above. When the mixed layer deepens again during
the following fall, the deeper anomalies are mixed backtoward the sea surface. This process, first noted by
Namias and Born (1970, 1974) and termed the ‘‘reemergence mechanism’’ by Alexander and Deser (1995),
occurs over large portions of the North Atlantic andNorth Pacific Oceans (Alexander et al. 1999, 2001; Bhattet al. 1998; Timlin et al. 2002; Hanawa and Sugimoto2004). The PDO SSTA pattern, generated by internal
atmospheric dynamics and/or the atmospheric bridge,
recurs in consecutive winters via the reemergencemechanism (Alexander et al. 1999, 2001, 2002), while thesummertime PDO signal (at the surface, at least) does not15 JUNE 2016

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correlated over three consecutive springs but is nearly
uncorrelated over three consecutive autumns.

2) OCEAN GYRE DYNAMICS

FIG. 4. Illustration of the reemergence of oceanic thermal anomalies. Correlation of the February–April (FMA) value of the PDO index(as in Fig. 1b, but determined from 3-month running means) with
ECMWF ORAS4 ocean temperatures (Balmaseda et al. 2013) for thesubsequent 3 yr, area averaged in (a) the Gulf of Alaska, (b) the
central Pacific, and (c) the western Pacific, for the years 1958–2014,
with the 57-yr linear trend removed from each area average. The grayline shows the climatological mean mixed layer depth as a function oftime of year at each location, so it repeats over the 3-yr period.

tend to recur (Nakamura and Yamagata 1999) and isinstead largely forced by contemporaneous air–sea fluxes.
The effects of reemergence can extend into a second year,
as evidenced in Fig. 4 by the cold season maxima during
both years 1 and 2.
Thus, the reemergence mechanism determines theeffective thermal inertia and the value of r in (1) as corresponding to deep winter mixed layers (Deser et al.
2003), enhancing PDO variability on interannual to decadal time scales (Newman et al. 2003; Schneider andCornuelle 2005). The impact of reemergence is evident in
the observed PDO lag autocorrelation structure (Fig. 1c).
For example, while the year-to-year PDO autocorrelation
is over 0.45 in late winter and spring, it drops below 0.3 inlate summer and autumn. Also, the PDO is significantly

Much of the large-scale dynamics within the North
Pacific Ocean involves two basin-wide circulations ofwater, a counterclockwise subpolar gyre and a clockwise
subtropical gyre, separated by a sharp meridional SST
gradient called the subarctic frontal zone (SAFZ). In thewestern Pacific, large SST variations associated with the
PDO occur within the SAFZ, specifically in the Kuroshio Extension (KE) and Oyashio frontal zones and themixed water region in between (Nakamura et al. 1997;
Nakamura and Kazmin 2003; Nonaka et al. 2006; see,e.g., Fig. 5 in Frankignoul et al. 2011). While in most of
the North Pacific, SST variability is driven primarily byatmospheric forcing (Smirnov et al. 2014), in this region
persistent warm (cool) SSTAs in winter tend to enhance(reduce) heat and moisture fluxes into the atmosphere(Tanimoto et al. 2003; Taguchi et al. 2012), a consequence of the dynamic adjustment of upper-ocean gyre
circulations that contributes to F in (1). The adjustment
occurs primarily through westward-propagating Rossby
waves excited by anomalous wind stress curl (Miller
et al. 1998; Deser et al. 1999; Seager et al. 2001; Schneider
et al. 2002; Qiu and Chen 2005; Taguchi et al. 2007).
These waves, whose sea surface height variations can bemeasured by satellite (Fig. 5b), take approximately 3–
10 yr to cross the basin guided by the KE jet (Sasaki andSchneider 2011; Sasaki et al. 2013), producing primarily
decadal variability in F with a red spectra without preferred spectral peaks (Frankignoul et al. 1997). Specifically, SSTAs result (Fig. 5a) from shifts in latitude andintensity of the fronts (Qiu 2003; Nakamura and Kazmin
2003; Taguchi et al. 2007), and modulations of the stability of the KE jet, the recirculation gyre (Qiu 2000, 2002;
Qiu and Chen 2005, 2010; Kelly et al. 2010; Kwon et al.
2010a), and thereby oceanic eddy-driven heat transports(Sugimoto and Hanawa 2011). The latitude of the shallow
Oyashio front, by contrast, may be more sensitive to local
wind forcing (Nonaka et al. 2006, 2008). Decadal SSTAs
in the KE and Oyashio regions have undergone longterm amplitude modulations, suggesting that this contribution from extratropical air–sea interaction to the PDO
may vary on multidecadal scales (Miyasaka et al. 2014).
An atmospheric response to PDO SST variations
could enhance PDO decadal variance, or lead to a preferred time scale, if the response projects onto F in (1)as a positive or delayed negative, respectively, feedbackon the PDO. A basic thermodynamic response exists:
the temperature difference between the atmosphericboundary layer and oceanic mixed layer decreases because of air–sea heat exchange, slowing subsequent heat

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FIG. 5. Illustration of the slow ocean (Rossby wave) dynamics
process driving PDO variability. (a) Time series of the SSTA in themixed-water region (MWR, solid) and the PDO index (with sign
inverted, dashed). The temperature index is based on the optimal
interpolation, blended, 1/ 48 SST analysis of Reynolds et al. (2007).
The MWR extends from the coast of Japan to 1508E, and between
368 and 428N. Both the MWR and PDO indices have been normalized by their respective standard deviations. The correlation
between MWR and PDO indices is 20.49. (b) Satellite-observed
sea surface height anomalies (cm), averaged between 338 and 358N.
The dotted line marks a westward phase speed of 3.7 cm s21 (Qiu
and Chen 2010). Sea surface temperature and sea surface heightanomalies have been detrended and smoothed with a 2-yr runningmean, with weights varying linearly as a function of lag.

exchanges that depend upon this difference, producing‘‘reduced thermal damping’’ (Barsugli and Battisti 1998)
and a related increase in temperature variability (Bladé1997). Stochastic wind forcing whose spatial scale corresponds to certain ocean advective scales might also resonantly enhance decadal SST variability without otherfeedbacks (Saravanan and McWilliams 1998).
The coupled ocean–atmosphere dynamical response to
PDO SSTAs has been less clear. Latif and Barnett (1994,
1996) suggested that western Pacific SST anomalies resulting from Aleutian low–forced Rossby wave adjustment of the subtropical gyre (described above) drive an
atmospheric response that, via the wind stress curl, reverses the sign of the subtropical gyre anomaly and thecorresponding central North Pacific SST anomaly. This
negative feedback in the western Pacific initiates the
opposite phase of a 20-yr PDO oscillation. However, thenecessary western Pacific atmospheric response appears
to be of the wrong sign to generate sustained oscillations(Schneider et al. 2002). In particular, when the Aleutian
low strengthens, it also shifts southward; as a result,

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the gyre circulation shifts equatorward, and the SST
anomalies subsequently cool rather than warm in the KE
region (Deser et al. 1999; Miller and Schneider 2000;
Seager et al. 2001). In addition, surface heat fluxes damp
SST anomalies in the KE region both in observations
and ocean model hindcasts (Seager et al. 2001;
Schneider et al. 2002; Tanimoto et al. 2003). Note alsothat the length of the Latif and Barnett model run was
relatively short; their results were not reproduced in alonger run of the model (Schneider et al. 2002), whereas
large model ensembles are now used to discern SSTAforced signals from undersampled atmospheric noise(e.g., Sardeshmukh et al. 2000; Deser et al. 2004).
Still, recent observational studies (Frankignoul et al.
2011; Taguchi et al. 2012), coupled model experiments(Wu et al. 2003; Kwon and Deser 2007; Taguchi et al.
2012), and observationally derived heuristic models(Qiu et al. 2007) suggest that the atmospheric response
to SSTAs in the KE and Oyashio frontal zones could
induce a modest atmospheric response in the anomalous
Aleutian low, which may be able to enhance the variability at decadal periods. Those SST frontal zones may
act to anchor the Pacific storm track (Nakamura et al.
2004), and its feedback forcing from synoptic-scaleeddies migrating along the storm track seems importantin maintaining a stationary atmospheric response (Taguchi
et al. 2012; Okajima et al. 2014). However, AGCM studies
continue to disagree on even the sign of the atmospheric
response to frontal zone SSTAs, with sensitivity to manyfactors including the simulated direct response to the lowlevel heating, downstream eddy feedbacks of the modulated storm track, and dependencies upon seasonality andbase state (Okajima et al. 2014; see also the review byKushnir et al. 2002). Also, Frankignoul et al. (2011) andTaguchi et al. (2012) observed an anomalous surface
Aleutian low in response to KE–Oyashio front SSTAs
that, however, was accompanied by different upper-levelanomalies. Some recent observational analyses and modelexperiments at finer resolutions (typically ;25 km) suggestthat a robust atmospheric response to KE–Oyashio front
SSTAs may involve significant changes in poleward heatand moisture transports by individual storms (O’Reilly
and Czaja 2015) that are not well captured by currently
typical climate model atmospheric resolution (;18;Smirnov et al. 2015), so the impact of ocean–atmosphere
coupling onto the PDO remains poorly understood.

3) SUMMERTIME AIR–SEA FEEDBACKS
During summer, because of both a shallower seasonal
thermocline and weaker atmospheric variability, thePDO tends to be less persistent and less confined to theoceanic frontal zones (Nakamura and Kazmin 2003),
although the spatial structure of the PDO is not15 JUNE 2016

NEWMAN ET AL.

fundamentally changed (Chen and Wallace 2015).
Instead, SST variability may be linked to anomalous
low-level cloudiness. Over the northeast (NE) Pacific,
for example, where summertime midtroposphericsubsidence is maintained by the climatological subtropical high (Miyasaka and Nakamura 2005), most of
the clouds develop under the capped inversion layer(Klein et al. 1995; Norris 1998; Wood 2012). Decadalenhancement of lower-tropospheric static stability,
due both to cooler SSTAs and stronger subsidence
associated with the intensified subtropical high, acts
to increase the low-level cloud amount, optical depth,
and planetary albedo, and vice versa (cf. Clement
et al. 2009). The resulting anomalous cloud radiative
forcing could enhance the underlying SSTA as positive
feedback. Over the northwest (NW) Pacific, low-levelcloudiness is climatologically high in summer, but is lessvariable than over the NE Pacific (Norris et al. 1998),
which is particularly the case over the subpolar oceanic
gyre. Still, decadal variability in low-level cloudinesstends to maximize around 358N to the south of the North
Pacific (NP) SAFZ. Schwartz et al. (2014) show that thesummertime coastal low clouds all along the West Coastvary in concert with PDO-related SST variations.

4) NORTH PACIFIC IMPACTS ON TROPICALVARIABILITY

While the atmospheric bridge primarily extends fromthe tropics to the extratropics, North Pacific SSTA variability may also influence the tropical Pacific. In the‘‘seasonal footprinting mechanism’’ (Vimont et al. 2001,
2003), wintertime weather noise alters surface heat fluxes
to create SST anomalies that in the subtropics persist
through the summer. These SST anomalies, which arelargely orthogonal to the PDO since they instead reflect
the second EOF of North Pacific SSTAs (Bond et al.
2003), associated with the North Pacific Gyre Oscillation(NPGO; Di Lorenzo et al. 2008), drive near-equatorial
zonal wind stress anomalies impacting ENSO variability(Vimont et al. 2003, 2009; Alexander et al. 2008, 2010; Di
Lorenzo et al. 2010; S.-Y. Wang et al. 2012). On longer
time scales, in some models the atmospheric response to
slowly varying KE SST anomalies extends deep into thetropics to affect decadal ENSO variability (Barnett et al.
1999; Kwon et al. 2010b).
A coupled extratropical–tropical mechanism for decadal variability proposed by Gu and Philander (1997)
involves subduction of North Pacific mixed layer temperature anomalies from the surface layer into thethermocline and their subsequent southward advection
by the subtropical cell (STC) to upwell at the equator.
However, observational analyses showed that subducted central North Pacific anomalies decay prior to4407

reaching the tropics (Deser et al. 1996; Schneider et al.
1999; Capotondi et al. 2003; Sasaki et al. 2010; Y. Li et al.
2012). Alternatively, variations in subtropical winds(which may also be noise driven) alter the strength of
the STC overturning circulation, changing its southward advection of relatively cold extratropical water
that, upwelling at the equator, drives tropical air–sea
feedbacks and hence decadal variability. This mechanism, evident in some models (Kleeman et al. 1999),
although possibly confined to within about 208 of theequator (Capotondi et al. 2005), is supported by observational analyses (McPhaden and Zhang 2002;
Zhang and McPhaden 2006). Note that even if thismechanism does not extend into the North Pacific it
still impacts the PDO by modifying its tropicalforcing.

4. The PDO as the sum of multiple processes
In this section we employ two approaches, empiricaland numerical modeling studies, to explore how PDO
processes combine to produce PDO variability and dynamics and, potentially, PDO regime shifts.

a. Empirical autoregressive models
Several recent studies extended the simple AR1model in (1) to incorporate additional PDO dynamical
processes discussed above. Using annually averaged(July–June) observed SSTAs, Newman et al. (2003)
found that (1) could be improved by including ENSO
forcing within F, extending the AR1 model to
PDO(n) 5 rPDO(n 2 1) 1 aENSO(n) 1 noise.

(2)

Note that r, which corresponded to a time scale ofabout 2 yr, included reemergence, amplifying the
low-frequency ENSO component of PDO variability
to produce a ‘‘reddened ENSO.’’ Schneider andCornuelle (2005) included both shifts in the North
Pacific Ocean gyres and explicit representation of
anomalous Aleutian low forcing in F to show that oninterannual time scales, random Aleutian low fluctuations and ENSO teleconnections were about equallyimportant in determining PDO variability with negligible contributions from ocean currents, while on decadal time scales, stochastic forcing, ENSO, andchanges in the gyre circulations each contributed approximately one-third of the PDO variance. The primary implication of these analyses is that, unlike
ENSO, the PDO is likely not a single phenomenon but
rather the sum of several different basin-scale processes (e.g., Schneider and Cornuelle 2005; Newman
2007, 2013; Alexander et al. 2008).

4408

JOURNAL OF CLIMATEAs (1) is extended into (2) by including observed,
nonstochastic forcing in F, it is no longer strictly
speaking an AR1 model, nor is it a closed model. Also,
while the patterns corresponding to these processes are
not identical and their characteristic time scales arequite different, they all project strongly onto the PDO
pattern. However, many common multivariate analysis
techniques cannot distinguish between spatially similar,
or nonorthogonal, patterns with differing patterns ofevolution.
These problems can be addressed by extending theAR1 model to many variables:
x(n) 5 Gx(n 2 1) 1 hs ,(3)

where x is now a multivariate state vector and hs represents noise. In the following, x represents maps of observed SSTAs covering the tropical (188S–188N) andNorth Pacific (208–708N) Oceans. The resulting multivariate AR1 model [linear inverse model (LIM); Penlandand Sardeshmukh 1995; Newman 2007; Alexander et al.
2008] yields patterns representing different dynamical
processes with different evolutions, which are independent but not orthogonal; that is, they have potentially similar spatial structures. In analogy with (1), eachpattern (each eigenmode of G) is associated with a timeseries that has its own value of r (the real part of its eigenvalue), but not all the patterns are static (some also
propagate with characteristic frequency given a nonzero
imaginary eigenvalue). Here, we extend the Newman(2007) LIM to finer spatial (28 3 28) and temporal (3month running mean) resolution (details of the approach,including strengths and weaknesses, can be found there
and the many papers cited therein). Similar to that study
and related LIM analyses (Compo and Sardeshmukh
2010; Newman 2013), the leading eigenmode’s pattern(not shown) is the departure of the local SST trend fromthe global mean SST trend and makes almost no contribution to the PDO. Results below are also little changed
using a linearly detrended dataset (Newman 2013).
Ordered by decreasing r, the three eigenmodes inFig. 6 represent dynamical processes with maxima in
the northern, central tropical–northern subtropical,
and eastern tropical Pacific, respectively; similar patterns from various analyses have been reported elsewhere (e.g., Barlow et al. 2001; Chiang and Vimont
2004; Guan and Nigam 2008; Compo and Sardeshmukh2010). The first eigenmode represents largely North
Pacific dynamics. The latter two represent interannualto-decadal tropical dynamics driving North Pacificvariability (Newman 2007), consistent with Schneider
and Cornuelle (2005); their tropical portions form asimple basis for ENSO evolution including its ‘‘flavors’’

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or diversity (e.g., Penland and Sardeshmukh 1995;
Trenberth and Stepaniak 2001; Takahashi et al. 2011;
Capotondi et al. 2015). Each eigenmode’s projection
on the PDO EOF yields time series (also in Fig. 6)that when summed result in a ‘‘reconstructed’’ PDO(Fig. 6g) that is quite similar to the full PDO (Fig. 6h),with 0.7 correlation that increases to over 0.8 whenboth are smoothed with the 6-yr low-pass filter used inFig. 1b. The residual between the two time series,
representing the contributions of other eigenmodes(not shown), is likely primarily noise since its decorrelation time scale is approximately 5 months.
The PDO appears to undergo rapid transitions between extended periods of opposite phase every few decades or so (e.g., Ebbesmeyer et al. 1991; Graham 1994;
Mantua et al. 1997; Minobe 1997; Fleming 2009; Minobe1999), as denoted by the green lines in Fig. 6h. Such‘‘regime shifts’’ (if significant; see Rudnick and Davis
2003) might represent sudden nonlinear changes between
relatively stable climate states. However, similar behavior is also well known to exist in aggregations of AR1processes (Granger 1980; Beran 1994), that is, in (3). So,
to the extent that the PDO represents an aggregation ofseveral basin-scale dynamical processes with differing,but substantial, projections onto the PDO pattern, each
PDO regime could result from different combinations ofprocesses (see also Deser et al. 2004), with apparent regime shifts due to randomly forced variations in the superposition of these processes (Newman 2007), as
captured by the reconstructed PDO in Fig. 6g. In otherwords, PDO climate regime shifts could be partly an artifact of measuring the multivariate North Pacific Ocean
climate system with a single index.
As a corollary, a PDO regime shift need not correspond to pronounced changes throughout the North
Pacific. This point is illustrated by comparing multidecadal SST change across 1976/77 relative to change
across 1969/70. The latter time is not typically identified with a Pacific regime shift (although see Bainesand Folland 2007), but it is when the time series of themost slowly varying PDO component ended a long
period of negative values (Fig. 6a). For the 1976/77
regime shift between 20-yr epochs (Fig. 7b), the welldocumented warming in the tropical Indo-Pacific andalong the west coast of North America is evident,
along with central northeastern Pacific cooling (e.g.,
Graham 1994; Miller et al. 1994b; Meehl et al. 2009).
However, North Pacific multidecadal cooling across
1969/70 (Fig. 7a) was stronger and extended farther
westward; note also a corresponding Atlantic signal.
The two figures together show that these multidecadal
shifts in tropical and North Pacific SSTs were not coincident but rather occurred over several years, and15 JUNE 2016

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4409

FIG. 6. Reconstructing the PDO as the sum of three different dynamical processes. Time series for the contributions to the PDO from the (a) second (North Pacific), (c) third (central Pacific ENSO), and (e) fourth [eastern
Pacific ENSO; showing the most energetic phase of this complex eigenmode (essentially, cosine phase), with theleast energetic phase (sine phase) not shown] eigenmodes and (b),(d),(f) the corresponding maps of the LIM
described in the text. Note that unlike EOFs, these eigenmodes are nonorthogonal. Contour intervals are the samein all three eigenmode maps; all eigenmodes are normalized to have unit amplitude. For all time series, positive(negative) values are drawn in red (blue). The LIM is determined in a reduced EOF space (with 25 degrees of
freedom) that retains about 85% of the SST variance in the tropics and North Pacific domains. (g) PDO reconstruction is the sum of the time series shown in (a),(c),(e). (h) PDO index time series (as in Fig. 1c, but with a 3month running mean smoothing applied). In the time series panels, thick black lines represent the application of thesame 6-yr low-pass smoother as in Fig. 1b, and vertical green lines indicate times of PDO regime shifts.

may not have corresponded to a coherent basin-wide
Pacific climate regime shift.

b. PDO representation by coupled climate models
Perhaps our most comprehensive tool for understanding how processes interact to produce the PDO isthe coupled general circulation model (CGCM). Here,

we assess a CGCM reproduction of the PDO and PDO
processes.
Figure 8 exemplifies the range of PDO patterns(defined according to Mantua et al. 1997) across thehistorical model runs from phase 5 of the Coupled
Model Intercomparison Project (CMIP5), in comparison with the observed pattern (Fig. 1a). All CMIP5

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FIG. 7. Epoch difference maps, showing SST differences between two adjacent 20-yr means centered on (a) 1968/69 and (b) 1976/77. Contour
interval is 0.18C. The adjacent 20-yr periods used for each epoch calculation are indicated by the corresponding color bars in Fig. 6h.

PDO patterns are available online (http://www2.cesm.
ucar.edu/working-groups/cvcwg/cvdp) (Phillips et al.
2014). Some models can depict the observed spatial
pattern (model A) while others lack the tropical connection (model B). Also shown are two PDO patterns,
chosen to be closest (A) and farthest (B) from the observations, generated from two ensemble members
taken from the same CGCM, the Community Earth
System Model–Large Ensemble (CESM-LE; Kay et al.
2015). Here, the differences are entirely due to internally generated variability, since all other aspects ofthe experiment (including the model and external
forcing) are fixed. We next use the Taylor diagram in
Fig. 2 to quantify the degree of resemblance of the PDO
pattern in each CMIP3, CMIP5, and CESM-LE model
simulation with the observations (HadISST). Recall
that the distance of each symbol from the reference(HadISST PDO) represents the RMS difference of eachmodel PDO pattern. The North Pacific SSTA pattern ofeach model ensemble member is broadly similar to theobservations, and is accompanied by a realistic SLP
anomaly that is consistent with the atmosphere forcing
the ocean and not vice versa (see also Sheffield et al.
2013; Yim et al. 2014), but Fig. 2 shows that the simulations are generally outside the range of the observational dataset and sampling uncertainty. Comparing theCMIP5 and CESM-LE values in Fig. 2 suggests that
PDO simulation uncertainty may be due more to differences between models than to differences between
realizations. Some modest improvement of the PDO
simulation from CMIP3 to CMIP5 is apparent (see also
Polade et al. 2013), primarily through the reduction ofoutliers.
As a result of the chaotic nature of the climate system,
even very small differences in initial conditions cause

simulations to diverge from one another and from nature.
Hence, in the absence of an externally forced signal, we
expect CGCMs to represent PDO variability statistics but
not the observed PDO time series. Figure 8c shows thisexpected range in CMIP5 historical PDO time series(smoothed with the 6-yr low-pass filter); in particular,
none reproduce the observed twentieth-century sequenceof PDO regimes. Additionally, the ensemble mean of all
the time series is near zero. This is also the case for the
CESM-LE ensemble mean, although the CMIP3 ensemble mean has a weak trend. Given that these model runs
are forced with the post-1850 history of radiative forcing,Fig. 8c suggests that the externally forced PDO signal has
been negligible, consistent with the empirical analysis in
section 4a, and that the PDO represents natural internalvariability.
We show the lag autocorrelation of the PDO (Fig. 9a),as well as the lagged correlations between the PDO andthe December–February (DJF) Niño-3.4 index (Fig. 9b)
or DJF PNA index (Fig. 9c), for each model ensemble
member over its respective period, compared to observations from 1901 to 2009. Most, but not all, simulated
PDO time series are too persistent. In all the models
during the winter, the PDO is too weakly correlated withboth the PNA and Niño-3.4, yet many of the models thenoverestimate the lagged correlation between these same
indices and the PDO the following spring and summer,
consistent with too strong PDO persistence or with errors in the North Pacific seasonal cycle.
Previous analysis showed that almost all CMIP3 models
underestimated the ENSO–PDO relationship comparedto the observations (Newman 2007; Oshima and Tanimoto
2009; Furtado et al. 2011; Lienert et al. 2011; Deser et al.
2012; Park et al. 2013). To investigate how the representation of tropical–extratropical interaction impacts the15 JUNE 2016

NEWMAN ET AL.

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FIG. 8. The PDO over the historical record as simulated by coupled CGCMs. (a),(b) As in Fig. 1a, but showing two
selected members of the historical CMIP5 ensemble that are (a) closest and (b) farthest from the reference pattern in Fig. 2.
(c),(d) As in (a),(b), but showing two selected members of the CESM-LE that are (c) closest and (d) farthest from thereference pattern in Fig. 2. (e) PDO time series from all ensemble members; all time series are smoothed with the Zhang
et al. (1997) filter (used in Fig. 1c). Thin gray lines represent each ensemble member, the thin black solid (dashed) line in
the CMIP5 panel represents model A (B), and the thick black line is the ensemble mean for each set of models.

PDO in the CMIP5 historical runs, we fit each PDO timeseries with the extended AR1 model:
PDO(n) 5 rPDO(n 2 1) 1 aENSO1(n)1 bENSO2(n) 1 «(n) ,(4)where PDO is the PDO time series, ENSO1 and ENSO2
are the time coefficients of the leading two EOFs oftropical Pacific (208S–208N) SSTAs, « is white noise, andn is the time step. This model, estimated for detrended
and normalized annual mean time series averaged from

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FIG. 9. Temporal relationships relevant to the PDO for the (top) CMIP3, (middle) CMIP5, and (bottom) CESM-LE (LENS) ensembles.
Shown are (a),(d),(g) the autocorrelation of the monthly PDO index; (b),(e),(h) the lagged seasonal correlation between the seasonal PDO
index and the DJF averaged Niño-3.4 index; and (c),(f),(i) the lagged seasonal correlation between the seasonal PDO index and the DJFaveraged PNA index. In all panels the thin gray lines indicate model correlations, the thick solid black line indicates correlations for indicesfrom the HadISST data, and the thick dashed line indicates correlations with indices from the ERSST.v3b data. In (c),(f),(i) the observed
DJF PNA time series is obtained from the twentieth-century reanalysis. Observed correlations are taken over the time period 1901–2009,
CMIP3 over 1900–99, CMIP5 over 1901–2004, and LENS over 1920–2005. For seasonal correlations, positive lags indicate that the Niño-3.4
or PNA index leads the seasonal PDO index, and the label along the abscissa indicates the season for which the PDO is defined.

July to June, follows the approach introduced in (2)but includes potential diversity in ENSO anomalies(Capotondi et al. 2015). ENSO1 and ENSO2 are determined separately for each model and dataset andthen annually averaged.

The extended AR1 models constructed from the observations and the CMIP5 runs have several notable differences. First, tropical forcing of the PDO is dominatedby ENSO1 in the observations, but not for most CMIP5
runs (Fig. 10b). Also, how well each extended AR1 model

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FIG. 10. Parameters for an AR1 model of the PDO time series [(4)] for CMIP5 models (bluebars) and observations (red bars). The AR1 model is determined from the PDO index and twoleading tropical PCs, ENSO1 and ENSO2, calculated as discussed in the text but for the period
1900–2000, and then averaged from July to June. (a) Unforced lag-1 autocorrelation, that is, r in(4). (b) Forcing coefficient for ENSO1, that is, a in (4). (c) Forcing coefficient for ENSO2, thatis, b in (4). (d) Correlation r between each model’s PDO index time series and the corresponding estimated PDO time series determined from the AR1 model.

represents each PDO time series is related to ENSO1
forcing strength: in Fig. 10, AR1 model performance r, thecorrelation of each PDO time series with its corresponding
estimate (4), and a are correlated at 0.7. On the other hand,
CMIP5 ENSO2 forcing is generally greater than for theobservations (Fig. 10c). These results indicate that thetropics–PDO linkage in the observations is different, bothquantitatively and qualitatively, from most CMIP5 models,
which may also be due to ENSO simulation errors (e.g.,
Bellenger et al. 2014; Capotondi et al. 2015). Also, note thatmost CMIP5 models overestimate r and underestimate
the noise forcing, suggesting that mechanisms causing thepersistence of the PDO such as oceanic vertical mixing,
SST reemergence, oceanic Rossby wave propagation, and/or ocean–atmosphere feedback in the midlatitudes play astronger role in CMIP5 models than in the observations.
Why these errors exist remains to be understood, but together they suggest that most CGCM PDOs may be more
independent of the tropics than is observed.

5. Decadal-to-centennial PDO variability
As with simpler AR1 models, the LIM may be used togenerate confidence intervals for power spectra, although

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unlike simpler AR1 models, LIM-generated variability
can include broadband spectral peaks (such as ENSO;
Newman et al. 2009; Ault et al. 2013). Figure 11 showsthat PDO spectra calculated from all four observational
datasets (black lines) and, for the most part, the CMIP5
simulations (both control and historical) all generally lie
within the LIM confidence intervals, quantified using 140
different 1750-yr realizations of (3) (e.g., Ault et al. 2013).
So too do the 1000-yr, externally forced last millennium
CMIP5 simulations that include estimates of explosive
volcanism, solar variability, and anthropogenic changes
to atmospheric composition and land use since 850, again
suggesting that the PDO largely represents internal, unforced variability, even on decadal-to-centennial (or dec–cen) time scales. However, given that the CMIP5 PDO
connections to the tropics are generally too weak and that
unforced CMIP5 models generally underestimate dec–
cen tropical SST variability (Ault et al. 2013; Laepple andHuybers 2014), the agreement between the CMIP5
power spectra and the LIM may be somewhat fortuitous.
That is, some ‘‘reddened ENSO’’ power in the observations is missing in the CMIP5 models, but its loss seems to
be compensated for by too-persistent internally generated North Pacific SSTAs.
Even relative to a univariate AR1 (red noise) process,no statistically significant decadal or multidecadal peaksin these spectra are detectable (Pierce 2001). Rather, the
PDO spectral power appears to continually increase
with decreasing frequency f, raising the possibility thatthe spectra represent not red noise but rather longmemory, 1/f noise (Keshner 1982), with potentiallypronounced regime behavior (e.g., Percival et al. 2001;
Overland et al. 2006; Fleming 2014; see also Fraedrichet al. 2004). However, note that the LIM spectrum also is
not flat over multidecadal time scales. In fact, spectral
slopes are characteristic not only of some nonlinear
systems but also aggregations of processes resulting
from a multivariate AR1 system driven by noise (Milotti
1995; see also Penland and Sardeshmukh 2012).
Spectra for several paleoreconstructions of the PDO,
made by combining proxies such as tree rings, corals,
and sediments (Table 1), stay below the upper LIM
confidence interval for decadal-to-multidecadal periods.
Many of these reconstructions have been used to suggest
the existence of PDO multidecadal spectral peaks,
particularly for periods of about 20 and 70 yr (e.g.,
Biondi et al. 2001), although note that none of the observational datasets shows a significant peak on thesetime scales. However, poor reproducibility between thevarious PDO reconstructions (Fig. 11d) calls into question their collective fidelity as paleo-PDO indices, evenfor the multidecadal time scales that multicentury record lengths should be able to resolve (Fig. 11e). Beyond

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profound differences in the various proxy networks used
to reconstruct the PDO, the reconstructions also reflect
differences in local climate variable responses to largescale atmospheric and oceanic variability, potential
seasonal biases, and geographic domain (e.g., Tingley
et al. 2012; St. George 2014). As most PDO reconstructions are based on tree-ring-width time series,
the statistical removal of biological growth-related
trends in these series (Cook et al. 1995; Jones et al.
2009 and references therein) may deflate the centennialscale variability, removing spectral power at the lowest
frequencies in Fig. 11c.
Because the PDO represents not one but many dynamical processes, it poses a unique challenge as a target
for proxy-based reconstruction. Thus far, reconstructions
have largely sought to reproduce and extend the PDO
index itself. A more nuanced approach that targets one ormore of the different processes that contribute to PDOrelated variability would likely lead to an understanding
about how such processes have varied in the past, andtheir potential interrelationships. In pursuing such an
approach, it is important to remember the different
spectral biases in different proxy types; for example, someproxies could be strongly correlated with PDO-related
variability simply because they are ‘‘climate integrators,’’
responding to ENSO-related forcing in the same way that
the North Pacific Ocean does (Newman et al. 2003; seealso Pederson et al. 2011). As it stands, the current disagreement among the existing PDO reconstructions is
problematic for any assessment of PDO variability and
PDO-related impacts over the last several hundred years(Kipfmueller et al. 2012).

6. Use of the PDO in climate diagnosis andprediction
In this section, we address both forecasting the PDO
and forecasting with the PDO, focusing on the need to
distinguish between PDO-correlated and PDO-predictableclimate impacts.

a. Retrospective analysis of PDO impacts
PDO dependence on other climate processes implies
that correlations with the PDO will be related to correlations with other climate indices. For example, Fig. 12shows the correlation of revised U.S. climate division(nClimDiv; Vose et al. 2014) cold-season precipitation(left panels) and temperature (right panels) anomalies
with the PDO, ENSO, and NPI indices. While there aresome interesting differences, the precipitation maps are
all quite similar, with ENSO having the most pronounced
signal. For cold-season temperatures (Figs. 12d–f), there
are notable differences between the PDO and ENSO

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FIG. 11. Comparison of observed, paleoclimate, and CMIP5 PDO spectra: (a) CMIP5 historical simulations (190
runs total) and forced last millennium (past 1000 yr) simulations (6 runs), (b) unforced control simulations (48 runs
total), and (c) paleoclimate (tree ring and other proxy based) reconstructions of the PDO. In (a)–(c), the thick blackline represents the HadISST PDO spectrum, and the three thin blacks lines show the other three observational
PDO spectra. In each case, only winter [November–March (NDJFM)] averages are used for consistency between
data types. All PDO reconstruction indices were normalized to unit variance over 1901–2000; all other indices werenormalized to the unit variance overall, not just the reference period. The gray shading and black lines show theupper and lower 95% confidence limits of the PDO power spectrum derived from 140 realizations of a LIM simulation [see (3)] each lasting 1750 yr. (d) Time series of each PDO reconstruction and the relative similarity of thereconstructions through time. The colored lines show the individual reconstructions themselves (left axis), while thegray shading shows the relative similarity (right axis), measured by the shared variance of the different indicesthrough time, or the fraction of the total variance shared by all reconstructions in the correlation matrix of all timeseries over a moving 40-yr window. The ratio is computed by dividing the leading eigenvalue of the reconstruction
correlation matrix by the total number of reconstructions available through time. (e) As in (d), but smoothed witha 21-yr running Gaussian filter.

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TABLE 1. Paleoreconstructions of the PDO used in Fig. 11. Paleoclimate reconstructions were all obtained from the NOAA paleoclimatology program and are publicly available. Each of the PDO reconstructions targets slightly different aspects of the PDO, and eachfollows its own conventions for normalization, so all indices are normalized to unit variance over the period 1901–2000.
Reconstruction

Time period

Proxy used

Season targeted

Biondi et al. (2001)
D’Arrigo and Wilson (2006)
Gedalof and Smith (2001)
MacDonald and Case (2005)
D’Arrigo et al. (2001)
Felis et al. (2010)
Shen et al. (2006)

1661–1991
1565–1988
1599–1983
993–1996
1700–1979
1873–1994
1470–1998

Tree rings
Tree rings
Tree rings
Tree rings
Tree rings
Coral (porites)
Historical documents

WinterMarch–May (MAM)
MAM
Annual (January–December)
WinterNovember–February (NDJF)
Annualcorrelation patterns; however, now the PDO pattern isquite similar to, and somewhat weaker than, the temperature correlations with the NPI. Since both ENSO
and the NPI lead the PDO (Fig. 3), these results suggestthat much of the PDO correlation patterns may not
represent a direct response to the PDO. Repeating theanalysis for the warm season (May–September) yields
similar relationships between the correlation patterns,
although all are somewhat weaker (not shown). A key
concern then is to determine additional predictive information from the PDO, and not merely duplicate
teleconnections from those processes that simultaneously act to force it.
Many studies that have explored historical PDO relationships with climate, especially hydrological quantities including precipitation, snowpack, streamflow, anddrought (e.g., Gershunov and Barnett 1998; Hamlet andLettenmaier 1999; McCabe and Dettinger 1999, 2002;
Gutzler et al. 2002; Brown and Comrie 2004; McCabe
et al. 2004; Stewart et al. 2005; Hunter et al. 2006;
Kurtzman and Scanlon 2007; Yu and Zwiers 2007;
Higgins et al. 2007; Hu and Huang 2009; Zhang et al.
2010; Goodrich and Walker 2011; Mehta et al. 2012; L. Li
et al. 2012; McCabe et al. 2012; Cook et al. 2014; Oakley
and Redmond 2014; Wang et al. 2014), are often focusedon the impact of PDO phase on ENSO teleconnections.
However, sorting ENSO responses by PDO phase may
have unanticipated pitfalls beyond double counting the
ENSO signal. On average, well over half of the largescale extratropical atmospheric seasonal-mean anomaly
occurring during an ENSO event is a consequence not of
ENSO, but rather of ever-present weather and internal
slowly evolving atmospheric anomalies such as blocking(e.g., Lau 1997). This ‘‘ENSO year but non-ENSO
forced’’ atmospheric anomaly also modifies North Pacific SSTAs through changes in surface fluxes, while simultaneously contributing to the seasonal-mean climate
anomaly downstream over North America (Pierce 2002).
(Obviously, such anomalies exist in non-ENSO years
as well, cf. NPI and PDO temperature correlations in
Fig. 12.) Additionally, like snowflakes, no two ENSO

events are alike; recent research (see review byCapotondi et al. 2015) suggests that diversity among
ENSO events may also drive different and/or asymmetricteleconnections across the North Pacific into North
America (e.g., Hoerling et al. 2001; Larkin and Harrison
2005; Wu et al. 2005; Mo 2010; Yu et al. 2012). Of course,
different teleconnections could drive variations in theatmospheric bridge with consequent PDO variations(e.g., An et al. 2007; Yeh et al. 2015), including on decadal
time scales (Yeh and Kirtman 2008). ENSO diversity isevident in separate ENSO SST composites based on highand low PDO years (see Fig. 13; also cf. the North Pacific
SSTAs for the two ENSO eigenmodes in Figs. 6d and 6f),
raising the possibility that this stratification may partly
capture ENSO diversity impacts on both PDO andENSO teleconnections, rather than PDO impacts on
ENSO teleconnections.

b. PDO prediction
Interest in PDO prediction is high due to its potential
climate impacts, especially related to decadal variation
and PDO regime shifts. From the earlier discussion,
however, we might expect that predicting regime duration, which may depend upon the current amplitude ofdifferent processes with different memory time scales,
could be more skillful than predicting regime shifts, which
may be largely randomly forced.
Some extended forecast skill resulting from oceanic
Rossby wave propagation (Schneider and Miller 2001)
may occur for western Pacific SSTAs within the SAFZ.
Recent studies established the multiyear predictabilityof the Kuroshio Extension speed (Nonaka et al. 2012; cf.
Nonaka et al. 2016) and stability (Qiu et al. 2014) thatenter the PDO forcing F in (1). How much this enhances
PDO predictability (see Figs. 5a,b) remains to be determined, but it may be related to the more slowly
evolving North Pacific component in Figs. 6a,b.
Given the PDO’s relationship with ENSO, PDO
forecast skill strongly depends on ENSO forecast skill,
especially for forecast leads of up to 1–2 yr (e.g.,
Alexander et al. 2008; Wen et al. 2012). For longer

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4417

FIG. 12. Cold season relationship between climate indices discussed in this paper and U.S. precipitation andtemperature anomalies determined from U.S. climate division data (Vose et al. 2014), for the years 1901–2014.
NDJFM U.S. precipitation anomalies correlated with (a) the PDO index, (b) the ENSO index, and (c) the NPI.
NDJFM U.S. temperature anomalies correlated with (d) the PDO index, (e) the ENSO index, and (f) the NPI.

forecast leads, largely unpredictable ENSO events act
mostly as high-amplitude noise for decadal forecasts(Newman 2013; Wittenberg et al. 2014). This may help
explain why decadal PDO forecast skill in CMIP5
hindcasts, which are initialized from the observed
oceanic state, is unimpressive (Guemas et al. 2012; Kimet al. 2012; Van Oldenborgh et al. 2012; Newman 2013;
Meehl and Teng 2014), although model bias also contributes (Guemas et al. 2012; Kim et al. 2014). Similarly, predicting the atmospheric impacts associatedwith the PDO may depend upon tropical forecast skill.
In fact, current GCM forecasts initialized with a PDO
SSTA alone have little atmospheric skill (Kumar et al.
2013; Kumar and Wang 2014), although additional skill

with higher-resolution models remains possible (e.g.,Jung et al. 2012).

7. The IPO and the PDO
One particular pattern of ENSO-like decadal variability, the IPO (Power et al. 1999; Folland et al.
2002; Parker et al. 2007; Dai 2012), is often comparedto the PDO. The precise definition of the IPO varies,
but typically it has been defined as the projection ofmonthly Pacific SST data upon a pattern representing
low-pass (;decadal) global SST variability (Parker
et al. 2007; Henley et al. 2015). The resulting IPO
monthly time series (Henley et al. 2015) is essentially

4418

JOURNAL OF CLIMATEVOLUME 29

FIG. 13. NDJFM SST ENSO composites separated by high and low PDO values, determined over the years 1948–2008 from the
ERSST.v3b SST dataset. Shown are composites of the top quintile (El Niño) of the ENSO index segregated by the (top left) top and(bottom left) the bottom halves of the PDO indices for the 12 cases, and the bottom quintile (La Niña) of the ENSO index segregated bythe (top right) top and (bottom right) the bottom halves of the PDO indices for the 12 cases. Each half of the quintile is determined byranking the PDO values of the quintile years. Contour interval is 0.28C.

identical to the leading PC of the monthly Pacific basin
SSTA variability, whether or not it has been low-pass filtered (r 5 0.99). Moreover, the unfiltered IPO time series
is very highly correlated to the ENSO PC used in Fig. 3(r 5 0.96), even when both are low-pass filtered (r 5 0.89).
However, the IPO is not identical to the PDO. While
North and South Pacific centers of action in the IPO
regression pattern are roughly equivalent (e.g., Henley
et al. 2015), the PDO North Pacific center is significantly
enhanced (Fig. 1a; see also the orange circle in Fig. 2).
Conversely, the South Pacific decadal oscillation (SPDO;
e.g., Chen and Wallace 2015), the leading PC in the 208–
708S Pacific domain, has hemispheric asymmetry butwith a South Pacific maximum in its regression pattern.
The SPDO also has a reemergence (during austral spring)and stronger ENSO forcing (Shakun and Shaman 2009).
In fact, the SPDO and IPO time series are actually morecorrelated (r 5 0.82) than are the PDO and IPO timeseries (r 5 0.74), although this difference diminishes with
low-pass filtering. The PDO and SPDO unfiltered (lowpass) PCs are also correlated, but less highly than each isto the IPO and ENSO, with r 5 0.5 (0.56).
It seems reasonable to suggest, based on the above
correlations as well as the discussion in sections 2 and 3,

that the IPO represents the reddened ENSO component,
driven by both interannual and decadal ENSO variability, which is coherent between the North and South Pacific (Di Lorenzo et al. 2015). The difference between the
PDO and IPO is then due to internal North Pacific processes, primarily because of atmospheric noise forcing of
the PDO, directly through anomalous surface fluxes andindirectly via westward-propagating Rossby waves thatdrive SST variability in the SAFZ. Seasonality differences between the PDO and SPDO, in sensitivity to
ENSO forcing and in reemergence, might also play a rolein PDO–IPO differences, as would South Pacific atmospheric noise forcing. Which index should be used forclimate diagnosis or prediction may thus depend upon the
problem at hand. Note that the previous discussion relating to double counting in diagnostic studies will also
apply (perhaps even more strongly) when using both
ENSO and the IPO as predictors.

8. Concluding remarks
The PDO is now a well-established climate index,
frequently used in correlation analyses to suggest
physical linkage between a particular variable of15 JUNE 2016

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4419

FIG. 14. Summary figure of the basic processes involved in the PDO.

interest and North Pacific Ocean variability. As summarized in Fig. 14, the PDO represents not a single
phenomenon but rather a combination of processes
that span the tropics and extratropics. It is therefore
important to distinguish climate impacts correlatedwith the PDO from climate impacts that are predictableby the PDO. Within this context, since much of the
PDO represents the oceanic response to atmosphericforcing, care should be taken when using the PDO as a‘‘forcing function’’ of nonoceanic responses without aconvincing argument for the physical forcing mechanism. For example, claiming that PDO drives contemporaneous changes in rainfall over western North
America may be more simply explained by both variables (PDO and rainfall) being driven by a common
forcing function (Pierce 2002) such as diverse ENSO
events and the internal variability of the midlatitude
atmosphere. A common forcing function must therefore be considered to be the first approximation for
explaining a discovered simultaneous correlation between nonoceanic variables and PDO, including whenreconstructing PDO-related variability into the pastwith proxy records. Caution is also needed when using
the PDO together with other indices in analyses where
the PDO depends upon those indices; determining
which portion of the PDO, and/or which PDO process,is legitimately an ‘‘independent’’ predictor is an important first step. Still, it is important to note that while
the PDO is generally not an independent predictor, italso may not be assumed to be entirely dependent uponother predictors.
Ultimately, climate models may offer the best hopefor establishing links with the PDO, because the historical record of PDO has limited degrees of freedom, aconsequence of PDO representing an ‘‘integrated intime’’ response to forcing. Of course, the issues presented above still need consideration when analyzing
model output. Moreover, while a realistic balance of
PDO processes must be simulated in CGCMs, it appears
that the current generation of models underestimates
the tropical forcing of the PDO in the North Pacific
Ocean. While models with particularly weak tropical–
PDO connections could still be useful for examiningsome aspects of internal North Pacific Ocean dynamics

4420

JOURNAL OF CLIMATE(e.g., Zhong et al. 2008; Giannakis and Majda 2012;
Zhang and Delworth 2015) and possible feedback to theatmosphere (Taguchi et al. 2012), their inability tocapture realistic tropical interactions with the North
Pacific may yield problematic conclusions about the
PDO and its role in the global climate system. In fact, (4)may provide a diagnostic foundation for process-based
analysis of future CMIP6 CGCMs, since it tests not only
how well ENSO variability is captured but also how
ENSO teleconnections and North Pacific memory anddynamical processes are simulated.
This paper has focused on the PDO, but of course it isbut one element of Pacific decadal variability (e.g., Di
Lorenzo et al. 2013, 2015). Note that many of the caveats
above should also be kept in mind when consideringother modes in the North Pacific. We have only justbegun to touch upon what this picture of the PDO as asum of processes means for the diagnosis of PDOrelated regional climate, ecological, and socioeconomic impacts; the reconstruction of the PDO and its
multidecadal impacts from proxy data over the last
several hundred years; and the relationship between the
PDO and global mean temperature variations including
the recent global surface warming ‘‘hiatus.’’ These issues
will be explored in a forthcoming companion paper.
Acknowledgments. The authors wish to thank Mike
Wallace, Mike McPhaden, Sasha Gershunov, Joe Barsugli,
and an anonymous reviewer for helpful comments thatsignificantly improved this paper. MN was supported by
NOAA/CPO (CDEP) and NSF AGS CLD 1035325. MA
and JDS acknowledge support from NOAA/CPO(ESM) and NASA. CD and ASP acknowledge support
from the NOAA MAPP Program. NCAR is sponsoredby the National Science Foundation. AJM was supported by NSF OCE1026607 and OCE1419306. SM and
HN were supported in part by the Japanese Ministry of
Education, Culture, Sports Science and Technology(MEXT) through Grant-in-Aid for Scientific Research2205 in Innovative Areas. SM is also supported by JSPS
KAKENHI Grants 15H01606, 26287110, and 26610146.
HN is also supported by MEXT through Grants-in-Aid
25287120 and 26241003 and through Arctic Challenge
for Sustainability Program, and by the Japanese Ministry of Environment through Environment Research andTechnology Department Fund 2-1503. NS was supported by NSF1357015; U.S. Department of Energy,
Office of Science, DOE-DESC000511; and JAMSTECIPRC Joint Investigations.
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