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35
Journal of Oceanography, Vol. 58, pp. 35 to 44, 2002
Review
Keywords:
⋅Regime shift,
⋅climate impacts,
⋅PDO,
⋅IPO,
⋅NPO,
⋅fishery oceanogra-
phy.
* Corresponding author. E-mail: mantua@atmos.washington.
edu
Copyright © The Oceanographic Society of Japan.
The Pacific Decadal Oscillation
NATHAN J. MANTUA1* and STEVEN R. HARE2
1University of Washington, Joint Institute for the Study of the Atmosphere and Oceans,
Seattle, WA 98195-4235, U.S.A.
2International Pacific Halibut Commission, P.O. Box 95009, Seattle, WA 98195-2009, U.S.A.
(Received 19 May 2001; in revised form 16 August 2001; accepted 16 August 2001)
The Pacific Decadal Oscillation (PDO) has been described by some as a long-lived El
Niño-like pattern of Pacific climate variability, and by others as a blend of two some-
times independent modes having distinct spatial and temporal characteristics of North
Pacific sea surface temperature (SST) variability. A growing body of evidence high-
lights a strong tendency for PDO impacts in the Southern Hemisphere, with impor-
tant surface climate anomalies over the mid-latitude South Pacific Ocean, Australia
and South America. Several independent studies find evidence for just two full PDO
cycles in the past century: “cool” PDO regimes prevailed from 1890–1924 and again
from 1947–1976, while “warm” PDO regimes dominated from 1925–1946 and from
1977 through (at least) the mid-1990’s. Interdecadal changes in Pacific climate have
widespread impacts on natural systems, including water resources in the Americas
and many marine fisheries in the North Pacific. Tree-ring and Pacific coral based
climate reconstructions suggest that PDO variations—at a range of varying time
scales—can be traced back to at least 1600, although there are important differences
between different proxy reconstructions. While 20th Century PDO fluctuations were
most energetic in two general periodicities—one from 15-to-25 years, and the other
from 50-to-70 years—the mechanisms causing PDO variability remain unclear. To
date, there is little in the way of observational evidence to support a mid-latitude
coupled air-sea interaction for PDO, though there are several well-understood mecha-
nisms that promote multi-year persistence in North Pacific upper ocean temperature
anomalies.
years. This situation, which originated with a strongly
anomalous winter in 1976–1977, has been termed a “cli-
matic regime”, following a regime shift in 1977. The 1977
change in Pacific climate was first reported by Nitta and
Yamada (1989) and Trenberth (1990), who described a
step-like shift in the mean state of winter sea level pres-
sure (SLP) in the North Pacific. Miller et al. (1994) pro-
vided the first detailed depiction of the climatic changes
and dubbed the 1976/77 North Pacific event a regime shift.
Biologists noted dramatic late-1970’s changes in
much of the biota around the North Pacific. Ebbesmeyer
et al. (1991) quantified the change in 40 “environmen-
tal” (climatic and biological) variables demonstrating a
statistically significant step between 1976 and 1977 in a
composite of the time series. It was observations on Pa-
cific salmon, however, specifically the catch history of
Pacific salmon going back 70 years, that provided the most
1. Introduction
Climate records from around the Pacific Basin con-
tain evidence for strong interannual to interdecadal vari-
ability, in special cases with remarkably large-scales
(O(104 km)) of spatial coherence (NRC, 1998). El Niño/
Southern Oscillation (ENSO) has long been known to be
the prominent source for hemispheric-scale interannual
climate variations for the Pacific and the global tropics
(Rasmussen and Wallace, 1983). In the last two decades
of the 20th Century, the extratropical Pacific Ocean was
in an almost continuous El Niño-like state despite the
absence of tropical El Niño events in a majority of those
36 N. J. Mantua and S. R. Hare
tantalizing evidence that a definite link existed between
interdecadal changes in North Pacific climate and North
Pacific fisheries. In a series of papers, Francis and Hare
focused on Alaska salmon production and its link to cli-
mate (Francis and Hare, 1994; Hare and Francis, 1995;
Francis and Hare, 1997), arguing that Alaska salmon pro-
duction was best characterized as alternating regimes,
where the transition from one regime to another was
abrupt.
The race to describe and understand interdecadal
changes in the Pacific accelerated through the 1990’s.
Latif and Barnett (1996) provided a comparison of the
low-frequency variability in observations with that in the
output from a coupled ocean/atmosphere model simula-
tion, and proposed a mechanism for Pacific Decadal Vari-
ability (PDV) with a near-20 year periodicity. Zhang et
al. (1997) offered a series of analyses teasing apart sub-
tle spatial differences between Pacific climate variability
at interannual versus interdecadal time scales. Mantua et
al. (1997) capitalized on the maturity of the rapidly evolv-
ing research, synthesizing and extending research results
from fishery, climate and hydroclimate studies, and
labeled the dominant pattern of PDV the Pacific
(inter)Decadal Oscillation (PDO). Other studies have
used other names for what we call the PDO, for example:
the Interdecadal Pacific Oscillation (IPO) of Power et al.
(1997, 1999a), and the North Pacific Oscillation (NPO)
of Gershunov and Barnett (1998).
The collective body of research suggested that three
main characteristics distinguished PDO from ENSO: first,
20th century PDO “events” persisted for 20-to-30 years,
while typical ENSO events persisted for 6 to 18 months;
second, the climatic fingerprints of the PDO were most
visible in the extratropics, especially the North Pacific/
North American sector, while secondary signatures ex-
isted in the tropics, and the opposite was true for ENSO;
and third, the mechanisms causing PDO variability were
not known, while causes for ENSO variability were rela-
tively well-understood (Zhang et al., 1997; Mantua et al.,
1997; NRC, 1998).
A PDO index developed by Hare (1996) and Zhang
(1996), also used by Mantua et al. (1997), is the leading
PC from an un-rotated EOF analysis of monthly, “re-
sidual” North Pacific sea surface temperature (SST)
anomalies, poleward of 20°N for the 1900–1993 period
of record (see lower panel of Fig. 1). “Residuals” are here
defined as the difference between observed anomalies and
the monthly mean global average SST anomaly (see Zhang
et al., 1997). A remarkable characteristic of this index is
its tendency for multiyear and multidecadal persistence,
with a few instances of abrupt sign changes. Based on a
variety of studies, sign changes beginning in 1925, 1947,
and 1977 have been labeled regime shifts (Hare and
Francis, 1995; Zhang et al., 1997; Mantua et al., 1997;
Minobe, 1997). These and other studies also provided
evidence that PDO variations had considerable influence
on climate-sensitive natural resources in the Pacific and
over parts of North America in the 20th Century.
Subsequent study has revealed several new and im-
portant wrinkles to a rapidly growing literature on the
general topic of PDV and on the nature of the PDO. Ac-
cumulating evidence suggests that the PDO mode of vari-
ability exhibits a robust symmetry in interdecadal climate
variations of the Northern and Southern Hemispheres (e.g.
White and Cayan, 1998; Garreaud and Battisti, 1999;
Dettinger et al., 2000), with signature responses in East
Asia, North, South and Central America, and Australia.
Historical records tracking aspects of Pacific marine eco-
systems suggest a strong association between PDO vari-
ability and Pacific salmon production (Beamish and
Bouillon, 1993; Beamish et al., 1999; Hare et al., 1999),
Pacific sea birds (Vandenbosch, 2000), Alaska ground fish
and zooplankton production in the central and eastern
North Pacific (Hollowed et al., 1998; Francis et al., 1998),
and Gulf of Alaska marine species assemblages (Anderson
and Piatt, 1999), to name just a few. Careful reconstruc-
tions of instrumental data have extended the PDO record
back to 1854 (Kaplan et al., 2000), and paleoclimate re-
constructions now provide an extended, albeit sometimes
contradictory, view of PDV and PDO behavior back to
1600 (cf. Minobe, 1997; Evans et al., 2000; Linsley et
al., 2000; Biondi et al., 2001; Gedalof and Smith, 2001).
Research into the dynamics of PDV has also pro-
duced numerous publications, yet at this time mechanisms
for PDO behavior remain mysterious (see Miller and
Schneider (2000) for a comprehensive review). In spite
of the remaining mysteries, a number of insights into
mechanisms favoring multi-year persistence of North
Pacific climate anomalies have recently come to light
(Schneider and Miller, 2001; Seager et al., 2001; Deser
(Clara Deser, NCAR, Boulder Colorado, personal com-
munication); and Barsugli and Battisti, 1998), indicating
promising prospects for PDV predictability at lead times
of one to a few years.
Mantua et al. (1997) proposed that the PDO repre-
sents a special class of PDV defined by a preferred spa-
tial pattern with a range of interdecadal time scales of
variability. We argue here that the case for a robust PDO
mode of PDV is, on balance, strengthened by the results
of recent studies, although many critical questions about
the PDO await answers. Whether there is a preferred PDO
time scale is critical for several reasons, including the
issue of mechanisms and how understanding those mecha-
nisms should aid the development of a PDO monitoring
and prediction system. Regardless of PDO predictability,
we also believe that recognition of PDO variability is
important because it clearly demonstrates that “normal”
climate conditions can vary over time periods compara-
The Pacific Decadal Oscillation 37
ble to the length of a human’s lifetime, and climate anoma-
lies that persist for one to a few decades can cause espe-
cially large impacts on ecosystems and societies.
For brevity, we will provide only a select review of
PDO research in the remainder of this article, and in do-
ing so will omit many valuable research results. We apolo-
gize here for those omissions, but hope that our survey
offers readers a solid foundation for the present state of
PDO research.
2. PDO Characteristics
2.1 Spatial patterns
Typical sea surface temperature, surface wind, and
sea level pressure anomaly patterns for warm phases of
the PDO are shown in the top panels of Fig. 1. During
warm PDO phases sea surface temperatures (SSTs) tend
to be anomalously cool in the central North Pacific coin-
cident with anomalously warm SSTs along the west coast
of the Americas. For November-to-March averages, warm
PDO sea level pressure (SLP) anomalies have low pres-
sures over the North Pacific which cause enhanced
counterclockwise winds, and high SLP over the northern
subtropical Pacific which cause enhanced clockwise
winds. Anomalously high SLP in the western tropical
Pacific and low SLP in the eastern tropical Pacific depict
a relatively weak negative phase of the Southern Oscilla-
tion (see Trenberth and Shea, 1987). PDO circulation
anomalies in the Northern Hemisphere extend through the
depth of the troposphere, and are well-expressed as per-
sistence in the Pacific North America (PNA)
teleconnection pattern described by Wallace and Gutzler
(1981) (not shown). Because all these patterns were de-
rived from linear analyses, climate anomalies associated
with cool phases of the PDO are simply opposites of those
for warm PDO phases (not shown).
Although the PDO mode of variability has been dis-
cussed widely in the literature, the more general quest
for understanding PDV is an area of very active research.
One important lesson is clear from the published litera-
ture: different analyses yield different descriptions of 20th
Century PDV. Some studies find evidence for distinct and
independent lobes of North Pacific SST variability em-
bedded within the canonical PDO pattern shown in Fig.
1. Nakamura et al. (1997) examined low-pass filtered
North Pacific SST data for the 1968–1992 period of record
and identified two independent centers of action, one en-
compassing the subtropical front north of Hawaii and the
other encompassing the subarctic front that defines the
Kuroshio/Oyashio Extension. Barlow et al. (2001)
analyzed Pacific SSTs for the 1945–1993 period of record
and identified a different pair of North Pacific SST modes,
each spatially correlated with the canonical PDO pattern
of Mantua et al. (1997). In contrast, Kaplan et al. (2000)
applied an optimal interpolation scheme to available SST
and SLP records for the 1854–1992 period of record, then
recovered the PDO pattern as the second leading mode of
co-variability between the five-year low-pass-filtered glo-
bal fields (the leading mode of co-variability was a trend
mode). While these results yield somewhat different pic-
Fig. 1. (top) Anomalous climate conditions associated with warm phases of the Pacific Decadal Oscillation (PDO), and (bottom)
November–March average values of the PDO index. Values shown are °C for sea surface temperature (SST), millibars for sea
level pressure (SLP) and direction and intensity of surface wind stress. The longest wind vectors represent a pseudostress of
10 m2/s2. Actual anomaly values for a given year at a given location are obtained by multiplying the climate anomaly by the
associated index value. Adapted and updated from Mantua et al. (1997).
38 N. J. Mantua and S. R. Hare
tures of past PDV, there remains a wealth of evidence in
support of spatial modes that generally resemble, if not
reproduce, the canonical PDO pattern (cf. Tanimoto et
al., 1993; Graham, 1994; Trenberth and Hurrell, 1994;
Latif and Barnett, 1994, 1996; Zhang, 1996; Hare, 1996;
Mantua et al., 1997; Minobe, 1997; Nakamura et al., 1997;
Enfield and Mestas-Nuñez, 1999; Folland et al., 1999;
Kaplan et al., 2000; Barlow et al., 2001; Tourre et al.,
2001).
2.2 Temporal scales of variability
Research aimed at identifying temporal scales of
PDV also yield a variety of results, again based on the
data examined and the analysis techniques employed. In
a pair of closely related studies, Minobe (1999, 2000)
applied Wavelet analysis to indices for boreal winter and
spring North Pacific SST and SLP and found PDO fluc-
tuations were most energetic at periodicities in the 15-to-
25 year and 50-to-70 year bands. Chao et al. (2000) ap-
plied Singular Spectrum Analysis to a persistence index
for North Pacific SST variations, and they found evidence
for oscillatory variations at 15-to-20 and near 70 year
periodicities. Tourre et al. (2001) used a Multi-Taper-
Method/Singular Value Decomposition (MTM/SVD)
technique to identify coherent patterns of low-frequency
20th Century Pacific SST and SLP variations from 30°S
to 60°N. The canonical PDO SST pattern shown in Fig. 1
is clearly reproduced by the spatial patterns of Tourre’s
et al. (2001) Interdecadal mode (which has peak variance
at 12-to-25 year periods), and somewhat similar to that
of their Decadal mode (which has peak variance at 9-to-
12 year periods) (see Tourre’s et al. (2001); figure 2).
2.3 Paleoclimate reconstructions
To better understand the long-term behavior of the
PDO, several studies report on proxy environmental re-
corders of PDO-related climate changes several hundred
years back in time. Minobe (1997) used Fritts’ (1991) tree-
ring based temperature reconstructions to project North
American air temperatures back to 1600. The leading EOF
had the same 50–70 peak periodicity as the instrumental
record from which the PDO was identified. Biondi et al.
(2001) used ring-widths from moisture stressed trees in
Southern California and Baja, Mexico, to create a paleo-
PDO time series to 1661; Gedalof and Smith (2001) used
tree-ring chronologies from a coastal transect spanning
northern California to the Gulf of Alaska to reconstruct a
PDO index to 1600. The PDO index was positively cor-
related with the dominant climate signal in the 20th Cen-
tury sections of these two dendrochronologies (Fig. 2, see
Table 1). Gedalof and Smith (2001) identified 11 regime
shifts in the PDO record since 1650 with the most recent
occurring in 1976/77. With average duration of a regime
being 23 years, they suggest that another shift is due
around the end of the century. While the two
dendrochronologies capture much of the interdecadal
variability in the instrumental PDO indices of Mantua et
al. (1997) and Kaplan et al. (2000) (Fig. 2), they also
exhibit periods in which they show little, if any corre-
spondence with each other. This situation warrants fur-
ther investigation, and highlights opportunities to narrow
the uncertainty of pre-instrumental PDV, perhaps through
multi-proxy reconstructions.
Fig. 2. 5-year running average plots of tree-ring based PDO reconstructions of Gedalof and Smith (2001) and Biondi et al.
(2001), along with Kaplan et al.’s (2000) COADS SST index for 1854–1992 and Mantua et al.’s (1997) SST-based PDO
index. Each time series has been normalized with respect to the available period of record, and they are plotted with an offset
for clarity.
Table 1. Correlation coefficients between the time series dis-
played in Fig. 2. Correlations were computed on the com-
mon period of record (1903–1981). Note that these time
series are 5-year running averages of the raw data series.
PDO Index Gedalof Biondi Kaplan
Gedalof 0.55
Biondi 0.58 0.31
Kaplan 0.77 0.58 0.31
The Pacific Decadal Oscillation 39
Results of PDO index reconstructions from outside
North America have also been published. Evans et al.
(2000) examined 15 tree-ring chronologies from mid-lati-
tude North and South America and found 20th Century
coherence in these records closely matched that in the
PDO index. Linsley et al. (2000) examined Sr/Ca vari-
ability in a long lived coral from Rarotonga and found a
strong PDO signal in the extracted coral SST history that
spans the period from 1726 to 1997. These last two proxy
records are of special interest because they substantiate a
robust PDO connection to tropical and southern hemi-
sphere climate (Evans et al., 2001).
3. PDO Impacts
3.1 Surface climate
Many of the climate anomalies associated with PDO
are broadly similar to those connected with ENSO varia-
tions (El Niño and La Niña), though generally not as ex-
treme (Latif and Barnett, 1996; Mantua et al., 1997;
Minobe, 1997). Correlations between the November–
April PDO index and the 0.5 degree gridded surface tem-
perature and precipitation data of Willmott and Matsuura
(2000) (see also Willmott and Robeson, 1995) are shown
in Fig. 3.
The correlations suggest the following patterns of
PDO precipitation anomalies: warm phases of the PDO
coincide with anomalously dry periods in eastern Aus-
tralia, Korea, Japan, the Russian Far East, interior Alaska,
in a zonally elongated belt from the Pacific Northwest to
the Great Lakes, the Ohio Valley, and in much of Central
America and northern South America; warm PDO phases
also tend to coincide with anomalously wet periods in
the coastal Gulf of Alaska, the southwest US and Mexico,
southeast Brazil, south central South America, and west-
ern Australia.
The correlations suggest the following patterns of
November–April PDO temperature anomalies: warm
phases of the PDO tend to coincide with anomalously
warm temperatures in northwestern North America, north-
ern South America, and northwestern Australia, and
anomalously cool temperatures in eastern China, Korea,
Japan, Kamchatka, and the southeast US and Mexico. It
is notable that Minobe (2000) and Cayan et al. (2001)
find that the most prominent PDO temperature signal in
North America is in the boreal spring, rather than winter
season.
Independent studies have confirmed PDO signals in
the Southern Hemisphere. Garreaud and Battisti (1999)
extended the study of Zhang et al. (1997) to the Southern
Hemisphere and identified a clear pattern of symmetric
atmospheric circulation changes associated with the PDO.
Dettinger et al. (2000) found evidence for a symmetric
pattern of PDO-related precipitation and water year (Oc-
tober-to-September) streamflow anomalies in the Ameri-
cas, wherein warm PDO (El Niño-like) periods tend to
have anomalously wet subtropics but dry tropics and
midlatitudes in both North and South America. Power et
al. (1997, 1999a, 1999b) examined interdecadal changes
in eastern Australian climate, finding warm PDO periods
to be associated with anomalously warm-dry conditions,
while cool PDO periods are associated with cool-wet con-
ditions, consistent with the correlation fields displayed
in Fig. 3.
Fig. 3. Correlations between November–April mean precipita-
tion (top) and temperature (bottom) and the November–
April mean PDO index shown in Fig. 1. Precipitation and
temperature data are the 0.5 degree grid climatologically
aided interpolation (CAI) fields produced at the University
of Delaware by Cort Willmott and collaborators (available
via the internet at http://climate.geog.udel.edu/, also see
Willmott and Robeson, 1995). Negative correlation coeffi-
cients are shaded in blues, positive correlation coefficients
are shaded in reds and yellows.
40 N. J. Mantua and S. R. Hare
4. Marine Ecosystems
In the past few decades a number of studies have
identified compelling evidence for connections between
PDV and variations in Pacific marine ecosystems.
Kawasaki (1991) has documented a remarkable 20th Cen-
tury coherence between interdecadal fluctuations in sar-
dine population off Japan, California, Chile and Peru (see
also Yasuda et al., 1999). Studies linking 20th Century
Pacific salmon catches in eastern Asia and western North
America to variability in the Aleutian Low have been
published by Beamish and co-workers (Beamish, 1993;
Beamish and Bouillon, 1993; Beamish et al., 1999).
The differing regional responses of salmon stocks
along the west coast of North America have been exam-
ined by Adkison et al. (1996) and Peterman et al. (1998).
Their findings indicated that Alaskan stocks showed a
strong uniform response to climate but British Columbia
stocks were mixed. Hare et al. (1999) extended the geo-
graphic scope to include stocks from Washington, Oregon
and California and analyzed catch records from the five
major salmon species. They identified an “inverse pro-
duction regime”, associated with the PDO, where the
warm phase of the PDO favors high production for Alaska
stocks and low production for Washington, Oregon and
California (WOC) stocks. The cool phase of the PDO has
the opposite effect on Alaska and WOC stocks. The es-
sence of the results of their analysis is illustrated in Fig.
4. The response of groundfish stocks to the PDO has
also been documented in several studies. A strong one
year jump in recruitment coincident with the 1976–77
regime shift was demonstrated for many commercially
exploited stocks in the Northeast Pacific by Beamish
(1993) and for sablefish in particular (McFarlane and
Beamish, 1992). Pacific halibut recruitment was shown
by Clark et al. (1999) to have undergone interdecadal
shifts closely matched to the phases of the PDO (Fig. 4).
Like Alaska salmon, halibut flourish during warm phases
of the PDO. Hollowed et al. (1998) assembled recruit-
ment time series for the major exploited groundfish and
pelagic species in Alaska and WOC. They found that,
while a large fraction of the species appeared to respond
more to ENSO events, several flatfish species (arrowtooth
flounder, Greenland turbot, Pacific halibut) exhibited
PDO-like recruitment histories. In one of the most thor-
ough documentations of the changes that have taken place
in the groundfish complex, Anderson and Piatt (1999)
assembled 45 years of small mesh trawl survey records
from the Gulf of Alaska. They show that the marine eco-
system underwent a transformation from one dominated
by lower trophic level forage species (e.g. capelin, shrimp,
sand lance) prior to the mid-1970s, to one dominated by
higher trophic level groundfish (e.g. gadids and flatfish)
since that time.
A number of other studies have shown impacts of
the PDO on other components of the marine and terres-
trial ecosystems of the North Pacific and North America.
At the plankton level, primary and secondary productiv-
ity responses to the climate shift of 1976–77 have been
documented by Venrick et al. (1987), Brodeur and Ware
(1992), Brodeur et al. (1996), Roemmich and McGowan
(1995), McGowan et al. (1998), and Mackas et al. (1998).
At the higher, non-piscivore, trophic levels, Piatt and
Anderson (1995) and Francis et al. (1998) discuss decadal
changes in marine mammal and pisciverous bird
populations, particularly in response to the climatic re-
gime shift of 1976–77. More recently, Vandenbosch
Fig. 4. A graphical depiction of the “Inverse Production Re-
gimes” of Hare et al. (1999). The bars represent loadings
from a principal component analysis (PCA) of 30 salmon
time series for the period 1925–1997. Regional definitions
are as follows: 1 - Western Alaska, 2 - Central Alaska,
3 - Southeast Alaska, 4 - British Columbia, 5 - Washington,
6 - Oregon, 7 - California. Three climate indices were in-
cluded in the PCA: Pacific Decadal Oscillation (PDO), Aleu-
tian Low Pressure Index (AL) and the El Niño-Southern
Oscillation (ENSO). The longest bar, Central Alaska pink
salmon, represents a correlation coefficient with a value of
0.855, and represents the correlation between that time se-
ries and the illustrated temporal component (score) from
the PCA.
The Pacific Decadal Oscillation 41
(2000) linked Hawaiian Island and Farallon Island seabird
population variability to phases of the PDO. Finally,
Cayan et al. (2001) document a long term change to an
earlier onset of spring in the western United States—as
measured by blooming times of lilac and honeysuckle
bushes—and variability in this timing also shows a high
correlation with the springtime PDO index. In perhaps
the broadest examinations to date of the widespread cli-
matic impacts on the ecosystems of the North Pacific,
Hare and Mantua (2000) conducted a principal compo-
nent analysis on a matrix of 100 climatic and biological
time series. The climatic time series were selected to rep-
resent the atmosphere and ocean across the North Pacific
while the biological time series ranged across all trophic
levels. The dominant principal component has the same
time trajectory as the PDO.
5. Dynamics and Predictability
The physical mechanism(s) behind the PDO are not
currently known. Some climate simulation models pro-
duce PDO-like oscillations (e.g. Latif and Barnett, 1994),
although often for different reasons (NRC, 1998). The
mechanisms giving rise to PDO will determine whether
skillful PDO climate predictions for up to one or more
decades into the future are possible. Even in the absence
of a theoretical understanding, PDO climate information
improves season-to-season and year-to-year climate fore-
casts for North America because of its strong tendency
for multi-season and multi-year persistence.
While causes for the PDO remain unclear, several
mechanisms promoting persistence in extratropical cli-
mate have been identified. Alexander et al. (1999, 2001)
detail simple mixed layer mechanisms that give rise to
the reemergence of subsurface thermal anomalies from
one winter to the next. Deser (Clara Deser, NCAR, per-
sonal communication) reports on the ability to reproduce
the observed multiyear autocorrelation structure of North
Pacific and North Atlantic SSTs with a simple entraining
mixed-layer model. Barsugli and Battisti (1998) use a sim-
ple model to demonstrate that local air-sea interactions
yield “differential thermal damping” on atmospheric
anomalies that redden the spectrum of variability and in-
crease the overall variance over that which would occur
in the absence of feedback.
The complimentary results of Seager et al. (in re-
view) and Schneider and Miller (2001 (in press)) offer
strong support for multi-year predictability for important
aspects of mid-latitude ocean variability. Both studies
report on the dynamic response of the thermocline in the
Kuroshio/Oyashio Extension (KOE) region to the inte-
grated wind-stress curl over the previous few years. Their
hypothesis suggests the following: in a situation with
enhanced westerly surface winds over the central North
Pacific, for instance due to an anomalously deep Aleu-
tian Low, the local intensification of the westerlies cool
the interior North Pacific via enhanced surface heat fluxes
and anomalous Ekman advection of the mean meridional
temperature field; this wind field also displaces the zero
windstress curl line to lower latitudes, thereby generat-
ing anomalous upwelling Rossby Waves at the latitude of
zero-windstress curl that slowly propagate to the west;
these upwelling Rossby waves eventually raise the
thermocline in the KOE region one to several years hence;
subsequent deep winter mixing in the KOE region trans-
mits the thermocline anomalies to the surface where they
eventually cool the SST. The end result is that the same
wind anomalies that generate negative SST anomalies in
the interior North Pacific eventually generate SST anoma-
lies of the same sign in the KOE region one to a few years
later. Any persistence in those winds will therefore result
in an amplified persistence in the North Pacific SST field.
6. Discussion and Conclusions
We have provided a review of late 20th Century stud-
ies of Pacific Decadal Variability (PDV), with particular
attention to a special case of PDV known as the Pacific
Decadal Oscillation (PDO). Much controversy now ex-
ists over how PDO works, and how it might best be moni-
tored, modeled and predicted. The stakes in PDO science
are high, as an improved PDO understanding offers even
sharper views of future climate and its attendant impacts
on resources than those now provided by ENSO science
alone.
We believe that the case for a robust PDO mode of
PDV is, on balance, strengthened by the results of recent
studies, while acknowledging the fact that many critical
questions about the PDO remain unanswered. Regardless
of PDO predictability, we also believe that recognition
of PDO variability is important because it clearly dem-
onstrates that “normal” climate conditions can vary over
time periods comparable to the length of a human’s life-
time, and climate anomalies that persist for one to a few
decades can cause especially large impacts on ecosystems
and societies.
Recent advances in understanding mechanisms for
persistence and slow changes in extratropical SST anoma-
lies offer improved confidence for PDV predictability at
lead times of one to a few years. Accurate PDO monitor-
ing and prediction may have practical benefits in both
seasonal and longer term climate forecasts for select re-
gions. Gershunov and Barnett (1998), for example, ar-
gued that combining PDO and ENSO information may
enhance the skill of empirical North American climate
forecasts.
The potential for skillful PDV predictions at lead
times beyond a few years hinges on the premise that un-
stable coupled ocean-atmosphere interactions and delayed
negative feedbacks contribute to PDV. Direct observa-
42 N. J. Mantua and S. R. Hare
tional evidence for these types of interactions, at least
outside the tropical Pacific, is tantalizing yet equivocal
(see NRC, 1998; Miller and Schneider, 2000). Proxy and
instrumental evidence for robust PDO impacts within and
around the Pacific Rim, both in the tropics and mid-lati-
tudes of the Northern and Southern Hemisphere, supports
the idea that causes for PDO variability originate in the
tropics (Evans et al., 2001).
The present day skill in PDO-related forecasts comes
from persistence. This skill disappears when there is an
unforeseen sign change in the PDO pattern. Such a
change—a flip from warm to cool PDO phases—may have
taken place in 1998, coincident with the demise of the
1997/98 El Niño and the beginning of the subsequent La
Niña episode (Hare and Mantua, 2000; Schwing and
Moore, 2000). However, because no one is certain how
the PDO works, it is not possible to say with confidence
that the 1998 changes in Pacific climate mark the begin-
ning of a 20-to-30 year long cool phase of the PDO.
The lack of PDO understanding presents a barrier to
both real-time monitoring and forecasting PDO regime
shifts. The research community’s experience with ENSO
demonstrated that improved understanding and predic-
tions came with the synergy of observational, theoreti-
cal, and modeling studies (NRC, 1996). Each of these lines
of PDO research have been identified as high priorities
by the ongoing International CLIVAR program (see
http://www.clivar.org/). PDO science is relatively new
compared to ENSO science, but insights into the PDO
came at a furious pace in the last decade of the 20th Cen-
tury. More insights into how PDO works, and how to pre-
dict PDO variations, are sure to follow throughout the
first decade of the 21st Century.
Acknowledgements
We thank S. Minobe, R. Allan and an anonymous
reviewer for providing insightful comments and construc-
tive suggestions for revising an early draft of this article.
We are also indebted to R. Francis, E. Miles, J. M.
Wallace, and W. Wooster for lively discussions that have
contributed to advancing the research into PDV and its
impacts on regional climate and natural resources, and T.
Mitchell for developing the mapping routines used to cre-
ate Fig. 3. This study was prompted by collaborations in
the University of Washington’s Climate Impacts Group,
an interdisciplinary effort within the Joint Institute for
the Study of the Atmosphere and Oceans and School of
Marine Affairs, and was funded under NOAA’s coopera-
tive agreement #NAI17RJ1232 and The Hayes Center.
This is JISAO contribution #862.
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