The North Pacific Gyre Oscillation:
Linking Decadal Variations in the Extratropics and Tropics
Introduction. Sea surface temperatures (SSTs) across the North Pacific Ocean exhibit strong decadal
variability (e.g., Namias 1969, Trenberth and Hurrell 1994; Latif and Barnett 1996; Barnett et al. 1999a). This
variability in North Pacific SSTs is also associated with low frequency changes in general weather patterns
over North America, particularly changes in storm tracks, temperatures, and precipitation. Because of these
teleconnections, understanding and predicting Pacific Decadal Variability (PDV) is a high priority in climate
dynamics research.
The Pacific Decadal Oscillation (PDO; Mantua et al. 1997) is one particular mode of climate
variability in the North Pacific that has received much attention in the climate literature. The PDO is formally
defined as the first empirical orthogonal function (EOF) of SST “residuals” (i.e., deviation of North Pacific
SSTs from the global-mean SST) in the North Pacific. Previous studies have linked PDO variability to
modulation of the El Niño-Southern Oscillation (ENSO) phenomenon, offering a link between decadal
variations in the North Pacific and in the tropical Pacific Ocean (eg., Barnett et al. 1999b; Pierce et al. 2000).
Other studies have examined atmospheric and oceanic mechanisms to understand and explain dynamics
underlying the PDO and
its role in tropicalextratropical interactions
(e.g., Alexander et al. 2002;
Deser et al. 2003; Deser et
al. 2004; Schneider and
Cornuelle 2005). However
the
mechanisms
of
tropical-extratropical
decadal interactions, if
any, are still unclear.
Recent modeling
studies (Di Lorenzo et al.,
2008) of decadal variations
in physical and biological
timeseries
in
the
Northeast Pacific find that
while the PDO is the
dominant mode of SST
and sea surface height
(SSH) variability (Figure
1), it fails to explain
FIG. 1. From Di Lorenzo et al. 2008. (a) (shading) Regression of the first (top; red box) and the
prominent low-frequency
second (bottom; blue box) principal component time series of SSHa variability in the northeast
Pacific Ocean onto SSHa (cm). (contours) Positive (black) and negative (white) wind stress curl
fluctuations
of
key
anomalies regressed onto the same indices. Black box and straight line indicate locales of oceanic
physical-biological
biological indices shown in (b) and (c). (b) (red box) The PDO index (black) superimposed on SSTa
variables (e.g. salinity,
(°C) from the California Cooperative Oceanic Fisheries Investigations (CalCOFI) records. (blue
nutrients and chlorophyll). box) The NPGO index (black) superimposed on salinity (psu), nitrate (μmol m-3), and chlorophyll
Instead, these fluctuations (std) anomalies from CalCOFI. (c) Same as (b) except for the records in the Gulf of Alaska from
are associated with an Line P observations. Both the PDO and the NPGO indices are standardized in all plots.
independent climate mode
of variability - the North
Pacific
Gyre
Oscillation
(NPGO) (Fig. 1). The
NPGO is isolated as the second mode of SSH anomaly (SSHa) variability in the central/eastern North Pacific
and physically represents the weakening and strengthening of the central and eastern branches of the
subtropical and subpolar gyres. The NPGO index is strongly correlated with long-term observations of
salinity and nutrients in the California Current and Gulf of Alaska,
which have been previously unexplained. Similar to the PDO, the
NPGO has a global-scale signature in both SST anomalies (SSTa)
and SSHa (Figure 2), including an equatorially-symmetric pattern.
This symmetric structure in SSTs suggests that the tropics are
involved in explaining the NPGO. This result allows us to
develop new hypotheses of mechanisms of decadal tropicalextratropical interactions, which are presented below and will be
tested in this proposed research.
FIG. 2. From Di Lorenzo et al. (2008).
Correlation of global SSHa from satellite
altimerty and global SSTa from the Smith
and Reynolds (2004) dataset with the NPGO
index. Note the equatorially symmetric
pattern in both correlation maps.
Linking the NPGO to other modes of variability in the
North Pacific. The NPGO can be linked to extratropical
atmospheric variability in the North Pacific. The North Pacific
extratropical atmosphere is characterized by relatively low sea
level pressure (SLP) across the far north (in the vicinity of the
Aleutian Low) and relatively high pressure in the subtropical
Pacific Ocean (e.g., near Hawaii). SLP anomalies (SLPa) between
this two regions constitute the North Pacific Oscillation (NPO), as
first identified by Walker and Bliss (1932) and later by Rogers
(1981,1990). The corresponding wind stresses associated with
this dipole in SLP forces the NPGO SSHa dipole in the northeast
Pacific Ocean. Hence, the NPGO
is the oceanic expression of the
NPO. To see this, we reconstruct
the NPGO index using an
autoregressive lag-1 (AR-1) model
forced by the NPO index:
NPGO(t+1) = αNPGO(t) +
γNPO(t)dt, where  is the lag-1
autocorrelation and  is the
coefficient of the forcing (the
NPO) found by least-squares
fitting. When we use this AR-1
model (see Figure 3b), the
reconstructed NPGO index closely
tracks the NPGO index, suggesting FIG. 3. (a) The NPO index (standardized), represented as SLPa at Hawaii. (b) The
in fact that the extratropical NPGO index (black; standardized) and the reconstructed NPGO index (green) from the
atmosphere in the North Pacific AR-1 model, forced only with the NPO index in (a) (r = 0.71, which is significant at the
99% level). (c) The NPGO index (black) and the KOE index (red; Taguchi et al. 2007),
forces the oceanic SSHa pattern.
lagged by 3 years (i.e., the NPGO leads the KOE by 3 years).
The adjustment process of
the northeast Pacific Ocean to
NPGO SSHa consequently excites westward-propagating oceanic Rossby waves. As these waves reach the
western boundary they modulate decadal variability in the Kuroshio-Oyashio Extension (KOE; Qiu 2000,
2002; Taguchi et al. 2007). This view is supported by Fig. 3c that shows strong correlation between the
NPGO index and the KOE index lagged by 3 years (i.e., the NPGO index leads the KOE index by 3 years).
Hence, variability in the northeast Pacific associated with the NPGO can be tied to the KOE with a common
driving mechanism -- the NPO.
Extratropical-Tropical Interactions. The oceanic expression of the NPO (i.e. the NPGO) exhibits a
global-scale equatorially-symmetric expression of SSTa and SSHa (Fig. 2) suggesting a link between the
tropics and extratropics. Previous studies have shown that NPO can affect the tropical Pacific Ocean by
imposing long-lasting SSTa on the subtropical ocean which then feedback onto the overlying atmosphere in
an ENSO-like manner
(i.e., the seasonal footprinting
mechanism; Pierce et al.
2000; Vimont et al. 2001,
2003). Figure 4 (middle
and bottom rows) clearly
describe this type of
footprinting
sequence.
The lag 0 correlations (Fig.
4, middle panels) show
both the NPO (in SLPa)
and the NPGO signature
in the northeast Pacific
Ocean (in SSTa). In the
following boreal winter
(Fig. 4, bottom row), a
clear ENSO-like signature
is evident, with a dipole in
SLPa across the tropical
Pacific Ocean and the
canonical
horseshoeshaped SSTa pattern in the
tropics. This footprinting
mechanism provides a link
from the extratropics to
Schneider and Di Lorenzo, in prep.
the tropics. However it is
FIG. 4. (left) Lag correlations between SLPa (hPa; from NCEP/NCAR reanalysis) and SLPa unclear if the tropics have
at Hawaii (hPa; from NCEP/NCAR reanalysis). Top row: For the boreal summer in year -1.
a feedback on the
Middle row: For the boreal spring in year 0. Bottom row: For the boreal winter in year +1.
extratropics by affecting
(right) Same as left except for SSTa (°C; from Smith and Reynolds (2004)).
the NPO. To explore
preliminary evidence of
the tropics affecting the
NPO, we look at the negative lags (Fig. 4, top row). During the boreal summer in year -1, the SLPa
correlation map shows positive correlations (r > 0.3) in the western tropical Pacific Ocean. Furthermore, the
SSTa correlation pattern shows a distinct warming across most of the tropical oceans but in particular
stronger correlations with warmer SSTa in the eastern tropical Pacific Ocean with high SLPa in Hawaii (Fig.
4, top row). Hence, there are indications that the NPO may not be purely a stochastic mode but may have a
deterministic component forced by the tropics. Furthermore, because the NPO and NPGO are directly
involved with decadal modulations of the KOE, there appears to be a link between the tropical Pacific Ocean
and the KOE. This link/hypothesis has not been explored in previous climate literature.
Hypotheses & Research Plan. Figure 5 graphically presents a synthesis of our previous results and the
avenues of research we would like to pursue. We use this figure to motivate our central research question:
Can we isolate new mechanisms underlying decadal extratropical-tropical interactions in the North Pacific? To address this
question, we formulate five hypotheses that we intend to test.
H1: The NPGO is forced by the NPO
and is related to changes in the KOE. This
hypothesis covers the right side of the
loop in Fig. 5 and will serve to
corroborate further the preliminary
evidence presented in this proposal
using additional observational datasets
and model simulations.
H2: The NPGO is part of the footprinting
mechanism that links North Pacific
extratropical forcing to the tropics This
hypothesis is supported by both Figs.
2 and Fig. 4.
Together with
hypothesis H1, we will construct a
coherent picture of how the
extratropical North Pacific influences
tropical Pacific climate variability.
H3: The NPO is not purely stochastic FIG. 5. Diagram synthesizing the concepts presented in this proposal and how our
but has a deterministic component forced in research will be organized. The diagram is constructed as a loop linking the extratropics
to the tropics via NPO-NPGO interactions and the seasonal footprinting mechanism
the western tropical Pacific Ocean. In from Vimont et al. (2003). The tropics then feed back into the extratropics via the
concert
with
the
previous western tropical Pacific Ocean and precursors there in the SLPa field. Two additional
hypothesis, we want to explain what avenues of research are also depicted in the diagram. One is the link between the
portion of the NPGO forcing NPGO and the KOE. The second is the assumption of stochastic forcing for the NPO.
together, this diagram suggests a quasi-decadal loop of variability for the Pacific
originates
from
extratropical Taken
Ocean basin.
atmospheric “noise” and what
portion is deterministic from other
influences. One area of interest to investigate the western tropical Pacific Ocean, where SLP changes precede
the canonical NPO signature the following year (Fig. 4). The NPO might also be linked to extratropical
forcing via large-scale planetary waves. This idea may be more difficult to explore but will be pursued if
evidence surfaces.
H4: ENSO dynamics forces the NPO precursor in the western tropical Pacific. The ENSO-like pattern that originates
through the SSTa footprinting mechanism (Fig. 4, bottom row) and that precedes the NPO-signature (Fig. 4,
top row) makes this hypothesis noteworthy. The interannual variability associated with ENSO could also
work as a modulation for the NPO.
These first four hypotheses effectively complete the “loop” presented in Fig. 5, and together suggest that the
links between the extratropics and the tropics could constitute a quasi-decadal loop of climate variability. In
addition to better understanding these tropical-extratropical interactions in PDV, evidence of increases in
NPGO activity in future climate scenarios (Di Lorenzo et al. 2008) motivates one final hypothesis for this
work.
H5: Components of the quasi-decadal loop of variability depicted in Fig. 5 will amplify in magnitude and variability in an
increasingly warmer climate. While examining the changes in PDV in these climate models, we can also assess the
degree to which each model contains the mechanisms and links hypothesized in Fig. 5.
Observational and Modeling Datasets. To examine our hypotheses, we plan on using a suite of
observational datasets, including:
• NCAR/NCEP and ERA-40 reanalysis products for atmospheric variables (SLP, winds,
geopotential heights, and temperatures used to further investigate the influence of the NPO on the
NPGO and potential links to the tropics;
• Smith and Reynolds (2004) SST dataset;
• Satellite data (e.g., AVISO) for SSTs and SSHs to complement the atmospheric variables and also
examine changes in the KOE.
Along with the observational datasets, we will also assess the degree to which hypotheses H1 - H4 (i.e., the
various components of Fig. 5) are true in a collection of model output. We currently have access to the
GFDL 2.0 model output, both coupled and AMIP simulations, for the pre-industrial (1861-2000) and 21st
century runs used in the IPCC report. We also plan on examining how the output from the NASA GISS
coupled ocean-atmosphere model (e.g., Schmidt et al. 2006) and the ocean GCM for the Earth Simulator
(OFES; Masumoto et al. 2004) agrees with our observational results. North Pacific ocean high-resolution
model hindcasts with the Regional Ocean Modeling System (ROMS, Haidvogel et al., 2008) will also be used
and compared to the other modeling/observational datasets. We also plan to perform sensitivity analyses to
different SSTa boundary conditions with a AGCM developed at International Center for Theoretical Physics
(ICTP) by Molteni (2003).
Collaborations. This research project will be done in close collaboration with Dr. Niklas Schneider in the
Department of Oceanoggraphy at the University of Hawaii at Manoa. I plan to spend two month at the
University of Hawaii to work with Dr. Schneider and interact with other scientists at the International Pacific
Research Center (IPRC), who are leading experts in KOE decadal variability (e.g. Qiu) and ENSO dynamics
(e.g. Timmerman). Also, the collaborative efforts will provide access to runs from OFES.
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