Papers on Pacific Decadal Oscillation
Posted by Ari Jokimäki on February 10, 2011
This is a list of papers on Pacific Decadal Oscillation. The list is not complete, and will most likely be updated in the future in order to make it more thorough and more representative.
Investigating the possibility of a human component in various pacific decadal oscillation indices – Bonfils & Santer (2010) “The pacific decadal oscillation (PDO) is a mode of natural decadal climate variability, typically defined as the principal component of North Pacific sea surface temperature (SST) anomalies. To remove any global warming signal present in the data, the traditional definition specifies that monthly-mean, global-average SST anomalies are subtracted from the local anomalies. Differences in the warming rates over the globe and the PDO region may therefore be aliased into the PDO index. Here, we examine the possibility of a human component in the PDO, considering three different definitions. The implications of these definitions are explored using SSTs from both observations and simulations of historical and future climate, all projected onto (definition-dependent) observed PDO patterns. In the twenty first century scenarios, a systematic anthropogenic component is found in all three PDO indices. Under the first definition—in which no warming signal is removed—this component is so large that it is also statistically detectable in the observed PDO. Using the second/traditional definition, this component is also large, and arises primarily from the differential warming rates predicted in the North Pacific and over global oceans. Removing the spatial average SST signal in the PDO region (in the third definition) partially solves this problem, but a human signal persists because the predicted pattern of SST response to human forcing projects strongly onto the PDO pattern. This illustrates the importance of separating internally-generated and externally-forced components in the PDO, and suggests that caution should be exercised in using PDO indices for statistical removal of “natural variability” effects from observational datasets.” Céline Bonfils and Benjamin D. Santer, Climate Dynamics, DOI: 10.1007/s00382-010-0920-1. [full text]
Pacific decadal oscillation hindcasts relevant to near-term climate prediction – Mochizuki et al. (2010) “Decadal-scale climate variations over the Pacific Ocean and its surroundings are strongly related to the so-called Pacific decadal oscillation (PDO) which is coherent with wintertime climate over North America and Asian monsoon, and have important impacts on marine ecosystems and fisheries. In a near-term climate prediction covering the period up to 2030, we require knowledge of the future state of internal variations in the climate system such as the PDO as well as the global warming signal. We perform sets of ensemble hindcast and forecast experiments using a coupled atmosphere-ocean climate model to examine the predictability of internal variations on decadal timescales, in addition to the response to external forcing due to changes in concentrations of greenhouse gases and aerosols, volcanic activity, and solar cycle variations. Our results highlight that an initialization of the upper-ocean state using historical observations is effective for successful hindcasts of the PDO and has a great impact on future predictions. Ensemble hindcasts for the 20th century demonstrate a predictive skill in the upper-ocean temperature over almost a decade, particularly around the Kuroshio-Oyashio extension (KOE) and subtropical oceanic frontal regions where the PDO signals are observed strongest. A negative tendency of the predicted PDO phase in the coming decade will enhance the rising trend in surface air-temperature (SAT) over east Asia and over the KOE region, and suppress it along the west coasts of North and South America and over the equatorial Pacific. This suppression will contribute to a slowing down of the global-mean SAT rise.” Takashi Mochizuki, Masayoshi Ishii, Masahide Kimoto, Yoshimitsu Chikamoto, Masahiro Watanabe, Toru Nozawa, Takashi T. Sakamoto, Hideo Shiogama, Toshiyuki Awaji, Nozomi Sugiura, Takahiro Toyoda, Sayaka Yasunaka, Hiroaki Tatebe, and Masato Mori, PNAS February 2, 2010 vol. 107 no. 5 1833-1837, doi: 10.1073/pnas.0906531107. [full text]
The Role of Tropospheric Rossby Wave Breaking in the Pacific Decadal Oscillation – Strong & Magnusdottir (2009) “The leading pattern of extratropical Pacific sea surface temperature variability [the Pacific decadal oscillation (PDO)] is shown to depend on observed variability in the spatiotemporal distribution of tropospheric Rossby wave breaking (RWB), where RWB is the irreversible overturning of potential vorticity on isentropic surfaces. Composite analyses based on hundreds of RWB cases show that anticyclonic (cyclonic) RWB is associated with a warm, moist (cool, dry) column that extends down to a surface anticyclonic (cyclonic) circulation, and that the moisture and temperature advection associated with the surface circulation patterns force turbulent heat flux anomalies that project onto the spatial pattern of the PDO. The RWB patterns that are relevant to the PDO are closely tied to El Niño–Southern Oscillation, the Pacific–North American pattern, and the northern annular mode. These results explain the free troposphere-to-surface segment of the atmospheric bridge concept wherein El Niño anomalies emerge in summer and modify circulation patterns that act over several months to force sea surface temperature anomalies in the extratropical Pacific during late winter or early spring. Leading patterns of RWB account for a significant fraction of PDO interannual variability for any month of the year. A multilinear model is developed in which the January mean PDO index for 1958–2006 is regressed upon the leading principal components of cyclonic and anticyclonic RWB from the immediately preceding winter and summer months (four indexes in all), accounting for more than two-thirds of the variance.” Strong, Courtenay, Gudrun Magnusdottir, 2009: J. Climate, 22, 1819–1833.
Tropical origins of North and South Pacific decadal variability – Shakun & Shaman (2009) “The origin of the Pacific Decadal Oscillation (PDO), the leading mode of sea surface temperature variability for the North Pacific, is a matter of considerable debate. One paradigm views the PDO as an independent mode centered in the North Pacific, while another regards it as a largely reddened response to El Niño-Southern Oscillation (ENSO) forcing from the tropics. We calculate the Southern Hemisphere equivalent of the PDO index based on the leading mode of sea surface temperature variability for the South Pacific and find that it adequately explains the spatial structure of the PDO in the North Pacific. A first-order autoregressive model forced by ENSO is used to reproduce the observed PDO indices in the North and South Pacific. These results highlight the strong similarity in Pacific decadal variability on either side of the equator and suggest it may best be viewed as a reddened response to ENSO.” Shakun, J. D., and J. Shaman (2009), Geophys. Res. Lett., 36, L19711, doi:10.1029/2009GL040313. [full text]
The 18.6-year period moon-tidal cycle in Pacific Decadal Oscillation reconstructed from tree-rings in western North America – Yasuda (2009) “Time-series of Pacific Decadal Oscillation (PDO) reconstructed from tree-rings in Western North America is found to have a statistically significant periodicity of 18.6-year period lunar nodal tidal cycle; negative (positive) PDO tends to occur in the period of strong (weak) diurnal tide. In the 3rd and 5th (10th, 11th and 13rd) year after the maximum diurnal tide, mean-PDO takes significant negative (positive) value, suggesting that the Aleutian Low is weak (strong), western-central North Pacific in 30–50°N is warm (cool) and equator-eastern rim of the Pacific is cool (warm). This contributes to climate predictability with a time-table from the astronomical tidal cycle.” Yasuda, I. (2009), Geophys. Res. Lett., 36, L05605, doi:10.1029/2008GL036880.
On the Pacific Decadal Oscillation and the Atlantic Multidecadal Oscillation: Might they be related? – d’Orgeville & Peltier (2007) “The nature of the Pacific Decadal Oscillation (PDO) is investigated based upon analyses of sea surface temperature observations over the last century. The PDO is suggested to be comprised of a 20 year quasi-periodic oscillation and a lower frequency component with a characteristic timescale of 60 years. The 20 year quasi-periodic oscillation is clearly identified as a phase locked signal at the eastern boundary of the Pacific basin, which could be interpreted as the signature of an ocean basin mode. We demonstrate that the 60 year component of the PDO is strongly time-lag correlated with the Atlantic Multidecadal Oscillation (AMO). On this timescale the AMO is shown to lead the PDO by approximately 13 years or to lag the PDO by 17 years. This relation suggests that the AMO and the 60 year component of the PDO are signatures of the same oscillation cycle” d’Orgeville, M., and W. R. Peltier (2007), Geophys. Res. Lett., 34, L23705, doi:10.1029/2007GL031584.
Secular variation of the Pacific Decadal Oscillation, the North Pacific Oscillation and climatic jumps in a multi-millennial simulation – Hunt (2007) “Outputs from a 10,000-year simulation with a coupled global climatic model for present climatic conditions have been used to investigate the behaviour of the Pacific Decadal Oscillation (PDO), the North Pacific Oscillation (NPO) and related phenomena. The analysis reveals a wide range of temporal variability for these Oscillations, suggesting that observations to date provide only a limited sample of possible outcomes. In addition, the simulation suggests that the current observed phase relation between the PDO and NPO may not be typical of longer-term variability. Climatic jumps appear to be a ubiquitous feature of climatic variability, and while, as observed, the most common interval between such jumps is about 20 years, intervals of up to 100 years occur in the simulation. The probability density functions of the PDO and NPO are very close to Gaussian, with the PDO being represented by an auto-regressive function of order one, while the NPO consisted of white noise. An FFT analysis of PC1 of the PDO revealed periodicities concentrated near 10 years, while for the NPO the principal periodicities were decadal to bidecadal. Global distributions of the distributions of the correlations between PC1 or the NPO and selected climatic variables were similar, and in agreement with observations. These correlations highlight the inter-relationships between these two Oscillations. The above correlations were not necessarily stable in time for a given geographical point, with transitions occurring between positive and negative extremes. Climatic jumps were identified with transitions of both the PDO and NPO, with magnitudes of importance as regards climatic perturbations. Spatial patterns of the changes associated with such jumps have global scales, and the need to consider the implications of these jumps in regard to greenhouse induced climatic change is noted.” B. G. Hunt, Climate Dynamics, Volume 30, Number 5, 467-483, DOI: 10.1007/s00382-007-0307-0.
A Pacific Decadal Oscillation record since 1470 AD reconstructed from proxy data of summer rainfall over eastern China – Shen et al. (2006) “Recent studies indicated that the spatial pattern and temporal variability of summer rainfall over eastern China are well correlated with the Pacific Decadal Oscillation (PDO). Here we used a data set of the drought/flood index (a proxy of summer rainfall) since 1470 AD to reconstruct the annual PDO index. The reconstruction indicates that the PDO is a robust feature of North Pacific climate variability throughout the study period, however, the major modes of oscillation providing the basic PDO regime timescale have not been persistent over the last 530 years. The quasi-centennial (75–115-yr) and pentadecadal (50–70-yr) oscillations dominated the periods before and after 1850, respectively. Our analysis suggest that solar forcing fluctuation on quasi-centennial time scale (Gleissberg cycle) could be the pace-maker of the PDO before 1850, and the PDO behavior after 1850 could be due, in part, to the global warming.” Shen, C., W.-C. Wang, W. Gong, and Z. Hao (2006), Geophys. Res. Lett., 33, L03702, doi:10.1029/2005GL024804.
Variations in the Pacific Decadal Oscillation over the past millennium – MacDonald & Case (2005) “Hydrologically sensitive tree-ring chronologies from Pinus flexilis in California and Alberta were used to produce an AD 993–1996 reconstruction of the Pacific Decadal Oscillation (PDO) and to assess long-term variability in the PDO’s strength and periodicity. The reconstruction indicates that a ∼50 to 70 year periodicity in the PDO is typical for the past 200 years but, was only intermittently a strong mode of variability prior to that. Between AD 1600 and 1800 there is a general absence of significant variability within the 50 to 100 year frequency range. Significant variability within in the frequency range of 50 to 100 years reemerges between AD 1500 and 1300 and AD 1200 to 1000. A prolonged period of strongly negative PDO values between AD 993 and 1300 is contemporaneous with a severe medieval megadrought that is apparent in many proxy hydrologic records for the western United States and Canada.” MacDonald, G. M., and R. A. Case (2005), Geophys. Res. Lett., 32, L08703, doi:10.1029/2005GL022478. [full text]
The Forcing of the Pacific Decadal Oscillation – Schneider & Cornuelle (2005) “The Pacific decadal oscillation (PDO), defined as the leading empirical orthogonal function of North Pacific sea surface temperature anomalies, is a widely used index for decadal variability. It is shown that the PDO can be recovered from a reconstruction of North Pacific sea surface temperature anomalies based on a first-order autoregressive model and forcing by variability of the Aleutian low, El Niño–Southern Oscillation (ENSO), and oceanic zonal advection anomalies in the Kuroshio–Oyashio Extension. The latter results from oceanic Rossby waves that are forced by North Pacific Ekman pumping. The SST response patterns to these processes are not orthogonal, and they determine the spatial characteristics of the PDO. The importance of the different forcing processes is frequency dependent. At interannual time scales, forcing from ENSO and the Aleutian low determines the response in equal parts. At decadal time scales, zonal advection in the Kuroshio–Oyashio Extension, ENSO, and anomalies of the Aleutian low each account for similar amounts of the PDO variance. These results support the hypothesis that the PDO is not a dynamical mode, but arises from the superposition of sea surface temperature fluctuations with different dynamical origins.” Schneider, Niklas, Bruce D. Cornuelle, 2005, J. Climate, 18, 4355–4373. [full text]
ENSO-Forced Variability of the Pacific Decadal Oscillation – Newman et al. (2003) “Variability of the Pacific decadal oscillation (PDO), on both interannual and decadal timescales, is well modeled as the sum of direct forcing by El Niño–Southern Oscillation (ENSO), the “reemergence” of North Pacific sea surface temperature anomalies in subsequent winters, and white noise atmospheric forcing. This simple model may be taken as a null hypothesis for the PDO, and may also be relevant for other climate integrators that have been previously related to the PDO.” Newman, Matthew, Gilbert P. Compo, Michael A. Alexander, 2003, J. Climate, 16, 3853–3857. [full text]
The Pacific Decadal Oscillation – Mantua & Hare (2002) “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 sometimes independent modes having distinct spatial and temporal characteristics of North Pacific sea surface temperature (SST) variability. A growing body of evidence highlights a strong tendency for PDO impacts in the Southern Hemisphere, with important 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 mechanisms that promote multi-year persistence in North Pacific upper ocean temperature anomalies.” Nathan J. Mantua and Steven R. Hare, Journal of Oceanography, Volume 58, Number 1, 35-44, DOI: 10.1023/A:1015820616384.
A Pacific Interdecadal Climate Oscillation with Impacts on Salmon Production – Mantua et al. (1997) “Evidence gleaned from the instrumental record of climate data identifies a robust, recurring pattern of ocean–atmosphere climate variability centered over the midlatitude North Pacific basin. Over the past century, the amplitude of this climate pattern has varied irregularly at interannual-to-interdecadal timescales. There is evidence of reversals in the prevailing polarity of the oscillation occurring around 1925, 1947, and 1977; the last two reversals correspond to dramatic shifts in salmon production regimes in the North Pacific Ocean. This climate pattern also affects coastal sea and continental surface air temperatures, as well as streamflow in major west coast river systems, from Alaska to California.” Mantua, Nathan J., Steven R. Hare, Yuan Zhang, John M. Wallace, Robert C. Francis, 1997, Bull. Amer. Meteor. Soc., 78, 1069–1079. [full text]
ENSO-like Interdecadal Variability: 1900–93 – Zhang et al. (1997) “A number of recent studies have reported an ENSO-like EOF mode in the global sea surface temperature (SST) field, whose time variability is marked by an abrupt change toward a warmer tropical eastern Pacific and a colder extratropical central North Pacific in 1976–77. The present study compares this pattern with the structure of the interannual variability associated with the ENSO cycle and documents its time history back to 1900. The analysis is primarily based on the leading EOFs of the SST anomaly and “anomaly deviation” fields in various domains and the associated expansion coefficient (or principal component) time series, which are used to construct global regression maps of SST, sea level pressure (SLP), and a number of related variables. The use of “anomaly deviations” (i.e., departures of local SST anomalies from the concurrent global-mean SST anomaly) reduces the influence of global-mean SST trends upon the structure of the EOFs and their expansion coefficient time series. An important auxiliary time series used in this study is a “Southern Oscillation index” based on marine surface observations. By means of several different analysis techniques, the time variability of the leading EOF of the global SST field is separated into two components: one identified with the “ENSO cycle-related” variability on the interannual timescale, and the other a linearly independent “residual” comprising all the interdecadal variability in the record. The two components exhibit rather similar spatial signatures in the global SST, SLP, and wind stress fields. The SST signature in the residual variability is less equatorially confined in the eastern Pacific and it is relatively more prominent over the extratropical North Pacific. The corresponding SLP signature is also stronger over the extratropical North Pacific, and its counterpart in the cold season 500-mb height field more closely resembles the PNA pattern. The amplitude time series of the ENSO-like pattern in the residual variability reflects the above-mentioned shift in 1976–77, as well as a number of other prominent features, including a shift of opposite polarity during the 1940s.” Zhang, Yuan, John M. Wallace, David S. Battisti, 1997: ENSO-like Interdecadal Variability: 1900–93. J. Climate, 10, 1004–1020. [full text]
Decadal Climate Variability over the North Pacific and North America: Dynamics and Predictability – Latif & Barnett (1995) “The dynamics and predictability of decadal climate variability over the North Pacific and North America are investigated by analyzing various observational datasets and the output of a state of the art coupled ocean–atmosphere general circulation model that was integrated for 125 years. Both the observations and model results support the picture that the decadal variability in the region of interest is based on a cycle involving unstable ocean–atmosphere interactions over the North Pacific. The period of this cycle is of the order of a few decades. The cycle involves the two major circulation regimes in the North Pacific climate system, the subtropical ocean gyre, and the Aleutian low. When, for instance, the subtropical ocean gyre is anomalously strong, more warm tropical waters are transported poleward by the Kuroshio and its extension, leading to a positive SST anomaly in the North Pacific. The atmospheric response to this SST anomaly involves a weakened Aleutian low, and the associated fluxes at the air–sea interface reinforce the initial SST anomaly, so that ocean and atmosphere act as a positive feedback system. The anomalous heat flux, reduced ocean mixing in response to a weakened storm track, and anonmalous Ekman heat transport contribute to this positive feedback. The atmospheric response, however, consists also of a wind stress curl anomaly that spins down the subtropical ocean gyre, thereby reducing the poleward heat transport and the initial SST anomaly. The ocean adjusts with some time lag to the change in the wind stress curl, and it is this transient ocean response that allows continuous oscillations. The transient response can be expressed in terms of baroclinic planetary waves, and the decadal timescale of the oscillation is therefore determined to first order by wave timescales. Advection by the mean currents, however, is not negligible. The existence of such a cycle provides the basis of long-range climate forecasting over North America at decadal timescales. At a minimum, knowledge of the present phase of the decadal mode should allow a “now-cast” of expected climate “bias” over North America, which is equivalent to a climate forecast several years ahead.” Latif, M., T. P. Barnett, 1996, J. Climate, 9, 2407–2423.. [full text]
A century and a half of change in the climate of the NE Pacific – Ware (1995) “Spectral analysis of twenty-one climate records indicates that NE Pacific temperatures and winter wind stress have fluctuated at four dominant time scales in this century: 2–3 years (quasi-biennial oscillation), 5–7 years (El Nin̈o-Southern Oscillation, ENSO), 20–25 years (bidecadal oscillation, BDO), and a poorly resolved, very-low-frequency (VLF) oscillation with a 50–75 year period. Forty-four per cent of the low-frequency variability in British Columbia air temperatures is associated with the strength of the Aleutian Low pressure system in winter. Only 42% of the ‘strong’ and 25% of the ‘moderate’ ENSO events in this century have produced large warm anomalies off BC. Interactions between the ENSO, bidecadal and very-low-frequency oscillations produce a pattern of alternating warm and cool climate states, with major warnings every 50 to 75 years. Since 1850 there have been seven warm periods, lasting an average of 11.4 years, and six cool periods lasting an average of 10.8 years. Sharp transitions from cool to warm climate states (as in 1977/78) occur when warming phases of the BDO and VLF oscillations coincide. Recent evidence suggests that the BDO may originate in either the tropical or the subtropical North Pacific. The NE Pacific has experienced a major warming since 1978. A long-range forecast suggests that the BDO and VLF oscillations peaked in 1989 and are currently in a cooling phase. Consequently, coastal temperatures should moderate for the rest of this century. A transition to the next cool climate state could occur about the year 2001. The forecast for moderating temperatures could begin the first phase of the recovery of the southern BC coastal chinook and coho salmon and herring stocks, which are currently at low abundance levels.” D.M. Ware, Fisheries Oceanography, Volume 4, Issue 4, pages 267–277, December 1995.
Causes of Decadal Climate Variability over the North Pacific and North America – Latif & Barnett (1994) “The cause of decadal climate variability over the North Pacific Ocean and North America is investigated by the analysis of data from a multidecadal integration with a state-of-the-art coupled ocean-atmosphere model and observations. About one-third of the low-frequency climate variability in the region of interest can be attributed to a cycle involving unstable air-sea interactions between the subtropical gyre circulation in the North Pacific and the Aleutian low-pressure system. The existence of this cycle provides a basis for long-range climate forecasting over the western United States at decadal time scales.” M. Latif and T. P. Barnett, Science 28 October 1994, Vol. 266 no. 5185 pp. 634-637, DOI: 10.1126/science.266.5185.634. [full text]
Decadal-scale regime shifts in the large marine ecosystems of the North-east Pacific: a case for historical science – Francis & Hare (1994) “There are two fundamental ways of doing science: the experimental-predictive and the historical-descriptive. The experimental-predictive approach uses the techniques of controlled experiment, the reduction of natural complexity to a minimal set of general causes, and presupposes that all times can be treated alike and adequately simulated in the laboratory. The historical-descriptive approach uses a mode of analysis which is rooted in the comparative and observational richness of our data, is holistic in its treatment of systems and events, and assumes that the final result being studied is unique, i.e. dependent or contingent upon everything that came before. We suggest that one of the real difficulties we have in understanding ecosystem properties is our inability to deal with scale, and we show how historical science allows us to approach the issue of scale through the interpretation of pattern in time and space. We then use the techniques of the historical-descriptive approach to doing science in the context of our own and other research on climate change and biological production in the North-east Pacific Ocean. In particular, we examine rapid decadal-scale shifts in the abundance and distribution of two major components–salmon and zooplankton – of the large marine ecosystem of the North-east Pacific, and how they relate to similar shifts in North Pacific atmosphere and ocean climate. We conclude that they are all related, and that climate-driven regime shifts, such as those we have identified in the North-east Pacific, can cause major reorganizations of ecological relationships over vast oceanic regions.” Robert C. Francis, Steven R. Hare, Fisheries Oceanography, Volume 3, Issue 4, pages 279–291, December 1994. [full text]
Linkage of ocean and fjord dynamics at decadal period – Ebbesmeyer et al. (1989) “At decadal period (1020 years), dynamic linkage was evident between atmospheric low pressure systems over the North Pacific Ocean and circulation in a Pacific Northwest fjord (Puget Sound). As the Aleutian low pressure center shifts, storms arriving from the North Pacific Ocean deposit varying amounts of precipitation in the mountains draining into the estuarine system; in turn, the fluctuating addition of fresh water changes the density distribution near the fjord basin entrance sill, thereby constraining the fjord’s vertical velocity structure. The linkage was examined using time series of 21 environmental parameters which covaried between the 2 regimes associated with cycling of the Aleutian Low between its eastern and westernmost winter positions. Observations from 1899 to 1987 suggest that, in the 20th century, approximately 5 cycles may have occurred between these regimes. Covariation in all but one of the time series (Puget Sound’s main basin salinity) occurred because of the high degree of correlation between parameters and the strong decadal cycles compared with long-term averages, interannual variability, and seasonal cycles. Basin salinity was relatively steady due to opposing influences of oceanic source water salinity and the addition of fresh water in each regime. However, the decadal signal for the other parameters characterizing Puget Sound water are apparently amplified twofold compared with that of the atmosphere over the North Pacific Ocean.” Ebbesmeyer, C.C., C.A. Coomes, G.A. Cannon, and D.E. Bretschneider, In Aspects of Climate Variability in the Pacific and the Western Americas, D.J. Peterson (ed.), Geophys. Monogr. 55, American Geophysical Union, Wash., D.C., 399–417 (1989).