Pacific decadal oscillation

PDO positive phase global pattern

The Pacific Decadal Oscillation (PDO) is a robust, recurring pattern of ocean-atmosphere climate variability centered over the mid-latitude Pacific basin. The PDO is detected as warm or cool surface waters in the Pacific Ocean, north of 20° N. Over the past century, the amplitude of this climate pattern has varied irregularly at interannual-to-interdecadal time scales. There is evidence of reversals in the prevailing polarity of the oscillation occurring around 1925, 1947, and 1977; the last two reversals corresponded with dramatic shifts in salmon production regimes in the North Pacific Ocean. This climate pattern also affects coastal sea and continental surface air temperatures from Alaska to California.

During a "warm", or "positive", phase, the west Pacific becomes cooler and part of the eastern ocean warms; during a "cool" or "negative" phase, the opposite pattern occurs. The Pacific (inter-)decadal oscillation was named by Steven R. Hare, who noticed it while studying salmon production pattern results in 1997.[1]

The Pacific Decadal Oscillation index is the leading empirical orthogonal function (EOF) of monthly sea surface temperature anomalies (SSTA) over the North Pacific (poleward of 20° N) after the global mean SST has been removed, the PDO index is the standardized principal component time series.[2] A PDO signal has been reconstructed to 1661 through tree-ring chronologies in the Baja California area.[3]

Mechanisms

Several studies have indicated that the PDO index can be reconstructed as the superimposition of tropical forcing and extra-tropical processes.[4][5][6][7] Thus, unlike ENSO (El Niño Southern Oscillation), the PDO is not a single physical mode of ocean variability, but rather the sum of several processes with different dynamic origins.

At inter-annual time scales the PDO index is reconstructed as the sum of random and ENSO induced variability in the Aleutian low, whereas on decadal timescales ENSO teleconnections, stochastic atmospheric forcing and changes in the North Pacific oceanic gyre circulation contribute approximately equally. Additionally sea surface temperature anomalies have some winter to winter persistence due to the reemergence mechanism.

ENSO teleconnections, the atmospheric bridge[8]
The atmospheric bridge during el nino

ENSO can influence the global circulation pattern thousands of kilometers away from the equatorial Pacific through the "atmospheric bridge". During El Nino events, deep convection and heat transfer to the troposphere is enhanced over the anomalously warm sea surface temperature, this ENSO-related tropical forcing generates Rossby waves that propagate poleward and eastward and are subsequently refracted back from the pole to the tropics. The planetary waves form at preferred locations both in the North and South Pacific Ocean, and the teleconnection pattern is established within 2–6 weeks.[9] ENSO driven patterns modify surface temperature, humidity, wind, and the distribution of clouds over the North Pacific that alter surface heat, momentum, and freshwater fluxes and thus induce sea surface temperature, salinity, and mixed layer depth (MLD) anomalies.

The atmospheric bridge is more effective during boreal winter when the deepened Aleutian low results in stronger and cold northwesterly winds over the central Pacific and warm/humid southerly winds along the North American west coast, the associated changes in the surface heat fluxes and to a lesser extent Ekman transport creates negative sea surface temperature anomalies and a deepened MLD in the central pacific and warm the ocean from the Hawaii to the Bering Sea.

SST reemergence[10]
Reemergence mechanism in the North Pacific.
Mixed layer depth seasonal cycle.

Midlatitude SST anomaly patterns tend to recur from one winter to the next but not during the intervening summer, this process occurs because of the strong mixed layer seasonal cycle. The mixed layer depth over the North Pacific is deeper, typically 100-200m, in winter than it is in summer and thus SST anomalies that forms during winter and extend to the base of the mixed layer are sequestered beneath the shallow summer mixed layer when it reforms in late spring and are effectively insulated from the air-sea heat flux. When the mixed layer deepens again in the following autumn/early winter the anomalies may influence again the surface. This process has been named "reemergence mechanism" by Alexander and Deser[11] and is observed over much of the North Pacific Ocean although is more effective in the west where the winter mixed layer is deeper and the seasonal cycle greater.

Stochastic atmospheric forcing[12]

Long term sea surface temperature variation may be induced by random atmospheric forcings that are integrated and reddened into the ocean mixed layer. The stochastic climate model paradigm was proposed by Frankignoul and Hasselmann,[13] in this model a stochastic forcing represented by the passage of storms alter the ocean mixed layer temperature via surface energy fluxes and Ekman currents and the system is damped due to the enhanced (reduced) heat loss to the atmosphere over the anomalously warm (cold) SST via turbulent energy and longwave radiative fluxes, in the simple case of a linear negative feedback the model can be written as the separable ordinary differential equation:

{\operatorname{d}y\over\operatorname{d}t}= v(t)- \lambda t

where v is the random atmospheric forcing, λ is the damping rate (positive and constant) and y is the response.

The variance spectrum of y is:

{G(w) = \frac{F}{w^2 + \lambda ^2 }}

where F is the variance of the white noise forcing and w is the frequency, an implication of this equation is that at short time scales (w>>λ) the variance of the ocean temperature increase with the square of the period while at longer timescales(w<<λ, ~150 months) the damping process dominates and limits sea surface temperature anomalies so that the spectra became white.

Thus an atmospheric white noise generates SST anomalies at much longer timescales but without spectral peaks. Modeling studies suggest that this process contribute to as much as 1/3 of the PDO variability at decadal timescales.

Ocean dynamics

Several dynamic oceanic mechanisms and SST-air feedback may contribute to the observed decadal variability in the North Pacific Ocean. SST variability is stronger in the Kuroshio Oyashio extension (KOE) region and is associated with changes in the KOE axis and strength,[7] that generates decadal and longer time scales SST variance but without the observed magnitude of the spectral peak at ~10 years, and SST-air feedback. Remote reemergence occurs in regions of strong current such as the Kuroshio extension and the anomalies created near the Japan may reemerge the next winter in the central pacific.

Saravanan and McWilliams[14] have demonstrated that the interaction between spatially coherent atmospheric forcing patterns and an advective ocean shows periodicities at preferred time scales when non-local advective effects dominate over the local sea surface temperature damping. This "advective resonance" mechanism may generate decadal SST variability in the Eastern North Pacific associated with the anomalous Ekman advection and surface heat flux.[15]

Dynamic gyre adjustments are essential to generate decadal SST peaks in the North Pacific, the process occurs via westward propagating oceanic Rossby waves that are forced by wind anomalies in the central and eastern Pacific Ocean. The quasi-geostrophic equation for long non-dispersive Rossby Waves forced by large scale wind stress can be written as the linear partial differential equation:[16]

{\partial h\over\partial t} -c{\partial h\over\partial x} = \frac{-\nabla \times \vec{\tau}}{\rho_0f_0}

where h is the upper-layer thickness anomaly, τ is the wind stress, c is the Rossby wave speed that depends on latitude, ρ0 is the density of sea water and f0 is the Coriolis parameter at a reference latitude. The response time scale is set by the Rossby waves speed, the location of the wind forcing and the basin width, at the latitude of the Kuroshio Extension c is 2.5 cm s−1 and the dynamic gyre adjustment timescale is ~(5)10 years if the Rossby wave was initiated in the (central)eastern Pacific Ocean.

If the wind white forcing is zonally uniform it should generate a red spectrum in which h variance increases with the period and reaches a constant amplitude at lower frequencies without decadal and interdecadal peaks, however low frequencies atmospheric circulation tends to be dominated by fixed spatial patterns so that wind forcing is not zonally uniform, if the wind forcing is zonally sinusoidal then decadal peaks occurs due to resonance of the forced basin-scale Rossby waves.

The propagation of h anomalies in the western pacific changes the KOE axis and strength[7] and impact SST due to the anomalous geostrophic heat transport. Recent studies[7][17] suggest that Rossby waves excited by the Aleutian low propagate the PDO signal from the North Pacific to the KOE through changes in the KOE axis while Rossby waves associated with the NPO propagate the North Pacific Gyre oscillation signal through changes in the KOE strength.

Impacts

Temperature and precipitation

PDO DJFM temperature pattern.
PDO DJFM precipitation pattern.

The IPO/PDO spatial pattern and impacts are similar to those associated with ENSO events. During the positive phase the wintertime Aleutian low is deepened and shifted southward, warm/humid air is advected along the North American west coast and temperatures are higher than usual from the Pacific Northwest to Alaska but below normal in Mexico and the Southeastern United States.[18]
Winter precipitation is higher than usual in the Alaska Coast Range, Mexico and the Southwestern United States but reduced over Canada, Eastern Siberia and Australia[18][19]
McCabe et al.[20] showed that the PDO along with the AMO strongly influence multidecadal droughts pattern in the United States, drought frequency is enhanced over much of the Northern United States during the positive PDO phase and over the Southwest United States during the negative PDO phase in both cases if the PDO is associated with a positive AMO.
The Asian Monsoon is also affected, increased rainfall and decreased summer temperature is observed over the Indian subcontinent during the negative phase.[21]

PDO Indicators PDO positive phase PDO negative phase
Temperature
Pacific Northwest, British Columbia, and Alaska Above average Below average
Mexico to South-East US Below average Above average
Precipitation
Alaska coastal range Above average Below average
Mexico to South-Western US Above average Below average
Canada,Eastern Siberia and Australia Below average Above average
India summer monsoon Below average Above average

Reconstructions and regime shifts

Observed monthly values for the PDO (1900–mar2014).
Reconstructed PDO Index (993-1996).

The PDO index has been reconstructed using tree rings and other hydrologically sensitive proxies from west North America and Asia.[3][22][23]

MacDonald and Case[24] reconstructed the PDO back to 993 using tree rings from California and Alberta. The index shows a 50-70 year periodicity but is a strong mode of variability only after 1800, a persistent negative phase occurring during medieval times (993-1300) which is consistent with La Niña conditions reconstructed in the tropical Pacific[25] and multi-century droughts in the South-West United States.[26]

Several regime shifts are apparent both in the reconstructions and instrumental data, during the 20th century regime shifts associated with concurrent changes in SST, SLP, land precipitation and ocean cloud cover occurred in 1924/1925,1945/1946 and 1976/1977:[27]

Predictability

NOAA's forecast products do not include the PDO at this time. Alexander et al., explored the prediction skill of a linear inverse modeling (LIM)[34] method to predict the PDO, LIM assumes that the PDO can be separated into a linear deterministic component and a non-linear component represented by random fluctuations.

Much of the LIM PDO predictability arises from ENSO and the global trend rather than extra-tropical processes and is thus limited to ~4 seasons. The prediction is consistent with the seasonal footprinting mechanism[35] in which an optimal SST structure evolves into the ENSO mature phase 6–10 months later that subsequently impacts the North Pacific Ocean SST via the atmospheric bridge.

Skills in predicting decadal PDO variability could arise from taking into account the impact of the externally forced[36] and internally generated[37] Pacific variability. The difference in phasing of the PDO can provide predictive power for regional climate anomalies, such as is the case for the American West's drought cycle.[38] The Great Salt Lake of Utah, for example, follows a distinct phase of this oscillation.[39]

Commercial climate forecasting vendors such as MW&A, utilize solar factors to predict the PDO index. This vendor also utilizes the PDO, along with other parameters to forecast drought patterns in the American West.

Related patterns

See also

References

  1. ↑ Mantua, Nathan J.; et al. (1997). "A Pacific interdecadal climate oscillation with impacts on salmon production". Bulletin of the American Meteorological Society 78 (6): 1069–1079. Bibcode:1997BAMS...78.1069M. doi:10.1175/1520-0477(1997)078<1069:APICOW>2.0.CO;2.
  2. ↑ Deser, Clara; Alexander, Michael A.; Xie, Shang-Ping; Phillips, Adam S. (January 2010). "Sea Surface Temperature Variability: Patterns and Mechanisms". Annual Review of Marine Science 2 (1): 115–143. doi:10.1146/annurev-marine-120408-151453. PMID 21141660. Cite uses deprecated parameter |coauthors= (help)
  3. 1 2 3 Biondi, Franco; Gershunov, Alexander; Cayan, Daniel R. (2001). "North Pacific Decadal Climate Variability since 1661". Journal of Climate 14 (1): 5–10. Bibcode:2001JCli...14....5B. doi:10.1175/1520-0442(2001)014<0005:NPDCVS>2.0.CO;2.
  4. ↑ Newman, M.; Compo, G.P.; Alexander, Michael A. (2003). "ENSO-Forced Variability of the Pacific Decadal Oscillation". Journal of Climate 16 (23): 3853–3857. Bibcode:2003JCli...16.3853N. doi:10.1175/1520-0442(2003)016<3853:EVOTPD>2.0.CO;2.
  5. ↑ Vimont, Daniel J. (2005). "The Contribution of the Interannual ENSO Cycle to the Spatial Pattern of Decadal ENSO-Like Variability". Journal of Climate 18 (12): 2080–2092. Bibcode:2005JCli...18.2080V. doi:10.1175/JCLI3365.1.
  6. ↑ Schneider, Niklas; Bruce D. Cornuelle (2005). "The Forcing of the Pacific Decadal Oscillation". Journal of Climate 18 (8): 4355–4372. Bibcode:2005JCli...18.4355S. doi:10.1175/JCLI3527.1.
  7. 1 2 3 4 Qiu, Bo; Niklas Schneider; Shuiming Chen (2007). "Coupled Decadal Variability in the North Pacific: An Observationally Constrained Idealized Model". Journal of Climate 20 (14): 3602–3620. Bibcode:2007JCli...20.3602Q. doi:10.1175/JCLI4190.1. Retrieved 2010-09-16.
  8. ↑ Alexander, Michael A; Ileana Bladé; Matthew Newman; John R. Lanzante; Ngar-Cheung Lau; James D. Scott (2002). "The Atmospheric Bridge: The Influence of ENSO Teleconnections on Air–Sea Interaction over the Global Oceans". Journal of Climate 15 (16): 2205–2231. Bibcode:2002JCli...15.2205A. doi:10.1175/1520-0442(2002)015<2205:TABTIO>2.0.CO;2.
  9. ↑ Liu, Zhengyu; Alexander Michael (2007). "Atmospheric bridge, oceanic tunnel,and global climate teleconnections". Reviews of Geophysics 45 (2): 2. Bibcode:2007RvGeo..45.2005L. doi:10.1029/2005RG000172. Retrieved 2010-09-20.
  10. ↑ Deser, Clara; Michael A. Alexander; Michael S. Timlin (2003). "Understanding the Persistence of Sea Surface Temperature Anomalies in Midlatitudes". Journal of Climate 16 (12): 57–72. Bibcode:2003JCli...16...57D. doi:10.1175/1520-0442(2003)016<0057:UTPOSS>2.0.CO;2.
  11. ↑ Alexander, Michael A.; Deser Clara (1995). "A Mechanism for the Recurrence of Wintertime Midlatitude SST Anomalies". Journal of Physical Oceanography 125 (1): 122–137. Bibcode:1995JPO....25..122A. doi:10.1175/1520-0485(1995)025<0122:AMFTRO>2.0.CO;2.
  12. ↑ Alexander, Michael A.; Penland, Cecile (1996). "Variability in a mixed layer ocean model driven by stochastic atmospheric forcing". Journal of Climate 9 (10): 2424–2442. Bibcode:1996JCli....9.2424A. doi:10.1175/1520-0442(1996)009<2424:VIAMLO>2.0.CO;2.
  13. ↑ Frankignoul, Claude; Hasselmann, Klaus (1977). "Stochastic climate models, Part II Application to sea-surface temperature anomalies and thermocline variability". Tellus 24 (4): 289–305. doi:10.1111/j.2153-3490.1977.tb00740.x.
  14. ↑ Saravanan, R.; McWilliams James C. (1998). "Advective Ocean–Atmosphere Interaction: An Analytical Stochastic Model with Implications for Decadal Variability". Journal of Climate 11 (2): 165–188. Bibcode:1998JCli...11..165S. doi:10.1175/1520-0442(1998)011<0165:AOAIAA>2.0.CO;2.
  15. ↑ Wu, Lixin; Zhengyu Liu (2003). "Decadal Variability in the North Pacific: The Eastern North Pacific Mode". Journal of Climate 16 (19): 3111–3131. Bibcode:2003JCli...16.3111W. doi:10.1175/1520-0442(2003)016<3111:DVITNP>2.0.CO;2.
  16. ↑ Jin, Fei-Fei (1997). "A Theory of Interdecadal Climate Variability of the North Pacific Ocean–Atmosphere System". Journal of Climate 10 (8): 1821–1835. Bibcode:1997JCli...10.1821J. doi:10.1175/1520-0442(1997)010<1821:ATOICV>2.0.CO;2. Retrieved 2010-10-07.
  17. ↑ Ceballos, Lina; Emanuele Di Lorenzo; Carlos D. Hoyos; Niklas Schneider; Bunmei Taguchi (2009). "North Pacific Gyre Oscillation Synchronizes Climate Fluctuations in the Eastern and Western Boundary Systems". Journal of Climate 22 (19): 5163–5174. Bibcode:2009JCli...22.5163C. doi:10.1175/2009JCLI2848.1. Retrieved 2010-10-07. Cite uses deprecated parameter |coauthors= (help)
  18. 1 2 Mantua, Nathan J.; Hare, Steven R. (1 January 2002). "The Pacific Decadal Oscillation" (PDF). Journal of Oceanography 58 (1): 35–44. doi:10.1023/A:1015820616384. Retrieved 24 May 2013.
  19. ↑ Power, S.; et al. (1998). "Australian temperature, Australian rainfall and the Southern Oscillation, 1910-1992: coherent variability and recent changes" (PDF). Australian Meteorological Magazine 47 (2): 85–101. Retrieved 8 April 2013.
  20. ↑ McCabe, G. J.; Palecki, M. A.; Betancourt, J. L. (11 March 2004). "Pacific and Atlantic Ocean influences on multidecadal drought frequency in the United States" (PDF). Proceedings of the National Academy of Sciences 101 (12): 4136–4141. Bibcode:2004PNAS..101.4136M. doi:10.1073/pnas.0306738101. PMC 384707. PMID 15016919. Retrieved 24 May 2013.
  21. ↑ Krishnan, R.; Sugi, M. (31 August 2003). "Pacific decadal oscillation and variability of the Indian summer monsoon rainfall". Climate Dynamics 21 (3-4): 233–242. Bibcode:2003ClDy...21..233K. doi:10.1007/s00382-003-0330-8.
  22. ↑ Shen, Caiming; Wei-Chyung Wang; Wei Gong; Zhixin Hao (2006). "A Pacific Decadal Oscillation record since 1470 AD reconstructed from proxy data of summer rainfall over eastern China". Geophys. Res. Lett. 33 (3). Bibcode:2006GeoRL..3303702S. doi:10.1029/2005GL024804. Retrieved 2010-10-26.
  23. ↑ D'arrigo, R.; Wilson R. (2006). "On the Asian Expression of the PDO". International Journal of Climatology 26 (12): 1607–1617. Bibcode:2006IJCli..26.1607D. doi:10.1002/joc.1326.
  24. ↑ MacDonald, G.M.; Case R.A. (2005). "Variations in the Pacific Decadal Oscillation over the past millennium". Geophys. Res. Lett. 32 (8). Bibcode:2005GeoRL..3208703M. doi:10.1029/2005GL022478. Retrieved 2010-10-26.
  25. ↑ Rein, Bert; Andreas Lückge; Frank Sirocko (2004). "AA major Holocene ENSO anomaly during the Medieval period". Geophys. Res. Lett. 31 (17). Bibcode:2004GeoRL..3117211R. doi:10.1029/2004GL020161. Retrieved 2010-10-26.
  26. ↑ Seager, Richard; Graham, Nicholas; Herweijer, Celine; Gordon, Arnold L.; Kushnir, Yochanan; Cook, Ed (2007). "Blueprints for Medieval hydroclimate". Quaternary Science Reviews 26 (19–21): 2322–2336. Bibcode:2007QSRv...26.2322S. doi:10.1016/j.quascirev.2007.04.020. Cite uses deprecated parameter |coauthors= (help)
  27. 1 2 3 Deser, Clara; Phillips, Adam S.; Hurrell, James W. (2004). "Pacific Interdecadal Climate Variability: Linkages between the Tropics and the North Pacific during Boreal Winter since 1900". Journal of Climate 17 (15): 3109–3124. Bibcode:2004JCli...17.3109D. doi:10.1175/1520-0442(2004)017<3109:PICVLB>2.0.CO;2.
  28. ↑ Minobe, Shoshiro; Atsushi Maeda (2005). "A 1° monthly gridded sea-surface temperature dataset compiled from ICOADS from 1850 to 2002 and Northern Hemisphere frontal variability". International Journal of Climatology 25 (7): 881–894. Bibcode:2005IJCli..25..881M. doi:10.1002/joc.1170.
  29. ↑ Hare, Steven R.; Mantua, Nathan J. (2000). "Empirical evidence for North Pacific regime shifts in 1977 and 1989". Progress in Oceanography 47 (2–4): 103–145. Bibcode:2000PrOce..47..103H. doi:10.1016/S0079-6611(00)00033-1.
  30. ↑ Trenberth, Kevin; Hurrell, James W. (1994). "Decadal atmosphere-ocean variations in the Pacific". Climate Dynamics 9 (6): 303–319. Bibcode:1994ClDy....9..303T. doi:10.1007/BF00204745.
  31. ↑ Yasunaka, Sayaka; Kimio Hanawa (2003). "Regime Shifts in the Northern Hemisphere SST Field: Revisited in Relation to Tropical Variations". Journal of the Meteorological Society of Japan 81 (2): 415–424. doi:10.2151/jmsj.81.415. Retrieved 2010-10-26.
  32. ↑ Chavez, Francisco P; John Ryan, Salvador E. Lluch-Cota, Miguel Ñiquen C. (2003). "From Anchovies to Sardines and Back: Multidecadal Change in the Pacific Ocean". Science 299 (5604): 217–221. Bibcode:2003Sci...299..217C. doi:10.1126/science.1075880. PMID 12522241. Cite uses deprecated parameter |coauthors= (help);
  33. ↑ Bond, N.A.; J. E. Overland; M. Spillane; P. Stabeno (2003). "Recent shifts in the state of the North Pacific". Geophys. Res. Lett. 30 (23). Bibcode:2003GeoRL..30wCLM1B. doi:10.1029/2003GL018597.
  34. ↑ Alexander, Michael A.; Ludmila Matrosova; Cécile Penland; James D. Scott; Ping Chang (2008). "Forecasting Pacific SSTs: Linear Inverse Model Predictions of the PDO". Journal of Climate 21 (2): 385–402. Bibcode:2008JCli...21..385A. doi:10.1175/2007JCLI1849.1.
  35. ↑ Vimont, Daniel J.; John M. Wallace; David S. Battisti (2003). "The Seasonal Footprinting Mechanism in the Pacific: Implications for ENSO". Journal of Climate 16 (16): 2668–2675. Bibcode:2003JCli...16.2668V. doi:10.1175/1520-0442(2003)016<2668:TSFMIT>2.0.CO;2.
  36. ↑ Meehl, Gerard A.; Aixue Hu; Benjamin D. Santer (2009). "The Mid-1970s Climate Shift in the Pacific and the Relative Roles of Forced versus Inherent Decadal Variability". Journal of Climate 22 (3): 780–792. Bibcode:2009JCli...22..780M. doi:10.1175/2008JCLI2552.1.
  37. ↑ Mochizuki, Takashi; Masayoshi Ishii; Masahide Kimoto; Yoshimitsu Chikamotoc; Masahiro Watanabec; Toru Nozawad; Takashi T. Sakamotoa; Hideo Shiogamad; Toshiyuki Awajia; Nozomi Sugiuraa; Takahiro Toyodaa; Sayaka Yasunakac; Hiroaki Tatebea; Masato Moric (2010). "Pacific decadal oscillation hindcasts relevant to near-term climate prediction". PNAS 107 (5): 1833–1837. Bibcode:2010PNAS..107.1833M. doi:10.1073/pnas.0906531107. Cite uses deprecated parameter |coauthors= (help)
  38. ↑ Wang; et al. (2009). "Recent rainfall cycle in the Intermountain Region as a quadrature amplitude modulation from the Pacific Decadal Oscillation". Geophysical Research Letters 36. Bibcode:2009GeoRL..36.2705W. doi:10.1029/2008GL036329.
  39. ↑ Wang; et al. (2012). "Multidecadal Drought Cycles in the Great Basin Recorded by the Great Salt Lake: Modulation from a Transition-Phase Teleconnection". Journal of Climate 25: 1711–1721. doi:10.1175/2011JCLI4225.1.

Further reading

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