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ENSO-forced variability of the Pacific Decadal Oscillation

ENSO-forced variability of the Pacific Decadal Oscillation . Matt Newman NOAA-CIRES CDC. See Newman, Compo, and Alexander, Dec. 2003, J. Climate . . Is the PDO independent of ENSO?. From Alexander et al 2002. Re-emergence. Depth (m). From Alexander et al 2002. Null hypothesis:.

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ENSO-forced variability of the Pacific Decadal Oscillation

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  1. ENSO-forced variability of the Pacific Decadal Oscillation Matt Newman NOAA-CIRES CDC See Newman, Compo, and Alexander, Dec. 2003, J. Climate.

  2. Is the PDO independent of ENSO?

  3. From Alexander et al 2002

  4. Re-emergence Depth (m) From Alexander et al 2002

  5. Null hypothesis: • PDO as an AR1 (red) process + ENSO forcing

  6. Data • Hadley Ice Sea Surface Temperature Analysis (1900-1999) and Reynolds SST (2000-2001) • Monthly mean SST anomalies determined from 1950-2001 annually-varying climatology • Same results (not shown) for 1950-2001 period with Reynolds Reconstructed SST

  7. Indices [1900-2001] • PDO (Pacific Decadal Oscillation) • Projection of North Pacific SST upon leading EOF of monthly North Pacific (north of 20N) SST for the years 1950-2001 (Mantua et al 1997) • ENSO (Leading tropical Pacific PC) • Projection of tropical Pacific SST upon leading EOF of monthly tropical Pacific (20N-20S, 120E-60W) SST for the years 1950-2001 • NPI (North Pacific Index) • Area-weighted SLP in region 30-65 N, 160E-140W (Trenberth and Hurrell 1994) • All indices smoothed with 3-month or 12-month running means; 102-yr linear trend removed

  8. Indices (note: not detrended here, but centered)

  9. r (JUN YR 0, DEC YR+1) r(FEB, FEB) r (DEC YR 0, JUN YR-1) “spring persistence barrier” Annual cycle of lead-lag correlation (1950-2001)

  10. Mar yr+1 Mar yr+2 Mar yr-1 r (Feb yr 0 PDO, Mar yr +1 PDO) r (Mar NPI, Dec PDO)

  11. Wavelet analysis (1900-2001)

  12. 4-9 yr band: r = 0.71 10-64 yrs: r = 0.61 40-64 yrs: r = 0.98 PDO and ENSO are coherent on both interannual and decadal periods

  13. variance of decadal averages : variance of seasonal averages PDO 0.54 ENSO 0.32 NPI 0.30

  14. A null hypothesis where July-June mean PDO index July-June mean ENSO index white noise applied to annual mean anomalies

  15. “Forecast” PDO vs. observed PDO r = 0.74 Forecasts are cross-validated Skill is significantly better (95% level) than AR1 model

  16. PDO Mean model PDO ENSO Denominator < 1 for T > 5 Power spectra (annual means) PDO response to ENSO forcing is redder than ENSO forcing

  17. ENSO-PDO relationship:A good test for coupled GCMs

  18. power*frequency power*frequency frequency (yr-1) Comparison of observed power spectra (gray shading) with CMIP2+power spectra[annual means]

  19. a >> b a << b “Null hypothesis” applied to CMIP2+ output

  20. Summary Slide • Null hypothesis of the PDO: • North Pacific SST integrates effects of ENSO via the atmospheric bridge • Re-emergence brings back ENSO-induced anomalies in succeeding winters (no summer/fall PDO) • PDO should be redder than ENSO, e.g.…

  21. North Pacific SST anomaly should be greater than Tropical Pacific SST anomaly on decadal timescales:

  22. Penultimate Slide • Consequences for analysis: • Don’t stratify ENSO into high/low PDO years? • Summertime PDO: just ENSO? • Spring PDO: more than ENSO? • What other “climate integrators” might redden ENSO? • Tree rings • Drought (soil moisture anomalies)

  23. Last Slide • What part of predictable North Pacific decadal variability is independent of the Tropics? • Data is limited • Coupled GCMS must reproduce not only ENSO power spectrum… • … but also [seasonal cycle of] a and b

  24. PDO forecast • Continued positive next year (assuming that we do not have a cold event in the Tropical Pacific) For more, see Newman, Compo, and Alexander, Dec. 2003, J. Climate, 16, 3853-3857.

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