# Presentation: 083434

## 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

- From Alexander et al 2002

- Depth (m)

## Null hypothesis:

- PDO as an AR1 (red) process
- + ENSO forcing

## 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

## 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

## Indices

- (note: not detrended here, but centered)

## Annual cycle of lead-lag correlation (1950-2001)

## Slide 10

## Wavelet analysis (1900-2001)

## PDO and ENSO are coherent on both interannual and decadal periods

## variance of decadal averages   :

- _variance of decadal averages _  :
- variance of seasonal averages
- PDO 	0.54
- ENSO	0.32
- NPI	0.30

## A null hypothesis

- where

- July-June mean PDO index

- July-June mean ENSO index

- white noise

- applied to annual mean anomalies

## “Forecast” PDO vs. observed PDO

*r *= 0.74

- Forecasts are cross-validated

***Skill is significantly better (95% level) than AR1 model***

## Power spectra (annual means)

***PDO response to ENSO forcing is redder than ENSO forcing***

## ENSO-PDO relationship:A good test for coupled GCMs

## Comparison of observed power spectra (gray shading) with CMIP2+power spectra[annual means]

## “Null hypothesis” applied to CMIP2+ output

## 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....

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

## 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)

## 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]  and 

## 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.