PDO research: a few new insights into lingering questions and a simple method

for 1 year forecasting

Nate Mantua

Climate Impacts Group

October 21, 2003

El NiñoEl Niño

La NiñaLa Niña

Recent equatorial sea surface temperature history

http://iri.columbia.edu/

Jan 1983

Jan 1992Nov 1987

Dec 1997

Dec 2002

mid-1998to early 2001

Nov 1988

An intense An intense Aleutian Low Aleutian Low warms and warms and stratifies the stratifies the coastal oceancoastal ocean

Typical winter winds and jet Typical winter winds and jet stream during El Niño wintersstream during El Niño winters

The Pacific Decadal OscillationThe Pacific Decadal Oscillation

• an El Niño-like pattern of climate variability

• 20 to 30 year periods of persistence in North American and Pacific Basin climate

• warm extremes prevailed from 1925-46, and again from 1977-98; a prolonged cold era spanned 1947-76

19981925 1947 1977

Zhang, Wallace, Battisti, 1997 Journal of Climate

• interannual SST pattern looks like tropical ENSO

• interdecadal SST pattern has more amplitude in the North Pacific, and a “washed-out” ENSO-like pattern in the tropics

• simultaneous correlations between ENSO indices and the PDO index are weak

A history of the PDO

warmcool

warmA history of ENSO

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

questions

• Is PDO variability truly independent of tropical ENSO variability?

• Can we predict the PDO index? If so, how far into the future, and how skillfully?

Cool season Aleutian Low variability: (Nitta and Yamada 1989, J. Met. Soc. Japan; Trenberth 1990,

BAMS)

North Pacific winter climate “shifted” in 1977

~ 75% of the variance in the NPI occurs at Periods < 20 yrs(Deser et al. in review)

19251946

SST and SLP epoch differences: (1943-76)-(1925-42)

From ZWB 97 Fig 14

Links between the tropics and North Pacific during winter

Deser, Phillips and Hurrell, submitted to J. Clim (submitted Aug 2003)

• What are the origins of Interdecadal variability in the wintertime Aleutian Low (as tracked by the NPI)?

• What are the links to the tropical IndoPacific region?

• Commonly used ENSO indices (CTI, SOI) do not have clear interdecadal shifts ~1925 and ~ 1947

• CTI-NPI relationship not clear 1925-45

Deser et al., Fig 2

Deser et al. Fig 7

• For P>9yrs, the strongest correlations between NPI and tropical SSTs are in the Indian Ocean, far western Pacific, and off equatorial eastern/central Pacific

Epoch Differences in Clouds and Precip Deser et al. Fig 12

Tropical Time Series coherent with NPI decadal variabilityDeser et al. Fig 9

Composite tropical index and the NPIDeser et al. Fig 14

• A composite tropical index derived as the leading PC from the 6 tropical indices

Power Spectra and coherence squared for NPI and Tropical Index

Note the elevated power and coherence for periods of ~4-8 years and periods > ~20 years

Deser et al. Fig 15

• Many similarities, yet also some clear differences between the canonical ENSO signatures and those associated with interdecadal shifts– SSTs and stratus decks in the E. Eq. Pacific; SPCZ clouds

and precip; North Pacific SSTs; western Eq. Pacific SSTs

Deser et al., Fig. 17

ENSO-forced variability of the PDO Newman et al. J. Climate Letters (in press)

• PDO variability reflects a “reddened” response to atmospheric “white noise” and ENSO forcing (at all time scales)

• Extend the Hasselmann model:

Pn = Pn-1 + n

To include an ENSO forcing term (En)

Pn = Pn-1 +En + n

Climate index lag-correlations, Fig 1 from Newman et al.

• PDO index has most memory starting in winter

• NPI has very little memory• NPI leads PDO in winter and

spring; NPI from one winter correlates with PDO in the next winter

• ENSO variability best correlated with PDO when it leads by a few months, mostly in summer and winter

JFM to FMA

FMA NPIvs NDJ PDO(+9 mo)

Wavelet Power Spectra and for PDO and ENSO:

Fig 2 from Newman et al.

• Variations in spectral power is similar for both indices, though PDO is “redder”

• Energy at P >10 years

• North Pac “reddens” atmos noise+ENSO

PDO 54%

ENSO 32%

NPI 30%

Simple model performance

• Forecast vs Observed correlations

• Modeled time series power spectra not as “red” as observed PDO

Forecast vs. ObservedAnnual PDO (July-June)

Power Spectra

Period

Full model R = .74

Ignore ENSO R = .53

ENSO only R = .54

Observedforecast

Take home messages from Newman et al.

• The null hypothesis for North Pacific decadal variability should include “re-emergence” + “ENSO forcing”

• The PDO is dependent on ENSO at all time scales• Predicting PDO may be related to predicting

ENSO variability– a 2-year lead-time predictability limit on ENSO? If so,

it wont be possible to predict interdecadal shifts in PDO

The Forecast?

• July 2002-June 2003 PDO = 1.07

• July 2002-Oct 2003 Niño3.4 ~ 0.57

• Forecast Niño3.4 for Nov-June ~ 0 to .5, let’s guess .3

The Result?

Forecast for July 2003-June 2002

PDO ~ 0.80

More questions

• Why the lack of variability at periods ~8 to 14 years in both the tropical and midlatitude climate indices?

• What are the causes for interdecadal variability in the tropical ENSO? Might they be predictable?

• Does the definition of ENSO need to be broadened to include interdecadal variability?– Recent decision by US research scientists uses Nino3.4

values as sole criteria for El Niño/La Niña

Yeh and Kirtman 2003, JGRExamine annual SST data for 1950-2000 using extended EOF analysis to identify cyclical patterns of variability

• North Pacific SST variability has 2 distinct time scales: (1) interannual and (2) interdecadal; the patterns of interannual and interdecadal variability differ

• Interannual variability in the tropical Pacific is in equilibrium with interannual variability in the North Pacific

• Decadal variability in the North Pacific leads tropical decadal variability by ~ 5-7 years