Guest post by Bob Tisdale
The first version of this post (The Common Misunderstanding About The PDO dated June 26, 2008) incorrectly described the method for calculating the Atlantic Multidecadal Oscillation. I originally intended to do a quick correction in agreement with my post The Atlantic Multidecadal Oscillation – Correcting My Mistake, but then I decided to expand this post.
Many climate change bloggers often note that global temperatures rise when the Pacific Decadal Oscillation (PDO) is positive and drop when the PDO is negative. They then make the assumption that it’s the PDO that causes global temperature to vary. To dispel this, let’s first examine what the PDO is.
THE PACIFIC DECADAL OSCILLATION
The Pacific Decadal Oscillation (PDO), Figure 1, is “derived as the leading PC of monthly SST anomalies in the North Pacific Ocean, poleward of 20N. The monthly mean global average SST anomalies are removed to separate this pattern of variability from any ‘global warming’ signal that may be present in the data.” The quote is from the JISAO website: http://jisao.washington.edu/pdo/PDO.latest
The main JISAO PDO webpage is here:
The semi-periodic variation in the PDO can be better seen when the data is smoothed with a 121-month running-average filter, Figure 2.
THE METHOD USED TO CALCULATE THE PDO
Nathan Mantua of the University of Washington and JISAO, in an email, described the process used to calculate the PDO. And it is a process:
“The full method for computing the PDO index came from Zhang, Y., J.M. Wallace, D.S. Battisti, 1997: ENSO-like interdecadal variability: 1900-93. J. Climate, 10, 1004-1020.
“They labeled this same time series “the NP index” (see their figs 5 and 6). The steps are listed below, and files described below can be found at: ftp://ftp.atmos.washington.edu/mantua/pdofiles/
* monthly 5×5 Hadley Center SST 1900-93
1. create monthly anomaly fields for all grid points
2. create a monthly mean global SST anomaly time series for all months, 1900-93, using gridpoints specified in file grid.temp.glob_ocean.977
3. create a “residual SST anomaly” field for the North Pacific by subtracting out the global mean anomaly from each North Pacific grid point in file grid.N_Pac_SST.resi.172 (20N-65N, only in Pacific Basin) for all months and locations
np_resi(mo,loc)= np_ssta(mo,loc) – global_mean(mo)
4. compute the EOFs of the North Pacific residual SST anomaly fields, and ignore all missing data point (set them to zeros)
5. the PDO index is the leading PC from the above analysis
6. for PDO index values post 1993, project observed ‘North Pacific residual SST anomalies’ onto the leading eigenvector (what we call the ‘PDO pattern’ of ssts) from the EOF analysis done in step 4. We now do this with the Reynold’s and Smith Optimally Interpolated SST (version 2) data.”
A link to the referenced Zhang et al (1997) paper is here:
The point of listing that multistep process was to show that the PDO is a statistically created dataset. Let’s look at what the PDO does not represent.
THE PDO DOES NOT REPRESENT NORTH PACIFIC SST ANOMALIES
SST anomalies for the North Pacific Ocean (20N-65N) and scaled PDO data are illustrated in Figure 3. The PDO does not represent SST anomalies for the North Pacific.
THE PDO DOES NOT REPRESENT DETRENDED NORTH PACIFIC SST ANOMALIES
The PDO is not calculated in the same fashion as the Atlantic Multidecadal Oscillation (AMO). NOAA ESRL calculates the AMO by detrending SST anomalies for the North Atlantic. Refer to The ESRL AMO webpage:
In Figure 4, the PDO (scaled) is compared to detrended North Pacific (North of 20N) SST anomalies (calculated the same as the AMO). While there are semi-periodic variations in detrended North Pacific SST anomalies, the PDO does not represent them.
THE PDO DOES NOT REPRESENT VARIATIONS IN THE DELTA T BETWEEN NORTH PACIFIC SST AND GLOBAL TEMPERATURES
Let’s subtract Global temperature anomalies (LST & SST) from North Pacific SST anomalies to see what that curve looks like. Refer to Figure 5. The PDO does not represent the difference between global temperature anomalies and North Pacific SST anomalies.
SO WHAT DOES THE PDO DESCRIBE?
The PDO represents a pattern of SST anomalies in the North Pacific. The operative word in that sentence is PATTERN. Figure 6 (from the JISAO PDO webpage) illustrates the warm and cool phases of the PDO. When the PDO is positive, SSTs in the eastern North Pacific are warmer than in the central and western North Pacific, and when the PDO is negative, the reverse is true.
Keep in mind, though, that the PDO data itself represents only the North Pacific, north of 20N, which I’ve blocked off in Figure 7. Figure 7 is a map of SST anomalies from April 14-21, 2008 that shows a negative PDO pattern. It’s from the NASA Earth Observatory webpage here:
Specifically, this linked page:
PDO VERSUS ENSO
There is also a popular belief that the sign of the PDO dictates whether El Nino or La Nina events dominate. There is, however, an analysis that contradicts that belief. Refer to:
And for those who enjoy PowerPoint presentations for the visuals:
In “ENSO-Forced Variability of the Pacific Decadal Oscillation”, Newman et al state in the conclusions, “The PDO is dependent upon ENSO on all timescales. To first order, the PDO can be considered the reddened response to both atmospheric noise and ENSO, resulting in more decadal variability than either. This null hypothesis needs to be considered when diagnosing and modeling ‘internal’ decadal variability in the North Pacific. For example, the observed spatial pattern of Pacific SST decadal variability, with relatively higher amplitude in the extratropics than in the Tropics, should be at least partly a consequence of a reddened ENSO response.”
In the introduction, Newman et al explain, “Anomalous tropical convection induced by ENSO influences global atmospheric circulation and hence alters surface fluxes over the North Pacific, forcing SST anomalies that peak a few months after the ENSO maximum in tropical east Pacific SSTs (Trenberth and Hurrell 1994; Alexander et al. 2002). This ‘atmospheric bridge’ explains as much as half of the variance of January-March seasonal mean anomalies of SST in the central North Pacific (Alexander et al. 2002). Furthermore, North Pacific SSTs have a multiyear memory during the cold season. Deep oceanic mixed layer temperature anomalies from one winter become decoupled from the surface during summer and then ‘reemerge’ through entrainment into the mixed layer as it deepens the following winter (Alexander et al. 1999). Thus, over the course of years, at least during winter and spring, the North Pacific integrates the effects of ENSO.” [Emphasis added]
They continue, “The prevailing null hypothesis of mid latitude SST variability posits that the ocean integrates forcing by unpredictable and unrelated weather, approximated as white noise, resulting in ‘reddened’ noise with increased power at low frequencies and decreased power at high frequencies (e.g., Frankignoul and Hasselmann 1977). In this paper, we propose an expanded null hypothesis for the PDO: variability in North Pacific SST on seasonal to decadal timescales results not only from red noise but also from reddening of the ENSO signal.”
Figures 8 and 9 are comparative graphs of the PDO and NINO3.4 SST anomalies, smoothed with 12-month and 121-month filters.
As discussed and illustrated, the PDO cannot directly explain global temperature variations because it represents a pattern of SST variability, not SST. And the Newman et al paper explains why the low frequency variations of the PDO are greater than ENSO. They write in their abstract, “Variability of the Pacific decadal oscillation (PDO), on both interannual and decadal timescales, is well modeled as the sum of direct forcing by El Nino-Southern Oscillation (ENSO), the ‘reemergence’ of North Pacific sea surface temperature anomalies in subsequent winters, and white noise atmospheric forcing.” [Emphasis added]
Do other areas of the Global oceans integrate the effects of ENSO like the North Pacific?
The links for the PDO data are included in the text of the post. HADISST NINO 3.4 SST anomaly data, HADISST North Pacific SST anomaly data, and the combined CRUTEM3+HadSST2 global temperature anomaly data are available through the KNMI Climate Explorer website: