>My post “Misunderstandings about the PDO – REVISED” also ran at WattsUpWithThat (WUWT) and received a number of comments, some agreeing, others disagreeing. A link to the WUWT version:
In that post, I illustrated how the Pacific Decadal Oscillation (PDO) is calculated, what it represents, and what it does not represent. I also quoted from a Newman et al (2003) paper that appears to have been controversial. Link to Newman et al:
Most bloggers do not have the time to sort through all of the comments, so I’ve reworded and expanded on a few of my replies from the WUWT version of the post. I’ve also added a few more illustrations to reinforce a specific point.
THE PDO LAGS ENSO
Bloggers many times note that El Nino events dominate ENSO during the positive phase of the PDO, and La Nina events prevail during the negative PDO phase. It would be difficult, however, for the phase of the PDO to dictate whether El Nino or La Nina events dominate an epoch if the PDO lagged ENSO. And the PDO does lag ENSO.
The calculation of the PDO is based on the methods used in the Zhang et al (1997) paper “ENSO-like Interdecadal Variability: 1900–93”. Refer to:
Zhang et al refer to the PDO as “NP”, and, for an ENSO index, they use the Cold Tongue Index (CT) in place of NINO3.4 SST anomalies, which are used more frequently now. The Cold Tongue Index represents SST Anomalies of 6S-6N, 180-90W, where NINO3.4 SST Anomalies represent the area of 5S-5N, 170W-120W. In Figure 7 of Zhang et al, they illustrate the cross-correlation functions between the Cold Tongue and the other time series they examined. Note how in the bottom cell NP (PDO) lags (CT) ENSO by approximately 3 months.
Zhang et al Figure 7
They wrote on page 1011 (pdf page 8), “Figure 7 shows the cross-correlation function between CT and each of the other time series in Fig. 5. The lag is barely perceptible for TP and G and it increases to about a season for G – TP and NP, confirming that on the interannual timescale the remote features in THE PATTERNS SHOWN IN Fig. 6 ARE OCCURRING IN RESPONSE TO THE ENSO CYCLE RATHER THAN AS AN INTEGRAL PART OF IT, consistent with the conclusions of Alexander (1992a,b) and Yulaeva and Wallace (1994).” [Emphasis added]
Their Figure 6 shows the spatial pattern for the North Pacific associated with the PDO:
Zhang et al Figure 6
They also observed the interdecadal variability of the PDO (NP), but did not appear to feel it conflicted with the above findings that the PDO occurs in response to ENSO. On page 1012 (pdf page 9) they wrote, “In summary, of the time series in Fig. 8, CT is most strongly dominated by the interannual variability associated with the ENSO cycle, while G – TP and NP exhibit the clearest evidence of interdecadal variability. This distinction is also evident in the autocorrelation functions shown in Fig. 9: CT’s negative sidelobe reflects the ENSO cycle, WHILE NP’S POSITIVE VALUES OUT TO LAGS OF 5 yr AND BEYOND REFLECT THE GREATER PROMINENCE OF INTERDECADAL VARIABILITY.” [Emphasis added]
Zhang et al Figure 9
THE PDO, IN AND OF ITSELF, DOES NOT RAISE AND LOWER GLOBAL TEMPERATURE ACCORDING TO ITS PHASE
The PDO represents a pattern of SST anomalies; it does not represent SST anomalies of the North Pacific. In the original post, I also wrote something to the same effect. Why is this important? The often-repeated comment by many bloggers is that global temperatures rise when the PDO is positive and global temperatures decline when the PDO is negative. The visual correlation exists for that argument, but it’s the average SST anomalies for the North Pacific that dictate whether the area is contributing to or impeding the rise or fall in global temperature.
The following SST anomaly map was cropped from Figure 7 from the original post. The larger version is here:
It shows the North Pacific, North of 20N. That is the only part of the Pacific Ocean expressed by the PDO. The PDO illustrates nothing more, only the pattern of SST variability for that area. The illustration is from the NASA Earth Observatory webpage here: http://earthobservatory.nasa.gov/IOTD/view.php?id=8703
Specifically, this linked page: http://earthobservatory.nasa.gov/images/imagerecords/8000/8703/sst_anomaly_AMSRE_2008105_lrg.jpg
The Area Represented By The PDO
The illustration shows SST anomalies (Correction/clarification: It shows the PDO) in a cool phase, which means the SST anomalies in the eastern North Pacific are cool while the SST anomalies in the central and western portions are warm. But note that the warm area is significantly larger than the cool area in the east. The average SST anomalies for the North Pacific north of 20N in that case are probably positive even though the PDO is in the cool phase. And if the average SST anomaly is positive, it is contributing more positive anomalies to the global average than “normal”.
The following graph illustrates the PDO and the temperature difference between the SST anomalies of the North Pacific north of 20N (the same area as the PDO) and Global Temperature anomalies. That second dataset is calculated as North Pacific SST anomalies minus Global Temperature anomalies. Note that between the early 1940s and the late 1970s, the PDO was below zero (in the cool phase) for the most part. But during that same period, North Pacific SST anomalies were greater than Global temperature anomalies, so that part of the Pacific Ocean was actually contributing positive anomalies to the global average–in other words, it was heating.
PDO vs North Pacific SST Anomalies Minus Global Temperature Anomalies
I do understand that while the PDO is in the cool phase, other parts of the Pacific are NORMALLY cooler than normal, like the eastern tropical Pacific, like the equatorial Pacific (the NINO areas), etc. And someone could try to argue that fact. The point is, the PDO only deals with a specific area of the North Pacific. Nothing else. There is another dataset to express the pattern of variability in the entire Pacific basin, and it’s called the Interdecadal Pacific Oscillation or IPO. And there’s another dataset for discussions of the pattern of variability for the global ocean called the “G” Time Series. If someone wants to discuss the eastern equatorial Pacific SST anomalies, there are the NINO indices and the Cold Tongue Index (CTI).
TEMPERATURES IN THE PACIFIC NORTHWEST DO CORRELATE WITH THE PDO
The SST anomalies in the Northeast portion of the North Pacific tend to agree with the phase of the PDO. That is, when the PDO is positive, the SST anomalies in the Northeast “coastal region” of the North Pacific also tend to be positive. Additionally, the Eastern North Pacific SST anomalies of that near coastal area impact Western North America Land Surface Temperature anomalies, as they should. The following graph confirms that fact:
Eastern Northeast Pacific SST Anomalies vs Pacific Northwest LST Anomalies
Hence, if the PDO is positive, Pacific Northwest land surface temperature anomalies tend to be above normal, and the reverse occurs when the PDO is negative. But the Pacific Northwest only represents a small portion of the globe.
Keep in mind, on the other side of the Pacific, the Western North Pacific SST anomalies have an impact on Eastern Asian Land Surface Temperatures, and here’s that graph:
Western North Pacific SST Anomalies vs Eastern Asian Land Surface Temperatures
The areas included in the two preceding graphs are:
Areas Used For Above Two Comparisons of SST and LST
ENSO, NOT THE PDO, DOMINATES THE PACIFIC
The KNMI Climate Explorer allows users to compute and illustrate EOFs of datasets. The following series of four maps show the EOFs (November through February) of HADSST SST anomalies (1850 to 2008) for the North Pacific, North of 20N, which is the area included in the PDO. The pattern shows a positive PDO.
North Pacific EOFs
But in the next set of four maps of that same EOF analysis, I’ve expanded the viewing area to the entire Pacific. The eastern equatorial Pacific dominates the maps. The ENSO regions are so dominant that the color scale shifts to accommodate it, muting the North Pacific.