Misunderstandings about the Pacific Decadal Oscillation
Guest post by Bob Tisdale
INITIAL NOTE
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.
INTRODUCTION
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:
http://jisao.washington.edu/pdo/
http://i41.tinypic.com/vrq7uq.jpg
Figure 1
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.
http://i39.tinypic.com/20svqsx.jpg
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/
“Data used:
* monthly 5×5 Hadley Center SST 1900-93
“Method:
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:
http://www.atmos.washington.edu/~david/zwb1997.pdf
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.
http://i43.tinypic.com/29fp8ad.jpg
Figure 3
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:
http://www.cdc.noaa.gov/data/timeseries/AMO/
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.
http://i42.tinypic.com/17pev8.jpg
Figure 4
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.
http://i42.tinypic.com/345kgsk.jpg
Figure 5
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.
http://i39.tinypic.com/20v1934.jpg
Figure 6
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:
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
http://i39.tinypic.com/262prfa.jpg
Figure 7
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:
http://www.cdc.noaa.gov/people/gilbert.p.compo/Newmanetal2003.pdf
And for those who enjoy PowerPoint presentations for the visuals:
http://www.cpc.noaa.gov/products/outreach/proceedings/cdw28_proceedings/mnewman_2003.ppt
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.
http://i41.tinypic.com/fd4vgz.jpg
Figure 8
##########
http://i41.tinypic.com/n14010.jpg
Figure 9
CLOSING
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?
SOURCES
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:
http://climexp.knmi.nl/selectfield_obs.cgi?someone@somewhere
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Frankly this seems very circular and dissembling.
To seek to dismiss the PDO, even as it ahs been driving the temps down and changing rain patterns in a very predictible patter seems to be motivated by something other than seeking answers. It seems much more like seeking to squelch things that invalidate AGW.
hunter,
You may say that but I couldn’t possibly do so.
Sorry for posting this on this thread but I would like to hear some discussion on a research project done on African droughts . Here is a quote from the National Geographic article:”More and more, it’s starting to look like the AMO is a big player affecting climate change around the Northern Hemisphere, including drought variability over Western Africa and western North America,” Overpeck said.”
http://blogs.nationalgeographic.com/blogs/news/chiefeditor/2009/04/extreme-droughts-west-africa.html
Another article, which I can not find at the moment, also connected this event to the Little Ice Age.
Correction:
hunter,
You may say that but I couldn’t possibly comment.
alf,
When the air around the globe is warming the air circulations move poleward.
When that air is cooling the air circulations move equatorward.
The desert areas shift polewad or equatorward accordingly.
Currently the air is cooling so areas south of the Sahara will get drier after a period of alleviation whilst the air was warmimg.
It’s not just the AMO or PDO. What matters is the net warming or cooling effect of all the oceans combined as regards their effect on the air. That dictates the latitudinal positions of all the weather systems in so far as they move beyond normal seasonal variability.
Willis Eschenbach provided an interesting graph on CA back in Sept of 2007.
UAH Trop temps with and without El Nino effect.
This is his image…
http://homepage.mac.com/williseschenbach/MSU_and_el_nino.jpg
This is the page on CA….
Anthony Watts at UCAR
http://www.climateaudit.org/?p=1999
This is the reply / comment containing the image…
http://www.climateaudit.org/?p=1999#comment-133427
This is his post image comment….
As you can see, eliminating the El Nino effect removes some of the swings, but does not affect the trend.
——————————-
Hence, it would appear that if the general trend still exists after removing the El Nino effect then that general trend comes not from the ENSO but rather from another source.
On the other hand. If I look at the PDO by itself the only potentially significant real agreement (even after adjustment for volcanic activity) I see occurs in 1999 with the ‘great shift’. While investigating the divergences I compared NINO3.4 0l.v2 SST with UAH lower trop temps and found a surprising level of agreement between the two.
At first I thought the was a good deal of disparity but then it became clear that what I was seeing was an apparent lag between the Nino3.4 and the global trop temps which, to me, is logical.
Prior to the shift in 1999 there appears to be very close agreement, after the shift the only real disparity was a greater drop in temps in the Nino than in the Lower trop. That again makes logical sense since the rest of the oceans did not have the same drop the Nino did and the Lower trop is effected by ocean temps as a whole; global SST and UAH lower trop temps in close agreement.
Though I am not totally ready to file for divorce from my thoughts on the PDO this blog entry by Bob Tisdale has invoked a further review into the facts and I thank him for that.
Tony Hansen: You asked, “What percentage of the whole Pacific would this be?”
The area of the Pacific North of 20N should be approximately 22 to 25% of the Pacific Ocean. If you want to check my math, the Latitude/Longitude Distance Calculator helps:
http://www.nhc.noaa.gov/gccalc.shtml
You asked, “Why would the scale not be symmetrical?” Good question. The years JISAO used as “typical” years and the base years of the anomalies would dictate the scale. Maybe it’s like my Figure 5, where the data fit better with a scale that ran from -0.3 to 0.4 deg C.
Thanks Bob
As luck would have it. The new paper out by Douglass and Christy ( Limits on CO2 Climate Forcing from Recent Temperature Data of Earth ) has a great graph of the NINO / UAH comparison. Had I seen it earlier I would have spared myself a bit of work this morning. It shows the lag I was talking about in my previous post.
Anyway.. . it appears on page 20 of the PDF found here…
http://www.pas.rochester.edu/~douglass/papers/E&E%20douglass_christy-color.pdf
Steven Garland: “My understanding, after reading this article, is that the PDO represents the SST anomaly for the North Pacific after subtraction of the global SST anomalies (please correct me if I am wrong).”
You asked for a correction, so here it is. If global SST anomalies are subtracted from the SST anomalies of the North Pacific (north of 20N), creating a residual, the resulting curve is very similar to “North Pacific SST Anomalies MINUS Global Temperature Anomalies” curve in Figure 5. A North Pacific Residual (North Pacific SST Anomalies MINUS Global SST Anomalies) versus PDO comparison is here:
http://i40.tinypic.com/w9d4k6.jpg
The calculation of the PDO involves many more steps than a simple residual, as noted above in the post.
Thanks Bob Tisdale for all your efforts to help us understand that the PDO is not a climate driver. I am going to try this in everyday language, so the science will probably get botched. The PDO is not a driver of anything in and of itself; it tells us what has happened re changing ocean temperatures (in the North Pacific) because of certain changes in the global climate system, most specifically through ENSO. It tells us this because scientists have developed the statistics so it can do so. It “represents a pattern of SST anomalies”, of “SST variabilites”. Now, If ENSO “provides” the main temperature changes that instruments record, moving the North Pacific temperature from warm to cool in short bursts that eventually add up to approximately 30 year periods of more warm vs more cool we know as the PDO, then I think you are directing our attention to what drives or causes ENSO variability.
Your graph, Figure 4, in “Revisiting Bratcher and Giese (2002)” amazingly (to my eyes) shows the step change caused by NINO3 SST anomaly from an average of 0.003 Deg C from 1950-1975 ratcheting up to an average of 0.045 Deg C from 1978-Present. Therefore, ENSO initiated the warm PDO. In your other work, I can see the influence of ENSO on the Indian Ocean.
It seems that you are pointing us to one of the most exciting areas of research. ENSO appears to be extremely influential. It certainly doesn’t have to “do everything”, but what makes it so important? It appears to be a collection of “small” TSI variabilities (without cloud cover) affecting at the very least SSTs, input from the Humbolt Current, changing pressure gradients, winds, mixing of deep oceanic layers with other layers and the surface, and, I imagine, the influence of the topography and geothermal heating — in the largest global ocean and the greatest ocean extent of the tropics. What drives ENSO seems to be an important place to begin with this dynamic system.
About the AMO. I don’t yet understand its difference from the PDO, but where you graph them, I notice the AMO leads the PDO most of the time. I think you have suggested that ENSO also significantly influences the AMO.
Stimulating research. You certainly get an interesting back and forth with other knowledgable researchers. I hope you generate a intense new focus of research energies — but how to get the grants for something important rather than see them go to line the pockets of the corporate-marxist-university-government-publication elites.
AlexB: You wrote, “Maybe I’m reading this wrong but my best assessment is that the author is arguing that the PDO cannot be responsible for warming or cooling as it is not a direct indicator of SST anomalies in the north pacific or difference between SST and LST anomalies.”
You missed a few points. I tried to provide basic information for those new to the PDO as well as advanced discussion material for those who are well-versed. The intent of the post was to define what the PDO is, illustrate how it is calculated, illustrate what it is not, and present the findings of the Newman et al study, which indicated that the PDO is dependent on ENSO. I expected Newman et al to be controversial, and it was. I also linked the Zhang et al study, which was the basis for the PDO. Then in follow-up comments, I noted how the Zhang study reinforces Newman inasmuch as it concludes the spatial pattern and multidecadal nature of the PDO are lagged responses to ENSO.
Regards.
Syl: You wrote, “The problem here is that though the PDO has a ‘period’ of about thirty years and thus supposedly cancels out after about sixty years, that leaves room for an odd number of oscillations in a century.”
As you’ll note in Figure 9, the NINO3.4 data also has periods, during which El Nino events or La Nina events dominate. The two referenced papers indicate the PDO is an aftereffect of ENSO, so I’ll address the ENSO portion of Figure 9. The “cancels out” portion of your comment only works if the frequency and amplitude of the positive events are equal to the frequency and magnitude of the negative events. They clearly are not. El Nino (positive) events are clearly dominant. During periods when El Nino events dominate, global temperatures rise, and global temperatures decline when La Nina events dominate. The only period when that doesn’t work is the 1960s+, but during the 60s, volcanic aerosols overrode ENSO.
“Lubos Motl (09:54:48) :
The statement that “PDO is influenced by ENSO” contradicts very basic physical principles. PDO is the slower process among the two, so it can’t possibly be measurably influenced by faster processes such as ENSO.”
This assertion is false. If I hit one of those heavy boxing bags in rapid succession in a pattern, first on one side, then on the other, that patterned sequence of rapid blows will cause a slower-moving oscillation in the punching bag. This is true whethor or not the punching bag is already oscillating due to someone else hitting it, too. When the earth revolves around the sun once a year, the gravitational force between the two causes an oscillation of the sun (albeit a lot smaller magnitude) with a much slower cycle. The fact that the PDO oscillates on a much slower cycle than ENSO does not disprove the proposition that the PDO is a causal function of ENSO.
gary gulrud: You quoted Nate Mantua and wrote, “IMHO Money quote from Bob’s Mantua link.”
Thanks for emphasizing that. I believe it might also help explain another phenomenon. If you were to compare Global Temp and NINO3.4 from 1978 to 2007, you’ll note that the global temperature response to El Ninos was much greater than it was to La Ninas. Has that swapped now that the PDO is in the negative mode? Global temperature definitely responded to the 2007/08 La Nina. It’ll be interesting to see how global temps respond to an El Nino now.
Stephen Wilde: You quoted Zhang et al, then commented, “They are clearly hedging their bets as to whether there is a seperate mode of variability which operates independently of the ENSO influence. They recognise that others do think that other modes exist, as do I.”
I don’t interpret the quote as Zhang et al hedging their bets. To me they’re downplaying the referenced studies. In fact, their final sentence emphasizes that. They wrote, “In view of these problems and the limited length of the historical record, we are not convinced that a formal modal separation involving interdecadal variability is meaningful.” In other words, the other studies tried to separate the modes and they see no reason for doing so.
Basil: You wrote, “Is anybody actually denying an ENSO influence on the PDO?”
Based on the comments on past posts here at WUWT and at other bogs, there are many who don’t even consider it. The standard comment goes something to the effect, “Well now that the PDO has shifted global temperatures will drop.” The PDO is the only driver of climate in their eyes. Maybe I need to introduce the “G” Time Series data to those who don’t know it exists. It’s also from the Zhang et al paper.
http://jisao.washington.edu/data_sets/G/
You wrote, “So the PDO must be something more than just ENSO,” then asked, “Do you agree with that, or not?”
I don’t disagree with it. Newman et al and Zhang et al both fail to address the impact of Meridional Overturning Circulation in the North Pacific. I seem also to recall reading about teleconnections between the North Atlantic and North Pacific. There are a multitude of other factors that can influence both ENSO and the PDO on yearly, decadal, and multidecadal bases.
hunter: You wrote, “Frankly this seems very circular and dissembling.
To seek to dismiss the PDO, even as it ahs been driving the temps down and changing rain patterns in a very predictible patter seems to be motivated by something other than seeking answers. It seems much more like seeking to squelch things that invalidate AGW.”
I am in NO way attempting to squelch things that invalidate AGW. If you were to read my other posts here at WUWT or at my blog, you’d note that my primary empahsis is to illustrate phenomenon that downplay AGW. My hope with this post was to redirect the understanding of primary driver from the PDO to ENSO.
Bob,
Good answers but for the moment I’ll continue my view that there is something more than ENSO variability involved in the PDO, probably independent variation within the oceans.
I have no problem with you emphasising the ENSO influence on short term variability within the background PDO cycle.
Ninderthana: If I understand your argument, referring to Figure 9, you would have expected the PDO from the mid-70s to early 2000s to have reacted more like the PDO did from the 1920s to the 1940s. From the 1920s to the 1940s, the PDO rose gradually until it peaked in the late 1930s. But the period from the mid-70s to the early 2000s had the sharp rise at the beginning, which you believe contradicts the Newman hypothesis. Did I get that right?
My first thought: There were differences between the two periods. The latter period had two major El Nino events, where the early period had none. The latter period also had two significant volcanic eruptions, where the early period had none.
In the following graph, for the period of January 1975 to February 2009, I subtracted NINO3.4 SST anomalies from the PDO and compared it to NINO3.4 SST anomalies (as a reference for timing only) and comapred them to Northern Hemisphere Sato Index data (used also only for timing so the Sato Index scaling is unimportant). Then I smoothed the data with a 12-month filter.
http://i40.tinypic.com/otd739.jpg
In looking at the graph, two anomalous periods stand out, but there may be more than two. The first occurred after the 1982/83 El Nino and the 1982 eruption of El Chichon. The difference between the PDO and NINO3.4 SST anomalies rose in an atypical fashion. Did the El Chichon eruption alter the normal effects of ENSO on the North Pacific and, if so, what changed? The second anomalous period occurred after the 1997/98 El Nino. Did the 1997/98 El Nino shift enough heat to the North Pacific to upset the normal SST anomaly patterns of the PDO?
Since I can’t answer those questions, I can’t dismiss the Newman et al hypothesis (assuming I’ve correctly interpreted your argument).
Hmmn. I’m not sure whether I’m one of the “those” bloggers, but assuming I am, my argument is a little different. It is simply that long term climate oscillations modulate the warming trend. Currently, we seem to be in a cooling phase – that suppresses the indicators of warming, but doesn’t change the build up of heat. When we switch back into a warming phase, global temperatures will increase at double the average rate for the century.
So is the PDO cause or effect? I’m not too concerned. But I do think it is an indicator of the global climate oscillation. That oscillation is not a driver of temperatures, but a modulator.
30 years of cooling? Yay! I’m alright. Let the kids fry.
bob
I also thank you Bob for bringing up this track. It helps all of us to focus on a very vital topic. The one area that I still have probem is your statement ” PDO cannot directly explain global temperature variations because it represents a pattern of SST variability, not SST.” What is a “pattern of SST anomlies’, what is its make up and why do these anomalies do not affect temperatures ? To me, PDO patterns explains local , hemispheric and ultimately global tempertures as well as the global warm and cool periods very clearly . It may not be the prime or first level driver but whether ENSO events are the prime driver is also up for debate as we really do not understand what triggers them . We do not even understand what triggers or moves the deep ocean currents .
I would love to hear your comments on the make up of AMO, Bob
vibenna: You wrote, “Currently, we seem to be in a cooling phase – that suppresses the indicators of warming, but doesn’t change the build up of heat. When we switch back into a warming phase, global temperatures will increase at double the average rate for the century.”
Reallly???!!!
Where is the build up of heat? Temperatures are dropping in the lower troposphere. Sea surface temperatures are dropping. Ocean heat content is either flat or dropping, depending on the dataset. So where exactly is this heat being stored for future release?
Bob Tisdale, please forgive my earlier post. Delete it if you can, at least from your mind. I still fell into the same trap as many others. I have some questions that were struggling to be formed in that garbled attempt, in part because I follow your research, and in part because the oceans seem to me to be the most significant storage, heat-cold mixing, heat-releasing, and heat-circulating source on earth. I will try again later. Thanks for you unwavering attempts to clarify the PDO. You better be careful; soon you will gain a reputation something like Leif’s.
matt v.: You asked, “What is a “pattern of SST anomlies’, what is its make up and why do these anomalies do not affect temperatures?
In the following SST anomaly map, I’ve cropped Figure 7 from the post so that it only shows the North Pacific, North of 20N. That is the only part of the Pacific Ocean expressed by the PDO. Nothing else. (Before you comment on that, read the rest of this reply, please.)
http://i39.tinypic.com/4r5oxx.jpg
The illustration shows SST anomalies 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 warm 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”.
Now scroll up to Figure 5. It illustrates the PDO, and it also shows 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.
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 the area highlighted in Figure 7. 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).
You wrote, “I would love to hear your comments on the make up of AMO, Bob”
I covered the AMO in a recent post here:
http://bobtisdale.blogspot.com/2009/04/atlantic-multidecadal-oscillation.html
Based on the way it is calculated, the AMO is in no way related to the PDO.