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
Discover more from Watts Up With That?
Subscribe to get the latest posts sent to your email.
Bob,
I have some results you might find interesting, showing maybe even more clearly the correlation/correspondence between ENSO and PDO.
If you are interested, email me at blcjr2 at gmail dot com and I’ll send it to you.
Basil
bob
Your comments seem at odds with the quote from this web page ? Especially the last paragraph.
My own research of warm and cool temperature anomalies along the west coast of Canada and the western half of Canada is that they correlate with warm and cool phases of PDO. How do you explain that? Warm and cool PDO levels correlate and thus explain local temperatures which become part of the make up of global temperaure anomalies
“The PDO is often quantified by the use of an index, referred to as the PDO Index. The PDO Index is calculated by spatially averaging the monthly sea surface temperature (SST) of the Pacific Ocean north of 20°N. The global average anomaly is then subtracted to account for global warming (Mantua, 2000). Normally only October to March values are used in calculating the PDO index because year-to-year fluctuations are most apparent during the winter months (Mantua, 2001).
When the PDO Index is positive, waters in the north central Pacific Ocean tend to be cool, and waters along the west coast of North America tend to be warm. The opposite is true when the PDO Index is negative (Null, 2002).
The effects of the PDO are most drastic in the Pacific Northwest. In this region, a positive, or warm phase PDO, generally correlates with lower than average rainfall and higher than average air temperatures. Likewise, a negative, or cool phase PDO, correlates with relatively high precipitation rates and low air temperatures (Null, 2002).
http://ffden-2.phys.uaf.edu/645fall2003_web.dir/Jason_Amundson/pdoindex.htm
Vibenna – and anyone else concerned that once the PDO turns positive again, global temperatures will accelerate – as Bob pointed out, there is no extra heat built up since 2002, though the oceans still hold 84% of the past century global warming signal (IPCC), but you need to check out a few related things:
1. the carbon dioxide model uses an unproven multiplier (x3) in relation to water vapour, and on its own, the gas even when doubled in concentration can warm things only a little (between 0.5 and 1.0 C);
2. the global rise between 1980-2000 was taken as proof of the multiplier, but…
3. in 2007/8 re-analysis of upper ocean heat content revised the heat stored in that period downward by x2;
4. the computer models ignored the PDO (it wasn’t invented when they were created and they still have not incorporated these long term cycles), but assumed the global fall from 1945-1980 was due to anthropogenic sulphur – in 2005 new analysis of satellite data showed this was a false assumption (admitted also by the IPCC in 2007) and that the human effects were localised – the global effect was therefore natural and due to cloud and aerosols (which i assume are linked to ocean cycles).
Finally, this demonstrates that the current suite of computer models upon which projections of future warming are based are seriously flawed – this is not yet admitted and efforts are being made to incorporate ocean cycles.
And for the link to global temperatures – do not underestimate the spatial patterning around Alaska – this has a knock-on effect, delayed by a few years, on the Beaufort Gyre and the dynamics of the Arctic Basin – the cooling of SSTs off Alaskan pacific coasts in late 2006 and the 2007/2008 cooling of the Alaskan shelf, these will reverse the gyre and expel warm water from underneath the sea-ice, and also lead to less cloud (Arctic cloud increased above the ice from 1982-2000 by 14%). Although GHG theory expects a stronger effect in the water-vapourless Arctic, the clouds and warm seawater are the dominant factors – and these have now reversed their trend, hence in 2008, there was 9% more sea-ice in the summer.
It will be interesting to see whether an El Nino will drive world temperatures up – this was predicted for 2007 by Hadley – and they got it spectacularly wrong – that prediction was based on an incipient El Nino signal at the end of 2006.
The following areas correlate with PDO
http://ffden-2.phys.uaf.edu/645fall2003_web.dir/Jason_Amundson/trends.htm
matt v.: You wrote, “Your comments seem at odds with the quote from this web page ? Especially the last paragraph.”
The final paragraph agrees with what I’ve written. The “Pacific Northwest” Coast of North America, which is land, lies adjacent to the eastern North Pacific Ocean. During the cool phase of the PDO the eastern North Pacific (ocean) is cooler, so the Pacific Northwest Coast of North America (land) would in turn be cooler. But again, the western and central portion of the ocean surface area included in the PDO are warmer when the eastern part of the North Pacific Ocean is cool. The area included in the PDO is only sea surface, not the adjoining landmass.
bob
You are correct. I mixed my east and west and forgot that our west coast[land] is Pacific ocean’s east side[water]. It is the effect of this eastern half of the Pacific ocean that correlates with our air temperatures in the western half of our continent as the previous post showed.Since the jet streams go west to east , it impacts the weather all the way to the Great Lakes and many times to our Atlantic coast. These are significant weather fronts .
Thanks again. I have no further comments.
matt v.: Regarding you last comment, I agree that the Eastern North Pacific SST anomalies of the area included in the PDO impact Western North America Land Surface Temperature anomalies. No doubt about it. In fact, here’s a graph to confirm that fact:
http://i41.tinypic.com/2ee8uj7.jpg
HOWEVER
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.
http://i42.tinypic.com/20ppslw.jpg
Looks like a wash then for the direct impacts of the PDO on land surface temp anomalies. Why? During the extremes of the PDO, if the Eastern Pacific is warm or cool, the Western Pacific has changed in the opposite direction.
Again, the effects on global temperatures of the Pacific are dictated by the entire Pacific basin, not only the area of the North Pacific included in the PDO.
Also, here’s a map of the areas included in those graphs.
http://i43.tinypic.com/2w3z0o4.jpg
Regards
BOB
Great graphs. They show clearly how ocean SST affect climate in the Pacific region. What would CRUTEM 3 GLOBAL and HADISST Western Pacific look like . Perhaps HADISST Western Pacific is even a better indicator of global temperatures because it is more vast?
The traditional 2 cool and 2 warm periods are clearly better apparent.
Bob
HADSST 2NH for NORTHERN HEMISPHERE OCEAN SURFACE TEMPERATURE ANOMALIES and HADCRUT 3GGL for GLOBAL SURFACE TEMPERATURE ANOMALIES have a super fit . This is like AMO[ATLANTIC SST ]and PACIFIC STT
combined which match global surface temperatures.
http://www.woodfortrees.org/plot/hadsst2nh/from:1976/to:2009/plot/hadcrut3gl/from:1976/to:2009
And the reason is, the trade wind blows all that hot surface water up against landforms to the East of us. While we freeze in uncovered and upwelled cold water, Japan melts in the warm waters we blew over their way.
matt v, your graph begs the question why we continue to discuss climate change, CO2, and the Sun, when we should be discussing weather pattern variation. What’s that line from the Wizard of Oz? Something about it’s right in your own backyard?
matt v., regarding your graph: Land surface temperatures follow sea surface temperatures. Always have, alway will. LST exaggerates SST a little and there’s some other noise, but LST is pretty much just along for the ride.
Regarding the AMO: It’s calculated (method used by NOAA ESRL) by detrending North Atlantic SST anomalies. They aren’t the same, but are based on the same data.
Someone on topic: I just posted the April 2009 SST anomalies:
http://bobtisdale.blogspot.com/2009/05/april-2009-sst-anomaly-update.html
Regards
Bob / Pamela
Thanks for the data. There is a world out there that still thinks Co2 is responsible for climate warming and climate change . If you tell a lie often enough the myth becomes stronger than the truth.
Bob, thanks for the reply.
You responded “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”
Bob, if you inverted one of the graphs they would look a lot more similar!
Steven Garland: You wrote, “Bob, if you inverted one of the graphs they would look a lot more similar!”
Here’s the residual graph again, so you don’t have to scroll up.
http://i40.tinypic.com/w9d4k6.jpg
But in this correct form, it shows that during the cool phase of the PDO (between the early 1940s and late 1970s) the SST anomalies for the North Pacific were greater than Global SST anomalies. This means that the North Pacific was actually adding to the global SST anomalies during this period; i.e., it was warming.
Remember, it’s the small Eastern portion of the North Pacific being cool that’s indicative of the cool mode. The anomalies of the much larger central and western portion of the North Pacific are elevated and they represent a much greater area.
It’s the typical “cool” pattern over the rest of the Pacific that dictates whether global temperatures fall, and that pattern is a product of ENSO. (It’s also dominated by ENSO.)
The reverse holds true when the PDO is in the warm mode.
Regards