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
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. If that is correct then I can’t really say I’m impressed. I would have thought it was obvious that any link between the PDO and global temperatures would most likely lie in the changes to ocean and air circulation patterns that the PDO might result in, not based on the method we use to measure it. I really don’t get the argument.
Well, here’s my take….
(1) The PDO is a descriptor of an underlying phenomenon that affects global temperature but is also an influence (below). But for the purposes of the climate models, it really doesn’t matter because the models don’t consider anything with a period of less than a century because the assumption is that these shorter periods will merely cancel out over the long term so can be ignored. Other temperature-affecting phenomena such as aerosols and volcanoes are not periodic and therefore are thrown into the mix by the models at random (volcanoes) or ad hoc (aerosols).
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. So the 20th century had two warmish phases but only one coolish phase thus the PDO didn’t have a chance to cancel out as ‘weather noise’.
The ‘weather noise’ will not cancel out for another twenty years or so, therefore the global temp rise so far must have part of its attribution handed over to the ocean which the models have not done.
(2) When I view the ‘overhead’ shots of negative PDO and positive PDO and where the warm SST’s vs the cooler SST’s in the northern pacific are, it reminds one more of a huge low vs a huge high with their counterclockwise vs clockwise energy flows.
ie, when the PDO is negative, the warm waters are in the middle (as would be the case under a gigantic high pressure system) and the cooler waters surround. the ‘flow’ would be clockwise and the ‘bottom’ of the ‘high’ would push equatorial waters westward and result in la nina conditions (cooler waters upwell to replace the displaced warmer water). When the PDO is positive that westward flow reverses or halts, allowing warmer water temperatures to build in the enso3.4 region giving el nino conditions.
But what would cause the PDO to ‘flip’ from positive to negative or vice versa? Surface winds and currents must play a roll here, no? Both from the ENSO region and from the periphery, especially the west coast of No Amer. These ‘highs’ and ‘lows’ drift and wobble and effect the flow of heat in the waters below (hence the noisy anomalies). ‘Drifts’ over areas of strong wind and current that are in place for other reasons may break up the flow around the ‘high’ or ‘low’ and wear it down and eventually there’s more warm/cold around the periphery than before and the pattern of wind between hot and cold, low and high pressure, eventually ‘congeals’ into the opposite pattern: the shift in the PDO.
So, basically, though ENSO does not directly dictate the PDO, it certainly influences it back. I’d say the PDO is the driver of ENSO here, but the feedback from prolonged ENSO trends, chaotic nudges, and the winds going along for the ride, gives us the flips and the flops in an amazingly consistent timeframe.
That’s my story and I’m sticking to it.
Stephen Wilde (12:31:32) : “I am conscious that ENSO in the form of El Nino and La Nina events seems to be wind driven to some extent but I am nevertheless unclear as to {1} whether the associated changes in SSTs cause further changes in the air as a positive feedback or {2} whether changes in the air are actually the drivers of the ENSO phenomenon.”
Good comment. I believe the former {1} is the case. My take on it is this: “normal or La Nina conditions” result in Earth-rotation-winds displacing a 1/2 meter high mound of water from the East and mid-Pacific (EP and MP) into the West Pacific (WP). This is obviously not a stable situation. Eventually (especially after a La Nina), the system pressure causes increasing amounts of cold water to well up in the EP. This lowers SST’s in the EP, which in turn cause two phenomena: higher seawater viscosity (see Note 2) and lower air T and humidity in the EP.
The air is thus denser and more difficult for the trade winds to move; the water is “stiffer” and also harder for the trade winds to move. This is effectively the same as an apparent decrease in wind velocities. The system can then reverse, spreading the heated water from the WP back across the MP and EP, into full El Nino conditions.
Note 1: radiation losses for highly differentiated EP/WP temperatures are theoretically higher than for more isothermal (average) conditions. However, I’m unsure at the moment whether the T^4 weighted average SST for a La Nina is greater or less than that for El Nino conditions.
Note 2: seawater viscosity is 100 times more sensitive than seawater density to temperature changes.
Good post.
But what would cause the PDO to ‘flip’ from positive to negative or vice versa? Surface winds and currents must play a roll here, no?
It follows the Southern Oscillation, an atmospheric oscillation (not dissimilar to the NAO) which is measured by a Tahiti/Darwin pressure differential.
The PDO may be said to be a result of the SO rather than a cause, in and of itself. One may even say it is a result of magnified Nino or Nina phases of the SO.
SO went positive in 1977 along with PDO. Don’t know where it is now, other than I guess it has probably recent gone cold.
P.S., I have just read two mentions that the AMO has gone into cold phase. And even some evil rumors going ’round about the the AO and NOA. Watts Up With That?
“Right now, despite the very negative PDO conditions, it appears that an El Nino is building. The Southern Oscillation Index has suddenly shifted to signal El Nino, the Trade Winds have slowed, Atmospheric Angular Momentum is now signaling El Nino and the Upper Ocean Heat Content is showing much warmer ocean temperature water ready to surface.”
Breathing in, breathing out. The oceans are ready to exhale.
Thar she blows! Or will it be merely a sigh.
So, what causes those 30 year phase changes ?
There is no mechanism whereby the much more rapid El Nino/ La Nina changes can cause such large background changes on that time scale.
We are now told that we cannot allocate the term ‘PDO’ to those phase changes because the PDO is simply an artifact arising from the SO and ENSO signals with no independent existence of it’s own.
I for one as well as many others have been using the term ‘PDO’ to refer to those 30 year phase changes in the Pacific so I think the best thing to do is either rename the phenomenon or change the definition of PDO.
There seems to be similar academic and media confusion between the more rapid SST changes and underlying multidecadal cycles in every ocean.
There are multidecadal phase changes in every ocean.
They have profound effects on global air temperatures.
They interact with each other and solar variations.
They seem to occur independently of changes in the air alone.
They render the effect of CO2, and other changes in the air alone, insignificant.
I nominate the term ‘Wildean Ocean Cycles’.
All: Excuse me for disappearing yesterday, but something unexpected came up and took me away from home until late last night.
TO THOSE WHO CONTINUE TO INSIST THAT THE PDO DRIVES ENSO: Let me refer you to the linked Zhang et al “ENSO-like Interdecadal Variability: 1900–93”. As noted above, the calculation of the PDO is based on the methods used in that paper. They found that the PDO lags ENSO by about a season and concluded that spatial pattern of the PDO is a response to ENSO, not vice versa. And they also found that the interdecadal variability of the PDO was a lagged response to ENSO. Refer to:
http://www.atmos.washington.edu/~david/zwb1997.pdf
Zhang et al refer to the PDO as “NP”, and they use the Cold Tongue Index (CT), which are SST Anomalies of 6S-6N, 180-90W, in place of NINO3.4 SST Anomalies of 5S-5N, 170W-120W. In Figure 7, 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. Their Figure 7 is copied here:
http://i39.tinypic.com/14o3beb.jpg
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:
http://i44.tinypic.com/112h3k8.jpg
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]
Their Figure 9 is here:
http://i41.tinypic.com/200zfk0.jpg
Regards
matt v: You wrote, “You and I have blogged on this topic before. If “The PDO represents a pattern of SST anomalies in the North Pacific” and if this pattern of anomalies are indicators of certain weather and climate conditions or patterns especially in the Northern hemisphere including land temperatures in North America which repeat over a period of time , then various levels of PDO can be used as predicters or indicators of past and future of climate patterns .It may not be a direct indicator of Pacific Ocean SST but it does reflect a multiple of things which originate from SST, does it not.?”
It does. In his paper “The Pacific Decadal Oscillation and Climate Forecasting for North America”, Mantua did list the temperature, precipitation, etc. correlations with the PDO, which would then aid in prediction. Refer to:
http://www.atmos.washington.edu/~mantua/REPORTS/PDO/PDO_cs.htm
You wrote, “You explain well, Bob, what PDO is not, but do not explain well what it is, what it can be used for and why it was developed in the first place. The definition in the CLOSING is far too confusing for any blogger.”
Sorry, but there are just so many topics one can cover in a post. Hopefully the above link will help explain what it can be used for. The method of calculating the PDO was developed to explain the “ENSO-like Interdecadal Variability: 1900-93” in the Global Oceans, the North Pacific and others areas. Refer to the Zhang et al paper:
http://www.atmos.washington.edu/~david/zwb1997.pdf
With respect to the closing discussion, does my above comment (April 29 @ur momisugly 03:42:15) help explain it better? That the spatial pattern and interdecadal variations of the PDO are lagged responses to ENSO?
Regards
Stephen Wilde: You wrote, “I am conscious that ENSO in the form of El Nino and La Nina events seems to be wind driven to some extent but I am nevertheless unclear as to whether the associated changes in SSTs cause further changes in the air as a positive feedback or whether changes in the air are actually the drivers of the ENSO phenomenon.”
Refer to Bill Kessler’s discussion on ENSO. Trade winds, SST gradients across the tropical Pacific, and ENSO depend on one another.
http://faculty.washington.edu/kessler/occasionally-asked-questions.html#q1
You also wrote, “I am also conscious that the PDO phase shifts are much larger events over longer multidecadal time scales,” and “Importantly there is no need for the two processes to be linked.”
As noted in my April 29 @ur momisugly 03:42:15 comment, above, Zhang et al found the PDO pattern and the decadal variability to be lagged responses to ENSO.
“Recent studies suggest that ENSO teleconnections with North American climate are strongly dependent on the phase of the PDO, such that the “canonical” El Niño and La Niña patterns are only valid during years in which ENSO and PDO extremes are “in phase” (i.e. with warm PDO+El Niño, and cool PDO+La Niña, but not with other combinations) (Gershunov and Barnett 1999, Gershunov et al. 1999, McCabe and Dettinger 1999).”
IMHO Money quote from Bob’s Mantua link.
Curious: You wrote, “You say ‘The PDO is independent of global temperature.’”
If you were to replace the North Pacific SST anomalies in Figure 3 with Global Temperature, and have global temperatures compared to the PDO, the same PDO value and pattern have occurred at various global temperatures. And I also showed in Figure 5 that the Delta T between global temperature and the North Pacific SST do not correlate. (Before you reply, continue to read this comment.)
You asked, “Are you saying that the PDO is not statistically correlated with the rate of change of global temperature anomaly…”
As noted in a number of posts at my blog, a scaled running total of Hadley Centre’s NINO3.4 SST anomalies will reproduce the global temperature anomaly curve.
http://i39.tinypic.com/2w2213k.jpg
The graph is from my post “Reproducing Global Temperature Anomalies With Natural Forcings”
http://bobtisdale.blogspot.com/2009/01/reproducing-global-temperature.html
The curve of a running total (not scaled) of PDO reflects the variations, but not the Global Temperature curve.
http://i39.tinypic.com/6iy3ro.jpg
Even if I could coax the global temperature anomaly curve from the PDO, it’s immaterial, because as I noted in my April 29 @ur momisugly 03:42:15 comment above, Zhang et al found the PDO pattern and the decadal variability to be lagged responses to ENSO, So any global temperature response to changes in the PDO are ultimately responses to ENSO.
Bob,
I’ve looked at the Zhang study and found the following towards the end:
“It is difficult to compare the present study with those
that have attempted to recover separate ‘‘modes’’ of
variability using techniques such as conventional EOF
analysis (Parker and Folland 1991; Miller et al. 1994;
Deser and Blackmon 1995), rotated EOF analysis (Kawamura
1994), or Principal Oscillation Pattern analysis
(Latif et al. 1997) because the interdecadal variability
that we have identified exhibits a spatial signature that
is similar, in so many respects, to that associated with
the ENSO cycle. The confusion is exacerbated by the
general upward trend in global-mean SST over the century,
which renders the results of conventional EOF
analysis difficult to interpret. 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.”
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 find it difficult to exclude the possibility that varying solar input to the oceans over several solar cycles has an effect on the energy characteristics within the oceans which in turn affects movement of large bodies of water within the oceans.
If we describe ALL the observed oceanic effects on the air as consequences of processes in the air driving the oceans (as you seem to suggest) then that presupposes a static oceanic background despite an ever changing solar effect on ocean dynamics over multiple decades or even centuries.
I think that treatment of the main body (as opposed to the surface) of the oceans as an unchanging monolith for energy absorption and release purposes is a major defect in AGW theory and the main defect in the point of view put forward at the start of this thread.
PDO, AMO, NAO et.al. might all be statistical artifacts derived from observational SST data but to treat them as such does in my view deter one from considering modes of variation within the main body of the oceans themselves.
We should seperate PDO, AMO, NAO et.al. from their SST counterparts and look into the possibility that they are indeed independently driven and so affect the net balance between the different SST modes and thereby drive global air temperatures as much as is necessary to explain observed temperature changes without needing to attribute a large forcing to CO2.
We could also group them together under the generic name that I have suggested.
Bob,
Serious scientists working in the field know that the PDO is just an index
that represents a PATTERN in the sea surface temperature anomalies in the Nth central Pacific over time. Thanks however, for taking the time to point this out to the wider public.
You seem to be totally convinced by Newman et al. arguements even though it is obvious from the data and timing arguments that Newman et al. cannot be true.
Newman et al. basically argue that the PDO is a sea surface annomaly
pattern that is produced by the long term integration by the Nth. Pacific Ocean of the ENSO signal + white noise.
However, Newman et al. cannot explain the following sequence of events
that can be ascertained from Gergis & Fowler (2006)
See:
http://www.springerlink.com/content/2242tp4610562j55/
http://www.soest.hawaii.edu/~timm/files/workshop2007/gerigs.ppt
1) The PDO flips from negative to postive
Bob, you know that these switches (as indicated by your own graphs) are abrupt climate events like the great PDO shift in the mid 70’s. Normally
the flip in the PDO index from one extreme to the other only takes a couple of years.
2) Historical records (both instrumental and PROXY) show that the
PDO stays in a positive phase for any where from 10 – 40 years.
3) At the start of the postive PDO period, the strength of El Nino events
are typically weak.
4) As the postive PDO persists, the average strength of the EL Nino events
get’s strong and stronger.
5) The PDO eventually flips from positive to negative
– again abruptly over a period of a few years.
6) The average strength of the El Nino events start to decrease and
continue to decrease while the PDO remains negative.
This is NOT, repeat NOT, the time sequence that you would expect to observe if the PDO was just a long term intergration of the ENSO signal.
In fact if Newman et . al.’s model was correct you would expect the
gradual 10 – 40 year intensifications of the El Ninos to lead to slow
buildup in the PDO strength. This does NOT take place.
Hence, I believe that Newman et. al. ‘s model is directly contradicetd by the observations and so is false.
Sorry, but I cannot agree with Newman et al.
Bob,
I’m still trying to understand your main point. Here’s an abstract of another paper. Is this consistent with what you are trying to say?
The Pacific decadal oscillation (PDO), defined as the leading empirical orthogonal function of North Pacific sea surface temperature anomalies, is a widely used index for decadal variability. It is shown that the PDO can be recovered from a reconstruction of North Pacific sea surface temperature anomalies based on a first-order autoregressive model and forcing by variability of the Aleutian low, El Niño–Southern Oscillation (ENSO), and oceanic zonal advection anomalies in the Kuroshio–Oyashio Extension. The latter results from oceanic Rossby waves that are forced by North Pacific Ekman pumping. The SST response patterns to these processes are not orthogonal, and they determine the spatial characteristics of the PDO. The importance of the different forcing processes is frequency dependent. At interannual time scales, forcing from ENSO and the Aleutian low determines the response in equal parts. At decadal time scales, zonal advection in the Kuroshio–Oyashio Extension, ENSO, and anomalies of the Aleutian low each account for similar amounts of the PDO variance. These results support the hypothesis that the PDO is not a dynamical mode, but arises from the superposition of sea surface temperature fluctuations with different dynamical origins.
http://ams.allenpress.com/perlserv/?request=get-abstract&doi=10.1175%2FJCLI3527.1
Let’s suppose that this is correct, that the PDO is not a “dynamical mode.” To borrow terms from my discipline, I think that is simply saying that PDO is endogenous, not exogenous, i.e. it is the result of other forces, not a external force itself. It doesn’t change the fact that the PDO has characteristic modes, or phases, i.e. cool phases and warm phases, of decadal or even multi-decadal time scale.
Moreover, if, as the abstract I cite, and the Newman, et al paper you cite (“The PDO is dependent upon ENSO on all timescales.”) are correct that ENSO plays a part in the decadal pattern of variation in the PDO, how does that constitute a disproof of the proposition that during a PDO cool phase La Nina’s dominate, while during a PDO warm phase El Nino’s dominate?
And what is the bottom line here? Are you denying that the apparent recent shift of the PDO into a cool phase may be indicative of a climate shift that is likely to persist for some period of time?
Regardless of whether it is endogenous, or exogenous, there is plenty of evidence for a multi-decadal variation in climate here, isn’t there?
Basil
Whoops. Remove NAO from previous post. That is primarily atmospheric rather than oceanic.
Stephen,
I have recently attempted to assess your comment on solar impacts:
“I find it difficult to exclude the possibility that varying solar input to the oceans over several solar cycles has an effect on the energy characteristics within the oceans which in turn affects movement of large bodies of water within the oceans.”
Please see the bottom graph on this page, which shows that ENSO is controlled somewhat by high and low sunspot periods.
http://www.stormx.com/agriculture/weather-risk/2009/04/solar-activity-lowest-in-almost-100-years-implications-for-climate-potentially-significant/
I live in the high mountainous plains of NE Oregon. Understanding the PDO is essential for farmers, especially for those who grow less than robust crops. The lows, highs, and precip can be forecasted based on what the PDO is doing. That information tells farmers what to plant and when. Unless they are just plain dumb and plant spring wheat every year hoping this is the year it won’t freeze the tender spring shoots. Gardens are just now being plowed and no garden veggie seeds are in the ground yet that I can see (unless you are just plain dumb…see above). For those who understand this, seeds are being germinated in damp paper towels, etc. And recipes are being dusted off for green tomato salsa. Even peas will be a bit late this year but with the cool temps, we should have another bumper crop. Fast growing corn should be planted instead of the 10ft tall variety. I would skip potatoes. The tubers will freeze in the ground this fall before they are ready to be dug up. I would also put in the short season stubby carrot, not the long variety.
I could go on. You can see that this is just the tip of the iceberg when it comes to understanding the PDO and local weather pattern variation. It is information that makes or breaks the backs of farmers. I wish weather reports and services would go back to the days of reporting this kind of information instead of “climate change” warnings.
Bob Tisdale
Thanks for the answers. You said ” As discussed and illustrated, the PDO cannot directly explain global temperatures variations because it represents a pattern of SST variability, not SST” I think that the pattern that PDO index represents does reflect a changing SST pattern in the Pacific which is a forecaster of certain weather patterns and tempertaures for North America and hence the northern Hemisphere and the globe . In this context the pdo can explain global temperatures , in my opinion. I still disagree with your statement. It may not be a direct explanation but it does still explain global temperatures in my opinion.
Ian Holton
I agree with your oservations about El Nino’s . Here is my count of El Nino’s and La Nina’s during positive and negative phases of PDO.
DURING NEGATIVE OR COOL PDO AND AMO PHASE [1944-1976]
Number of La Nina’s
STRONG 2
MODERATE 4
WEAK 2
————————
Total 8
Number of El Nino’s
STRONG 1
MODERATE 3
WEAK 4
———————-
Total 8
DURING POSITIVE OR WARM PDO AND AMO PHASE [1977—2007]
Number of La Nina’s
STRONG 2
MODERATE 2
WEAK 3
————————-
Total 7
Number of El Nino’s
STRONG 3
MODERATE 2
WEAK 8
————————
Total 13
Lubos: You wrote, “If you averaged over 1-2 years, the ENSO index would give you an equally pronounced sin-like signal like PDO gives at the multidecadal scale.”
For you I’ve generated two more graphs. Here’s the PDO and ENSO 3.4 SST anomalies smoothed with a 25-month filter. The NINO3.4 data produces a signal that is more equally balanced than the PDO, but both have underlying variations as well.
http://i39.tinypic.com/23t5pi9.jpg
The second is a curve of the PDO MINUS NINO3.4 SST anomalies, as suggested by Carl Wolk. I will agree that the additional variations in the North Pacific have to influence the tropical Pacific, but…as noted in Zhang et al, the PDO pattern and the decadal variability are lagged responses to ENSO. Then the additional long-term variability in the North Pacific (referred to as the PDO) would be a decadal component of ENSO, providing feedback to the basic ENSO signal.
http://i40.tinypic.com/2mop4dj.jpg
Regards
Stephen Wilde: You wrote, “Well in my opinion something external to ENSO is causing those phase shifts and imposing them onto the ENSO cycle.”
Bratcher and Giese believe the shifts originated in the subsurface South Pacific. Refer to “Revisiting Bratcher and Giese (2002)”:
http://bobtisdale.blogspot.com/2009/04/revisiting-bratcher-and-giese-2002.html
The WUWT post of it is here:
http://wattsupwiththat.com/2009/04/10/revisiting-bratcher-and-giese-2002/
Also:
The long-term variations in the detrended North Pacific SST anomalies and the AMO (detrended North Atlantic SST anomalies) agree quite well. Are they climatically coupled?
http://i43.tinypic.com/2sbqtrk.jpg
And ENSO influences the AMO as discussed in my post “There Are ENSO Induced Step Changes In The North Atlantic”:
http://bobtisdale.blogspot.com/2009/02/there-are-also-el-nino-induced-step.html
And ENSO Influences AMOC. The following graph was taken from my post “Atlantic Meridional Overturning Circulation Data”:
http://i33.tinypic.com/5cyglz.jpg
Then there are what appear to be 100-year cycles in ~80% of the Southern Ocean (green curve).
http://i41.tinypic.com/qsjwwp.jpg
That’s from my post “A Closer Look At The ERSST.v3b Southern Ocean Data”:
http://bobtisdale.blogspot.com/2009/04/closer-look-at-ersstv3b-southern-ocean.html
Basil: You asked, “Moreover, if, as the abstract I cite, and the Newman, et al paper you cite (“The PDO is dependent upon ENSO on all timescales.”) are correct that ENSO plays a part in the decadal pattern of variation in the PDO, how does that constitute a disproof of the proposition that during a PDO cool phase La Nina’s dominate, while during a PDO warm phase El Nino’s dominate?”
It doesn’t disprove it at all. BUT, why not think of the domination of La Nina or El Nino events as dictating the sign of PDO? That would be more consistent with those papers and with the Zhang et al.
I keep seeing three solar cycles per PDO phase, six per cycle. All you need is something alternating each solar cycle, to put two of one type of solar cycle and one of the other into each phase of the PDO, thus giving alternating warming and cooling phases. Well, the shape of cosmic ray peaks alternate each cycle from pointed to broadened. And cosmic rays are tentatively linked to clouds and albedo. It isn’t enough to satisfy Leif, but the bare bones of a mechanism are there.
===========================================
Jeremy Ross, PhD: Thanks for the link. As soon as I catch up with replying to comments, I’ll enjoy reading it.
Let me pose something for you and for others to ponder. First, a scaled running total of Hadley Centre NINO3.4 SST anomalies reproduces the global temperature anomaly curve. Refer to:
http://bobtisdale.blogspot.com/2009/01/reproducing-global-temperature.html
Second, Newman et al wrote, “Thus, over the course of years, at least during winter and spring, the North Pacific integrates the effects of ENSO.”
Could the similarities of the scaled running total and global temperature anomalies indicate that additional portions of the global ocean also integrate the effects of ENSO?
I think this is still skating round the issue.
That issue is whether the ENSO cycle is sufficient on it’s own to account for observations without requiring any seperate input from independent oceanic variations.
From the links provided I get the impression that there is doubt and that there are differences of opinion on the matter.
For my part I cannot envisage there being no independent input from ocean variability.
Nor can I see how ENSO variability alone could give rise to phase shifts at 30 year intervals with a complete cycle of twice that.
It seems to me that the logical solution is that ENSO contributes air induced variations over short periods of time, seperate oceanic cycles contribute their own independent variations over longer periods of time and the general underlying trend is dependent on slow long term changes in solar output over many solar cycles.
On that basis one can explain the short term climate consequences of individual El Nino and La Nina events, the 30/60 year longer term variations in the dominance of El Nino and La Nina and also the slow rise in global air temperatures since the little ice age.
Indeed, one can also account for the stepped upward temperature movement when each positive longer term oscillation leaves the air temperatures a little higher than the one before as during the 20th Century.
Now if the sun were to go into a long term cooling trend (albeit very slow) then I would expect to see stepped downward temperature movements develop but it would take two more 30 year phase shifts for that to become apparent.
In the meantime a coincidence of weaker solar cycles and negative oceanic phases (especially if in all the oceans at the same time) would be capable of giving us a large a downward shift in temperatures as we saw a large upward shift in temperatures during the recent warming when we had both a more active sun and positive oceanic phases.
‘Large’ being a relative term of course. I’m quite sure that natural forcings can do far more than our records to date have ever noted.
Bob Tisdale (08:50:12) : It doesn’t disprove it at all. BUT, why not think of the domination of La Nina or El Nino events as dictating the sign of PDO? That would be more consistent with those papers and with the Zhang et al.
Okay, I must admit I misunderstood your:
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.
I now read you to be saying that the order of “causation” is reversed. However, without going back and looking at the Newman paper, or the Zhang paper, the paper I cited allows for decadal or multi-decadal influences on the PDO besides ENSO. In fact, if you are saying that “The PDO is dependent ONLY upon ENSO on all timescales” — then a disagreement remains. I would, for instance, leave room for various other influences, such as the lunar nodal cycle, solar, and atmospheric changes related to changing LOD influencing the multi-decadal climate regimes indicated by the PDO (which, as you know, is not limited to the instrumental period, but is indicated in proxy data).
Is anybody actually denying an ENSO influence on the PDO? I think the real debate, if there is one, is whether ENSO dominates the PDO to the point that we can ignore the concept of regime shift and long periods where climate regimes so dominate global temperatures that we cannot isolate the AGW influence from climate regime trends. I.e., if PDO can be reduced to ENSO, and if ENSO is basically an intradecadal phenomenon (say a 5-6 year cycle), then the multidecadal trend that began in the mid 1970’s must owe to AGW influences, and cannot be attributed in any significant measure to climate regimes.
Frankly, showing a relationship between PDO and ENSO at the intradecadal scale is trivial. In spectral terms, Nino 3.4 and PDO both show a strong peak at ~5.6 years. But the PDO shows multi-decadal trends that are not evident in the Nino 3.4 series. So the PDO must be something more than just ENSO.
Do you agree with that, or not?
Thanks, by the way. Regardless of where you come out on this, it has been a stimulating discussion of the kind that has helped me clarify some things about this issue.
Basil