<\/p>\n
A bit of information about the PDO<\/strong><\/p>\n The name “PDO” was coined by Mantua et al (Mantua, Nathan J. et al. (1997), “A Pacific interdecadal climate oscillation with impacts on salmon production”, Bulletin of the American Meteorological Society 78 (6): 1069\u00e2\u20ac\u201c1079).<\/p>\n Wikipedia (http:\/\/en.wikipedia.org\/wiki\/Pacific_decadal_oscillation) notes: “a long-lived El Ni\u00c3\u00b1o-like pattern of Pacific climate variability. While the two climate oscillations have similar spatial climate fingerprints, they have very different behavior in time.”<\/p>\n JISAO has produced a PDO monthly index, from 1900 to the present, with the PDO index defined as the leading principal component of North Pacific monthly sea surface temperature variability pole-ward of 20N for the 1900-93 period. JISAO presents this graphic to depict the difference between “warm phase” (left) and “cool phase” (right) PDO states: (click on any graphic to view at original resolution)<\/p>\n <\/p>\n Wood For Trees includes the JISAO PDO index among their sea surface temperature indexes. (http:\/\/www.woodfortrees.org\/plot\/jisao-pdo) It is pretty clear from inspection of the above graphic that here is rather poor correlation between the PDO index and Earth’s recent temperature history. But let’s suppose that the PDO index influences Earth’s average surface temperature by a cumulative, rather than an immediate effect. That is, let’s suppose that an integral of the PDO index over time can influence Earth’s average surface temperature. What could we look at to evaluate the cumulative effect of the PDO over time? I can think of two reasonable choices: 1) the continuous time integral of the PDO from 1900 forward, and 2) the trailing average (over a specified time, say 25 – 30 years) of the PDO. The figure below shows the cumulative total (the integral) of the monthly PDO index starting in 1900, along with the Hadley HADCRUT4 temperature history. The mid 1940’s peak in temperature correlates well with the peak in the cumulative PDO index, and the slight decline in temperature from the mid-1940’s to the mid 1970’s tracks the cumulative PDO index almost perfectly, while the rapid warming post 1976 corresponds perfectly to a rapid increase in the cumulative PDO index. Finally, the recent “plateau” in temperature corresponds to the leveling off and decline in the cumulative PDO index.<\/p>\n The graph below shows an adjustment of the HADCRUT4 data based on the cumulative PDO index.<\/p>\n The adjusted HADCRUT4 data no longer has a significant mid-1940’s peak in temperature, no decline in temperature from the mid 1940’s to the mid 1970’s, and slower warming since the mid 1970’s. All of which seems more consistent with estimates of the historical man made GHG forcing. (The constant of 0.00125 was chosen to maximize consistency with that forcing.)<\/p>\n One doubt about the cumulative PDO index is that it implicitly assumes the state of the PDO index in 1900 (and all times since then) continues to influence temperatures today. To avoid this assumption (and limit the period of influence), a trailing average of the PDO index can be used instead. The graph below shows the 25 year trailing average along with the HADCRUT4 historical record.<\/p>\n The graph begins in 1925 because the PDO index data starts in 1900, and 25 years is needed to generate the first data point for the 25 year trailing average. Once again, the variation in the HADCRUT4 trend seems to track the trailing average of the PDO index quite well. The graph below shows an adjustment of the HADCRUT4 data using the 25 year trailing average of the PDO.<\/p>\n Once again, the adjusted temperature trend appears to reasonably follow the historical man made GHG forcing. Finally, the graph below shows the slope of the adjusted temperature trend since 1975.<\/p>\n Observations and Comments<\/strong><\/p>\n Unlike like the AMO index, which tracks the Earths average temperature closely, the PDO index itself is much more variable, and does not correlate well with average temperature. However, it is clear that a long term cumulative measure of the PDO index correlates quite well with changes in the slope of the temperature trend over the recent past. It is of course possible that this is just coincidence, but the correlation is awfully good, so a causal relationship seems plausible (and IMO likely).\u00c2\u00a0 Nobody appears to suggest that the PDO is driven by man-made GHG forcing or man made aerosol effects; it is by all accounts a natural process.<\/p>\n Assuming there is a causal relationship, what mechanism is responsible? The honest answer is that I do not know, but it must be related to gradual changes in ocean heat content in the first few hundred meters of the ocean, since this is the part of the ocean which is substantial enough in thermal mass to account for relatively long cumulative effects, and which also has immediate influence on Earth’s average surface temperature. Changes in heat content at great ocean depth would not be expected to have direct influence on the Earth’s surface temperature except on multi-century time scales. So I would expect variations from any secular trend in ocean heat content for the top ~300 meters to be a reasonable measure of the cumulative influence of the PDO over 2 – 3 decades, with much less (and much slower) influence of the cumulative PDO on heat content at greater depth.<\/p>\n Since the state of the PDO tends to persist over multi-decade periods, and since 1998 the index has been mostly negative, it seems likely that the influence on the trend in global temperatures will continue to be negative, at least for the next 15-20 years. Based on past behavior, if the 25 year trailing average index reaches -0.6, then the trend in temperatures could be reduced by about 0.12C over that period, compared to what that trend would otherwise have been. Unless the PDO changes unexpectedly to consistently positive, we can reasonably expect the recent slow rate of global temperature increase to continue for some time. My personal SWAG is a rate of warming over the next 15-20 years of about 0.06-0.07 C per decade, with a true (underlying) secular trend of about 0.12C per decade.\u00c2\u00a0 If this happens, then the IPCC’s climate model projections are going to look even worse in the coming years than they do today.<\/p>\n","protected":false},"excerpt":{"rendered":" Over the past several months there have been a number of exchanges at The Blackboard about natural cyclical or pseudo-cyclical contribution to Earth’s temperature history. With the primary point of contention being whether or not natural cyclical variation has misled us about how much of the post 1975 warming has been due to man made … Continue reading Pseudo-Cyclical Contribution of the PDO to Earth’s Recent Temperature History<\/span>
\n“The prevailing hypothesis is that the PDO is caused by a ‘reddening’ of the ENSO combined with stochastic atmospheric forcing. A PDO signal has been reconstructed to 1661 through tree-ring chronologies in the Baja California area.”
\nand goes on to list several proposed physical mechanisms for the PDO, none of them related to human activities. So it seems unlikely that human GHG forcing is a causal influence on the state of the PDO. The PDO is described by JISAO (http:\/\/jisao.washington.edu\/pdo\/) as:<\/p>\n![\"pdo_warm_cool3\"](\"http:\/\/rankexploits.com\/musings\/wp-content\/uploads\/2013\/02\/pdo_warm_cool3.jpg\")
<\/a><\/p>\n
\n<\/a><\/p>\n
\n<\/a><\/p>\n<\/a><\/p>\n<\/a><\/p>\n<\/a><\/p>\n<\/a><\/p>\n