Study: Record cold and snowy North American winters not due to climate change – within normal variation

There have been several papers published in the last decade that have been trying to claim that global warming is causing declining Arctic sea ice, and that in turn is causing increased snowfall and colder winters in the Northern Hemisphere. There was lots of wailing and screaming about the “polar vortex” and even the White House got into the act by releasing a ridiculous video that claimed that weather feature was a direct result of climate change. But, this new study suggests that those extreme winters were within range of normal variations. Of course, things like that don’t make headlines or becomes tools for political points with the White House. Here’s the video by White House Science adviser Dr. John Holdren.

That video was soooo bad, that many people in the know pointed out its flaws even before it finished playing that 2 minutes, even going so far as to claim it was nothing more than quickly cobbled together propaganda like something from the HotWhopper (aka Miriam O’Brien) school of schlock. A judge agreed, as this May 9 2016 story from the Washington Times reports:

Obama White House showed ‘bad faith’ in global-warming case, judge rules

The White House showed “bad faith” in how it handled an open records request for global warming data, a federal court ruled Monday, issuing yet another stinging rebuke to the administration for showing a lack of transparency.

For President Obama, who vowed to run the most transparent government in U.S. history, Judge Amit P. Mehta’s ruling granting legal discovery in an open records case — the third time this year a judge has ordered discovery — is an embarrassing black eye.

In this most recent case, the Competitive Enterprise Institute was trying to force the White House office of science and technology policy to release documents backing up Director John C. Holdren’s finding that global warming was making winters colder — a claim disputed by climate scientists.

Dr. Judith Curry on her blog notes that:  “a 2014 paper published in Science by Tim Palmer titled Record breaking winters and global climate change argued that rising greenhouse gas emissions may have played a role in the severe 2013-2014 winter in the U.S. midwest via a mechanism whereby global warming has caused changes in the location of the jet stream tied to warming of the surface waters in the tropical west Pacific.”

There is now this new paper by Sigmond and Fyfe Published in Nature Climate Change which rebuts Tim Palmer’s notion the notion that  tropical Pacific changes have increased the probability of the unusually cold winters observed in recent years.

Tropical Pacific impacts on cooling North American winters 

Michael Sigmond & John C. Fyfe

Abstract. The North American continent generally experienced a cooling trend in winter over the early 2000s. This cooling trend represented a significant deviation from expected anthropogenic warming and so requires explanation. Previous studies indicate that climate variations in the tropical Pacific contributed to many mid-latitude climate variations over the early 21st Century. Here we show using  large ensembles of fully-coupled, partially-coupled and uncoupled model simulations that in northwest North America the winter cooling was primarily a remote response to climate fluctuations in the tropical Pacific. By contrast, in  central North America the winter cooling appears to have resulted from a relatively rare fluctuation in mid-latitude circulation that was unrelated to the tropical Pacific. Our results highlight how decadal climate signals – both remote and local in origin – can together offset anthropogenic warming to produce continental scale cooling.

The paper can be seen here: http://www.nature.com/nclimate/journal/vaop/ncurrent/full/nclimate3069.html

Curry writes: (bold mine) The methods used in the paper combine global observations, and a series of climate model experiments that include fully coupled, partially coupled (constrained by observations) and uncoupled (atmosphere only) simulations. Excerpts:

Having established that the models used in our study reproduce the observed linkages between the tropical Pacific and North America, we now investigate their simulated trends in the early 21st Century. First we employ the ensemble of fully-coupled CanESM2 simulations to explore potential links between variations in the simulated ΔSAT (surface air temperature) trend and variations in the simulated SAT trend averaged over NWNA (northwest North America). [There is] a positive trend in ΔSAT  (corresponding to a weakening zonal temperature gradient) and a relatively small cooling trend over NWNA.  This indicates that had the simulated tropical Pacific variability in the model been aligned in time with that observed that it would have likely simulated the  observed cooling over NWNA. This is our first line of evidence that the observed winter  cooling over NWNA from 2001-2002 to 2013-2014 was a remote response to decadal  changes in the tropical Pacific. Our second line of evidence is obtained from an  ensemble of partially-coupled CanESM2 simulations where surface wind stress in the  tropics is constrained to follow its observed monthly evolution from January 1979.  In all of these so-called “pacemaker” experiments the observed trade wind intensification and associated ΔSAT increase is associated with cooling surface temperatures over NWNA. These results are consistent with relationships that  exist between winter-to-winter fluctuations and 14-year trends in ΔSAT and NWNA SAT.

Indeed, it appears that the Aleutian Low weakening observed over the early 21st Century, arguably the most pronounced feature in recent Northern Hemisphere SLP (sea level pressure) trends, can be attributed to tropical Pacific climate variations. Moreover, the associated cold air  temperature advection was sufficient to overcome the externally-forced warming to cause cooling over NWNA. By contrast, tropical Pacific related SLP trends over the east coast of North America drove southwesterly winds to produce warming, rather than  cooling, over CNA (central North America). Hence, tropical Pacific variability cannot explain the observed winter  cooling over CNA.

It is clear that tropical Pacific SAT trends played an important role in early 21st Century mid-latitude climate trends, particularly over NWNA. We ask if this is due to cooling in the central to east Pacific, or due to warming in the west tropical Pacific? To address this question we employ uncoupled simulations using CanESM2. We perform control runs with  prescribed SST and sea-ice averaged over the period between 1997 and 2007, and simulations that use the control SST and sea-ice except over the key regions.

In  response to warming in only the west tropical Pacific a slight warming of CNA is found, contradicting the notion that west tropical Pacific warming was responsible  for the observed CNA winter cooling. Our analysis indicates that tropical Pacific changes during the early 21st Century produced a warming impact over CNA.

Using our uncoupled simulations, we  investigate the speculation [of Palmer] that the probability of these unusually cold winters may have increased due to the tropical Pacific changes. To arrive at statistically robust conclusions, we extended the uncoupled control simulation and the simulation with SST changes in the tropical Pacific region, where the  SST and sea-ice fields were taken from observations. We find that it is  less likely to find a colder winter in the perturbed simulation than in the control simulation. Hence, our results contradict the notion that tropical Pacific changes have increased the probability of the unusually cold winters observed in recent years.

If the recent CNA winter cooling cannot be attributed to tropical variability, then what  was its cause? To address this question we return to the large ensemble of coupled  CanESM2 simulations and note that in three of 100 ensemble members a stronger than observed CNA winter cooling was simulated. Figure 6 shows the average simulated  climate trends over the five ensemble members with the largest CNA winter cooling.  As in observations this cooling is the result of northerly winds associated with a ridge of  increased SLP over the west coast of North America and CNA. The composite shows that outside the mid-latitudes there are no climate trends that are substantially different from the forced response. This indicates that the observed CNA  winter cooling over the early 21st Century was not a response to decadal changes in  the tropical Pacific, but was instead the result of a local internally-generated fluctuation  in circulation that was unrelated to the tropical Pacific.

Our analysis shows how remotely and locally generated decadal climate variations  have offset anthropogenic warming to produce North American winter cooling over the  early 21st Century. Similar to the slowdown in the rise of global-mean surface temperature this cooling is very likely to be a temporary feature, as future decadal  variations could be opposite in sign and amplify anthropogenic warming of North  American winters.

In other words, the AGW signal that Michael Mann claims to have teased out, and we can see on TV news reports, is in fact overwhelmed by business as usual natural variation. It might be visible in the future, but right now the evidence is about as scarce as the hairs on Mann’s head.

Added: lest we forget this parody created at the time by Minnesotans for Global Warming, here it is again.