by Judith Curry
Record breaking trade winds may have led to hiatus in global surface average temperatures.
A relevant paper was published in Nature Climate Change last week, that is creating some buzz:
Recent Walker circulation strengthening and Pacific cooling amplified by Atlantic warming
Shayne McGregor, Axel Timmermann, Malte F. Stuecker, Matthew H. England, Mark Merrifield, Fei-Fei Jin & Yoshimitsu Chikamoto
Abstract. An unprecedented strengthening of Pacific trade winds since the late 1990s has caused widespread climate perturbations, including rapid sea-level rise in the western tropical Pacific, strengthening of Indo-Pacific ocean currents, and an increased uptake of heat in the equatorial Pacific thermocline. The corresponding intensification of the atmospheric Walker circulation is also associated with sea surface cooling in the eastern Pacific, which has been identified as one of the contributors to the current pause in global surface warming. In spite of recent progress in determining the climatic impacts of the Pacific trade wind acceleration, the cause of this pronounced trend in atmospheric circulation remains unknown. Here we analyse a series of climate model experiments along with observational data to show that the recent warming trend in Atlantic sea surface temperature and the corresponding trans-basin displacements of the main atmospheric pressure centres were key drivers of the observed Walker circulation intensification, eastern Pacific cooling, North American rainfall trends and western Pacific sea-level rise. Our study suggests that global surface warming has been partly offset by the Pacific climate response to enhanced Atlantic warming since the early 1990s.
Press Release
From the University of New South Wales news release Atlantic warming turbocharges Pacific trade winds:
New research has found rapid warming of the Atlantic Ocean, likely caused by global warming, has turbocharged Pacific Equatorial trade winds. Currently the winds are at a level never before seen on observed records, which extend back to the 1860s.
The increase in these winds has caused eastern tropical Pacific cooling, amplified the Californian drought, accelerated sea level rise three times faster than the global average in the Western Pacific and has slowed the rise of global average surface temperatures since 2001.
“We were surprised to find the main cause of the Pacific climate trends of the past 20 years had its origin in the Atlantic Ocean,” said co-lead author Dr Shayne McGregor from the ARC Centre of Excellence for Climate System Science (ARCCSS) at the University of New South Wales.
“It highlights how changes in the climate in one part of the world can have extensive impacts around the globe.”
The record-breaking increase in Pacific Equatorial trade winds over the past 20 years had, until now, baffled researchers.
Originally, this trade wind intensification was considered to be a response to Pacific decadal variability. However, the strength of the winds was much more powerful than expected due to the changes in Pacific sea surface temperature.
Another riddle was that previous research indicated that under global warming scenarios Pacific Equatorial Trade winds would slow down over the coming century.
The solution was found in the rapid warming of the Atlantic Ocean basin, which has created unexpected pressure differences between the Atlantic and Pacific. This has produced wind anomalies that have given Pacific Equatorial trade winds an additional big push.
Many climate models appear to have underestimated the magnitude of the coupling between the two ocean basins, which may explain why they struggled to produce the recent increase in Pacific Equatorial trade wind trends.
“It will be difficult to predict when the Pacific cooling trend and its contribution to the global hiatus in surface temperatures will come to an end,” Prof England said.
Critiques
Andy Revkin at DotEarth has a post with comments from several climate scientists, excerpts:
Kevin Trenberth: There is no question that the changes going on are global in scope and the Walker circulation linking the Pacific and Atlantic plays an important role, but I have to be very very skeptical to say the Atlantic is the driver. So my quick reaction to this paper is why the word “cause”? There is no doubt that the processes they describe are involved but what sets them off is another matter.
Carl Wunsch, wrote this note stressing the importance of putting this work in broader context with the broader flow of climate science: Science (lowercase) requires thought, independent calculations, discussion, time, and context. [This] proposal isn’t crazy, but figuring out how these things are connected will take months and years. Causality in anything remotely as complicated as the climate system is an exceedingly difficult concept, and I would argue anyone who instantly says “yes this must be right” or “no this is wrong” or even that “this is important” cannot possibly know what he is talking about.
Kerry Emanuel: It is very difficult to deduce causality in a complex system, even by doing numerical experiments. The hiatus temperature pattern (Figure 1a of the paper) is interesting and worth trying to understand. This paper does not advance us toward that goal.
JC reflections
Well, it is good to see papers working to unravel natural climate variability and the teleconnection patterns. With the overwhelming focus on anthropogenic forced climate change, natural climate variability has received short shrift. With the growing prominence of the ‘pause’ or ‘hiatus’, natural climate variability is starting to get more attention.
The MacGregor et al. paper identify a connection between the circulation patterns in the Atlantic and tropical Pacific Oceans. Trenberth, Wunsch and Emanuel all hit on the key problem with paper – claims of causality. Kerry Emanuel states: It is very difficult to deduce causality in a complex system, even by doing numerical experiments. I wholeheartedly agree with this statement; I just wish there was some acknowledgement of this difficulty in context of the highly confident statements about attribution of global warming.
This paper suffers from a chicken and egg problem, and there is really no way to sort out causality. Especially when it is so difficult to disentangle forced from intrinsic climate variability (previous post by Marcia Wyatt).
I think a better way to look at this is in context of a networked sequence of teleconnection and circulation patterns; the Stadium Wave paper is an example. Marcia Wyatt describes this as exceedingly complex dance between atmospheric circulations, sea ice and ocean circulations that evolve in a spatially and temporally ordered manner. The complex dance metaphor is more apt than attempting to identify causality here; at best MacGregor et al. have identified a temporal sequence of circulation patterns that may be linked.
With regards to the stadium wave, I tried to infer whether the pattern that MacGregor et al. found was consistent with the stadium wave. See in particular Figure 3a, which plots North Atlantic and Pacific surface temperature anomalies since 1900. Of particular interest is the divergence since about 1995, which looks similar to the divergence from ~1925-1940. Both periods are characterized by the beginning of the warm phase of the Atlantic Multidecadal Oscillation (AMO).
So there is little question in my mind that the AMO is a, if not the, lynchpin to understanding multidecadal natural climate variability. What sets the tempo of the AMO, how the AMO interacts with forced climate variability, and how the AMO interacts with global circulation patters, are to my mind probably the most important topics in climate dynamics research.
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