New Study Sees Atlantic Warming Behind a Host of Recent Climate Shifts




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 analyze 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 centers 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.

Kevin Trenberth of the National Center for Atmospheric Research sent an initial reaction to the overall thesis:

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.

I think the warming Indian Ocean is also a big factor in how ENSO develops these days. It is clearly now a bigger player than 20 or more years ago because it has warmed more than the other oceans, which get relief from ocean dynamics (ENSO in the Pacific and the Thermohaline circulation in Atlantic act as regulators of heat build up). The unusual thing about the current El Nino is the failure to cool off in the west Pacific and Indian Ocean as the central and eastern Pacific warmed, so the Southern Oscillation has not really switched. i.e. the SO part of ENSO has not yet happened. One interpretation is that the amount of warm water is so great that the normal change in winds and currents has just brought more warm water to the surface.

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, who’s currently a visiting professor of physical oceanography and climate at Harvard, wrote this note stressing the importance of putting this work in broader context with the broader flow of climate science:

As a colleague said to me “Just because it was published in Nature or Science doesn’t mean it’s wrong.” 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.

Insert, 3:15 p.m. | Kerry Emanuel of the Massachusetts Institute of Technology has added this very critical view:

It is very difficult to deduce causality in a complex system, even by doing numerical experiments. It is simply physically wrong to specify sea surface temperatures and observe the atmospheric response for any phenomenon whose time scale is more than a few months. I know of no climate scientist who would do experiments fixing land temperatures and observing the atmospheric response, and with good reason. If one would not do that for land, one should not do that for the ocean, certainly for features that evolve over years…and for exactly the same reason that one would not fix land temperatures.

There is, alas, a long, sad history of specified SST experiments and I, for one, hope that these go the way of argyle socks.

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.

Here’s the University of New South Wales news release:

Atlantic warming turbocharges Pacific trade winds

Record breaking trade winds may have led to hiatus in global surface average temperatures


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.

It may even be responsible for making El Nino events less common over the past decade due to its cooling impact on ocean surface temperatures in the eastern Pacific.

“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.
“The rapid warming of the Atlantic Ocean created high pressure zones in the upper atmosphere over that basin and low pressure zones close to the surface of the ocean,” said Prof Axel Timmermann, co-lead and corresponding author from the University of Hawaii.

“The rising air parcels, over the Atlantic eventually sink over the eastern tropical Pacific, thus creating higher surface pressure there. The enormous pressure see-saw with high pressure in the Pacific and low pressure in the Atlantic gave the Pacific trade winds an extra kick, amplifying their strength. It’s like giving a playground roundabout an extra push as it spins past.”

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.
