Atlantic ocean and apparent climate shifts

Andrew C Revkin presents reactions from researchers on a recent study about climate changes.

Using climate models and observations, a fascinating study in this week’s issue of Nature Climate Change points to a marked recent warming of the Atlantic Ocean as a powerful shaper of a host of notable changes in climate and ocean patterns in the last couple of decades — including Pacific wind, sea level and ocean patterns, the decade-plus hiatus in global warming and even California’s deepening drought.

The study, “Recent Walker circulation strengthening and Pacific cooling amplified by Atlantic warming,” was undertaken by researchers at the University of New South Wales and University of Hawaii. Here’s the 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. Our study of a series of climate model experiments suggests that global surface warming has been partly offset by the Pacific climate response to enhanced Atlantic warming since the early 1990s.

Initial response

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 skeptical to say the Atlantic is the driver.

I think the warming Indian Ocean is also a big factor in how El Niño Southern Oscillation (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.

The unusual thing about the current El Niño 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. 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.

Academic reply

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. Currently the winds are at a level never before seen on observed records.

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 Niño 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.

Another perspective

“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.”

While active, the stronger Equatorial trade winds have caused far greater overturning of ocean water in the West Pacific, pushing more atmospheric heat into the ocean, as shown by co-author and ARCCSS Chief Investigator Prof Matthew England earlier this year. This increased overturning appears to explain much of the recent slowdown in the rise of global average surface temperatures.

Importantly, the researchers don’t expect the current pressure difference between the two ocean basins to last. When it does end, they expect to see some rapid changes, including a sudden acceleration of global average surface temperatures.

“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.