New Paper Confirms the Drivers of and Processes behind the Atlantic Multidecadal Oscillation

The new paper by McCarthy et al. (2015) Ocean impact on decadal Atlantic climate variability revealed by sea-level observations has gained some attention around the blogosphere.    McCarthy et al. (2015) was discussed by Jo Nova here, at ReportingClimateScience here and LiveScience here.  Also see the University of Southampton press release Global climate on verge of multi-decadal change.

As could be expected, the alarmist mainstream media have so far chosen to ignore a paper that discusses an upcoming multidecadal natural suppression of global warming…probably because indicates the slowdown in global warming should continue and it implies the natural variability of the North Atlantic contributed to the global warming we have seen since the mid-1970s.

QUICK OVERVIEW OF THE ATLANTIC MULTIDECADAL OSCILLATION

The Atlantic Multidecadal Oscillation (AMO) is a mode of natural variability that reveals itself in the sea surface temperatures of the North Atlantic. It is normally portrayed by detrending the sea surface temperature anomalies of the North Atlantic. See Figure 1, which includes monthly the sea surface temperature anomalies of the North Atlantic (top graph) and the detrended data, the AMO (bottom graph).  The detrended (AMO) data are often smoothed with multiyear filters.

Figure 1

Note:  I borrowed the graphs in Figures 1 and 2 from my upcoming book, which I’ve been working on for more than a year.

According to the NOAA Frequently Asked Questions about the Atlantic Multidecadal Oscillation webpage, the AMO can enhance global warming or suppress it.

Looking at the comparison graph of global sea surface temperatures and those of the North Atlantic, Figure 2, we can see that the sea surface temperature anomalies of the North Atlantic often run in parallel with the global data. (The data have been smoothed in that illustration.) At those times, it isn’t enhancing global warming or suppressing it.

Figure 2

During the early cooling period from the late 1870s to about 1910, the North Atlantic data dropped at about the same rate as the global data, so the North Atlantic sea surface temperatures did not enhance or suppress that cooling. Keep in mind though that the global data is sampled very poorly during that early cooling period, so the comparison may not be too realistic at those times.  From 1910 to about 1920, the data show the surfaces of the North Atlantic warmed more slowly than the global data, so the North Atlantic was suppressing the global warming during that initial part of the early warming period. Then, from 1920 to about 1940, the surfaces of the North Atlantic warmed at a much faster rate than they did globally. This overcame the initial deficit and allowed the North Atlantic to enhance the global warming for the entire early warming period of 1910 to 1940.

We see similar responses during the mid-20^th Century cooling period and the late warming period. That is, the North Atlantic sea surface temperatures run in parallel with the global data for the initial 10 to 15 years of those periods.  It’s only after those initial periods that the North Atlantic either cools or warms more rapidly than the global data, which then enhances the cooling or warming.

INTRODUCTION TO MCCARTHY ET AL. (2015)

McCarthy et al. created a new index based on the U.S. east coast tidal-gauge measurements of sea level north and south of Cape Hatteras, from Florida to Boston. They used the new sea level based index as a proxy for variations in ocean circulation of the North Atlantic. See their “accumulated sea level index” shown in blue in their Figure 3, which is also my Figure 3.

Figure 3

McCarthy et al. confirmed the belief that (1) the North Atlantic Oscillation (a sea level pressure-based index that reflects changes in wind patterns there), (2) ocean circulation in the North Atlantic (the flow of warm tropical waters northward by the Gulf Stream) and (3) the Atlantic Multidecadal Oscillation (AMO) are linked.

The abstract of McCarthy et al. reads:

Decadal variability is a notable feature of the Atlantic Ocean and the climate of the regions it influences. Prominently, this is manifested in the Atlantic Multidecadal Oscillation (AMO) in sea surface temperatures. Positive (negative) phases of the AMO coincide with warmer (colder) North Atlantic sea surface temperatures. The AMO is linked with decadal climate fluctuations, such as Indian and Sahel rainfall¹, European summer precipitation², Atlantic hurricanes³ and variations in global temperatures⁴. It is widely believed that ocean circulation drives the phase changes of the AMO by controlling ocean heat content⁵. However, there are no direct observations of ocean circulation of sufficient length to support this, leading to questions about whether the AMO is controlled from another source⁶. Here we provide observational evidence of the widely hypothesized link between ocean circulation and the AMO. We take a new approach, using sea level along the east coast of the United States to estimate ocean circulation on decadal timescales. We show that ocean circulation responds to the first mode of Atlantic atmospheric forcing, the North Atlantic Oscillation, through circulation changes between the subtropical and subpolar gyres—the intergyre region⁷. These circulation changes affect the decadal evolution of North Atlantic heat content and, consequently, the phases of the AMO. The Atlantic overturning circulation is declining⁸ and the AMO is moving to a negative phase. This may offer a brief respite from the persistent rise of global temperatures⁴, but in the coupled system we describe, there are compensating effects. In this case, the negative AMO is associated with a continued acceleration of sea-level rise along the northeast coast of the United States^{9, 10}.

The last two sentences are noteworthy. Good news: the surface temperatures of the North Atlantic are going to suppress global warming for the next couple of decades…after enhancing them for the past 3+ decades.  The bad news:  there will likely be an accelerated rise in sea level from Cape Hatteras to Boston during that time.

HEADING THE CO2-OBSESSED OFF AT THE PASS

I suspect the true blue believers in catastrophic human-induced global warming will attempt to downplay the role of the AMO by citing the curious paper Steinman et al. (2015), which clearly illustrated model failings, even though they were attempting (and failing) to make other points.  See the posts:

• On Steinman et al. (2015) – Michael Mann and Company Redefine Multidecadal Variability And Wind Up Illustrating Climate Model Failings
• Another Couple of Notes about Michael Mann’s “Faux Pause” Post and Steinman et al. 2015

We’ve illustrated and discussed how poorly climate models simulate sea surface temperatures in the posts:

• Alarmists Bizarrely Claim “Just what AGW predicts” about the Record High Global Sea Surface Temperatures in 2014
• IPCC Still Delusional about Carbon Dioxide

For more information on the Atlantic Multidecadal Oscillation, refer to the NOAA Frequently Asked Questions About the Atlantic Multidecadal Oscillation (AMO) webpage and the posts:

• An Introduction To ENSO, AMO, and PDO — Part 2
• Multidecadal Variations and Sea Surface Temperature Reconstructions

[My thanks to Marcel Crok and blogger Alec aka Daffy Duck for the heads-up.]