- In this post I compare the de-trended HadCRUT4 global temperature reconstruction with the Atlantic Multidecadal Oscillation index (AMO).
- The AMO fluctuates between cold and warm phases on a quasi 66 year cycle, 33 years warming followed by 33 years cooling, and is modulated by strengthening and weakening of the Atlantic Meridional Overturning Circulation (AMOC) . The AMO has been in warm phase since around 1995 with consequences for climate in the circum N Atlantic domain.
- There is a high degree of co-variance with R^2 = 0.52, but with occasional periods where the AMO and HadCRUT4 are out of phase, for example in the 1950s (Figure 1). Fluctuations in AMOC and the AMO can explain much of but not all the cyclical variance in the global temperature record. Much of the global warming since 1850 also correlates with upwards trending North Atlantic sea surface temperatures (NA SSTs).
- The AMO is now due to enter it’s cooling phase that may continue for 20 or more years. Global temperatures may therefore continue to trend sideways or down, unless greenhouse gas emissions have the power to overcome nature.
Figure 1 De-trended HadCRUT4 compared with the AMO index that is based on de-trended NA SSTs. AMO index from NOAA. (AMO unsmooth long)
All climate watchers should be familiar with the oscillating pattern of temperature change since 1850 (Figure 2). The temperature record fluctuates above and below the trend line (linear regression). Subtracting the regression from the actual record de-trends the data and these residuals allow the pattern to be seen more clearly. A quasi 66 year long cycle emerges (Figure 3). I am certainly not the first person to observe this and to recognise that the distribution of residuals matches the Atlantic Multidecadal Oscillation (Figure 1) (R^2 = 0.52) e.g. .
Figure 2 HadCRUT4 temperature data oscillate around the liner regression (solid red line). Subtracting the linear regression from HadCRUT4 de-trends the data as shown in Figure 3.
Figure 3 Subtracting the linear regression from the actual HadCRUT4 data provides this distribution of residuals and the quasi 66 year cycle emerges. The dates for the peaks and troughs are labelled. Starting in 1878+33=1911+33=1944+32=1976+33=2009.
So why is any of this important? The answer lies in how this distribution of residuals matches the pattern of the Atlantic Multidecadal Oscillation (Figure 1). There is a stronger degree of co-variance than the healthy R^2 value of 0.52 suggests. There is a high level of correspondence between the major peaks and troughs. But also some periods where the AMO and HadCRUT4 are out of phase. This is not surprising since the AMO is not the only ocean cycle or the only factor influencing climate. But it does seem to be an important variable.
So what is the AMO and what does this correlation between the AMO and HadCRUT4 mean? NOAA have this interesting FAQ page on the AMO. A few selected quotes follow:
What is the AMO? The AMO is an ongoing series of long-duration changes in the sea surface temperature of the North Atlantic Ocean, with cool and warm phases that may last for 20-40 years at a time and a difference of about 1°F between extremes. These changes are natural and have been occurring for at least the last 1,000 years.
How much of the Atlantic are we talking about? Most of the Atlantic between the equator and Greenland changes in unison. Some area of the North Pacific also seem to be affected.
What are the impacts of the AMO? The AMO has affected air temperatures and rainfall over much of the Northern Hemisphere, in particular, North America and Europe. It is associated with changes in the frequency of North American droughts and is reflected in the frequency of severe Atlantic hurricanes. It alternately obscures and exaggerates the global increase in temperatures due to human-induced global warming.
How does the AMO affect rainfall and droughts? Recent research suggests that the AMO is related to the past occurrence of major droughts in the Midwest and the Southwest. When the AMO is in its warm phase, these droughts tend to be more frequent and/or severe (prolonged?). Vice-versa for negative AMO. Two of the most severe droughts of the 20th century occurred during the positive AMO between 1925 and 1965: The Dustbowl of the 1930s and the 1950s drought. Florida and the Pacific Northwest tend to be the opposite – warm AMO, more rainfall.
To summarise the informed opinion of Americas finest climate scientists working at NOAA. The AMO is a natural cycle. It affects rainfall patterns over the N hemisphere, including the frequency of severe hurricanes, and the occurrence of drought in the Midwest and Southwest that correlate with the warm phase of the AMO. The AMO shifted to warm phase around 1995 (Figure 1).
Figure 1 shows that the AMO can explain much of the structure in the temperature record. But can it explain the overall warming trend? NOAA also report the “raw” N Atlantic SST data (N. Atlantic SST averages, unsmoothed & not detrended 1856 to present) upon which the AMO index is based and these are plotted together with HadCRUT 4 in Figure 4. We see that from 1856 to 1972 there is very close agreement between the NA SSTs and HadCRUT4, but then the trends diverge. The most simple explanation is that the atmosphere is warming at twice the rate of the NA SST (0.51˚C v 0.26˚C per century). I do have an alternative explanation, but that is a story for another day or for the comments.
Figure 4 Comparison of North Atlantic SST with HadCRUT4.
The AMO and Global Temperatures
Why should the AMO show such a high degree of co-variance with global lower troposphere temperature reconstructions? At one level the explanation is simple. 71% of HadCRUT4 is based on SSTs and the temperature of the North Atlantic is included in that data. But on the other hand, the North Atlantic is only a small portion of global SSTs. The UK Met Office has just published a report on the impacts of ocean cycles on the climate system  (Big Changes Underway in the Climate System?), and they say this:
AMO variability is thought to be associated with the flow of water that carries heat northward within the North Atlantic Ocean, known as the Atlantic Meridional Overturning Circulation (AMOC; Knight et al. 2005). Variability in AMOC strength alters the amount of heat that is transported, in turn changing the average surface temperature of the North Atlantic Ocean (Gulev et al., 2013).
The AMO, therefore, is a reflection of the strength of the AMOC and since the AMOC is a part of the global conveyer belt of thermohaline circulation, if the AMOC changes pace one can presume that the flow of thermohaline circulation must also change pace with knock on effects to the climate system in multiple ways. In the North Atlantic, a more active AMOC transports more heat to higher latitudes and with it more water vapour, hence warmer and wetter climate. One can speculate that changes to the AMO and AMOC may also result in a change to the amount and distribution of global cloud cover. Juraj Vanovcan writing on WUWT  also observes a negative correlation with the Arctic Sea ice – no great surprise there.
When will the AMO revert back to its cold phase? Since the AMO is quasi cyclical, this is difficult to answer precisely, but the warm phase peak should have been around 2009 (Figure 3) and the cold phase trough is due around 2042. Between now and then, the N Atlantic should begin to cool and the Arctic sea ice begin to recover. If history repeats, a shift to the cooling phase of the AMO may see global surface temperatures trending sideways or down for another two decades or more. A look at current SSTs shows a large area of cool water stretching from Ireland to Newfoundland (Figure 5). This is something to watch in the months and years ahead.
Figure 5 SST anomalies for 17 September 2015. Does that large area of cool water in the N Atlantic portend a shift in the AMO towards its cool phase?
- Quasi cyclical changes in the Atlantic Multidecadal Oscillation (AMO) correspond to quasi cyclical changes in global surface temperatures. The AMO is modulated by changes in the pace of the Atlantic Meridional Overturning Circulation (AMOC). All of these phenomena can be linked to cyclical changes in the pace of global thermo-haline circulation.
- While it is difficult to predict, the AMO should be heading into its cooling leg (note that the cooling leg begins past the peak of the warm phase). The Met Office report  provides supporting evidence for this from multiple sources.
- Both the US NOAA and UK Met Office now seem to be fully aware of the role played by natural cyclical changes in the oceans in modulating the decade scale cyclical climate change we observe on Earth. And yet both organisations seem content to mislead the public and politicians by seeking to explain all variations from the recent average at specific locations by means of anthropogenic global warming.
 UK MET Office Big Changes Underway in the Climate System?
 Juraj Vanovcan on WUWT European climate, Alpine glaciers and Arctic ice in relation to North Atlantic SST record.