Guest Post by Bob Tisdale
Pierre Gosselin at NoTrickZone provided an introduction to a recently published paper in his post IPCC Scientist Mojib Latif Sees North Atlantic Cooling Over Next Decade…Confirms Oceans Play Crucial Role. The paper is Klöwer et al. (2014) Atlantic meridional overturning circulation and the prediction of North Atlantic sea surface temperature. It is a study of the cause of the multidecadal variability of the North Atlantic sea surface temperatures (known as the Atlantic Multidecadal Oscillation). They find that multidecadal variations in Atlantic Meridional Overturning Circulation, driven by the North Atlantic Oscillation, precede the changes in North Atlantic surface temperatures. Their findings suggest the “present warm phase of the AMO is predicted to continue until the end of the next decade, but with a negative tendency”.
The abstract of Klöwer et al. (2014) reads:
The Atlantic Meridional Overturning Circulation (AMOC), a major current system in the Atlantic Ocean, is thought to be an important driver of climate variability, both regionally and globally and on a large range of time scales from decadal to centennial and even longer. Measurements to monitor the AMOC strength have only started in 2004, which is too short to investigate its link to long-term climate variability. Here the surface heat flux-driven part of the AMOC during 1900–2010 is reconstructed from the history of the North Atlantic Oscillation, the most energetic mode of internal atmospheric variability in the Atlantic sector. The decadal variations of the AMOC obtained in that way are shown to precede the observed decadal variations in basin-wide North Atlantic sea surface temperature (SST), known as the Atlantic Multidecadal Oscillation (AMO) which strongly impacts societally important quantities such as Atlantic hurricane activity and Sahel rainfall. The future evolution of the AMO is forecast using the AMOC reconstructed up to 2010. The present warm phase of the AMO is predicted to continue until the end of the next decade, but with a negative tendency.
In his blog post, Pierre Gosselin has translated portions of the press release for the paper, which clarifies what the authors mean with the last sentence of the abstract. Co-author Dr. Mojib Latif of GEOMAR Helmholtz Centre for Ocean Research Kiel says, according to Pierre:
Our model tells us that the phase with a rather high surface temperatures in the North Atlantic will continue also over the coming decade, however with a lightly negative trend.
And as Pierre also notes, a slight negative trend means cooling. It’s also important to note that Dr. Latif was discussing surface temperatures of the North Atlantic, not the Atlantic Multidecadal Oscillation Index, which is detrended North Atlantic sea surface temperature data.
To further clarify the impact of this on global sea surface temperatures, see Figures 1 and 2. Figure 1 compares the sea surface temperature anomalies from January 1975 to present for the North Atlantic and for the rest of the global oceans. The warming rate of the North Atlantic doubled that of the rest of the global oceans in that time, and that additional warming is a response to the warming phase (not warm phase) of the Atlantic Multidecadal Oscillation. So it’s blatantly obvious that the North Atlantic contributed to the warming of global sea surface temperatures since the mid-1970s.
Sometime after the turn of the century the Atlantic Multidecadal Oscillation peaked. Due to the volatility of the data and the short time frame, it’s tough to determine when it peaked. But for illustration purposes, Figure 2 compares the same two sea surface temperature data subsets starting in 2003. The surface of the North Atlantic has cooled slightly over that time, while the surfaces of the rest of the global oceans show very little warming.
Proponents of the hypothesis of human-induced global warming like to claim that the additional warming of the North Atlantic was caused by manmade forcings. The CMIP5 climate models (the models used by the IPCC for their 5th Assessment Report) contradict that claim. I illustrated this quite plainly about a year ago in the post Questions the Media Should Be Asking the IPCC – The Hiatus in Warming. There I wrote:
[Start quote from earlier post.]
We can illustrate the Atlantic Multidecadal Oscillation using the method recommended by Trenberth and Shea (2006), and it was to subtract global sea surface temperature anomalies (60S-60N, excludes the polar oceans) from sea surface temperature anomalies of the North Atlantic (0-60N, 80W-0). They used HADISST data and so have I. In the time-series graph in Figure 3, I’ve also smoothed the AMO data with a 121-month running average filter. As shown by the blue curve, the North Atlantic has a mode of natural variability that causes its sea surface temperatures to warm and cool at rates that are much greater than the variations in the surface temperatures of the global oceans. And we can see that the variations occur over multidecadal time periods (thus the name Atlantic Multidecadal Oscillation). Keep in mind that the Atlantic Multidecadal Oscillation is responsible for some (but not all) of the warming of land surface temperatures in the Northern Hemisphere during the more recent warming period, according to the climate scientists at RealClimate. (See also Tung and Zhou (2012) Using data to attribute episodes of warming and cooling in instrumental records.)
If we subtract the modeled global sea surface temperatures from the modeled sea surface temperatures of the North Atlantic (shown as the red curve in Figure 3), we can see that the forced component of the CMIP5 models (represented by the multi-model ensemble mean) does not simulate the observed multidecadal variations in the North Atlantic. That is, there is very little difference between the modeled variations in global and North Atlantic sea surface temperature anomalies. The comparison also strongly suggests that the Atlantic Multidecadal Oscillation is NOT a response to manmade greenhouse gases (or aerosols) used by the climate modelers to force the warming (or cooling) of sea surface temperatures of the North Atlantic.
[End quote from earlier post.]
Back to Klöwer et al. (2014):
The abstract of Klöwer et al. (2014) notes the surface temperatures of the North Atlantic impact hurricanes and Sahel rainfall. The North Atlantic sea surface temperatures also have strong influences on rainfall patterns in North America, on land surface temperatures throughout the Northern Hemisphere and on Arctic sea ice (because the Arctic Ocean is open to the North Atlantic).
For those new to the topic of the Atlantic Multidecadal Oscillation (AMO) 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
The HADSST3 data presented in Figures 1 and 2 and the HADISST data in Figure 3 are available through the KNMI Climate Explorer. The North Atlantic surface area percentage is presented in the NOAA webpage Volumes of the World’s Oceans from ETOPO1.