Musings on the AMO - Highlighted Article
- Posted On:
- Jun 13, 2025 at 6:00 AM
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From: Watts Up With That
By: Andy May
Date: May 26, 2025
We hear a lot about the AMO, or the Atlantic Multidecadal Oscillation. How much does it influence the global mean surface temperature or GMST? Exactly what is the AMO? These are the issues we will discuss. First let’s look at various definitions of the AMO.
Enfield, et al.: “The AMO index [is a] ten-year running mean of detrended Atlantic SSTA [sea surface temperature anomaly] north of the equator.”
Gray, et al.: Uses detrended raw tree-ring measurements to demonstrate a strong and regular 60-100 year variability in basin-wide (0-70°N) North Atlantic sea surface temperatures (SSTs) that has been persistent for the past five centuries.
Trenberth & Shea: “To deal with purely Atlantic variability, it is highly desirable to remove the larger-scale global signal that is associated with global [anthropogenic] processes, and is thus related to global warming in recent decades … Accordingly, the global mean SST has been subtracted to derive a revised AMO index.”
NCAR uses the Trenberth & Shea method, but NOAA uses the original methodology and detrends the North Atlantic temperatures using a least squares linear trend. We will also use the original Enfield and Gray method in this post.
The reason for the AMO SST 60-70-year pattern is unknown, but according to Gray et al. it extends back to 1567AD, so it is a natural oscillation of some kind. Some have speculated that it is a result of the thermohaline circulation in the North Atlantic or a “combination of natural and anthropogenic forcing during the historical era.” (Mann, Steinman, & Miller, 2020). But while interesting these ideas are speculative. Further if the oscillation has existed since 1567, it seems unlikely that it is caused by human CO2 and aerosol emissions.
It is clear that “global” warming is mostly an extra-tropical Northern Hemisphere phenomenon. This is discussed here in figures 1A & 1B and here in the discussion around figure 1, which is also shown as figure 1 below. Regions outside the extratropical Northern Hemisphere don’t change temperature as quickly or as drastically. (continue reading)