The AMO – Staying Positive and Defying Expectations

November 13, 2017

As I started to prepare for AIR’s 2017 Atlantic Basin Hurricane Season in Review report today, I decided to check on the Atlantic Multidecadal Oscillation (AMO) Index. This index is a measure of the anomalous warmth of the Atlantic Ocean and it correlates strongly with hurricane activity. The correlation is so high, in fact, that it provided a rationale for the Warm Sea Surface Temperature (SST) catalog that AIR released in 2009. I had checked the monthly averages earlier in the season but not since Harvey, Irma, and others in the parade of major hurricanes that devastated parts of the Caribbean and the U.S. coast.

I saw that the October value was 0.436 (e.g., almost a half degree Celsius warmer than normal for October) and thought to myself, “that seems high”. A quick check confirmed my suspicion that it was indeed the highest October value on record ever—since 1856! That fact, combined with the expectation from more than just a few expert climate scientists for it to be on the other side of the x-axis, prompted me to write this blog.

What Will the AMO Do in Any Given Year?

While more than one definition of the AMO Index exists, my go-to (and the one I discuss in this blog) is formulated by NOAA’s Earth System Research Laboratory. But whichever formulation you use, the AMO is so called because it changes on multi-decadal time scales. Figure 1 shows that the Index was negative in the first 25 years or so of the last century, and then positive through the mid-1960s, then negative after that to about the mid-1990s, positive since. Its above-zero values were used to explain, at least in part, the very active 2004 and 2005 seasons, and prompted some to think such activity would continue for the foreseeable future. And basinwide, for the most part it has, despite the U.S. major hurricane drought that ended only recently.

ACM Figure 1
Figure 1. Annual average and 11-year smoothed values of Atlantic Multidecadal Oscillation. (Source: AIR)

In the early part of the current decade, many believed this most recent positive phase was reaching its end. Indeed, by some measures (as noted earlier, there are different ways of calculating the index) it was almost negative in 2009. But, it is important to remember that, just because it is called an oscillation, does not mean that it executes a regular predictable cycle in the short term (think El-Niňo Southern Oscillation).

In fact, much of the record shows very short-lived negative dips (e.g., in 1884, 1894, 1929, and numerous ones from the 1930s to the 1950s) only for it to rebound to positive for at least several more years in each of those instances. While a smoothed version (the red dotted line in Figure 1) more-clearly shows the multi-decadal variability of the Index, its value in any given year is still a challenging forecast. A case in point is this year and last, in which the index has been about as positive as it has ever been, even compared to what is was in 2004-05.

Better AMO Forecasting

The strong correlation between the index and hurricane activity in the Atlantic Basin is great motivation to improve forecasting in a to medium time frame (1-10 years). But accurate forecasting hinges on understanding the responsible mechanisms. There are two camps in this area:

  • Camp 1: Those who believe the AMO is driven internally, by regular and naturally occurring changes in the North Atlantic branch of the Thermohaline Circulation,
  • Camp 2: Those who believe the AMO is not a true oscillation at all, but simply variability that is externally driven by anthropogenic emissions and other aerosols

Using numerical climate models, the second camp points out that the AMO oscillated with much less amplitude before the industrial revolution. However, because there were no AMO records at that time, this theory is based entirely on numerical model output. Models and observations do indicate that changes in atmospheric circulation—which induce changes in clouds, atmospheric dust, and surface heat flux—are largely responsible for the tropical portion of the AMO. Many argue that the larger fluctuations and the prolonged negative index from the 1960s to the 1980s were the result of sulfate aerosols blocking sunlight, which led to cooling of North Atlantic waters. But climate models still have insufficient resolution, for example, to accurately simulate the Gulf Stream, which plays a significant role in the northward transport of salt water—a mechanism supporting the argument of the first camp. In fact, climate models have been unable to capture the amplitude of the oscillation, let alone the inter-annual variability.

The AMO for 2018

So, the jury is still out on the dominant cause of the oscillation (or variability, depending on which camp you belong to), and that may be unfortunate with respect to being able to forecast the AMO from one year to the next. Until a verdict is reached, persistence may be the best indicator of next year’s index. Applying that forecast technique, it may be safe to say that the index will remain positive for one more year through 2018, because for it to become negative, it would have to drop by more than 0.3 degrees Celsius in one year. And that has only happened three times in the last 160 years—which is less than a 2% chance.

A Retrospective on 10 Years of Modeling Hurricane Risk in a Warm Ocean Climate

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