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Facts About The AMO

January 7, 2013

By Paul Homewood




We often hear about the Pacific Decadal Oscillation, El Ninos etc. But there is also an oscillation in the North Atlantic, which has significant climatic impacts too, the Atlantic Multidecadal Oscillation, or AMO.

We, perhaps, know hear less about this. NOAA have a FAQ page on the topic, which I have reprinted below.



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 phase are we in right now?

Since the mid-1990s we have been in a warm phase.

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.

How does the AMO affect Florida?

The AMO has a strong effect on Florida rainfall. Rainfall in central and south Florida becomes more plentiful when the Atlantic is in its warm phase and droughts and wildfires are more frequent in the cool phase. As a result of these variations, the inflow to Lake Okeechobee – which regulates South Florida water supply – changes by 40% between AMO extremes. In northern Florida the relationship begins to reverse – less rainfall when the Atlantic is warm.

How important is the AMO when it comes to hurricanes – in other words – is it one of the biggest drivers? Or Just a minor player?

During warm phases of the AMO, the numbers of tropical storms that mature into severe hurricanes is much greater than during cool phases, at least twice as many. Since the AMO switched to its warm phase around 1995, severe hurricanes have become much more frequent and this has led to a crisis in the insurance industry.

Does the AMO influence the intensity or the frequency of hurricanes (which)?

The frequency of weak-category storms – tropical storms and weak hurricanes – is not much affected by the AMO. However, the number of weak storms that mature into major hurricanes is noticeably increased. Thus, the intensity is affected, but, clearly, the frequency of major hurricanes is also affected. In that sense, it is difficult to discriminate between frequency and intensity and the distinction becomes somewhat meaningless.

If the AMO (in part) affects hurricanes – what drives the AMO?

Models of the ocean and atmosphere that interact with each other indicate that the AMO cycle involves changes in the south-to-north circulation and overturning of water and heat in the Atlantic Ocean. This is the same circulation that we think weakens during ice ages, but in the case of the AMO the changes in circulation are much more subtle than those of the ice ages. The warm Gulf Stream current off the east coast of the United States is part of the Atlantic overturning circulation. When the overturning circulation decreases, the North Atlantic temperatures become cooler.

Can we predict the AMO?

We are not yet capable of predicting exactly when the AMO will switch, in any deterministic sense. Computer models, such as those that predict El Niño, are far from being able to do this. What is possible to do at present is to calculate the probability that a change in the AMO will occur within a given future time frame. Probabilistic projections of this kind may prove to be very useful for long-term planning in climate sensitive applications, such as water management.

Is the AMO a natural phenomenon, or is it related to global warming?

Instruments have observed AMO cycles only for the last 150 years, not long enough to conclusively answer this question. However, studies of paleoclimate proxies, such as tree rings and ice cores, have shown that oscillations similar to those observed instrumentally have been occurring for at least the last millennium. This is clearly longer than modern man has been affecting climate, so the AMO is probably a natural climate oscillation. In the 20th century, the climate swings of the AMO have alternately camouflaged and exaggerated the effects of global warming, and made attribution of global warming more difficult to ascertain.


So to summarise:-

  1. The AMO is a natural phenomenon, known to have existed for at least 1000 years, (and, no doubt, much longer).
  2. Since the mid 1990’s, the AMO has been in the warm phase, (together with the last part of the warm PDO up to about 2005).
  3. The AMO affects temperatures in both N America, and Europe. Warm AMO = higher land temperatures.
  4. Droughts in the Midwest and Southwest  tend to be much worse during the warm AMO.
  5. There are more hurricanes during the warm AMO.



I wonder why we never hear any of this from the fraudsters experts?

  1. January 7, 2013 3:39 pm

    Is it just me or is the picture at the top of the article a broken link?

  2. John F. Hultquist permalink
    January 7, 2013 7:29 pm

    The link under the chart works.

    I wonder about the date of the text. The claim about severe hurricanes being at least twice as many [that link does not work] seems to have failed. The crisis in the insurance industry has more to do with human stupidity than anything else so the “attribution” is clearly human, but not AGW. Some Ryan Maue provided charts here:

    • January 7, 2013 8:45 pm

      It’s probably only a generalised statement, John.

      I think Ryan, Chris Landsea and others have commented that hurricane levels currently are “low” by climatological standards (i.e considering AMO).

  3. January 8, 2013 11:23 am

    Today, 8th Jan at 08.00 on UK Radio 4 news

    “The met office says it does not believe global warming will be as severe as it had previously predicted.”

    Nothing on the BBC website though.

    • January 8, 2013 1:16 pm

      Does that make the met office deniers?

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