Atlantic Multidecadal Oscillation (AMO)
Atlantic Multidecadal Oscillation (AMO) FAQ's
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.
Red and blue colored dots represent positive and negative correlations of Northern Hemisphere summer rainfall with the AMO index. When the AMO is positive (warm Atlantic) there is less rainfall over most of the United States and northeastern South America, and more rainfall in southern Alaska, northern Europe, west Africa and Florida.
Most of the Atlantic between the equator and Greenland changes in unison. Some area of the North Pacific also seem to be affected.
Since the mid-1990s we have been in a warm phase.
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.
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.
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's 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.
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.
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.
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.
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.