The Atlantic multidecadal oscillation 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 15-40 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.
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 or prolonged. Vice-versa for negative AMO (cool phase). 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 as much as 40% between AMO extremes. In northern Florida the relationship begins to reverse — less rainfall when the Atlantic is warm.
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.