Atlantic Multidecadal Oscillation (AMO) is a sequence of long period changes in the sea surface temperature of the North Atlantic Ocean, with cool and warm phases that may last for 60-80 years at a time and a difference in temperature of approximately 0.556°C between extremes. These changes are natural in occurrence and have been happening for at least the last 1,000 years. Since the mid-1990s the world has been in a warm phase. The AMO is a natural form of climate variability, explaining short and long phases in climate caused by natural, large scale features. Other modes of climate variability include the Northern Annular Mode, Pacific Decadal Oscillation, and the North Atlantic Oscillation.
The AMO impacts air temperatures and rainfall over much of the Northern Hemisphere, especially in North America and Europe. It is associated with changes in the frequency of North American droughts and is associated in the frequency of severe Atlantic hurricanes. It also 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, severe, and prolonged. 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. Sea surface temperatures have shown warm phases roughly during: 1860-1880 and 1930-1960 and cool phases during 1905-1925 and 1970-1990.
During warm phases of the AMO, at least twice as many tropical storms mature into severe hurricanes as during cool phases. Since the AMO switched to its warm phase around 1995, the particularly strong hurricanes have become much more frequent. The frequency of weak category storms, tropical storms, and weak hurricanes are largely unaffected by the AMO. However, the number of weak storms that mature into major hurricanes noticeably increase. Thus, the intensity is affected, specifically by increasing the frequency of major hurricanes. Ocean and atmosphere models that interact with each other show that the AMO cycle involves changes in the south to north circulation of the ocean. 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. Currently, predicting when the AMO will switch is not applicable. Computer models, such as ones that predict El Niño, are far from being able to do this. However, what is possible to do currently is to calculate the probability that a change in the AMO will occur within a given future time frame.
The present day model is that underlying currents in the Atlantic produce the temperature changes, but volcanic activity, airborne particles emitted by humans, and dust from Africa may contribute. In addition, we can’t yet predict when exactly the AMO will switch from warm to cold or vice versa. Since a cool ocean surface absorbs more heat from the atmosphere than a warm ocean surface, the AMO can also either mask the impacts of anthropogenic climate change or make them appear worse than they actually are. Measurements of the AMO cycles have only been going on for the last 150 years, which is not long enough to conclude the causes. However, studies of tree rings and ice cores have shown that oscillations similar to those by technology based observations, have been occurring for at least the last millennium. This is clearly longer than humans have been affecting climate, so it seems safe to conclude that the AMO is a natural climate oscillation.