The Atlantic multidecadal oscillation (AMO) is a mode of natural variability occurring in the North Atlantic Ocean and which has its principle expression in the sea surface temperature (SST) field. While there is some support for this mode in models and in historical observations, controversy exists with regard to its amplitude, and in particular, the attribution of sea surface temperatures in the tropical Atlantic in areas important for hurricane development.
The Atlantic Multidecadal Oscillation should not be confused with the North Atlantic Oscillation.
The AMO signal is usually defined from the patterns of SST variability in the North Atlantic once any linear trend has been removed. This detrending is intended to remove the influence of greenhouse gas-induced global warming from the analysis. However, if the global warming signal is significantly non-linear in time (i.e. not just a smooth increase), variations in the forced signal will leak into the AMO definition. Consequently, correlations with the AMO index may alias effects of global warming.
In models, AMO-like variability is associated with small changes in the North Atlantic branch of the Thermohaline Circulation, however historical oceanic observations are not sufficient to associate the derived AMO index to present day circulation anomalies.
Climate impacts worldwideEdit
The AMO index is correlated to air temperatures and rainfall over much of the Northern Hemisphere, in particular, North America and Europe such as North Eastern Brazilian and African Sahel rainfall and North American and European summer climate. It is also 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 Dust Bowl of the 1930s and the 1950s drought. Florida and the Pacific Northwest tend to be the opposite — warm AMO, more rainfall.
Relation to Atlantic hurricanesEdit
The frequency of weak-category storms — tropical storms and weak hurricanes — is not strongly correlated with the AMO. However, during warm phases of the AMO, the numbers of tropical storms that mature into major hurricanes is significantly greater than during cool phases, at least twice as many. Since the AMO switched to its warm phase, circa 1995, major hurricanes (Category 3 or above on the Saffir-Simpson Hurricane Scale) have become much more frequent — compared to the 1970s and 1980s — and this has led to a crisis in windstorm insurance coverage and cost. Based on the typical duration of negative and positive phases of the AMO, the current warm regime is expected to persist at least until 2015 and possibly as late as 2035.
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 — the reservoir for 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.
Prediction of AMO shiftsEdit
There is no demonstrated predictability for 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.
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