Atlantic Multidecadal Oscillation
Learn more about Atlantic Multidecadal Oscillation
The Atlantic multidecadal oscillation (AMO) is a hypothesised 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 observations, controversy exists in regard to its amplitude, and in particular, the attribution of sea surface temperatures in the tropical Atlantic in areas important for hurricane development.
The AMO signal is usually defined from the patterns of SST variability in the North Atlantic once a linear trend has been removed. This detrending is intended to remove the influence of greenhouse gas induced global warming from the analysis, leaving a purely natural variation. 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 (Mann and Emanuel, EOS, 2006). Consequently, correlations with the AMO index may alias affects 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 worldwideThe AMO index is correlated to 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 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 hurricanes
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 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. However, if part of the recent uptick is related to global warming, the current warm regime may last significantly longer.
 Florida 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 — 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 shifts
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.
- Andronova, N. G. and M. E. Schlesinger, 2000: Causes of global temperature changes during the 19th and 20th centuries. Geophys. Res. Lett., 27, 2137-2140.
- Delworth, T. L. and M. E. Mann, 2000: Observed and simulated multidecadal variability in the Northern Hemisphere. Climate Dynamics, 16, 661-676.
- Enfield, D.B., A.M. Mestas-Nunez, and P.J. Trimble, 2001: The Atlantic Multidecadal Oscillation and its relationship to rainfall and river flows in the continental U.S. Geophys. Res. Lett., 28: 2077-2080.
- Goldenberg, S.B., C.W. Landsea, A.M. Mestas-Nuñez and W.M. Gray, 2001: The recent increase in Atlantic hurricane activity : Causes and implications. Science, 293:474-479.
- Gray, S. T., J. L. Graumlich, J. L. Betancourt, and G. T. Pederson, 2004: A tree-ring based reconstruction of the Atlantic Multidecadal Oscillation since 1567 A.D. Geophys. Res. Lett., 31, L12205, doi:10.1029/2004GL019932.
- Kerr, R. A., 2000: A North Atlantic climate pacemaker for the centuries. Science, 288, 1984-1986.
- Kerr, R. A., 2005: Atlantic climate pacemaker for millennia past, decades hence? Science, 309, 43-44.
- Knight, J. R., R. J. Allan, C. Folland, M. Vellinga, and M. E. Mann, 2005: A signature of persistent natural thermohaline circulation cycles in observed climate. Geophys. Res. Lett., 32, L20708, doi:10.1029/2005GL024233.
- McCabe, G. J., M. A. Palecki, and J. L. Betancourt, 2004: Pacific and Atlantic Ocean influences on multidecadal drought frequency in the United States. Proc. Nat. Acad. Sci., 101, 4136-4141.
- Sutton, R. T. and L. R. Hodson, 2005: Atlantic forcing of North American and European summer climate. Science, 309, 115-118.