Upper: AMO index: the ten-year running mean of detrended Atlantic sea surface temperature anomaly (SSTA, °C) north of the equator. Lower: Correlation of the AMO index with SSTA everywhere.

What is the AMO?

The Atlantic multidecadal oscillation (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 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. Visualisation of the SST of the ocean just before Hurricane Bob (August 14, 1991) Sea surface temperature (SST) is the water temperature at 1 meter below the sea surface. ... For other uses, see Atlantic (disambiguation) The Atlantic Ocean is Earths second-largest ocean, covering approximately one-fifth of its surface. ...

Climate impacts worldwide

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 Dust Bowl of the 1930s and the 1950s drought. Florida and the Pacific Northwest tend to be the opposite — warm AMO, more rainfall. A drought or an extreme dry periodic climate is an extended period where water availability falls below the statistical requirements for a region. ... This article is about weather phenomena. ... Global mean surface temperatures 1856 to 2004 Mean temperature anomalies during the period 1995 to 2004 with respect to the average temperatures from 1940 to 1980 Global warming is an increase in the average temperature of the Earths atmosphere and oceans. ... Dust storm approaching Stratford, Texas The Dust Bowl, also known as the Dirty Thirties, was a series of dust storms caused by a massive drought and decades of inappropriate farming techniques that began in 1930 and lasted until 1941. ...

The AMO temperature index vs. the frequency of major hurricanes in the Atlantic.

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. Saffir-Simpson Hurricane Scale - Wikipedia, the free encyclopedia /**/ @import /skins-1. ...

Florida rainfall

The upper panel shows the AMO index since 1860 (°C). The lower panel shows the smoothed anomaly of central Florida rainfall (shaded curve) and the amount of water flowing into Florida's Lake Okeechobee. Both are expressed as a percentage of their long-term average.

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. Lake Okeechobee, locally referred to as Lake O, The Big Lake, or simply The Lake is a freshwater lake in the U.S. state of Florida. ...

Prediction of AMO shifts

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.

References

• 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.
• Kerr, R. A., 2000: A North Atlantic climate pacemaker for the centuries. Science, 288, 1984-1986.
• Kerr, R. A., 2005: Atlantic climate pacemaker for millenia 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.
• Sutton, R. T. and L. R. Hodson, 2005: Atlantic forcing of North American and European summer climate. Science, 309, 115-118.

External links

Frequently asked questions about the AMO

Probabilistic projection of future AMO regime shifts