The Atlantic multidecadal oscillation (AMO) is a mode of variability occurring in the North Atlantic Ocean and which has its principal 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.
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Definition
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 2008, new models revealed that global warming should reduce the frequency of hurricanes overall, while intensity might increase in some areas. Because reliable records of hurricane strength and frequency only extend back to approximately 1970, researchers have faced difficulty in developing reliable models.[1]
Mechanisms
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 worldwide
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.
Climate models suggest that a warm phase of the AMO strengthens the summer rainfall over India and Sahel and the North Atlantic tropical cyclone activity.[2] Paleoclimatologic studies have confirmed this pattern—increased rainfall in AMO warmphase, decreased in cold phase—for the Sahel over the past 3,000 years.[3]
Relation to Atlantic hurricanes
The frequency of major hurricanes is not strongly correlated with the AMO. However, during warm phases of the AMO, the number of minor hurricanes (category 1 and 2) have seen a modest increase.[4] The hurricane activity index is found to be highly correlated with the Atlantic multidecadal oscillation.[4] If there is an increase in hurricane activity connected to global warming, it is currently obscured by the AMO quasi-periodic cycle.[4] 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. Enfield et al. assume a peak around 2020.[5]
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 are only about 130-150 years of data based on instrument data which are too few samples for conventional statistical approaches. With aid of multi –century proxy reconstruction, a longer period of 424 years was used by Enfield and Cid–Serrano as an illustration of an approach as described in their paper called "The Probabilistic Projection of Climate Risk".[6] Their histogram of zero crossing intervals from a set of five re-sampled and smoothed version of Gray et al (2004) index together with the Maximum Likelihood Estimate gamma distribution fit to the histogram, showed that the largest frequency of regime interval was around 10–20 year. The cumulative probability for all intervals 20 years or less was about 70% [7]
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. Enfield and colleagues have calculated the probability that a change in the AMO will occur within a given future time frame, assuming that historical variability persists. Probabilistic projections of this kind may prove to be useful for long-term planning in climate sensitive applications, such as water management.
Assuming that the AMO continues with its quasi-cycle of roughly 70 years, the peak of the current warm phase would be expected in c. 2020,[8] or based on its 50–90 year quasi-cycle, between 2000 and 2040 (after peaks in c. 1880 and c. 1950).[5][relevant? ]
References
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This article includes a list of references or external links, but its sources remain unclear because it has insufficient inline citations. Please help to improve this article by introducing more precise citations where appropriate. (January 2009) |
- ^ Emanuel, K; Sundararajan, R Agoth and Williams, J (2008). "Hurricanes and Global Warming". Bull. Amer. Meteor. Soc 89: 347–367. ftp://texmex.mit.edu/pub/emanuel/PAPERS/Emanuel_etal_2008.pdf.
- ^ Zhang, R.; Delworth, T. L. (2006). "Impact of Atlantic multidecadal oscillations on India/Sahel rainfall and Atlantic hurricanes". Geophys. Res. Lett. 33: L17712. doi:.
- ^ Shanahan, T. M.; et al. (2009). "Atlantic Forcing of Persistent Drought in West Africa". Science 324 (5925): 377–380. doi:.
- ^ a b c Chylek, P. & Lesins, G. (2008), "Multidecadal variability of Atlantic hurricane activity: 1851–2007", Journal of Geophysical Research 113: D22106, doi:
- ^ a b Enfield, David B.; Cid-Serrano, Luis (2009). "Secular and multidecadal warmings in the North Atlantic and their relationships with major hurricane activity". International Journal of Climatology Forthcoming: n/a. doi:.
- ^ http://www.usclivar.org/Newsletter/Variations_V3N3/Enfield.pdf
- ^ For additional comments and citations see AMO, The Key Global Climate Indicator.
- ^ Curry, Judith A. (2008), "Potential Increased Hurricane Activity in a Greenhouse Warmed World", in MacCracken, Michael C.; Moore, Frances; Topping, John C., Sudden and disruptive climate change, London: Earthscan, pp. 29–38, ISBN 1844074781, "Assuming that the AMO continues with a 70-year periodicity, the peak of the next cycle would be expected in 2020 (70 years after the previous 1950 peak)."
Further reading
- Andronova, N. G.; Schlesinger, M. E. (2000). "Causes of global temperature changes during the 19th and 20th centuries". Geophys. Res. Lett. 27: 2137–2140. doi:. http://www.agu.org/pubs/crossref/2000/2000GL006109.shtml.
- Delworth, T. L.; Mann, M. E. (2000). "Observed and simulated multidecadal variability in the Northern Hemisphere". Climate Dynamics 16: 661–676. doi:.
- Enfield, D. B.; Mestas-Nunez, A. M.; Trimble, P. J. (2001). "The Atlantic Multidecadal Oscillation and its relationship to rainfall and river flows in the continental U.S.". Geophys. Res. Lett. 28: 2077–2080. doi:. http://www.agu.org/pubs/crossref/2001/2000GL012745.shtml.
- Goldenberg, S. B.; et al. (2001). "The recent increase in Atlantic hurricane activity: Causes and implications". Science 293: 474–479. doi:.
- Gray, S. T.; et al. (2004). "A tree-ring based reconstruction of the Atlantic Multidecadal Oscillation since 1567 A.D.". Geophys. Res. Lett. 31: L12205. doi:.
- Hetzinger, Steffen; et al. (2008). "Caribbean coral tracks Atlantic Multidecadal Oscillation and past hurricane activity". Geology 36 (1): 11–14. doi:.
- Kerr, R. A. (2000). "A North Atlantic climate pacemaker for the centuries". Science 288 (5473): 1984–1986. doi:.
- Kerr, R. A. (2005). "Atlantic climate pacemaker for millennia past, decades hence?". Science 309 (5731): 41–43. doi:.
- Knight, J. R. (2005). "A signature of persistent natural thermohaline circulation cycles in observed climate". Geophys. Res. Lett. 32: L20708. doi:.
- McCabe, G. J.; Palecki, M. A.; Betancourt, J. L. (2004). "Pacific and Atlantic Ocean influences on multidecadal drought frequency in the United States". PNAS 101: 4136–4141. doi:.
- Sutton, R. T.; Hodson, L. R. (2005). "Atlantic forcing of North American and European summer climate". Science 309: 115–118. doi:.
- Knight, J. R.; C. K. Folland, and A. A. Scaife (2006). "Climate impacts of the Atlantic Multidecadal Oscillation". Geophys. Res. Lett.: L17706. doi:.
- "Climate change: the next ten years" by Fred Pearce and Michael Le Page, New Scientist, 13 Aug. 2008, pp. 26–30.
External links
- Frequently asked questions about the AMO
- Probabilistic projection of future AMO regime shifts
- AMO Data from 1856 - Present
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