Flood Risk and Vulnerability Analysis Project - Atlantic Climate ...

One of the reasons for this increase (there are likely others) in storm activity is the gradual
warming of Atlantic sea surface temperatures and of the water temperatures south of
Newfoundland. While this increase in sea surface temperature does not necessarily trigger the
development of more tropical storms and hurricanes, it does allow more intense hurricanes to
develop over the tropics. Then, approaching over the warmer waters east of the United States,
they can maintain their intensity for a longer period of time as they do not encounter enough
cold water to weaken significantly.
The Atlantic Multi-decadal Oscillation (AMO) is an ongoing series of long-duration cycles in the
sea surface temperature of the North Atlantic Ocean, with cool and warm phases that may last
for 20-40 years at a time. While the development and phase shift of the AMO is unable to be
predicted by current weather models, it is possible to calculate the probability that a change in
the AMO will occur over a given period of time. Since higher sea surface temperatures are
associated with positive phases of the North Atlantic Oscillation (NAO), as will be explained
more fully in later sections, anticipating phase shifts of the NAO would provide clues regarding
general hurricane intensity changes.
While there is no scientifically proven link between the rise in sea surface temperatures and
tropical storm activity, there appears to be a weak correlation between warm phases of the
AMO and an increase in the occurrence of minor (Category 1 or 2) hurricanes (Chylek, P. &
Lesins, G, 2008). Based on data from NOAA (the US National Oceanic and Atmospheric
Association), there appears to be a significant increase in the frequency, at least twice as often,
of weak storms maturing into severe hurricanes during warm phases of the AMO.
Since the mid 1990‟s, the AMO has been in a warm phase, which has been correlated with a
significant increase in the frequency of tropical storms and hurricanes in the Atlantic Ocean, as
shown in Figure 3-18. The number of named storms, hurricanes and major hurricanes (defined
as Category 3 or higher) reached an all-time high in the 2000‟s decade, see Figure 3-18, and
the last two years (2010/11) have been very active years as well. Based on the typical 60-70
year duration (Enfield, David B.; Cid-Serrano, Luis (2010)) of the negative (cool) and positive
(warm) phases of the AMO, the current warm phase is expected to peak around 2020.
Therefore, it is expected that tropical cyclone activity over the Atlantic Ocean will continue to
increase for the next 10-15 years under the positive phase of the AMO.
Based on research conducted in the United States by the National Centre for Atmospheric
Research (NCAR) and the Georgia Institute of Technology in 2011 (Holland et al, 2011),
advanced dynamical and statistical modeling techniques have been implemented to determine
the amount that tropical cyclone activity will increase through 2050. Results have shown that
North Atlantic hurricanes will experience an accelerated increase in numbers from 1-3 percent
per decade from the present up to 4-10 percent per decade leading up to 2050. Only a modest
increase in mean intensity, on the order of 2 m/s (7.2 kph), is expected, but a significant
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