Environmental Engineering Reference
In-Depth Information
trend (Landsea et al., 2010). The tentative picture that emerges is of Atlan-
tic cyclone frequency being strongly modulated by internal multi-decadal
variability, with levels of activity as high in the mid-20th century and the
late 19th century as in the most recent decade. Additional clarification of
tropical cyclone records outside of the Atlantic are needed to provide a
clearer global picture of trends and variability.
Recent studies with relatively high-resolution global atmospheric models
and with dynamical and statistical downscaling techniques have supported
this picture. Several of these studies have successfully simulated the trend
in Atlantic storm frequency in recent decades when provided with bound-
ary conditions, especially ocean temperatures, as observed over this period
(Knutson et al., 2008; LaRow et al., 2008; Zhao et al., 2009). When these
or similar models are used to simulate the tropical storm statistics consistent
with climate projections for the late 21st century, they consistently predict
responses that cannot be obtained by extrapolating recent Atlantic frequency
trends (e.g., Sugi et al., 2002; Oouchi et al., 2006; Bengtsson et al., 2007;
Knutson et al., 2008; Zhao et al., 2009). Averaged over the globe, models
typically predict that the frequency of tropical cyclones will stay roughly
unchanged or be reduced by 0-10% per degree C global warming. Within
the Atlantic basin, there is more variance in the results, with projections
either increasing or decreasing frequencies by as much as 25% per degree
C global warming.
New high-resolution studies are appearing regularly, so this picture is
likely to evolve, but results to date suggest that the recent increase in Atlantic
activity is likely due to an increase in tropical Atlantic ocean temperatures
relative to the rest of the tropics and not to the absolute increase in tempera-
tures (i.e., Vecchi et al., 2008). Consistently, the spread in projections for
the future may have less to do with the details of the downscaling approach
and more to do with the spatial pattern of tropical ocean warming projected
by different global climate models, with those models that project larger
(smaller) warming in the Atlantic than in other ocean basins resulting in an
increase (decrease) in Atlantic storm activity (Zhao et al., 2009).
Insights into likely changes in storm intensity are primarily guided by the
theory of the maximum intensity that tropical cyclones can attain in a given
environment (Emanuel, 1987), by idealized modeling studies (Knutson and
Tuleya, 2004), and a small number of attempts at dynamical and statistical
downscaling (Emanuel et al., 2008). A consensus picture emerges in which
storms are expected to become more intense on average, roughly by 1-4%
per degree global warming, as measured by maximum wind speeds (Knutson
et al., 2010), or by 3-12% per degree for the cube of this wind speed, of-
