The Future of Hurricanes and Tropical Cyclones
Julia Albert
May 2nd, 2018
Introduction:
The 2017 hurricane season consisted of an immense amount intense hurricanes, causing
massive amounts of damage along the coastal regions in the Mid-Atlantic. The destruction
caused by Hurricane Harvey, Hurricane Irma and Hurricane Marie left the equatorial regions
devastated. This hurricane season left the US with $200 billion dollars’ worth of damages, most
of the damage being link to Harvey, Irma, and Marie. This is concerning because most of the
US’s densely populated areas are found in coastal regions. Scientists wonder whether the 2017
hurricane season signals warnings for future hurricane seasons and climate change.
The causes of the increase in hurricane intensity is linked to an increase in sea surface
temperature (SST). There is debate regarding the causes of the global surface temperature
increase- whether the increase in hurricane intensity is linked to naturally forced climate
variability or anthropogenic climate change. It is suggested that the Atlantic Multidecadal
Oscillation (AMO) is the cause of the global sea surface temperature increase. Others suggest
that the increase in Greenhouse Gases released into the atmosphere from anthropogenic forcing
has caused the increase in oceanic warming.
The current issue faced when determining the significance of the increase in hurricane
intensities and frequencies is lack of observational data. It is inconclusive whether the increase in
hurricane intensity is linked to better hurricane modeling methods, but global warming is the
most plausible cause.
Background:
Tropical hurricanes and cyclones obtain their energy from warm tropical oceans. The
hurricane season runs from June through November. Cyclones and hurricanes form where sea
surface temperature is warm, causing the oceanic surface to turn into water vapor and is then
released into the upper atmosphere. This process lowers atmospheric pressure, which causes
winds at ocean surface to spiral inward and pick up speed. The increase in SST directly effects
cyclone formation and behavior. The factors that influence hurricane intensity are sea surface
temperatures, wind shear, warm moist air, and the projected storm track.
One driver of global sea surface change is the Atlantic Multidecadal Oscillation
(AMO). The AMO is the natural variation in sea surface temperatures between phases of
oceanic cooling and warming. These intervals of oceanic cooling and warming transition every
25-40 years. When the AMO is in a warming phase, the Atlantic Ocean and atmosphere
temperatures warm, causing an increase in hurricane frequency. The effects of AMO warming
period occurred during the 1940’s to late 1960’s, when there was an above average number of
hurricanes. The AMO then transitioned into a cooling phase that lasted until 1994. Although
natural variation may have caused this decrease in SST, The IPCC (2013) questions whether
anthropogenic aerosol forcing (along with natural variability) caused the decrease in Atlantic
hurricane activity during the 1970s and 1980.
Other scientist such as Kerry Emanuel theorize that current hurricane variation is directly
affected by human caused climate warming. In 1983, Emanuel suggest that the upper-limit
increase in hurricane intensity is directly related to global warming. Emanuel projects that future
hurricanes will not increase in frequency, but in intensity. Atmospheric and oceanic circulation
variability is a complex system that is influenced by multiple variables. Hurricane variation and
intervals have been observed on a multi decadal time scale since 1940. Scientists use various
simulations and methods to obtain a better understanding of hurricane patterns and frequencies,
including studying the paleoclimate. Reconstructing paleo-hurricanes during the Holocene
allows better insight to past hurricane interval, but high resolution data is rare.
Results:
According to the National Oceanic and Atmospheric Administration (NOAA), “Sea
surface temperature has been consistently higher during the past three decades than at any other
time since reliable observations began in 1880”. Figure 1 shows the increase in the average sea
surface temperature from 1880-2015.
Figure 1: Average Global Sea Surface Temperature, 1880-2015 (NOAA)
During the past 35 years, global sea surface temperature has increased .5 degrees Celsius
(Webster et al. 2005). This half of a degree Celsius creates a massive increase in hurricane
intensity, causing more water to be vaporized. Not only does the hurricane intensity pick up, but
the storm surge also increases creating massive floods in coastal regions. The intensity of storm
surge is a negative externality of human caused warming that increases in sea level from ice cap
depletion (CSIRO, 2015; NOAA, 2016). According to the NAOO, “Since 1993, however,
average sea level has risen at a rate of 0.11 to 0.14 inches per year—roughly twice as fast as the
long-term trend [1880-2015].” It is very likely that human GHG emissions has influential impact
on the oceanic and atmospheric systems.
In order to measure hurricane activity and intensities it is necessary to use indexes to
show correlations in graph trends. Measuring average hurricane intensities requires indexes such
as the Power Dissipation Index (PDI; Emanuel 2005, 2007) and the Accumulated Cyclone
Intensity Index (Bell et al. 2000; Camargo and Sobel 2005; Bell and Chelliah 2006). These
indexes are used to show the relationship between hurricane intensity and SST. The ACE and
PDI index are fairly recent climate modeling indexes used since the start of satellite detection.
The Power Dissipation Index (PDI) is "the sum of the maximum one-minute sustained
wind speed cubed, at six-hourly intervals, for all periods when the cyclone is at least tropical
storm strength” (Kunreuther 2011). PDI is also referred to the totality of storm intensity,
frequency, and duration and provides a measure of total hurricane power over a hurricane season.
Using this index allows a timeline of hurricane activity since 1950. The Accumulated Cyclone
Energy Index (ACE) is another index that measures the cyclone intensity and is used to show the
relationship between storm trends and sea surface temperature. Total ACE Index value for an
entire hurricane season by adding the values for all named storms (including subtropical storms,
tropical storms, and hurricanes). Both of these indexes show the same trend in hurricane activity
from 1950-2010, as shown in the figure below:
Figure 2: Time series of (top) PDI and (bottom) ACE with and without the adjustment in
Landsea (1993).
The ACE and PDI indexes show closely related patterns in hurricane intensity trends.
Combining the trend in the PDI index from 1949-2010 with the trend in global sea
surface temperature from 1949-2010 demonstrates a positive correlation. Figure 3
shows changing cyclone intensity for most of the mid- to late 20th century, followed by a
noticeable increase since 1995. These trends are shown with associated variations in sea surface
temperature in the tropical North Atlantic for comparison. Figure 3 demonstrates two distinct
increases in SST from 1910-1940 and from 1970 until present. There is a noticeable drop in SST
from 1880-1910.
Figure 3: North Atlantic Tropical Cyclone Activity According to the Power Dissipation Index,
1949–2015
According to Figure 4, the ACE Index from 1949-2015 shows that cyclone intensity
noticeably increased during the past 20 years. Since 1950, most of the active years have occurred
since mid-1990. There is also noticeably high levels of cyclone activity during the 1950s and
1960s. For this indicator, the NOAA converted to a scale where 100 equals the median value
over a base period from 1981 to 2010. The thresholds in Figure 4 show if the ACE Index for a
given year is close to normal, significantly above normal, or significantly below.
Figure 4: North Atlantic Tropical Cyclone Activity According to the Accumulated Cyclone
Energy Index, 1950–2015
Discussion:
Figures 1-4 are consistent with one another, and demonstrate the same trends in hurricane
intensity and SST. According to the NCA, “the intensity, frequency, and duration of North
Atlantic hurricanes, as well as the frequency of the strongest (Category 4 and 5) hurricanes, have
all increased since the early 1980s.” The ACE and PDI time series analyses show that the mid1970’s-1980 demonstrate increase in hurricane intensity. Some studies suggest that natural
variability, which includes the Atlantic Multidecadal Oscillation, is the dominant cause of the
warming trend in the Atlantic since the 1970s. Although the AMO’s cooling and warming
phases correlates with hurricane frequency, the warm phase’s intervals increase in number of
major hurricanes (Figure 5). Figure 5 shows that the number of tropical storms and hurricanes
has increased since 1940, along with the number of major hurricanes.
Figure 5: Annual number if tropical cyclones formed in the Atlantic Basin from 1851-2006
(NOAA)
Although natural variation may have caused this decrease in SST from 1960-1970, the
IPCC (2013) questions whether anthropogenic aerosol forcing (along with natural variability)
caused the decrease in Atlantic hurricane activity during the 1960s and 1970 (Figure 3), rather
than natural variability in climate. The current global sea surface temperature trend (Figure 1)
shows continual increase in SST, and according to Knuston (2011), “Future projections indicate
that greenhouse warming will cause the globally averaged intensity of tropical cyclones to shift
towards stronger storms, with intensity increases of 2–11% by 2100.” Knuston also states that
the models that predict an increase in hurricane intensity also project decreases in globally
averaged frequency of tropical cyclones by ~6-34%. The models used to project hurricane
variability need improvement, some lacking consistency in their climate projections. Changes in
Tropical Cyclone Number, Duration, Intensity in a Warming Environment (Webster et al. 2005)
states that “attributions of the 30-year trends to global warming would require a longer global
data record and, especially, a deeper understanding of the role of hurricanes in the general
circulation of the atmosphere and ocean, even in the present climate state.” The lack of
observational data creates an inconclusive understanding of the trends of hurricane intensity and
frequency.
Conclusion
The effects of anthropogenic global warming is increasingly prevalent in today’s climate
system. The release of human caused GHG into the atmosphere is changing the chemical
composition of our atmospheric and oceanic climate system, creating areas of increased climate
extremities. The increase in solar irradiance combined with the CO2 trapped in Earth’s
atmosphere has increased the stratospheric temperature, and changes the pH balance in the
oceans to become more acidic. Although natural climate variability influences the changes in
Earth’s climate, anthropogenic forcing has increased the extent of the variability. The melting of
ice caps has caused sea levels to rise, increasing the storm surge intensity. Although the causes of
the SST increase are inconclusive due to lack of data, the general warming of Earth’s climate
system from human climate change is prevalent factor towards the extreme weather. If the SST
continues to increase as suggested in Figure1, we will enter a climate system of extreme
hurricane intensity.
References
Emanuel, K.A. 2016 update to data originally published in: Emanuel, K.A. 2007. Environmental
factors affecting tropical cyclone power dissipation. J. Climate 20(22):5497–5509
IPCC (Intergovernmental Panel on Climate Change). 2013. Climate change 2013: The physical
science basis. Working Group I contribution to the IPCC Fifth Assessment Report.
Cambridge, United Kingdom: Cambridge University Press.www.ipcc.ch/report/ar5/wg1
Knutson, Thomas R., et al. “Tropical Cyclones and Climate Change.” Nature Geoscience, vol. 3,
no. 3, 2010, pp. 157–163., doi:10.1038/ngeo779.
NOAA (National Oceanic and Atmospheric Administration). 2016. Extended reconstructed sea
surface temperature (ERSST.v4). National Centers for Environmental Information.
www.ncdc.noaa.gov/data-access/marineocean-data/extended-reconstructed-sea-surfacetemperature-ersst.
NOAA (National Oceanic and Atmospheric Administration). 2016 update to data originally
published in: NOAA. 2009. Sea level variations of the United States 1854–2006. NOAA
Technical Report NOS CO-OPS
053.www.tidesandcurrents.noaa.gov/publications/Tech_rpt_53.pdf.
Villarini, Gabriele, and Gabriel A. Vecchi. “North Atlantic Power Dissipation Index (PDI) and
Accumulated Cyclone Energy (ACE): Statistical Modeling and Sensitivity to Sea Surface
Temperature Changes.” Journal of Climate, vol. 25, no. 2, 2012, pp. 625–637.,
doi:10.1175/jcli-d-11-00146.1.
Webster, P. J. “Changes in Tropical Cyclone Number, Duration, and Intensity in a Warming
Environment.” Science, vol. 309, no. 5742, 2005, pp. 1844–1846.,
doi:10.1126/science.1116448.
Yang, Chih-Yuan, and Chia-Chien Chang. “Measuring U.S. Hurricane Risk Associated with
Natural Climate Cycle and Global Warming Effects.” Asia-Pacific Journal of Risk and
Insurance, vol. 7, no. 2, 2013, pp. 1–26., doi:10.1515/apjri-2013-0002.