ACCUMULATED CYCLONE ENERGY AND TROPICAL CYCLONE TRACKS:
AN IN-DEPTH ANALYSIS OF THE ANOMALOUSLY INACTIVE 2013 ATLANTIC HURRICANE SEASON
by
Thomas Vaughan
A Senior Honors Project Presented to the
Honors College
East Carolina University
In Partial Fulfillment of the
Requirements for
Graduation with Honors
by
Thomas Vaughan
Greenville, NC
May 2015
Approved by:
Thomas Rickenbach, Ph.D.
Department of Geography, Planning, and Environment, Thomas Harriot College of Arts and Sciences
ACCUMULATED CYCLONE ENERGY AND TROPICAL CYCLONE TRACKS:
AN IN-DEPTH ANALYSIS OF THE ANOMALOUSLY INACTIVE 2013 ATLANTIC
HURRICANE SEASON
Thomas A. Vaughan
East Carolina University
Department of Geography, Planning, and Environment
Abstract
The 2013 season was forecast by the National Oceanic and Atmospheric Administration
(NOAA) Climate Prediction Center (and many other seasonal forecast centers) to be much more
active in terms of tropical cyclone activity than it turned out to be. The season was characterized
by tropical cyclone activity that was well below normal and produced significantly fewer named
storms than expected. This study investigates the reasons behind the poor forecast by analyzing
the differences in the 2013 season compared to climatology from the previous 18 years (19952012), a very active Atlantic hurricane period associated with a multidecadal oscillation in the
thermohaline circulation (Klotzbach & Gray, 2008).
Specifically, this study focuses in large part on the analysis of accumulated cyclone
energy (ACE), which is used by NOAA to determine how “active” an individual tropical cyclone
is throughout its life cycle, and how “active” a season is as a whole. ACE is calculated by using
the formula ACE = 10-4Σv2 where v is the estimated sustained maximum wind speed measured
in knots. This is calculated every six hours, typically at 0000, 0600,1200, and 1800 UTC. For
this study, the 2013 tropical cyclone tracks were mapped using ArcGIS software and the ACE
for all 2013 was calculated using data collected from NOAA. The Atlantic basin is then
subdivided into three regions where tropical cyclones typically form throughout the season: East
Atlantic (15°W-45°W), Mid-Atlantic (45°W-75°W) and Gulf of Mexico/Immediate Eastern U.S.
Seaboard (75°W - 105°W). For each region, the total ACE for the 2013 season as well as the
number of hurricane days is calculated. Then, the ACE values are calculated for each individual
2
month (June-December). These values are compared with “typical” averages and analyzed.
Additionally, the physical tracks of 2013 Atlantic tropical cyclones are analyzed and compared
to those of a “typical” season using ArcGIS. This information quantifies the extent to which
tropical cyclones in 2013 formed in anomalous locations or took anomalous paths compared to
the 1995-2012 average.
It can be concluded from this study that the total ACE for 2013 was well below the
average ACE value of a “typical” season. When analyzed by month, ACE for the Atlantic basin
generally followed the expected climatology trend with a peak in September, but values were
much smaller than climatology. It was found that 2013 ACE for the East Atlantic was about
average and as expected, however ACE values from the Mid-Atlantic and Gulf of
Mexico/Immediate Eastern U.S. Seaboard were well below average. Climatology suggests that
most tropical cyclone activity should be in the Mid-Atlantic region, but this was not the case for
2013 when most activity was located in the East Atlantic. Additionally, it was found that the
number of hurricane days in 2013 was well below average, with the Mid-Atlantic region being
particularly anomalous with no hurricane days in 2013.
This study will be a useful resource for meteorologists and climatologists to continue
analyzing the 2013 season, and will serve as a basis for determining possible causes of the
anomalous geographic distribution of tropical cyclone activity in 2013. The information herein
will also be beneficial in observing long-term trends and improving seasonal outlooks in the
future.
3
Table of Contents
Abstract ...........................................................................................................................................2
Introduction ....................................................................................................................................5
Review of Literature ......................................................................................................................8
Accumulated Cyclone Energy (ACE) .........................................................................................8
2013 NOAA Atlantic Hurricane Season Outlook .....................................................................12
2013 Klotzbach/Gray Hurricane Season Outlook.....................................................................13
Observed Tropical Cyclone Activity in 2013 ...........................................................................14
Multidecadal Variability ...........................................................................................................15
Data and Methods ........................................................................................................................17
Results ...........................................................................................................................................20
Discussion......................................................................................................................................25
Conclusion ....................................................................................................................................27
Acknowledgements ......................................................................................................................29
References .....................................................................................................................................30
4
Introduction
Prediction of seasonal hurricane activity in the Atlantic basin prior to the official start of
hurricane season, which runs from June 1 to November 30, has important implications on many
aspects of society. Each year various meteorological groups issue these forecasts, which are then
used by the general public as an outlook of how much activity is expected for the season and
how strong the storms may be. The two most prominent of these groups include the National
Oceanic and Atmospheric Administration’s (NOAA) Climate Prediction Center (CPC) and
Colorado State University’s Tropical Meteorology Project, headed by Phillip Klotzbach and
William Gray. These probabilistic seasonal outlooks are intended to provide an overall level of
activity and intensity, which can be used by residents, businesses, and government agencies
along the Atlantic coastline to prepare for the upcoming season. Additionally, NOAA, the
Federal Emergency Management Agency (FEMA), the National Hurricane Season (NHC), the
Small Business Administration, and the American Red Cross can use the seasonal hurricane
outlook to improve their hurricane preparedness information products (NOAA Predicts
Active…, 2013).
It should be noted that these forecasts are not meant to provide a seasonal hurricane
landfall forecast or predict activity levels impacting any particular region due to the fact that
tropical cyclone landfalls are highly influenced by weather patterns present when a storm is
approaching land. This information can only be predicted several days in advance with
reasonable accuracy (NOAA Predicts Active…, 2013).
Creating these seasonal outlooks can prove to be challenging, particularly because the
Atlantic basin (which includes the Atlantic Ocean, Caribbean Sea, and the Gulf of Mexico)
experiences the largest interannual variability of seasonal hurricane activity than any other place
5
on Earth (Gray, 1992). In general, there are five factors that are used primarily in creating these
forecasts, “including two slowly varying global-scale climate factors (1) the phase of the
stratospheric quasi-biennial oscillation (QBO) and (2) the presence or absence of a moderate-tostrong El Niño event, and three persistent regional-scale factors: (3) 200-mb zonal wind; (4) sea
level pressure anomalies in the Caribbean basin; and (5) the anticipated June-September rainfall
in the western Sahel region of Africa” (Gray et al. 1992).
There are several points of uncertainty with seasonal outlooks, and they are not
considered absolute predictions. Predicting El Niño and La Niña events, sea-surface
temperatures, vertical wind shear, moisture, and stability is an ongoing scientific problem with
limited-skill forecasts months in advance. To further complicate matters, there can be many
different combinations of tropical cyclone activity associated with the same set of climatological
conditions (i.e. a certain set of conditions can result in many weaker storms or fewer stronger
storms). Despite these challenges, climatologists and other atmospheric scientists use the best
information available to make a prediction, and usually are able to do so with reasonable
accuracy (NOAA, 2013).
Although no seasonal outlook is perfect, in 2013 the seasonal outlooks for tropical
cyclone activity in the Atlantic produced by NOAA and Klotzbach/Gray turned out to be
particularly erroneous. They both predicted above-normal tropical cyclone activity in the
Atlantic for 2013, but the overall tropical cyclone activity was actually well below average,
producing significantly fewer named storms than expected. The purpose of this study is to begin
exploring the causes of the anomalously inactive 2013 Atlantic hurricane season by analyzing
the differences in the 2013 season compared to climatology from the previous 18 years (19952012), a very active Atlantic hurricane period associated with a multidecadal oscillation in the
6
thermohaline circulation, defined in Review of Literature (Klotzbach & Gray, 2008). This
project serves as a post analysis of the 2013 season by analyzing the season’s accumulated
cyclone energy (ACE), an index used by NOAA to determine how “active” an individual tropical
cyclone is throughout its life cycle, and how “active” a season is as a whole. Analysis is done as
a whole Atlantic basin as well as by defined regions in order to determine the anomalies in the
geographic distribution of cyclone energy, track locations, and intensities in 2013.
There are many ongoing research projects related to the 2013 Atlantic hurricane season in
order to better determine why the seasonal outlooks may have been so inaccurate. However, this
project is unique in that it uses the accumulated cyclone energy index and physical track
locations to analyze the season’s activity geographically, which has not been done previously. It
attempts to differentiate the regions of the Atlantic basin that saw 2013 tropical cyclone activity
as expected, and which regions did not. This information can then be used as an aid in
determining what may have gone wrong with the forecasts by allowing atmospheric scientists to
specially focus their research on the anomalous regions of activity in the Atlantic in 2013.
7
Review of Literature
Accumulated Cyclone Energy (ACE)
One of the initial challenges of analyzing overall seasonal tropical cyclone activity is
determining the most efficient way to quantify this measurement. There are several methods of
calculating tropical cyclone activity which are used by various entities.
One of these is net tropical cyclone activity (NTC), which incorporates the number of
named storms, number of hurricanes, number of ‘intense’ hurricanes, number of named storm
days, and number of ‘intense’ hurricane days. These are generally viewed by the scientific
community as independent parameters but they tend to be strongly statistically correlated (Bell et
al. 1999).
An additional index that may be used to measure overall seasonal tropical cyclone
activity is hurricane destruction potential (HDP). HDP is calculated by summing the squares of
observed (estimated) maximum sustained wind speeds every six hours that a storm is classified
as hurricane strength, defined as maximum wind speeds greater than 64 knots or 74 mph (Bell et
al. 1999).
The National Oceanic and Atmospheric Administration has determined that the best way
to measure total overall seasonal tropical cyclone activity is by utilizing an index called
accumulated cyclone energy (ACE). The ACE index is a modified version of HDP and is
“defined as the sum of the squares of the maximum sustained surface wind speed (knots)
measured every six hours for all named storms while they are at least tropical storm strength”
(NOAA, 2014). Including tropical storms, defined as having a maximum sustained wind speed
greater than 34 knots (39mph), in the index allows scientists to better analyze tropical cyclone
activity during years of few hurricanes, as in 2013. Additionally, the value is typically
8
multiplied by 10-4 in order to make it easier to analyze the index values. The resulting formula to
calculate ACE is:
where 𝒗 is maximum sustained wind speed measured in knots and the unit for ACE is 104 kt2.
ACE can be calculated for each storm individually to gage the intensity of a certain
tropical cyclone. Additionally, the ACE values for each storm can be summed together to
quantify intensity for an entire hurricane season.
In their analyses, NOAA compares ACE values for each season to a climatology based
on the thirty year period 1981−2010. This time period that averaged 12.1 named storms, 6.4
hurricanes and 2.7 major hurricanes (category 3-5) each year. They define above-normal, belownormal, and near-normal hurricane seasons based on the following Tables 1 and 2. Colorado
State University’s Tropical Meteorology Project also uses a climatological average from
1981−2010 in their forecasts (Klotzbach and Gray, 2013).
Above-Normal Season
ACE greater than 111 (x 104 kt2)
≥120% of 1981-2010 ACE median
Near-Normal Season
ACE between 66 and 111 (x 104 kt2)
ACE less than 66 (x 104 kt2)
≤ 71.4 % of 1981-2010 ACE median
Below-Normal Season
OR
ACE greater than 66 (x 104 kt2)
Meeting all 3 conditions to the right
Including 2 of these 3
conditions:
 13+ named storms
 7+ hurricanes
 3+ major hurricanes



< 9 named storms
< 4 named hurricanes
<1 major hurricane
Table 1. NOAA Definitions of “Above-Normal,” “Near-Normal,” and “Below-Normal” based on ACE
Source: National Oceanic and Atmospheric Administration
9
Table 2. Typical Hurricane Season Statistics by Activity Level
Source: National Oceanic and Atmospheric Administration
In a 2006 study, Bell and Chelliah divided the Atlantic basin into 3 regions, Extratropics,
Gulf of Mexico, and Main Development Region (MDR), to analyze ACE values from each
region for seasons 1950 to 2004. The boundaries of their defined regions are shown in Figure 1
below. Note that these boundaries are different that the ones defined in the present study, but a
similar concept is employed—to analyze hurricane activity by area of the Atlantic.
Figure 1. Boundaries of areas identified in Bell and Chelliah, 2006
Source: Bell and Chelliah, 2006
10
Figure 2. ACE Values from 1950 – 2004 by Region as defined by Bell and Chelliah, 2006.
Source: Bell and Chelliah, 2006
In their study, when a tropical cyclone moved from one region to another throughout its
lifecycle, the entire ACE for that storm was attributed to the region in which it reached tropical
storm status. Bell and Chelliah concluded that 71% of total ACE was accounted for in the main
development region, peaking in the August-September-October months. They found that
extratopics accounted for 25% of total ACE, while the Gulf of Mexico accounted for only 5% of
ACE totals. Perhaps the low values for the Gulf of Mexico are a result of their decision to
attribute ACE for an entire storm to the region in which it reached tropical storm status.
Therefore, their results may not serve as a true representative of tropical cyclone intensity in the
regions, underrepresenting the Gulf of Mexico and overstating the tropical cyclone presence in
the main development region. As a result, in the current study of the 2013 season, ACE is
attributed to the region in which it occurs, regardless of where the storm gained tropical storm
status. In the present study, it is possible for ACE from a single storm to be broken into up to
three parts, with each part of the track adding to the ACE total in its own respective region.
11
2013 NOAA Atlantic Hurricane Season Outlook
The original seasonal outlook for the 2013 Atlantic hurricane season published by the
National Oceanic and Atmospheric Administration was released on May 23, 2013. The product
was the result of collaboration of hurricane experts from the National Hurricane Center (NHC)
and the Hurricane Research Division (HRD), and included the North Atlantic Ocean, Caribbean
Sea, and Gulf of Mexico.
The report summarizes that “climate signals and evolving oceanic and atmospheric
conditions, combined with dynamical and statistical model forecasts, indicate that an above
normal Atlantic hurricane season is likely in 2013. This outlook calls for a 70% chance of an
above-normal season, a 25% chance of a near-normal season, and only a 5% chance of a below
normal season” (NOAA, 2013). NOAA sites ongoing increased tropical cyclone activity in the
Atlantic since 1995 (agreeing with Bell and Chelliah, 2006), continued above average seasurface temperatures in the Atlantic and Caribbean, and neutral El Niño conditions (El Niño
typically suppresses hurricane activity as outlined in Gray et al. 1992) as some reasons for the
above-normal forecast. Their forecast of 70% chance of an above-normal season means that
they predicted a 70% chance of the above-normal parameters outlined in Table 1 and Table 2.
In a news release following the release of the outlook, NOAA cites “improvements to
forecast models, data gathering, and the National Hurricane Center communication procedure for
post-tropical cyclones” (NOAA Predicts Active 2013 Atlantic Hurricane Season, 2013) in the
2013 forecasting scheme. It also cites new Doppler radar improvements aboard NOAA’s
Hurricane Hunter aircraft and new computing technology to be unveiled in July 2013 that will
run an upgraded Hurricane Weather Research and Forecasting (HWRF) model to improve
intensity forecast guidance (NOAA Predicts Active 2013 Atlantic Hurricane Season, 2013).
12
2013 Klotzbach/Gray Hurricane Season Outlook
The 2013 Atlantic hurricane season forecast provided by the Colorado State University
Tropical Meteorology Project, headed by Phillip Klotzbach and William Gray was released on
April 10, 2013, nearly a month before the initial release by the National Oceanic and
Atmospheric Administration. Like NOAA’s forecast, they cite warming Atlantic sea-surface
temperatures and an unlikely El Niño as the major factors in predicting “enhanced activity when
compared with the 1981-2010 climatology” (Klotzbach and Gray, 2013). The Klotzbach and
Gray forecast is more detailed than NOAA’s, with specific’s located in Figure 3 below.
Figure 3. 2013 Atlantic Hurricane Season Forecast, Colorado State University Tropical Meteorology Project
Source: Klotzbach and Gray, 2013
13
Observed Tropical Cyclone Activity in 2013
The seasonal outlooks provided in early 2013 by NOAA and Klotzbach/Gray turned out
to be incorrect. According to the National Hurricane Center’s Annual Summary for 2013, the
season did not produce above-normal tropical cyclone activity. Instead, it produced well below
average activity. Despite slightly more named storms than average (14), only two reached
hurricane strength. No hurricanes and only one tropical storm, Andrea, made landfall in the
United States causing one fatality. Additionally, no storm reached category 3 status or higher in
the Atlantic basin for the first time since 1994. Total seasonal ACE in 2013 was 36, the lowest
since 1994, only 39% of the 1981-2010 median of 92, and only 22% of the predicted value of
165 by Klotzbach and Gray. Meanwhile, three tropical storms affected Mexico, resulting in
widespread flooding and more than 50 casualties. Summary statistics of each Atlantic storm in
2013 are shown in Table 3.
Table 4. 2013 Atlantic Hurricane Season Summary
Source: Blake, 2014
14
Multidecadal Variability
Bell and Chelliah pointed out in their 2006 study that that the 1995—2004 period was the
most active Atlantic hurricane decade on record since 1945. Goldenberg et al. 2001 and
Klotzbach 2006 also discuss a “considerable increase in Atlantic basin tropical cyclone activity
since 1995” (Klotzbach and Gray, 2008). Despite some disagreements in the scientific
community regarding the interpretation of data, increased tropical cyclone activity since 1995 in
the general consensus. The 1995 through present trend of high levels of tropical cyclone activity
closely mirrors the period of high activity seen from the late 1940s through the mid-1960s
(Goldenberg et al 2001; Klotzbach and Gray 2006; Klotzbach and Gray 2008).
Some suggest that the increase in activity is due to natural variability, while others point
to human-induced global warming changes or a combination of factors. In 2008, Klotzbach and
Gray published a paper that suggests that there is multidecadal variability observed in Atlantic
sea-surface temperatures and sea-level pressure fields. This may be a viable cause of periods of
years with increased tropical cyclone activity, followed by years of below-average tropical
cyclone activity, resulting in a cycle over time associated with a multidecadal variation in the
thermohaline circulation (THC) or the Atlantic multidecadal oscillation (AMO). These are
essentially cycles of fluctuation in sea surface temperatures and ocean currents in the Atlantic,
and little is understood regarding the exact causes.
Klotzbach and Gray 2008 points out that the variation in hurricane activity can be shown
by subtracting sea-level pressure anomalies from sea-surface temperature anomalies. The results
of this procedure are shown in Figure 4.
Due to the results produced in Klotzbach and Gray 2008, for the purposes of this study,
the climatological period will consist of an eighteen year average from 1995—2012 rather than
15
the thirty year climatological period of 1981—2010 used by NOAA and Klotzbach/Gray in their
seasonal hurricane outlooks. This is done in an attempt to compare the 2013 Atlantic hurricane
season with the most recent years amongst its positive-trending AMO/THC cohort. By
analyzing the 2013 season against a climatology of 1995—2012 rather than a thirty year average,
we can better understand how 2013 “fits” in the larger puzzle of overall tropical cyclone activity
that would be expected given average activity in more recent years.
Figure 4. Multidecadal Variability in
AMO/THC
Standardized values of North Atlantic
sea-surface temperature anomalies
from 1880—2004. Horizontal lines
indicate average values.
Standardized values of North Atlantic
sea-level pressure anomalies from
1880—2004. Horizontal lines
indicate average values.
Sea-surface temperature anomalies
minus sea-level pressure anomalies
produce a simulation of the strength
of the AMO from 1880—2004.
Alternating periods of positive
activity and negative activity are
present, with horizontal lines
indicating average values. It is shown
that the current period is a positive
AMO period, indicating increased TC
activity.
Source: Klotzbach and Gray, 2008
16
Data and Methods
In order to calculate ACE for 2013 and the climatological period 1995—2012,
observation data from 1995—2013 containing storm status, latitude, longitude, atmospheric
pressure, and maximum wind speed is collected from the National Oceanic and Atmospheric
Administration’s National Hurricane Center Data Archive HURDAT2 database. The
HURDAT2 database, which was last updated in April 2014 by Chris Landsea, James Franklin,
and Jack Beven, includes all observations made for each tropical cyclone every six hours (00, 06,
12, and 18Z) and at landfall and points of maximum intensity. It supersedes the original
HURDAT database, which was disseminated in 2012, to include a new format, non-developing
tropical depressions, and best track wind radii.
Because of the new format of HURDAT2 to include landfall observations and points of
maximum intensity, quite a bit of data manipulation is needed to remove these points from the
dataset, as ACE is only calculated every six hours at 00, 06, 12, and 18Z. Additionally, because
ACE is only calculated for tropical cyclones at tropical storm or hurricane status (35kt winds or
higher), every observation with wind speeds below 35kts has to be deleted from the dataset. All
manipulation of HURDAT2 data is done using Microsoft Excel.
Once the data is in a usable format and all disqualified observations are deleted, ACE can
be calculated in Excel using the formula outlined by NOAA, reproduced below:
where 𝒗 is maximum sustained wind speed measured in knots and the unit for ACE is 104 kt2.
17
This running calculation is made in Excel first for 2013 storms individually, then each
storm is summed to a total seasonal ACE for 2013. The same procedure is followed for the years
1995—2012 to produce a climatology of ACE values with which to compare the 2013 ACE.
Next, in order to analyze total seasonal ACE by region of the Atlantic, three equal-sized
regions are established. Each region is bounded to the north by the 45°N parallel and to the
south by the 7°N parallel, and is 30° of longitude wide. Region 1 includes the East Atlantic from
15°W-45°W, region 2 includes the Mid-Atlantic from 45°W-75°W, and region 3 encompasses
the Gulf of Mexico and immediate eastern U.S. seaboard from 75°W - 105°W. The longitude
values in the HURDAT2 dataset must be manipulated into a usable format (for example, 45°W
must be changed to -45 using Excel in order to make calculations). ACE values are then
organized into the three regions by latitude and longitude in Excel, producing a unique ACE
value for each of the three regions that, when added together, equals the total ACE of the
Atlantic basin for each year, from 1995—2013. The 1995—2012 values for each year are then
averaged together to create a climatological average for each region’s total ACE.
In order to further analyze ACE for the total Atlantic and each region, the values for 2013
and 1995—2012 are then broken up by month from June—December (note the inclusion of
December, while hurricane season officially ends November 30 of each year). This will provide
a comparison of 2013’s tropical cyclone activity (as well as the modified climatological period)
with the expected trend of peak activity during the months of August—October (Bell and
Chelliah, 2006).
Summary statistics are also calculated using Microsoft Excel. A line plot is made to
show ACE values each year from 1995—2013, and the standard deviation is calculated.
18
Finally, the number of hurricane days is calculated for each of the three regions in 2013
and compared to the climatological averages. A ‘hurricane day’ is defined as the number of
calendar days when there is a storm with maximum sustained winds at 75kts or higher present in
the area of study (i.e. the Atlantic basin or region 1, 2, or 3).
As an additional analysis, ESRI’s ArcGIS software is used to map the 2013 Atlantic
tropical cyclones, as well as every tropical cyclone from 1995—2012. In order to do this,
shapefiles are created from the manipulated HURDAT2 data using ArcGIS. The shapefiles are
then uploaded into the software, producing maps that can be used to spatially analyze the 2013
Atlantic hurricane season and visualize the regions 1, 2, and 3 defined above in the ACE
analysis.
19
Results
The maps of tropical cyclone tracks made from HURDAT2 data using ArcGIS are shown
in Figure 5. Region 2 contains the most tropical cyclone observation points in the climatology,
as well as the most yellow and red points (tropical storms and hurricanes). In 2013, most yellow
points and all red points fall into regions 1 and 3.
Figure 5.
Tracks of all Atlantic
tropical cyclones from
1995—2012 (top) and
tracks of 2013 Atlantic
tropical cyclones (bottom).
Green dots represent low
pressure centers, blue
represents a tropical
depression (a low pressure
area that is accompanied
by thunderstorms that
produce a circular wind
flow with maximum
sustained surface wind of
33 knots or less), yellow
represents a tropical storm
(34 to 63 kt winds), and
red indicated hurricane
status (64 kt winds or
greater).
20
Figure 6. Ace Values for each Atlantic Hurricane Season from 1995—2013
Figure 6 shows a plot of every ACE value from 1995—2013 for the entire Atlantic basin.
Although large fluctuations are present between consecutive years of the study, such as from
1996—1997 and from 2005—2006, it is shown that ACE for 2013, 36, was still well below the
average ACE of 140. Additionally, seasonal ACE from 2013 was the lowest value amongst the
years studied, with the total ACE for 2013 falling between 1.5 and 2 standard deviations from the
mean.
The results of dividing
ACE by region can be seen in
Figure 7. Note that ACE for
2013 from Region 1 is fairly
normal and consistent with what
an average ACE value would be
from Region 1 during a typical
Atlantic hurricane season during
Figure 7. Total Seasonal ACE by Region
the climatological period.
21
However, ACE from Regions 2 and 3 are significantly below the average values from those
regions. ACE is particularly low for Region 2. Note that a typical Atlantic hurricane season
during the climatological period would expect the majority of ACE to come from Region 2,
while in 2013, Region 2 produced the lowest ACE of any of the regions. Therefore, Region 2
stands out as particularly anomalous during the 2013 season.
Figure 8 shows a
comparison of ACE values
for the entire Atlantic by
month, while Figure 9 shows
the same information sorted
by individual regions. For
the Atlantic basin as a whole,
Figure 8. ACE by month for the entire Atlantic basin
ACE peaks during the month
of September during both the climatological period and the 2013 season, although values for
2013 remain much lower than average values. During the hurricane season which excludes
December, 2013 saw the lowest ACE in the month of August. This is somewhat unusual and
will be discussed in the next section.
When displayed by region, again Region 1 seems to follow the trend of what should be
expected of an average Atlantic hurricane season during the climatological period, closely
aligning with the climatology and peaking during the month of September. In fact, Region 1 saw
ACE values that were above average during the beginning and end of the season, including the
months of July, November, and December. The ACE from December being above average is
particularly interesting since the official end of the Atlantic hurricane season is November 30.
22
Region 2 sees a large drop off in ACE
values from Region 1, which is unusual as this
is the region in which the most ACE typically
occurs. Additionally, Region 2 does not
produce a distinct trend of a peak in activity
during August-September-October in 2013 as
the climatology suggests should occur. Of
particular note, ACE for the month of August
from Region 2 was zero.
Region 3 produces results between that
of Region 1 and Region 2. In Region 3, ACE
for 2013 is again mostly well below the
climatological values for the region, but the
results are not as outstanding as they were for
Region 2. The exception for Region 3 is the
month of June, in which ACE in 2013 was
slightly above average. July saw no ACE in
Region 3, however, nor did November or
December, while August’s ACE was extremely
low.
As a final gauge of tropical cyclone
Figure 9. Monthly ACE organized by Region
activity in 2013, the total number of hurricane
days was calculated for the climatological period as well as for 2013. Like the other parameters
23
and ACE, this calculation is done for the Atlantic basin as a whole, as well as by region. The
results, located in Table 5, show that the total number of hurricane days in 2013 was 6, just 18%
of the mean of 32.7 days. All of the hurricane days in 2013 were observed from Regions 1 and
3, with 3 days each. This does not follow the climatology, which suggests a peak in hurricane
days should occur in Region 2, with an average of 18.5 days.
Table 5. Hurricane Days for the Atlantic basin and by Region
24
Discussion
As Figure 5 shows, the strongest storms occurring during 2013 occurred in Regions 1
and 3, which is opposite of where we would expect the strongest storms to occur (Region 2)
based on the climatological tracks. Additionally, only one tropical storm made landfall in the
United States in 2013, which is unusual given the general climatological track pattern. Figure 7
supports this claim, as ACE in 2013 was lowest in Region 2 while slightly higher in Regions 1
and 3, although it remained lower than average for every region.
The analysis of monthly ACE for all regions (Figure 8) shows that the months of August,
September, and October fell well below average ACE values for the climatological period, which
suggests that some atmospheric, oceanic, or other conditions caused abnormal behavior during
the peak of the 2013 Atlantic hurricane season when we tend to expect the most activity.
Although ACE from the beginning and end of the season remains below average, the differences
in observed ACE in 2013 and the climatology is not as great as it is for the middle of the season.
A large difference can be seen between the graphs of monthly ACE for Region 1 and
Region 2 (Figure 9). Although still slightly below average overall, ACE for Region 1 more or
less follows the climatology of what is expected of ACE in this region during an average year.
However, a major drop off in ACE is seen for Region 2. As Figure 9 shows, there was no ACE
in June, August, or December from Region 2, with very low ACE for the remaining months of
July, September, and October. According to the climatology, ACE for September in Region 2
typically accounts for the largest single contributor to the overall ACE total of a season. The
ACE for September in 2013 in Region 2, however, was extremely low (roughly only 7% of an
average year’s value). Region 3 also produced underwhelming ACE from 2013 when compared
25
to the climatology, although the values are not quite as anomalous as in Region 2 given that
typical ACE from Region 3 is much lower than Region 2 anyway.
Besides ACE, another interesting statistic from the 2013 Atlantic hurricane season is that
there were only 6 days in which there was a storm classified as hurricane strength somewhere in
the Atlantic basin. This is much lower than the average number of hurricane days, and none of
the 2013 hurricanes reached category 3 or higher, meaning they were both weaker category
storms.
As Figure 6 shows, there is considerable variability in ACE on a year-to-year basis,
which is to be expected. Although 2013 saw the lowest ACE value compared to previous years
and fell within 1.5 and 2 standard deviations of the mean, it is not beyond the realm of
possibilities for this to happen, particularly given the fairly low ACE values in 1997, 2002, 2006,
2007, and 2009. What is so surprising about 2013, however, is the fact that the forecast was for
an above-average season. Perhaps if the forecasts by NOAA and Klotzbach/Gray had called for
a below-average season, or even an average season, there would not be as much discussion about
2013’s uniqueness. However, since all of the major seasonal forecasts called for a season with
above-average activity yet 2013 produced tropical cyclone activity in which the opposite
occurred, questions are raised. Despite being the season with the lowest tropical cyclone activity
in recent years, 2013’s low ACE and low activity is to be expected from time to time. The focus,
then, should not be entirely on comparing the 2013 season with previous recent years, but instead
should be on determining why the seasonal forecasts were wrong.
26
Conclusion
Based on results from geographic and statistical analysis, it can be concluded that the
total ACE for 2013, 36, was well below the average ACE value of 140. Seasonal ACE from
2013 was the lowest value amongst the years studied, and falls between 1.5 and 2 standard
deviations from the mean. Monthly ACE for the Atlantic generally followed the expected
climatology trend, with a peak in September, but values were much smaller than climatology.
The 2013 ACE for Region 1 (15°W to 45°W) was about average and as expected, while Region
2 (45°W to 75°W) was particularly well below average and Region 3 (75°W to 105°W) was also
well below average. Maps of tracks and intensities suggest the most tropical cyclone activity
should be in Region 2, but this was not the case for 2013, (most activity was in Region 1). The
number of hurricane days in 2013, 6, was well below the average of 33. The number of
hurricane days is typically greatest in Region 2, but 2013 produced 0 hurricane days in this
region.
Overall, ACE in 2013 was much lower than average. Region 2 was particularly inactive
in 2013, which is opposite of what would be expected because this region typically sees the most
tropical cyclone activity of the three regions according to the climatology.
Future research should focus on determining what atmospheric or oceanic conditions
caused abnormally low ACE and the abnormal geographic distribution of tropical cyclone
activity in 2013. Additionally, because the climatological period of this study involved an active
period of the AMO/thermohaline circulation, it would be interesting to compare the 2013 season
with a climatology of an inactive Atlantic hurricane period associated with the multidecadal
oscillation in the thermohaline circulation. At the time of publication, it can also be noted that
2014 was also a fairly inactive hurricane season, but unlike 2013, most tropical cyclone activity
27
seemed to occur in Region 2. Another possible research area could be to determine the factors
could have caused this change. There are some that suggest that, due to low activity in both
2013 and 2014, we could be transitioning back into another inactive period of Atlantic tropical
cyclone activity associated with the multidecadal oscillation of the thermohaline circulation.
Although two years of data is not enough to determine if this is true, if a low-activity trend
continues in the next few years, this is certainly within the realm of possible explanations for
recent inactive seasons.
The goal of case study analyses such as the 2013 Atlantic hurricane season is to find out
what mistakes were made in creating the seasonal hurricane outlooks, learning from these
mistakes, and correcting them in future seasonal outlooks. This will produce more accurate and
reliable seasonal hurricane forecasts in the future.
28
Acknowledgements
This study comprises Thomas Vaughan’s Senior Honors Project. It was conducted under
the direction of Dr. Thomas Rickenbach of East Carolina University’s Department of
Geography, Planning, and Environment with the support of the East Carolina University Honors
College.
29
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