|Description||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 (1995-2012), 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 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.||en_US