Nineteen Atlantic tropical storms 3 consecutive years: a very rare event

The 2012 Atlantic hurricane season closes this Friday with another top-five tally for named storms--nineteen. This is the third consecutive year with nineteen named storms in the Atlantic, which is a remarkable level of activity for a three-year period. The closest comparable three-year period of activity occurred during 2003 - 2004 - 2005, when each season had fifteen-plus named storms. Since 1851, only two seasons--2005 (28 named storms) and 1933 (20 named storms)--have been busier than 2010, 2011, and 2012.


Figure 1. Preliminary tracks of the nineteen named storms from 2012. Image credit: National Hurricane Center.

How rare are 3 consecutive top-five hurricane seasons for named storms?
It is tremendously rare to get three consecutive top-five years in a database with a 162-year record. This would occur randomly just once every 34,000 years--assuming the database were unbiased, the climate were not changing, and a multi-year climate pattern favorable for active seasons were not present. However the database IS biased, the climate IS changing, and we have been in an active hurricane period that began in 1995. So, which of these factors may be responsible for recording three consecutive years with nineteen named storms? It is well-known that prior to the arrival of geostationary satellites in December 1966 and aircraft hurricane reconnaissance in 1945 that tropical storms in the Atlantic were under-counted. Landsea et al. (2004) theorized that we missed up to six named storms per year between 1851 - 1885, and up to four between 1886 - 1910. Landsea (2007) estimated the under-count to be 3.2 named storms per year between 1900 - 1965, and 1.0 per year between 1966 - 2002. Other studies have argued for lower under-counts. So, if we assume the highest under-counts estimated by Landsea et al. (2004) and Landsea (2007), here would be the top ten busiest Atlantic hurricane seasons since 1851:

2005: 28
1887: 25
1933: 23
1995: 20
2012, 2011, 2010, 1969, 1936: 19

So, 2012, 2011, and 2010 would still rank as top-five busiest seasons since 1851, but the odds of having three consecutive seasons with nineteen named storms would drop from a 1-in-34,000 year event to "only" a 1-in-5800 year event. More recently, Landsea et al. (2010) showed that the increasing trend in North Atlantic tropical storm frequency over the past 140 years was largely due to the increasing trend in short‐lived storms (storms lasting 2 days or less, called “shorties”), after the 1940s (Figure 2, top). They did not detect a significant increasing trend in medium‐ to long‐lived storms lasting more than 2 days. They wrote that “while it is possible that the recorded increase in short‐duration TCs [tropical cyclones] represents a real climate signal, we consider it is more plausible that the increase arises primarily from improvements in the quantity and quality of the observations, along with enhanced interpretation techniques.” Villarini et al. (2011), in a paper titled, "Is the recorded increase in short-duration North Atlantic tropical storms spurious?", agreed. They attempted to correlate increases in tropical Atlantic sea surface temperatures in recent decades to the increase in short-lived Atlantic tropical storms, and were unable to do so. They wrote: using statistical methods combined with the current understanding of the physical processes, we are unable to find support for the hypothesis that the century‐scale record of short‐lived tropical cyclones in the Atlantic contains a detectable real climate signal. Therefore, we interpret the long‐term secular increase in short‐duration North Atlantic tropical storms as likely to be substantially inflated by observing system changes over time. These results strongly suggest that studies examining the frequency of North Atlantic tropical storms over the historical era (between the 19th century and present) should focus on storms of duration greater than about 2 days. So, let's do that. If we look during the past three hurricane seasons at how many "shorties" were observed, we see that a large number that stayed at tropical storm strength for two days or less: six storms in 2010, six in 2011, and seven in 2012. This leaves the hurricane seasons of 2010, 2011, and 2012 with twelve to thirteen tropical storms that lasted more than two days. This doesn't stand out that much when looking at trends since 1878 (Figure 2, bottom); there are now 25 years in the 135-year record with twelve or more long-lived tropical cyclones. However, there are no previous occurrences of three consecutive years with at least twelve long-lived tropical storms, so 2010, 2011, and 2012 still represent an unprecedented level of tropical storm activity in the historical record, and we would expect such an event to occur randomly about once every 157 years. That's a pretty rare event, and it is possible that climate change, combined with the fact we are in an active hurricane period that began in 1995, contributed to this rare event.


Figure 2. Atlantic tropical cyclones between 1878 - 2012 that spent two days or less at tropical storm strength (top) and more than two days at tropical storm strength or hurricane strength (bottom.) Figure updated from Villarini, G., G. A. Vecchi, T. R. Knutson, and J. A. Smith (2011), "Is the recorded increase in short-duration North Atlantic tropical storms spurious?", J. Geophys. Res., 116, D10114, doi:10.1029/2010JD015493.

References
Landsea, C. W., C. Anderson, N. Charles, G. Clark, J. Dunion, J. Fernandez‐Partagas, P. Hungerford, C. Neumann, and M. Zimmer (2004), "The Atlantic hurricane database re‐analysis project: Documentation for 1851–1910 alterations and additions to the HURDAT database," in Hurricanes and Typhoons ‐ Past, Present, and Future, edited by R. J. Murnane and K. B. Liu, pp. 178–221, Columbia Univ. Press, New York.

Landsea, C. W., (2007), "Counting Atlantic tropical cyclones back to 1900," Eos, 88(18), 197-202.

Villarini, G., G. A. Vecchi, T. R. Knutson, and J. A. Smith (2011), "Is the recorded increase in short-duration North Atlantic tropical storms spurious?", J. Geophys. Res., 116, D10114, doi:10.1029/2010JD015493

Jeff Masters