This year marks the 100th anniversary of the slowest Atlantic hurricane season on record--1914,
which had no hurricanes and only one tropical storm. Is it possible that the 2014 hurricane season could match 1914 for the lowest activity ever recorded, with Hurricane Arthur ending up as our only named storm? I think that is highly unlikely, even though the atmospheric and oceanic conditions in the Atlantic are looking hostile for development for the coming two weeks.A re-analysis of Atlantic tropical cyclones finds only one storm in 1914
In 2005, a reanalysis effort
was made of all Atlantic tropical cyclones between 1911 - 1914, using historical weather maps, ship reports, and newspaper accounts. I talked to the leader of the reanalysis project, NHC's Dr. Chris Landsea, about the 1914 reanalysis. He told me, "We went into the re-analysis process for 1914 knowing that this was the quietest year on record, with only one tropical storm and no hurricanes. I thought for sure we'd find some storms that were missed, since so many of the other years we re-analyzed came up with new storms that were missed. But when we analyzed the data and looked for missing storms, we couldn't find any. The year 1914 remained with just one named storm--truly a remarkable year in the annals of the Atlantic hurricane database."
The only tropical storm of 1914 developed in the Bahamas on September 15--the latest formation date for an Atlantic season's first storm in the official HURDAT database, which goes back to 1851. The storm moved slowly northwestward and made landfall near the Florida/Georgia border on September 17. Two other systems were formally considered for inclusion into the hurricane database in 1914: a potential late-October tropical depression that was identified in the Western Caribbean, but was too weak to be considered a tropical storm; and a storm that brought gale-force winds on September 30 - October 1 to the coasts of Alabama, Mississippi, and the Florida Panhandle, but which was deemed to be an extratropical storm. The reanalysis effort found that the only other Atlantic hurricane season that did not produce any hurricanes was 1907. Figure 1.
Top: August - October 1914 departure from average of relative humidity at middle levels of the atmosphere (near the 700 mb pressure level, which is roughly 10,000 feet above the surface.) Bottom: August - October 1914 departure from average of sea level pressure. The August - October peak part of hurricane season in 1914 had a very dry atmosphere with a relative humidity 4 - 8% lower than average (yellow, orange and red colors), and was dominated by high pressure (1 - 2 mb higher than average.) Images plotted using the NOAA/ESRL 20th Century Reanalysis
.Reasons for the Exceptionally quiet hurricane season of 19141) El Niño.
Not surprisingly, 1914 was an El Niño year, judging by the Southern Oscillation Index Archives
from Australia's Bureau of Meteorology. It is well-know that during an El Niño event, an atmospheric circulation that brings strong upper-level west-to-east winds over the tropical Atlantic typically sets up, and these winds tend to create high wind shear, discouraging tropical storm formation. 2. Sea Surface Temperatures (SSTs.)
Ocean temperatures in the Main Development Region (MDR) for Atlantic hurricanes, from the coast of Africa to Central America, between 10°N and 20°N, including the Caribbean, were -0.4°C (-0.7°F) from average during August - October 1914, according to the Hadley Centre SST data set (HadSST2)
. This ranks as the 12th coolest such departure from average since 1900, and this sort of temperature anomaly would have definitely tended to squelch tropical storm formation by limiting the amount of heat energy available to developing storms. The record coldest SST anomaly in the MDR since 1900 was -0.8°C during the 1913 hurricane season
, which was a very quiet year with six named storms and four hurricanes, all of which were Category 1 hurricanes.3. Dry air and high pressure.
It is well known that dry air at middle levels of the atmosphere (near the 700 mb pressure level, which is roughly 10,000 feet above the surface) discourages tropical storm formation. During August - October 1914, there was plenty of dry air over the Caribbean and Gulf of Mexico which would have made it difficult for tropical storms to form. The Atlantic was also dominated by higher than average pressure, so an atmosphere featuring large-scale dry, sinking air associated with persistent high pressure systems was likely in place during the 1914 hurricane season. Why the quiet hurricane season of 1914 will be hard to duplicate
Is it possible in our new climate and with the modern weather observing system to have just one named storm in an Atlantic hurricane season? It's not impossible, but the odds of such an event are much lower than they were in 1914, for two reasons:
1) The globe has warmed about 0.8°C (1.4°F) since 1914. That extra heat in the oceans makes it much more difficult to maintain a Sea Surface Temperature (SST) anomaly of -0.4°C during the peak part of hurricane season in the Main Development Region (MDR) for hurricanes, as occurred during the 1914 hurricane season. The MDR SST anomaly has averaged about -0.2°C in June and July 2014, and I would be surprised if it managed to sink as low as -0.4°C and maintain that level of coolness during all of August, September, and October. The quietest Atlantic hurricane seasons typically occur in years when MDR SSTs are much below average, so the extra heat in the ocean due to global warming should cut down on the number of seasons with very low named storm counts.
2) Back in 1914, we did not have weather satellites or the Hurricane Hunters, which meant that weak, short-lived tropical storms far out at sea may have been missed. 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). The researchers did not detect a significant increasing trend in medium‐ to long‐lived storms lasting more than 2 days. Looking at Figure 2, it looks quite plausible that 2 - 4 of these "shorties" were missed in 1914, due to the inferior observing system. 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. Figure 2.
Atlantic tropical cyclones between 1878 - 2013 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.
My conclusion: we will see a Tropical Storm Bertha in the Atlantic in 2014, but don't expect it until mid-August.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
Have a great weekend, everyone, and if the Atlantic stays quiet, I'll wait until Monday to make a new post.