Saturday, July 26, 2014

"The Record Quiet Hurricane Season of 1914: Could it Happen Again in 2014?"

From Wunderblog:
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 1914
1) 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....