Published: September 15,2015
(MORE: Strong El Nino to Last Through Spring 2016)
A weather.com analysis of NOAA's best-track dataset from 1950-2014 finds that Atlantic hurricane seasons during a strong El Nino see the last named storm end two to three weeks earlier – a mean date of Oct. 18 – than the long-term average date of Nov. 5.
Four of the five previous Atlantic hurricane seasons with a strong El Nino since 1950 – 1997, 1987, 1982 and 1965 – saw the season-ending named storm fizzle by Oct. 18 or earlier. Only 1972 had a later end during a strong El Nino, only due to a pair of subtropical storms.
(MET 101: What is a subtropical storm)
For analysis, we defined the "end of the season" as the date in which the season's final named storm, which had at least been tropical or subtropical storm strength, was downgraded to a post-tropical cyclone or remnant.
While the official Atlantic hurricane season runs from June 1 through Nov. 30 to capture 97 percent of all Atlantic tropical activity, only one out of every two seasons featured a November named storm, dating to 1950, according to the NOAA data.
A strong El Nino hurricane season was defined as one during which a three-month running mean sea-surface temperature (SST) anomaly of 1.5 degrees Celsius or greater was measured at some point in the strip of ocean used to define El Nino and its counterpart, La Nina.
This earlier ending to the season was much less distinct with weaker El Ninos.
Average
date the last named storm of the Atlantic hurricane season (that was at
least tropical storm-force) was downgraded from a tropical cyclone for
strong, moderate and weak El Nino seasons (since 1950). The average date
(1950-2014) is also shown.Moderate-strength El Nino hurricane seasons – defined by a three-month running SST anomaly from 1 to 1.5 degrees C – ended Oct. 31, about six days earlier than average.
Weak El Nino seasons – defined by a three-month running SST anomaly of 0.5 to 1 degrees C – only saw a two-day shift earlier than the long-term average.
While not the sole driver of the atmospheric circulation, increased wind shear in the Atlantic Basin in stronger El Nino years plays a role. (See the "What About Hurricane Season" section in our latest El Nino update)
Persistent, stubborn wind shear and dry air have thwarted tropical cyclones from intensifying or lasting very long in the Atlantic basin this season.
(MORE: No N. Hemisphere Trop. Cyclones Near Atlantic Season's Peak a First in 38 Years)
Named storm tracks in the 2015 Atlantic hurricane season through September 14.This doesn't imply, however, that hurricane seasons haven't ended early in non-El Nino seasons.
In both 1993 and 1960, the season's last named storm fizzled in late September. The same was true in the 1983 season, when the strong 1982-1983 El Nino fizzled after early summer, but still only spawned four named storms that season. (Though, one of those was the highly-destructive Alicia in the Houston metro area).
With wind shear – the difference in wind speed and/or direction with height – typically rising from mid-September on, it only stands to reason that this hostile ingredient will only become more of a tropical cyclone squelcher in the weeks ahead.
(MORE: Tropical Update | Hurricane Central)
"El Nino normally peaks in late November and December, so, on average, it has a greater effect on the second half of the hurricane season," says senior hurricane specialist Bryan Norcross (Wunderblog). But that just means that the odds of a named storm are lower. On any one day, there are many still pockets in the Atlantic Ocean or Gulf of Mexico that are favorable for tropical development."
Furthermore, while the Atlantic may struggle to maintain long-lived tropical cyclones from the deep tropics during this strong El Nino, more systems spinning up much closer to the U.S., such as we saw with Ana and Bill early in the season, could still occur into October.
The bottom line on the 2015 Atlantic hurricane season, says Norcross: "It's not over until it's over."
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