Hurricane Alex (developed from an area of low pressure which began in the Atlantic near Bahamas in January 2016, and developed subtropical and later tropical storm characteristics to the south of the Azores) clearly way outside the usual Jun-Nov hurricane season. How is it possible that such an energetic event forms during the North Atlantic winter? Clearly, this kind of hurricane is not common (previous one in 1938). What are the oceanic and atmospheric conditions necessary for the formation of Hurricane Alex?
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1$\begingroup$ strongest El Nino/? $\endgroup$– f.thorpe ♦Commented Jan 16, 2016 at 23:07
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1$\begingroup$ @aretxabaleta - It originated as a extratropical storm. Quite common during El Nino winters $\endgroup$– user1066Commented Jan 17, 2016 at 0:48
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$\begingroup$ @aretxabaleta - I believe there maybe a MJO contribution as well. I am having a peer review that before posting an answer here :-) $\endgroup$– user1066Commented Jan 18, 2016 at 9:41
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1$\begingroup$ Of related note: Tropical Storm Zeta reached 65 mph in the Atlantic in 2005, though was actually a 2004 hold-over. $\endgroup$– JeopardyTempestCommented Jun 30, 2017 at 5:36
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3 Answers
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Alex was an out of season freak event that bucked the trend and broke all the rules. As hurricanes go, it was somewhat weak, and could never have strengthened beyond category 1. Three conditions made it happen (i) there was an unusually strong vortex over Cuba which kick-started the rotation, (ii) the high level atmospheric shear, which usually kills off fledgling hurricanes, just didn't appear on this occasion, (iii) the storm initially followed the Gulf Stream Drift, and hence gained energy from higher than usual sea surface temperatures - though still a lot cooler than normal. After the hurricane reached central north Atlantic it veered off to the northwest, where it met much colder sea-surface temperatures. It was this cold water that killed it off.
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Alex was a weird development, being only the second ever recorded in any month to form north of 30°N and east of 30°W, the only other being Hurricane Vince in 2005. It developed over waters cooler than normally required for tropical development at 20-22°C, however upper level air was remarkably cold for the latitude above where Alex developed, giving a strong temperature gradient which allowed convection to consolidate and form a warm core tropical hurricane. For references see Bob Henson at Wunderground Astounding Alex..., Alex Becomes... Temperatures in the upper atmosphere in that area were extremely cold, down to around minus -60 degrees Celsius, or minus -76 degrees Fahrenheit. Andrew Freeman, Mashable Specifically the upper troposphere (6 miles up).Liam Dutton
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1$\begingroup$ do not know anything about upper air temperatures around the Bahamas but I am looking at the 500 hPa temperature around that area here - earth.nullschool.net/#2016/01/16/0300Z/wind/isobaric/500hPa/…. Is -7.5 C very cold ? $\endgroup$– user1066Commented Jan 18, 2016 at 3:26
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$\begingroup$ From my understanding the cold upper level air which helped to make it a hurricane was encountered to the south of the Azores, not around the Bahamas. $\endgroup$– SivCommented Jan 18, 2016 at 13:51
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$\begingroup$ Ok that is around -17 C. $\endgroup$– user1066Commented Jan 18, 2016 at 14:22
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1$\begingroup$ @gansub: From SPC's sounding climatology (and looking at Miami) it looks like -7.5 in the Bahamas in mid-October is about normal. -17 sounds pretty cold for October... even Oakland's normals show well above that. Only Seattle is that cold. And, as might be expected, water temps off the Azores should be a bit warmer than off the US Pac coast. $\endgroup$ Commented Jun 30, 2017 at 5:59
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Tropopause temperatures were unusually cold. This allowed a steep enough lapse rate despite cool sea surface temperatures of 68 degrees F, and was one of the factors that helped Alex to form despite such cool sea surface temperatures. Hurricanes are heat engines with the hot source at the ocean surface and the cold sink at the tropopause. Thus an unusually cold tropopause can compensate to some extent for sea surface temperatures lower than the usual minimum value of 78 degrees F required for hurricane formation.
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$\begingroup$ can you illustrate your answer with actual numbers from NOAA or ERA interim? $\endgroup$– user1066Commented Jun 30, 2017 at 11:45
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$\begingroup$ See www.nhc.noaa.gov/data/trc/AL012016_Alex.pdf. According to this report, in addition to unusually cold troposphere temperatures, reduced wind shear also helped out. $\endgroup$ Commented Jul 20, 2017 at 19:20