At the turn of the hurricane season
August 25, 2013
The Atlantic hurricane season has been, by the reckoning of most, been very quiet. This feeling is to some extent exacerbated by the unanimous predictions of above average activity that preceded the season’s start. Some people may argue against this notion by pointing out that having four named storms in the book before the end of August connotates a season that is anything but quiet; indeed that was significantly ahead of the climatological norm. However, in terms of aggregate activity, the Atlantic basin has been rather exceptionally lacking in activity. Interestingly, by similar measures so its western hemisphere counterpart has not been notably active, either. The tranquility in the Atlantic is set to end soon.
While there have been five tropical storms in the Atlantic so far this year, none have sustained that status for more than three days or have come very close to being a hurricane. As such, they don’t add up to much in terms of aggregate activity. One measure of this is Accumulated Cyclone Energy (ACE). It is calculated by squaring the maximum wind speed of a tropical storm/hurricane at the six hour “synoptic time” intervals. The number is made manageable by dividing by 10,000 and while the resulting unit is technically knots2,, the reduced number is referred to simply as a unit of ACE. To understand the numbers, it is helpful to remember that a 50 knot tropical storm persisting for one day generates 1 unit of ACE while a major hurricane with maximum wind speed of 100 knots generates 1 unit of ACE every six hours, 4 units per day.
As I write this, the Atlantic hurricane season only has roughly 7.6 units of ACE to its name. The year-to-date normal is 23 units. Again, the short-lived nature of the tropical storms not to mention the absence of hurricanes, let alone major ones, is to account for this. Having such a low sum this deep into the season is not without precedent, even during the post-1994 active period. At this point in 2002, the season just had 3 units in the books and 2001 had just fewer than 9. Such low figures were more normal in the 1981-1994 time-frame; as 1984 did not see a tropical storm until August 29, it had 0 units at this time.
Often times, a lack of activity in the Atlantic is off-set by high activity in the Pacific. Such was the case in 1984 when five major hurricanes had formed in the eastern Pacific before the calendar turned to September; 71 units of ACE to its credit going into the final week of August. During the active period of the Atlantic basin, such inequities exist during the El Niño Seasonal Oscillation, which tends to suppress tropical cyclone activity in the Atlantic while enhancing that of the Pacific. Some times the El Niño is readily apparent in the amount of eastern Pacific activity, as was the case in 2006, when the first ever (and to date, only) Central Pacific category 5 hurricane, Ioke, helped contribute to a whopping 90 units of ACE being tallied before the final week of August.
2013 does not fit into that paradigm, however. Rather than El Niño, we have what some jokingly call “La Nada”, the oscillation is in a neutral state that does not favor either the west Atlantic or easter Pacific basin. Furthermore, while the eastern Pacific has featured six hurricanes so far, none have became major hurricanes. It is the furthest we have gone into the season without a major hurricane in the western hemispher since 2003 when Fabian, the third hurricane of that season, attained major status on August 30th. Going back to the beginning of reliable records for the eastern Pacific basin, the latest date for the first major hurricane in either basin is September 7th (Floyd, 1981). Combined the two basins have accounted for 53 units of ACE, the fifth lowest total since 1980.
Using that season to date figure to try to predict the future is no straight-forward; wide variations in the spacing of activity make many seasons “front-loaded” or “back-loaded” with activity relative to the present. Also, there is a wide range of variation across the seasonal sums of ACE for both basins. There is no “magic number” that is guaranteed to be the total for any given season. While the two basins tend to compensate for each other and have a stable average total over the long term, that relationship is only clear over the long term. I will, however, proceed cautiously in presenting those seasons with comparably low levels of activity in both basins to give a sense of what may be expected for the rest of the season:
|Season||Atlantic ACE pre
|Pacific ACE pre
|Atlantic total for
Despite the average-below average starts to these four seasons, the only to finish distinctly below-average in terms of ACE was 2002, a season in which moderate El Niño conditions were present.** 1981 is a perfect fit for NOAA’s definition of a season with average activity (but was a busy season by 1980’s standards). 2003 had three hurricanes at this point, but three major hurricanes were yet to come. The director of the National Hurricane Center, Rick Knabb, tweeted about the 2001 hurricane season a few days ago. The first hurricane, Erin, did not arrive until September 9th but the season ended with four major hurricanes in the ledger.
I suspect 2013 will bear resemblance to 2001. Doing so would put the season in line with the pre-season predictions for storm numbers, albeit with a bit lower ACE than anticipated. While we had a bit of a false alarm a bit over a week ago ***, it appears that the turn of the season is upon us and activity is about to pick up substantially. Forecast models are indicating that a tropical wave currently exiting Africa may become a tropical cyclone in five to six days time in the deep tropical Atlantic, approximately 1000 miles east of the Lesser Antilles. This afternoon’s Tropical Weather Outlook from the National Hurricane Center gives the wave a 20% chance of becoming a tropical cyclone in the next five days. Given the time of year, such a system would have to monitored closely due its relative close-in development. Furthermore, the GFS model is currently showing development of the subsequent wave as well.
If one had to summarize the reason for the quiet season to date in one graphic, it would be hard to do better than this:
The graphic comes from NOAA’s Tropical Cyclone Formation Probability product. As vertical instability is a necessary component of formation and maintenance of tropical cyclones, it is one of the key data inputs for the product. As the graphic shows, instability in the tropical Atlantic has been below normal for the entirety of the season. Its relative absence has been a key factor in the shortened lives of Tropical Storms Dorian and Erin as well as the lack of additional tropical cyclone formation. While it has not yet reached a climatologically normal level, it is as high as it’s been all season.
While I have been composing this post, the sixth tropical cyclone of the season has formed in the Bay of Campeche. A WC-130 is en-route to determine whether it is a tropical storm. I cannot help but be reminded of what I call the “phony season” part of the 2005 hurricane season. From mid-July to mid-August there was a succession of insignificant storms, which mostly struggled in hostile conditions in the tropical Atlantic. The last of those was the tropical cyclone that seemed poised for development, Tropical Depression Ten. About a week later, a tropical storm, Jose. similar to our newly formed Tropical Depression Six, formed in the Bay of Campeche and the real season was back with a vengeance****. Hostile conditions do not last forever.
Will we have our first major hurricane landfall since Wilma in that season without equal? Will we see our first category five hurricane since Felix, nearly six years ago? Those are questions for which we will have to wait for the answers. While we are free to hope that the balance of the season passes without incident, such an occurrence is not probable. We must be prepared.
*** See, for example, the Colorado State University forecast team’s prediction for tropical cyclone activity from August 16 to August 29. Anticipating development of Invests 92 and 94 as well as Erin being longer-lived, the forecast called for above average levels of ACE for the two week forecast period. That forecast will go down as big bust.
**** Jose, of course, is the name that Tropical Depression Ten would have received had it become a tropical storm. And there was a time when it appeared the remnant bits, future Tropical Depression Twelve / Katrina, would get that name. Great example of the hazard of bandying about a name for a disturbance or tropical depression before the name is actually earned.