With October nearly done circling the drain, I figure it is about time to bring out the broom : Northern Hemisphere tropical cyclone activity is at historically low levels .
In fact, September 2007 suffered the lowest ACE since 1977 ! Even scarier, so far 2006 and 2007 have the lowest October ACE since 1976 and 1977. And, unnaturally, Sept-Oct 2007 is the lowest since 1977.
Yet, the tropical cyclone season was not shaping up to be such a ghastly bust. For about a week in June, NH ACE was exceeding climatology but then bit the proverbial dust until mid-August when a noticeable comeback ensued. It has been downhill since.
So, a naysayer over at the Huffington Post or the Daily Green may wonder why we use such metrics such as ACE/PDI or Tropical Cyclone days when we could use better metrics like number of category 5′s making landfall or storms that have intensified the fastest or perhaps number of pumpkins. There are even some spooky hints that the 2007 (Atlantic) Tropical Cyclone season is being “spun” to appear “dead” and inconsistent with the predominant trend. Also, a “storm pundit” would remind that us that the year is not in the grave and we may see hyper-activity (a.k.a. global warming proof) to come.
Here is what needs to happen to reach the 1970-2006 mean:
- 140 Tropical Cyclone Days — on average, 40 TC days (sigma of 16 days) occur through the end of the year. Slightly more than 70 TC days occurred during 1984, 1992, and 1997. Thus, 140 TC days would be at least a 6 sigma event
- 50 Hurricane Days — on average, 16 Hurricane days occur through the end of the year. Slightly more than 30 hurricane days occurred during 1984, 1990, 1992, and 1997. Thus, 50 hurricane days is more likely, only a 4 sigma event.
- Record level ACE exceeding 1997 ENSO enhanced output — just to reach the yearly mean — about a 4 sigma event.
Yet with nature, I wouldn’t bet my last carbon credit against an extreme event. We may need some poleward-moving, late-season tropical cyclones to save us from the winter of 1977-1978. Art Bell will have to warn the “indigenous dog-people” to collect additional firewood…