The North Atlantic hurricane season has nearly come to an end. As November progresses, the chance of another storm developing becomes smaller. Climatology (last 60 years) tells us that roughly 4 in 10 years see a November storm formation including 4 in 2005 (Beta, Gamma, Delta, Epsilon), Hurricane Michelle (2001), Hurricane Lenny (1999), and Hurricane Kate (1985). Jeff Masters from the Weather Underground has an image of previous early-November storm tracks especially clustered in the Western Caribbean.
So, what has the 2008 season wrought in the North Atlantic and how well did the seasonal prognosticators fare?
Even with the expected post-season tinkering of the real-time storm tracks by the folks at the National Hurricane Center, we can provide fairly accurate preliminary numbers. The community at Wikipedia constantly updates many interesting facts about the ongoing 2008 hurricane season.
Total Named storms (34 knots + one-minute maximum sustained winds): 15
Total Hurricanes (64 knots +): 7
Total Major Hurricanes (96 knots +): 4
Accumulated Cyclone Energy: 132
The respective forecasts made by CSU (Klotzbach and Gray), NOAA, as well as the UK Met Office came in quite close to the actual experienced storm activity. Before handing out trophies, please keep in mind that forecast “skill” is a function of many forecasts over longer time periods. Each of the forecasting outfits prefers to use different techniques and variables to calculate their storm numbers, so we will have to wait until each completes their post-season analysis to determine if they were “right for the right reason” or got lucky.
Now, to answer the question: how active was the 2008 hurricane season, we need to define climatology. This is where the tricksters can play pranks on the public. Where is the beginning point of the analysis? How well do we trust the frequency and the estimated intensities of each storm? What metric do we use – number of tropical storms, number of hurricanes, ACE (accumulated cyclone energy), Power Dissipation, or perhaps some complicated statistical measure? All of these questions are entangled in the debate surrounding whether anthropogenic climate change is indeed a modulating influence upon current and future Atlantic hurricane activity.
A well-accepted metric which convolves storm frequency, intensity, and duration is called accumulate cyclone energy (ACE) and is calculated very simply: take the maximum sustained winds reported by the NHC every 6-hours for all storms (> 34 knots), square this value, and sum over the entire lifetime, then divide by 10,000. In 2007, even though there were also 15 storms, the ACE was only 72 compared to 132 for 2008 with the same number of named storms. This is partially because the storms in 2008 were much longer lived especially Bertha.
Here are three different views of the Atlantic hurricane climatology depending upon what period you look at. The data is from the NHC Best Tracks without any corrections to the intensity data.
Thus, since 1995, Atlantic hurricane activity measured by ACE is hugely variable with feast (i.e. 2005) and famine (1997). 2008 ACE is nearly equivalent to 2006 and 2007 combined, but about half as what was experienced in the record 2005 season. The choice of 30-years is a particular favorite for many researchers in the tropical cyclone community (1978-2007). The second image clearly shows the nearly stepwise increase in ACE between 1994 and 1995. In this reference frame, 2008 ranks as one of the more active years of the past 30. Now, back up to 1944, when admittedly the intensity (and detection) data is somewhat less reliable. However, since the ACE metric is the convolution of an entire year’s worth of storm lifecycle information, and is most sensitive to higher wind speeds, the track data points prior to satellite observation (~1970s) are probably sufficient for this exercise.
Final verdict: When encapsulated in the recent active period in North Atlantic activity (1995-2007), 2008 experienced normal or expected activity as measured by ACE. In terms of a long-term climatology, either the last 30 or 65 years, 2008 is clearly an above average year.
Note: for the Climate Audit seasonal forecasters, especially those that showed exemplary skill (however you wish to measure it), please fill us in on your methodology and perhaps provide guidance for 2009. Or, for those feeling shame about being “blown off track”, time to think of good excuses.
Also, a new Science perspective has been published by Vecchi et al. (2008) entitled Whither Hurricane Activity? More on that later…