Wednesday, October 17, 2012

Weather Whys Wednesday: Autumn Atlantic Hurricanes and ACE

As what's left of Rafael shifts out to sea, I figured we'd come back into focus on the Tropical Atlantic. You might be saying to yourself: Self, we're really up to the "R" storm? And the answer of course, would be "Yes." It's fascinating to me how hurricane seasons like this, ones that are generally quiet at home, can sneak up on you as being incredibly active. But is it really? Let's discuss.

Atlantic Tropical Climatology - Credit: NHC/NOAA
By now, you've probably all seen the chart at left, which details the climatology of hurricane season. Of course, we peak around September 10th, then drop off before seeing that secondary peak in early to mid October. Much like climatology suggests, this October has been fairly busy. The image below shows the origins of tropical cyclones between October 21 and 31 (so basically the rest of this month) so you have an idea where formation is favored going forward. And you can see that it's predominantly in the Caribbean and Bahamas, with a few in the Atlantic, as well as the Gulf. The reason is that we still have warm oceans (albeit, they're moving downward in overall temperature), and often times you can get disturbances and fronts that end up in the Gulf or Caribbean. Wind shear is generally too strong in the northern latitudes to sustain any storms up the coast (of course Hazel occurred in mid-October). But all in all, we are winding down here.

Locations of tropical cyclone formation from 10/21-31
Credit: NHC/NOAA
Let's contextualize this season some more. A short while back, I posted about ACE (accumulated cyclone energy) being basically meteorological sabermetrics. According to Dr. Ryan Maue, this season's ACE is currently at 112.53. Thus far, we're up to 17 storms. 2011 saw 20 and 2010 saw 21 named storms. ACE last year was 127.1125. In 2010, it was 166.885. An average Atlantic hurricane season sees an ACE of 104 for the season. So we are slightly above normal, despite being 6-7 storms above normal for the season (depending on what climatology set you use). So despite the insane activity of the last three years, the cumulative intensity of those storms has been anything but overly impressive.

Anecdotally, I have noticed the National Hurricane Center has been extremely proactive in recent years in naming storms that would have been completely missed in the pre-satellite era. The NHC is doing the climatological record a very good service by doing this. We will now have a very thorough record of storms...and one that is more representative of reality. However, the trouble with this sort of progress is that we may no longer be comparing apples to apples, and hurricane history may become a science of apples vs. oranges. Looking at the last three seasons in a vacuum, it's clearly evident that 2010 was more intriguing than either 2011 or 2012, when taken as a whole. Of course, Irene and Isaac have made the last two seasons far more memorable in the US. But it will be interesting to see if ACE per storm averages continues dwindling in the coming seasons as the NHC becomes more liberal with naming systems.