Wednesday, August 01, 2012

Weather Whys Wednesday: The Eastern Caribbean Graveyard

As talk of the tropics inevitably heats up now that we've entered August, and as the just recently named TD 5 gets everyone on the internet's attention, let's talk about a region we actually associate with big time hurricanes. You've heard of the Bermuda Triangle in relation to ships and aircraft. But what about the Eastern Caribbean Graveyard?

Historical Hurricane tracks centered in the "Eastern
Caribbean Graveyard." Credit: NOAA
No Wikipedia page exists for this region. It doesn't mystify or capture people's imaginations. But it is an interesting anomaly in the world of meteorology. We shared the cool historical hurricane tracking tool with you yesterday on Facebook, so let's put it to use. Based on research from Owen Shieh at Oklahoma, I tried to take an approximate midpoint of this "graveyard" at about 14.75 N and 67.5 W. Using the hurricane tracking tool, I plotted all Cat 1-5 hurricanes recorded in a 200 nautical mile diameter circle in that area and compared it to a similar 200 nm circle just south of Cuba, east of the Yucatan and north of Honduras. Not surprisingly, 64 hurricanes have been recorded in the Eastern Caribbean, as compared to 107 of them in the Northwest Caribbean. You'd also find over 100 hurricanes just east of the Bahamas and north of the Greater Antilles and nearly 80 hurricanes in the Windward Islands. This not a scientific comparison by any means, but it should give you a rough idea as to how climatologically anomalous the Eastern Caribbean is. Worth noting, if you just look at tropical storm strength systems, the comparison holds up as well, with under 100 in the Graveyard and well over 100 in the other regions mentioned above.

So what causes the Graveyard to exist? Well, you can read the research above for a lot of good examples and details. Shieh's research found that one of the more permanent features in the Caribbean is the Caribbean Low Level Jet. It's an enhanced area of lower level winds (about 3,000 feet up or so) that's oriented primarily in the Central Caribbean during the summer, peaking in intensity in July. It's considered almost a semi-permanent feature of this region, meaning it's there almost all the time. Because winds converge there, there has to be compensation nearby, and winds actually diverge in the Eastern Caribbean, creating subsidence (or sinking air) that suppresses clouds and convection. Storms in the Eastern Caribbean are more likely later in hurricane season, as this effect dwindles. During El Niño events, this effect can be enhanced somewhat, though that research and linkage is still very limited.
Tropical Depression 5 Official Forecast Wednesday Evening
Credit: NOAA/NHC
So naturally, while this is more than just an urban legend or an untested theory, it doesn't always stand the test of time. And as of this evening, the National Hurricane Center is forecasting Tropical Depression Five to ultimately traverse the Eastern Caribbean Graveyard, survive, and become Hurricane Ernesto. Whether this occurs or not, we shall see. But it will be yet another interesting test of the Graveyard...in a developing El Niño no less! But now you know a bit more about this phenomenon.