Why no hurricanes for Puerto Rico this year?

A brief update on the tropics today:

A strong extratropical low pressure system (993 mb) is just south of the Azores Islands today, in the far eastern Atlantic. This storm is generating winds of 40-45 mph, and has the potential to make the transition to a tropical storm later in the week as it moves slowly westward. Water temperatures beneath it are 21-22C and wind shear is a low 5-10 knots. However, strong wind shear associated with an appraoching trough is expected to impact the storm on Wednesday, which may not leave it enough time to make the transition to Tropical Storm Zeta.

Why no hurricanes for Puerto Rico this year?

I've plotted an image (Figure 1) showing every stretch of Atlantic coast placed under a hurricane or tropical storm warning during the Hurricane Season of 2005. The phenomenal number of landfalling storms in 2005 led to warnings being hoisted for the entire Atlantic coast from Plymouth Massachusetts southwards to Costa Rica. A small section of Guatemala and Honduras escaped warnings, but nonetheless received damage from heavy rains from at least one tropical storm. Most of the Caribbean islands also suffered blows from at least one hurricane, with the notable exception of the area around Puerto Rico--the northeastern Caribbean
--where no storms occurred. Why did this region escape the wrath of the Hurricane Season of 2005?


Figure 1. Map of all coastal areas subjected to hurricanes warnings (red) and tropical storm warnings (yellow) during the Hurricane Season of 2005.

One obvious possibility is that wind shear levels in the northeast Caribbean happened to be higher than average. This is the theory advanced by Dr. Bill Gray of Colorado State University in his summary of the 2005 hurricane season. However, when one plots the average wind shear over the various portions of the Atlantic for 2005 (Figure 2), only the western Caribbean shows near average levels of wind shear. The ocean areas on either side of Puerto Rico--the eastern Caribbean and the tropical Atlantic--both show below-average levels of wind shear.


Figure 2. Historical average wind shear levels (black lines) and observed wind shear for the 2005 hurricane season (blue lines). The Gulf of Mexico, eastern Caribbean, and tropical Atlantic all had wind shear levels well below average for much of the 2005 hurricane season.

Another possibility is that large amounts of African dust inhibited tropical storm formation in the northeast Caribbean. This is likely a significant factor, since many large clouds of African dust tended to push across the Atlantic and into the northeast Caribbean between late July and early September of 2005, the prime part of hurricane season for this region. I can recall in particular that the dry air associated with Tropical Depression Ten helped kill this storm as it approached the northeast Caribbean in mid-August. Unfortunately, the dry air diluted enough by the time it reached the Bahamas that Hurricane Katrina was able to form from the remnants of Tropical Depression Ten.


Figure 2. Visible satellite image of one of the many clouds of African dust that crossed the Atlantic during the Summer of 2005.

Jeff Masters