Bermuda Tropical Cyclones


Following from my previous post, I’m looking into where Tropical Cyclones (TCs) that impact Bermuda form, how they track, and when during Hurricane Season they impact the island. Using NOAA’s Historical Hurricane Tracks mapping tool, I found a list of TCs that came within 75 nautical miles of Bermuda. I made sure they were at least tropical storm strength (ie. maximum sustained winds of >34 kts) and got their track coordinates from the National Hurricane Center’s latest HURDAT2 dataset. I did this for all cases fitting the above criteria for seasons between 1950 and 2014.

This turned up 29 cases in 65 years which corresponds to a TC coming within 75 nm of Bermuda every 2.25 years. For this period, the median date of a TC within 75 nm of Bermuda is September 11th, with an inter-quartile range (ie. the range that includes 50% of the cyclones) from August 15th to September 27th. This coincides with the peak in Atlantic Hurricane Season Activity which is around September 10th.

Of the three main genesis regions for all Atlantic TCs, the area between the Bahamas and Bermuda (SWATL), and the Main Development Regions (MDR) are responsible for cyclones that pass near Bermuda. No cyclones that formed in the Caribbean between 1950 and 2014 tracked within 75 nm of Bermuda – however, there have been cases where a cyclone has formed in the Caribbean and tracked near Bermuda prior to 1950. For all Atlantic TCs, the Caribbean is the most frequent genesis location, meanwhile the SWATL and the MDR each had less TC formation than the Caribbean, but had similar numbers between the two. For TCs that passed near Bermuda, about two times more formed in the SWATL than in the MDR.

If we class the SWATL as west of 60W, and the MDR east of 60W, then we see that the cyclones that form in the SWATL are on average weaker than those that form in the MDR by the time they pass within 75 nm of Bermuda. On average, the cyclones that formed in the SWATL passed Bermuda with maximum sustained winds near 53 kts (strong tropical storm strength), while those that formed in the MDR passed Bermuda with maximum sustained winds near 77 kts (hurricane strength).

Genesis Locations
The genesis locations of TCs that passed within 75 nm of Bermuda with maximum sustained winds over 34 kts, between 1950 and 2014. The red and white bulls-eye over the SWATL is where the majority of Bermuda TCs form. A secondary peak in genesis is in the MDR.

The tracks of TCs that pass near Bermuda are generally straightforward: if they form in the SWATL they either get picked up by strong mid-latitude flow and propelled northeastward toward Bermuda, or they meander in the area and dissipate. However, if they formed in the MDR, they track west-northwestward around the Bermuda-Azores high, then recurve to the north into a weakness in the Bermuda-Azores ridge near Bermuda’s longitude. Because TCs that form in the MDR often have days over warm open waters, they tend to be stronger by the time they make it to Bermuda, whereas TCs that form in the SWATL are already very close to the island and so don’t have as long to strengthen before passing. These SWATL cyclones are often also afflicted by dry continental airmasses from North America.

Bermuda Tracks
The track density plot of the 29 tropical cyclones that passed within 75 nm of Bermuda from 1950 to 2014.

With the 2015 Atlantic Hurricane Season now under way, please keep an eye to the tropics through the National Hurricane Center‘s and the Bermuda Weather Service‘s forecast and warning products. With lower that normal sea surface temperatures through the MDR, warmer than normal sea surface temperatures in the SWATL, and an El Nino in place to increase wind shear across the MDR – it is quite possible that the SWATL will be more active than normal. This increases the chances for a TC to pass near Bermuda this season.

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