by Michael Keller

Though millions of people in the Western Hemisphere might have already had a suspicion that October was a busy month for tropical storms, the National Hurricane Center has put some official numbers on it.

The center says five tropical storms formed in October. Two developed into hurricanes, including Hurricane Sandy, which ravaged parts of New Jersey and New York. This puts the month’s activity well above the long-term average of two named storms and one hurricane.

Perhaps more important, though, is activity for the year to date. The center says the combined strength and duration of all tropical storms so far this year is about 30 percent above the long-term average, which is calculated for the years 1981-2010. There have been 19 tropical cyclones, 10 of which have turned into hurricanes. One of those, Michael, in early September turned into a major hurricane with 115 mph maximum winds.

The Atlantic Hurricane season typically spans from June to the end of November, though 2012 saw an early start with two named tropical storms in May.

Greenland Blocking

Meanwhile, meteorologists have been shining a light on the unusual path that Hurricane Sandy took before it made landfall along the East Coast.

The storm traveled southeast to northwest as its center came ashore just south of Atlantic City, N.J. But typically, the prevailing winds north of the subtropical region would send a storm eastward out into the North Atlantic, where cold seawater would stop fueling the system.

Dr. Jeffrey Masters, cofounder and meteorology director of Weather Underground, says an atmospheric phenomenon called blocking is to blame.

From Masters’s blog post on the event:

A strong ridge of high pressure parked itself over Greenland beginning on October 20, creating a “blocking ridge” that prevented the normal west-to-east flow of winds over Eastern North America. Think of the blocking ridge like a big truck parked over Greenland. Storms approaching from the west (like the fall low pressure system that moved across the U.S. from California to Pennsylvania last week) or from the south (Hurricane Sandy) were blocked from heading to the northeast. Caught in the equivalent of an atmospheric traffic jam, the two storms collided over the Northeast U.S., combined into one, and are now waiting for the truck parked over Greenland to move. The strength of the blocking ridge, as measured by the strength of the North Atlantic Oscillation (NAO), was quite high–about two standard deviations from average, something that occurs approximately 5% of the time.”

Over at his American Geophysical Union Earth science blog, Dan Satterfield writes, “October or November hurricanes recurve into the Atlantic because of a much stronger fall jet stream, but the Greenland block turned Sandy into the coast. The track of Sandy was very RARE. Nearly unheard of actually, especially for this time of year.”

Top Image: Preliminary Atlantic Tropical Cyclone Tracks. Courtesy NHC/NOAA.

Michael Keller is the Managing Editor of Txchnologist. His work has appeared online and in newspapers, magazines and books, including the graphic novel Charles Darwin’s On the Origin of Species. Reach him at mkeller@groupsjr.com.