By Dr. Jeff Masters

Published: 4:50 PM GMT on November 30, 2012

The long and highly destructive hurricane season of 2012 has finally drawn to a close. The hurricane season of 2012 will long be remembered for spawning Hurricane Sandy–a freakish storm that was the largest, most powerful, and second most destructive Atlantic hurricane on record. But this year’s hurricane season had a number of unique attributes, making it one of the most bizarre seasons I’ve witnessed. Despite featuring a remarkable nineteen named storms–tied for the third highest total since record keeping began in 1851–this year’s hurricane season had just one major hurricane. That storm was Hurricane Michael, which stayed at Category 3 strength for a scant six hours. This is the least number of major hurricanes in a season since the El Niño year of 1997, which had only Category 3 Hurricane Erika. There were no Category 4 or 5 hurricanes in 2012, for just the 3rd time since the active hurricane period we are in began in 1995. The only two other years since 1995 without a Category 4 or stronger hurricane were the El Niño years of 2006 and 1997. Both of those seasons had around half the number of named storms of 2012–nine in 2006, and eight in 1997. The relative lack of strong storms in 2012 helped keep the Accumulated Cyclone Energy (ACE) down to 128, about 30% above average.

Figure 1. Hurricane Sandy at 10:10 am EDT October 28, 2012. Image credit: NASA/GSFC.

A near-average year for number of tropical cyclones hitting the U.S.
Since the active hurricane period we’ve been in began in 1995, the U.S. has averaged getting hit by 4 named storms per year, with an average of 1.7 of these being hurricanes, and 0.6 being major Category 3 and stronger hurricanes. This year, we were hit by 3 named storms (Beryl, Debby, and Isaac). One of these was a hurricane (Isaac). Sandy didn’t count as a hurricane strike on the U.S., since it transitioned to an extratropical cyclone a few hours before landfall. No major hurricanes hit the U.S., making 2012 the 7th consecutive year without a major hurricane strike. The only other time we’ve had a streak that long occurred between 1861 – 1868, during the decade of the Civil War.

Figure 2. Vertical instability over the tropical Atlantic in 2004 – 2012 (blue line) compared to average (black line.) The instability is plotted in °C, as a difference in temperature from near the surface to the upper atmosphere (note that the same scale is not used in all the plots, making the black climatological line appear different, when it is really the same for each plot.) Thunderstorms grow much more readily when vertical instability is high. Instability was near average during the August – October peak of hurricane season in 2004 – 2009, but was much lower than average during the hurricane seasons of 2010 – 2012. There was an unusual amount of dry, sinking air in the tropical Atlantic during 2010 – 2012, and the resulting low atmospheric instability reduced the proportion of tropical storms that have intensified into hurricanes. Vertical instability from 2004 – 2011 is taken from NOAA/RAMMB and for 2012 from NOAA/SSD.

Unusually stable air over the Tropical Atlantic in 2012
For the third consecutive hurricane season, 2012 featured an unusual amount of dry, sinking air over the tropical Atlantic and Caribbean Sea. Due to warmer-than-average sea surface temperatures and an active African Monsoon that generated plenty of African waves, a remarkably high number of tropical storms managed to form, but the unusually stable air in the hurricane genesis regions made it difficult for the storms to become strong. When we did see storms undergo significant intensification, it tended to occur outside of the tropics, north of 25°N, where there was not as much dry, sinking air (Sandy’s intensification as it approached landfall in Cuba was an exception to this rule.) If we look at the last nine hurricane seasons (Figure 2), we can see that the hurricane seasons of 2010, 2011, and 2012 all featured similar levels of highly stable air over the tropical Atlantic. This is in marked contrast to what occurred the previous six years. The past three seasons all featured a near-record number of named storms (nineteen each year), but an unusually low ratio of strong hurricanes. Steering patterns the past three years also acted to keep most of the storms out to sea. Is this strange pattern something we’ll see more of, due to climate change? Or is it mostly due to natural cycles in hurricane activity? I don’t have any answers at this point, but the past three hurricane seasons have definitely been highly unusual in a historical context. I expect the steering currents to shift and bring more landfalling hurricanes to the U.S. at some point this decade, though.

Figure 3. Sea water floods the Ground Zero construction site at the World Trade Center, Monday, Oct. 29, 2012, in New York City. Image credit: AP.

Most notable events of the Hurricane Season of 2012
Hurricane Sandy was truly astounding in its size and power. At its peak size, twenty hours before landfall, Sandy had tropical storm-force winds that covered an area nearly one-fifth the area of the contiguous United States. Since detailed records of hurricane size began in 1988, only one tropical storm (Olga of 2001) has had a larger area of tropical storm-force winds, and no hurricanes has. Sandy’s area of ocean with twelve-foot seas peaked at 1.4 million square miles–nearly one-half the area of the contiguous United States, or 1% of Earth’s total ocean area. Most incredibly, ten hours before landfall (9:30 am EDT October 30), the total energy of Sandy’s winds of tropical storm-force and higher peaked at 329 terajoules–the highest value for any Atlantic hurricane since at least 1969. This is 2.7 times higher than Katrina’s peak energy, and is equivalent to five Hiroshima-sized atomic bombs. At landfall, Sandy’s tropical storm-force winds spanned 943 miles of the the U.S. coast. No hurricane on record has been wider; the previous record holder was Hurricane Igor of 2010, which was 863 miles in diameter. Sandy’s huge size prompted high wind warnings to be posted from Chicago to Eastern Maine, and from Michigan’s Upper Peninsula to Florida’s Lake Okeechobee–an area home to 120 million people. Sandy’s winds simultaneously caused damage to buildings on the shores of Lake Michigan at Indiana Dunes National Lake Shore, and toppled power lines in Nova Scotia, Canada–locations 1200 miles apart!

Figure 4. Hurricane Isaac lit up by moonlight as it spins towards the city of New Orleans, LA, on August 26, 2012. The Suomi National Polar-orbiting Partnership (NPP) satellite captured these images with its Visible Infrared Imaging Radiometer Suite (VIIRS). The “day-night band” of VIIRS detects light in a range of wavelengths from green to near-infrared and uses light intensification to enable the detection of dim signals. Image Credit: NASA/NOAA, Earth Observatory.

Hurricane Isaac hit Louisiana as a Category 1 hurricane with 80 mph winds on August 28, but the storm’s massive wind field brought a storm surge characteristic of a Category 2 hurricane to the coast. A storm surge of 11.1 feet was measured at Shell Beach, LA and higher surges were reported in portions of Louisiana. Fortunately, the new $14.5 billion upgrade to the New Orleans levee system kept the city dry. Isaac killed 9 people in the U.S., and 29 in the Caribbean.

Hurricane Ernesto hit Mexico’s Yucatan Peninsula as a Category 1 hurricane with 85 mph winds on August 7. The storm killed 12 and did at least $250 million in damage.

Tropical Storm Debby formed on June 23, the earliest formation date on record for the season’s 4th storm. The previous record was Dennis, on July 5, 2005. Debby killed seven and did over $300 million in damage, but helped relieve drought conditions over Northern Florida and Southern Georgia.

Tropical Storm Beryl, which made landfall on May 28 near Jacksonville Beach, FL with 70 mph winds, was the strongest tropical storm to make landfall in the U.S. prior to June 1. Beryl killed two but did minimal damage.

Nadine lasted for 21.75 days as a named storm, the 5th longest-lasting tropical storm in the Atlantic basin.

It was the 3rd year in a row with 19 named storms.

No named storms existed during the month of July and November, but we still managed big numbers.

Only 7 seasons have had more hurricanes than 2012.

The season had two named storm before the official June 1 start of hurricane season, only the 3rd time that has occurred.

Eight named storms formed in August, which tied 2004 for the most to form in that month.

Typhoon Bopha a threat to the Philippines
In the Western Pacific, where typhoon season commonly brings several storms in December, we have impressive Typhoon Bopha. Bopha is expected to head west-northwest and intensify over the weekend, potentially arriving in the Philippines on Tuesday as a powerful Category 3 typhoon. Bopha formed at an unusually low latitude for a tropical cyclone–near 4°N. Storms forming that close to the Equator don’t get much help from the Earth’s spin to get spinning, and it is rare to see a tropical cyclone forming southwards of 5°N.

The Colorado State University hurricane forecast team, led by Phil Klotzbach and Bill Gray, has a more in-depth summary of the 2012 hurricane season.

Jeff Masters

from:    http://www.wunderground.com/blog/JeffMasters/article.html