Monday, December 2, 2013

Wimpy hurricane season a surprise puzzle

Brian McNoldy in the Washington Post: It was a hurricane season almost without hurricanes. There were just two, Humberto and Ingrid, and both were relatively wimpy, Category 1 storms. That made the 2013 Atlantic hurricane season, which ended Saturday, the least active in more than 30 years — for reasons that remain puzzling.

...In 2013, there were 13 tropical storms, a typical number, but for the first time since 1994 there were no major tempests in the Atlantic. The last time there were only two hurricanes was 1982. -The quiet year is an outlier, however, in the recent history of Atlantic cyclones. The National Oceanic and Atmospheric Administration notes that 2013 was only the third calmer-than-average year since 1995.

...While such an inactive season is not unprecedented, forecasters usually do a creditable job of anticipating one. Why was this season so inactive? What did the forecasts miss? Although there are some hypotheses, it is not entirely clear. We may have to wait another couple of months, but there are some possible explanations.
  • Dry air : Even over the long three-month window of August to October, the vast majority of the tropical Atlantic was dominated by drier-than-normal air, especially in the deep tropics off the coast of Africa. Dry air can quickly weaken or dissipate a tropical cyclone, or inhibit its formation.
  • Stable air: The average temperature profile in the region was less conducive to thunderstorm growth and development during the core months, which means that the amount of rising air in the region may have been reduced as well.
  • Weak African jet: Tropical waves, the embryos of many tropical cyclones, have their origins over continental Africa. A persistent feature called the African easterly jet stream — a fast-moving river of air in the low and middle levels of the atmosphere — extends from Ethiopia westward into the tropical Atlantic Ocean....
 Hurricane Ingrid on September 14, 2013, shot by NASA

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