It was a hurricane season almost without hurricanes. There were just two: Humbertoand Ingrid, and both were relatively weak Category 1 storms. That made the 2013 Atlantichurricane season, which ended Saturday, the least active in more than 30 years--forreasons that remain puzzling.
Heading into the hurricane season, all signs pointed toward a very active one.Forecast teams predicted seven to nine hurricanes, with higher-than-average ACE ( 累积气象学中用于衡量飓风强度指数的指标) . Only the European Center for Medium-Range Weather Forecasting predicted about 80 percent of average ACE, but itstill said there would be six or seven hurricanes.
Why was this season so inactive? What did the forecasts miss? Although there aresome hypotheses, it is not entirely clear, but in the meantime, there are some potentialexplanations.
    Major signals such as the E1 Nino Southern Oscillation (ENSO), surface pressureand sea-surface temperature all pointed to an average to above-average season. But therewere some possible suppressing factors.
    Even over the long three-month window of August to October, the vast majority ofthe tropical Atlantic was dominated by drier-than-normal air, especially in the deeptropics off the coast of Africa. Dry air can quickly weaken or dissipate a tropical cyclone (热带气旋) , or inhibit its formation.
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 risingair in the region may have been reduced as well.
Tropical waves, the embryos of many tropical cyclones, have their origins overcontinental Africa. A persistent feature called the African easterly jet stream ( 东非东风急流 ) --a fast-moving river of air in the low and middle levels of the atmosphere--extendsfrom Ethiopia westward into the tropical Atlantic Ocean. It breaks down into discretewaves, and every few days another wave leaves the coast. Some are barely noticeable,while others become tropical storms.
    During the height of the hurricane season, most tropical cyclones form fromdisturbances off the coast of Africa.
    Winds in the jet normally cruise along at 20 to 25 mph at an altitude of 10,000 feetfrom August to October, but this year they were about 12 to 17 mph weaker. One would expectthat to have a big impact on the amplitude ( 振幅 ) of easterly waves and the hurricaneseason.
    How was the season's inactivity related to climate change? It's not accurate toassociate any particular season with climate change. The reason is that intra- and inter-seasonal variability is so large that any subtle signals of influence from climate change areoverwhelmed.