Lackluster Hurricane Season Could Stay That Way
Though these numbers aren't dramatically low for late September, meteorological conditions over the tropical Atlantic will continue to give incipient disturbances little chance to flourish in the remaining days of the month. In fact, I wouldn't be surprised to see even the latest predictions for a quieter-than-normal season fall short.
Keep reading for more on the outlook for the rest of the 2009 hurricane season...
Stronger-than-average wind shear in the Atlantic Ocean has no doubt hindered the development of tropical cyclones thus far. These changes in wind speed and direction with height have been able to dismantle many of the budding disturbances that have dared to evolve into coherent systems. It's safe to say that the current El Nino episode -- the warming of the waters in the central and eastern Pacific Ocean -- is consistent with the lack of hurricane activity in the Atlantic.
Another, though not necessarily unrelated, reason for our relatively benign hurricane season can be linked to the unusual dryness of the atmosphere over the Atlantic, particularly at altitudes where air is often pulled into a developing storm. Research has shown that the penetration of relatively dry air into the core of a tropical system can have devastating effects on the health of its circulation. By generating cool, dry columns of downward-moving air in the lower levels of a storm, this process can block the intake of warm, humid air that is essential for its survival.
The plot below shows the relative humidity anomalies (departures from average) in the Atlantic at about one mile in altitude for the month of August. The regions shaded in yellow and red indicate conditions much drier than normal. Though a time-averaged plot like this does not necessarily yield insight into what happened during any single event, a case-by-case look at the tropical systems this year pretty much tells the same story; that is, just about every disturbance we've tracked has had to fight off the choking effects associated with the inhalation of a nearby layer of dry air originating from Africa.
Unless the air over the tropical Atlantic moistens up, and the winds become more favorable for storm development, the final hurricane count will be underwhelming. The door is slowly closing on the 2009 season.
| September 23, 2009; 10:45 AM ET
Categories: Tropical Weather
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