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Hurricane Season Finally Kicks Into Gear

By Joe D'Aleo
Monday, August 31, 2009

Hurricane Bill (shown below), reached Category 4 not once but twice as it swung northwest and north precariously close to the coastline. Danny, a much weaker system followed a similar path with rain as its major impact for the coast.


The overall environment with a warm Atlantic Multidecadal Oscillation (AMO) and cold Pacific Decadal Oscillation (PDO) is conducive for storms threatening Florida and the east coast, however, the weak to moderate El Nino is not.

Although La Ninas predominate in cold PDO eras, El Ninos do occur although they tend to be weaker and briefer. In the warm PDO modes, El Ninos are much more common and stronger.


El Ninos act to weaken Atlantic sector storms by firing up eastern Pacific systems and increasing the shear across the Atlantic. However although the ocean has been in a very clear El Nino mode, the atmosphere as reflected by the Southern Oscillation Index (SOI) has been much less locked in El Nino.

Since 1995, the Atlantic has become twice as active on average as the prior 25 years, similar to the period from 1930s to 1960s.  This is due to a shift to the ‘warm” mode of the multi-decadal scale oscillation in the Atlantic Ocean.  Most of the storms making landfall during the past 12 years have impacted the Mid-Atlantic region, Florida and the Gulf of Mexico.  However, though not yet realized, history tells us that the risk has also increased for more populated areas to the north New York City/(Long Island and New England).

It appears the Pacific plays a role too. The cold mode of the PDO (in place this summer) favors New York and New England landfalls in large part because it favors La Nina. We had a strong La Nina this past winter into the early spring but it has in recent months, warmed in the eastern tropical Pacific. It is unclear whether that will save the east coast and the northeast one more year. 

You can see La Nina years when the Atlantic is warm produced 15 landfalling east coast storms in 9 years, 11 were major hurricanes, 9 affecting the northeast directly on second or third landfall. We discussed the 1938 direct hit hurricane in a 2008 story here.

Christopher Landsea (HRD) and Roger Pielke Jr. estimated if a storm like the Hurricane of '38 were to happen today, it would be the sixth costliest of all-time. See much more about the east coast here and New England here.



Last year, the El Nino like warming in the eastern Pacific may have protected the east coast and Klotzback and Gray and NOAA think this summer’s stronger El Nino will do the same. Let us hope they are correct. You can see La Ninas make the entire Gulf and Atlantic coastline at risk. El Ninos affect mainly the Gulf.

Given the cold Pacific, by next spring and summer, we should see a return to La Nina. That should increase the threat to the east coast again. Given the eastern trough developing by next week and unconvincing SOI, we can’t entirely rule out this summer, early fall. Also given some similarity to 1969 in a number of indices, the Gulf also is not out of the ballgame. Camille occurred in late August 1969. Then again, storm chances in the western GOM, diminish as we move into September climatologically.