Re: El Nino and hurricanes?

Question

I've read (and noted over the years) that there is a correlation between El Nino years and a simultaneous increase of East Pacific and Central Pacific hurricanes and a decrease in Atlantic hurricanes. Last season, 2006, was such a year. I can understand the increase in eastern and central Pacific storms from the increased sea temperatures but has anyone in science
postulated or hypothesized or theorized why the decrease in Atlantic storms, being that the Atlantic/Caribbean/Gulf of Mexico is between 300 to 358 degrees east of the closest range of El Nino but prevailing winds both at the surface and at upper atmosphere are continually east to west?

Answer 1

This is how I understand it. In order to get an increase in organized tropical systems, you must have warm waters and weak wind shear with height. During an El Nino tropical season, with the warmer ocean waters in the very low latitudes, and a less amplified polar jet or flow over the northern latitude, the thermal gradient between higher and lower latitudes will tend to be a little stronger. Thus, there will be a tendency for a stronger mid to upper level sub-tropical jet or flow to exist over the Atlantic and Gulf. This position of the flow is often influenced in part by the position of the four corners high. This will mean there will be an increase in number of mid level shortwave energy moving through the Atlantic and the Gulf regions. The wind speed shear in the mid levels associated with these weak waves will tend to keep tropical system from becoming too organized. Thus you will see weaker and smaller areas of strong upper highs pressure system over the Atlantic. These large areas or bubble of high pressure with weak mid level vertical wind shear are areas where these storms will be most favorable to form with the right sea surface temperatures.

Answer 2

it is due to the higher wind shear over the tropics which inhibits tropical storm development.

from red cross...

"Weather experts say that warm El Nño events are characterized by decrease in the number of tropical storms and hurricanes in the Atlantic, Gulf of Mexico and the Caribbean Sea because of the increased wind shear. The wind shear associated with El Niño essentially cuts a storm off during its development, by hindering the formation of a vertical ring of clouds. The shear actually shaves off some of the clouds, which creates a slant in the cloudwall. When a storm is slanted, the heat energy released from condensation is spread out over a larger area and doesn't necessarily feed back into the storm system to strengthen it."

Answer 3

El Nino conditions lead to stronger summertime high-level wind shear over the low latitudes in the Atlantic basin. This wind shear makes it more difficult for tropical storms to strengthen there, which is why El Nino is anticorrelated with strong hurricane seasons in the Atlantic.

Now, the reason for that increased wind shear in the first place is more complicated, but it basically has to do with enhancing of the subtropical jet. This jet goes the opposite direction of the prevailing easterlies of the tropics felt at the surface. This is the "shear" referred to earlier.

Answer 4

This is much too broad to be answered in this format.
Go to this site to see some of the recording being done and click on the El Nino and La Nina tab at the top. You will get links that have great details on the effects.
http://www.pmel.noaa.gov/tao/

Click on 'the impacts of el nino' under the 'basics' heading at the second link for details about areas around the world