I've spoken with a lot of people recently who ask me why we haven't had many hurricanes in the Atlantic this year.  This is especially noticeable when contrasting with last year's record hurricane season.  During the 2005 hurricane season, the Atlantic saw 28 named storms and 15 hurricanes, which broke several records (Check here for a complete list: http://www.wunderground.com/hurricane/record2005.asp).  This year we've had only 9 named storms and 5 hurricanes so far.  Furthermore, no hurricanes have come anywhere near the U.S. coastline (with the exception of Ernesto, which was a hurricane for a brief period of time, but thankfully weakened to tropical storm status by the time it reached Florida).  Hurricanes Florence, Gordon, Helene, and Isaac have all formed deep in the Atlantic, and haven't hit any heavily populated areas.

So where have all the storms gone?  To answer this we first need to look at the elements needed to form a tropical storm/hurricane:

•  Sea surface temperatures of 80F or higher.
•  Atmospheric instability.
•  A moist atmosphere below 500mb.
•  A near-surface disturbance to get things rolling.
•  An area of formation at least 500km North or South of the Equator.
•  Low vertical wind shear.

So what's lacking this year?  Sea surface temperatures are one of the most critical requirements, and above-average SSTs continue from last year into this year.  See the current temperature map here:  http://www.osdpd.noaa.gov/PSB/EPS/SST/data/usatlant.cf.gif.  Instability and a moist atmosphere with surface disturbances have also been present, as several typical storms have formed in the Atlantic this year.  The National Hurricane Center have watched several of these storms, but most of them have just fizzled out.  Formation 500km North of the Equator isn't typically an issue...storms will form just about anywhere, that requirement is in place because typically storms that are closer than 500km from the Equator are not long-lived because of a lack of Coriolis Force (http://en.wikipedia.org/wiki/Coriolis_force).

That last requirement (low vertical wind shear) is missing though.  Vertical wind shear is defined as a difference in wind direction or wind speed as you ascend upwards into the atmosphere.  Ironically wind shear is good for extra-tropical storms like those that are common in the continental U.S., as it helps convection of moist air.  However tropical storms are much different because they typically last for days instead of minutes or hours.  A strong hurricane will grow to the upper edge of the troposphere, and with such a large area of circulation any bit of vertical wind shear is detrimental.  What happens here is that once a storm intensifies, it starts expanding upwards.  If there is more than about 10 knots of wind shear, the upper part of the storm will be blown away from the lower part.  To see this visually, check out a picture I took while flying to the Caribbean in May of 2006: http://flickr.com/photos/mikepj/158827442/.  Here you can see a storm in a high-shear environment.  The wind at lower levels are travelling in a different direction as the upper levels, so as this cloud builds toward the upper levels of the troposphere, the top part of the cloud is blown in a different direction (to the left from this viewpoint).  This season several smaller storms have formed in the Atlantic, only to be blown apart as they build into the upper troposphere.

So what is causing this wind shear to develop?  There could be several causes, but the most prominent is the development of a weak El Nino event in the Pacific.  During an El Nino year, a Pacific jet stream will often develop around the same latitudes as the Caribbean.  A jet stream brings a lot of wind shear with it, and even though the El Nino event is happening 2,000 miles away, the effects can be profound.

What I am wondering at this point is what this will mean for the Atlantic hurricane season next year.  Typically, a large hurricane will cool down ocean water in it's wake, and without many hurricanes this year to cool down the tropics, there's a possibility that the sea surface temperatures will be even higher next year.  Assuming El Nino ends sometime next spring, this could be setting up a very active 2007 Hurricane Season.