Since Hurricane Fred in mid-September, we have not had any named tropical cyclones in the Atlantic basin. The culprit for the slow 2009 Atlantic Hurricane Season is westerly wind shear that has torn apart almost every tropical cyclone that has developed in the Atlantic. This is an El Nino year, and El Nino is well known to increase shear:
(1) Warm El Nino waters in the Pacific Ocean increases thunderstorms over that region.
(2) Upper outflow from those thunderstorms become upper westerly winds across the Atlantic tropics
(3) Upper westerlies are in direct opposition to tropical latitude low-level easterlies (also called trade winds), increasing shear tropics-wide in the Atlantic.
The above circulation is also called the Walker Circulation. Another effect due to enhanced Pacific storms is a rise in the upper atmospheric pressure in the region of enhanced storms. Relatively lower pressures to the northeast of the Pacific (i.e. in the Atlantic) thus allow for significant upper trough patterns during the Atlantic Hurricane Seaon, and we are defenitely seeing this right now. The upper troughs help to recurve storms out to sea, and also allow westerly shear to be very widespread.
For some recent El Nino Atlantic Hurricane Seasons, here are the dates when the season ended (when the last NAMED tropical cyclone dissipated that season)
2006: Season ended on October 2, last storm was Isaac
2002: Season ended on October 4, last storm was Lili
1997: Season ended on October 17, last storm was Grace*
1993: Season ended on September 21, last storm was Harvey
1992: Season ended on October 26, last storm was Frances*
1991: Season ended on November 2, last storm was Unnamed**
* Storm developed from extratropical system
** Unnamed storm also known as the "Perfect Storm," also developed from extratropical system
One trend I notice from the above is that during El Nino years, the season tends to end in late September/early October. For El Nino seasons that end later than this, the storms are produced by extratropical cyclones that transition to tropical ones.
On September 15, I forecasted in my last post (Trends in the 2009 Atlantic Hurricane Season) that as we head into October and November, "upper trough will be king," and it sure is now. Upper troughs and westerly shear will continue to suppress late season Atlantic tropical cyclone activity. I am standing by my September 15 prediction that we may see 1 to 2 more systems that could originate from extratropical systems. Otherwise, it seems 2009 like other El Nino seasons is effectively over.
Regardless of any predictions made though, all should always be prepared. These are just my opinions on what I have observed.
