Cookie asked me a good question on the comments page about whether the 2005 season could happen again, with this question in mind I've decided to do another blog entry on this very topic, that being what did cause 2005 to be so busy?Well in my mind there are 2 main reasons, but a 3rd reason which is why this season came in above the likes of other super above average seasons such as 1933 and 1887...[b]Reason 1:[/b] Weak El nino of the 2nd half of 2004 pushed Atlantic tropical SST's even higher up then they were in the season of 2004. This was then followed by a slack Azores/Bermuda high during the spring of 2005 which allowed further warming upto record temperatures by the time we reached May. Such warm temperatures, esp in the Western Atlantic is what helped aid so many hurricanes that season in generally favourable conditions[b]Reason 2:[/b]During the summer of 2005 a rather unusual occurence occured, the Pacific monsoonal trough spread a lot further east then it usually would, getting well into the Caribbean. This meant lower then normal surface pressures for a good part of the western basin. This also meant a good breeding ground for tropical cyclones and therefore it should not be surprising that most of the storms formed west of 50-60W. Another point worth making with this reason is 3 of the top 10 deepest pressures in the basin were recorded in 2005. This is not by chance and also purely because of the monsoonal trough being present which meant pressure was lower then normal even before any system was present. Usually any monsoonal will not tend to last long before retreating back westwards but the fact it lasted for so long was unusual. It is worth noting this is why the WPAC basin tends to be so active and thats why 2005 had a similar number of storms in the Atlantic as in the Westr Pacific.[b]Reason 3:[/b]This reason is somewhat hypothetical in nature simply because I don't have the data in front of me, but here we go anyway!The NE part of the basin was very active in the Autumn of 2005 with quite a few systems that developed tropical features in parts of the Atlantic that are not typically warm enough to give tropical systems during Autumn. The above normal SST's helped but they clearly weren't the full picture...and thus what was the cause?Well to put it simply the driving force for deep convection to form in the first place is the lapse rates, the difference between the surface and the upper levels. I can only imagine that during Autumn of 2005 the uppers in this part of the world were below normal, this therefore meant surface temperatures could be that bit lower and still allow deep enough convection for the system to be tropical. However these systems are for another blog thats for sure!So those are the three reasons in my eyes, the ENSO state clearly is always going to be important but given it was a neutral season for most of it, the other three factors probably had a larger direct part in why the hurricane season of 2005 had so many storms as it did.Its worth comparing 2005 to 1933 and 1887 as well, and more to the point look at the maps and see where the storms were:1887:[url="http://weather.unisys.com/hurricane/atlantic/1887/track.gif"]http://weather.unisys.com/hurricane/atlantic/1887/track.gif[/url]1933:[url="http://weather.unisys.com/hurricane/atlantic/1933/track.gif"]http://weather.unisys.com/hurricane/atlantic/1933/track.gif[/url]2005:[url="http://weather.unisys.com/hurricane/atlantic/2005/track.gif"]http://weather.unisys.com/hurricane/atlantic/2005/track.gif[/url]The key thing to note is just how few storms there were east of 40E, indeed 1887 only had 1 whilst 1933 has none. Compare that to 2005 which had 5-7 storms. What does this mean?Possibly either 1933 or 1887 could have been every bit as active at least in terms of numbers of storms. The simple truth is we'll never know but because data was both lower in terms of quantity and sometimes quality, plus our understanding has improved loads (I'm sure there were systems in 2005 that would have been thought as just extratropical systems, esp in Autumn, afterall remember the old 26.5C rule, which is in theory not actually true) its quite possible they had as many storms as 2005 and some may have been stronger then currently estimated.So in conclusion, the 2005 was indeed an amazing season and had exceptionally positive factors which helped it be super active, however there have been other very active seasons in the last 150 years which may have been nearly as busy.