[Hurricane John of 2006 about to strike Baja California.]
NASA has a new analysis of why the 2006 hurricane season in the Atlantic was much tamer than expected by seasonal forecasters. Interestingly, some of what the agency says contradicts what I've been hearing from other sources. Notably:
1. NASA says that El Nino quashed Atlantic storms in two ways: "a sinking motion in the middle and upper atmosphere and increased wind shear in the Caribbean." By contrast, earlier this week in a talk at the AMS meeting in San Antonio, National Hurricane Center forecaster James L. Franklin said that El Nino had indeed been associated with sinking motions and drier, more stable air, but not with wind shear. "Basically, we think it was the thermodynamics, rather than the kinematics," he said according to my notes.2. NASA says that the 2006 Atlantic sea surface temperatures were significantly cooler than they were in 2005, by as much as 2 degrees (I'm assuming Fahrenheit). But yet again, Franklin in his talk said he didn't think that cooler sea temperatures could explain why 2006 was so much less active than 2005.
I don't know what to make of these discrepancies. Maybe we just don't know what the real causes for the less active 2006 season were.
There is at least one reason for distrusting NASA, though. The NASA analysis repeatedly claims that there were 25 storms (like John, pictured above) in the Northeast Pacific, which saw a much more active season than the Atlantic in 2006. But no matter how you count those Northeast Pacific storms, I only get 21 at max, and it seems like a more reasonable number is 18, which is the number of storms that actually received names. So NASA seems way off in this respect.
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Unitl there is general agreement on understanding all of the factors that go into hurricane formation, most of the arguements are very close to ad hoc.
I'm betting that NASA was including the Central Pacific TCs in that count of 25, and perhaps just inadvertently referred to them as Northeast Pac storms.
Looking at the Central & Eastern Pacific in whole, I can not escape the conclusion that El Nino played a strong to leading roll in the activity of these basins (Ioke & perhaps even 91C standing out the most to me). Given this, and the well-documented correlation between active east Pac years and subdued activity over in the Atlantic (and vice versa), I suspect that the argument for El Nino to have played a leading roll in the relatively tame 2006 Atlantic season to be a strong one.
As you know, it is well-documented that increased thunderstorm activity related to El Nino events increase the shear over the tropical Atlantic.
The 06 Atlantic season was pretty close to normal, at least by storm counts and intensities. The unusual thing was that no significant storms threatened major land areas so damage and media coverage was minimal. It was still anomalous in that total storm activity was well below the (recent) trendline, and forcasts, but not by nearly as much as it seemed to the general public.
Again 2005's 28th unnamed subtropical cyclone is forgotten...
NASA's 2006 Epac storm total is the simple sum of tropical depressions in both the East pacific and the Central pacific. It is not unusual to include the Central pacific storms in the East pacific.
Everyone seems to have their own rules for the "numbers game", but I'd venture to say that what is probably most important is the trend, not the actual number of storms in any given year or how one chooses to tally the totals.
I'd also have to say that scientists probably have not figured all this stuff out -- and probably never will figure out all of it.
If they had, the scientists could pack their bags and head to Bermuda for a permanent (retirement) vacation -- well-deserved after what they have had to endure over the past few years (from the Bush administration and American media, not the storms).