There's no other possible conclusion: The 2007 Atlantic hurricane season is having an identity crisis.
On the one hand, this has been the year that started out like gangbusters with its first two hurricanes, Dean and Felix, reaching Category 5 intensity. And it has also been a year in which we've seen a lot of ominous records being set when it comes to hurricane rapid intensification. Felix, Humberto, and now most recently Hurricane Lorenzo have each set different types of rapid intensification records. (For details see here.)
But if the 2007 Atlantic hurricane season has been behaving like a lion in some respects, it has been a lamb in others. Of the 13 named storms this year, just four have attained hurricane status, and Humberto and Lorenzo only attained it very briefly before landfall (save for their rapid intensifications, these too would have been tropical storms). That 4:13 ratio is definitely on the low end when considered in the context of the past five years. By contrast, the ratio of hurricanes to total storms was 7:16 in 2003, 9:15 in 2004, 15:28 in the record year of 2005, and even 5:10 in the less active El Nino year of 2006.
In short, not even a third of the named storms have attained hurricane strength this year, whereas over the previous four years, something much closer to a half have become at least this strong. (Note, however, that this year's Tropical Storm Karen may be upgraded to hurricane status in post-season reanalysis.) One reason for the relative quietude is that despite La Nina conditions in the Pacific ocean, we're still seeing lots of strong vertical wind shear over the Atlantic. Ingrid, Karen, Melissa -- all have been done in by powerful shear, which has separated the storms' thunderstorms from their centers and thereby prevented organization and development.
So what seems to be happening is that while we're seeing many initial disturbances with hurricane potential -- a lot of which have become tropical depressions or tropical storms -- we're also seeing very few storms actually live up to that potential due to an unfavorable environment in terms of atmospheric winds. However, for the few storms that have found themselves in a friendly environment -- Dean, Felix, Humberto, Lorenzo -- intensification has been rapid and in some cases record-setting.
In a prior Storm Pundit post, I noted my plan to ask some top hurricane specialists whether they think that hurricane intensification rates ought to change under global warming (a subject that appears to have been little studied as yet). I heard back from MIT's Kerry Emanuel, who explained the idea in the context of a hurricane's maximum potential intensity.
There is a theoretical speed limit for hurricanes, and Emanuel believes this is changing due to global warming -- essentially meaning that hurricanes may become more intense on average in favorable conditions. In addition, Emanuel told me by email that he thinks hurricane rates of intensification should also change -- i.e., if the potential intensity increases, so should the potential rate of intensification (once again, under idealized conditions). However, Emanuel cautioned that this is merely a theoretical outlook: He knows of no study that has yet related observed storm intensification rates to this theoretical expectation.
So in sum, we find ourselves staring down a hurricane season that poses a puzzle: On the one hand seemingly ominous; on the other, seemingly humdrum. The hurricane-global warming debate continues apace, and at least thus far, 2007 isn't providing anyone with a clear victory.
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