Five named storms before July 15! Obviously we're headed for
some kind of record year. Or are we? And what does all of this have
to do with planetary warming?
The answers aren't clear. Hurricane (and tropical storm) data
are notoriously noisy from year-to-year, and the way that we gather
these statistics hasn't been constant. Prior to 1945, most
hurricane information came from merchant shipping in the Atlantic.
It's likely that many storms were missed, especially those that
didn't cross major shipping lanes.
After World War II, we began to send out aircraft (called
Hurricane Hunters), but these missions weren't launched unless
there was some evidence of a storm, i.e. weather reports from an
island or a ship. Only since the mid-1960s has weather satellite
technology been sufficient to detect all storms.
Since World War II the Hurricane Hunters have detected an
apparent long-term decline in the average maximum winds measured in
a given year. Apparently hurricanes are getting weaker.
Important word, that: "apparently." It's possible that the
decline in average maximum winds is real. Some scientists (not this
one) think global warming increases the frequency or intensity of
El Nino, the big Pacific Ocean weathermaker. El Nino activity and
hurricane intensity in the Atlantic are highly anticorrelated.
The same result -- an apparent decline in maximum winds -- would
obtain if we were now naming more tropical storms than we used to.
In that case, the apparent lack of any trend in long-term frequency
would be wrong; the true picture would be a slight decline.
It's complicated, okay?
Equally convoluted is the relationship between the number of
early-season storms and overall annual activity.
One would think that it would be very straightforward. The peak
of hurricane season is around September 12. If numbers tend to fall
off smoothly on both sides of that peak, then a large number of
early-season storms would signal a banner year.
But hurricanes aren't evenly distributed at all throughout their
season. Rather, they tend to "bloom," with multiple storms
occurring simultaneously (or nearly so). When that occurs, the
storms in each pod tend to be on pretty similar tracks -- which is
why Florida got whacked four times last year in six weeks. A worse
shellacking actually took place in northwestern Mexico in 1933, as
six storms hit in fairly close proximity.
The relationship between early and entire-season hurricanes is
non-existent. Only 3 percent of the year-to-year variation in total
number of tropical cyclones per year (since 1945) is explained by
the number observed before August 1. Statistically speaking, that
number is indistinguishable from zero.
Further, there is no relationship between the number of severe
hurricanes ("category 3" or higher) and early-season activity.
There is a pretty good reason for all of this, and it can be
found in a study of recent Hurricane Dennis.
Dennis was an extremely threatening Category 4 hurricane on
Sunday morning, July 10, a mere six hours prior to landfall. But,
as it approached the Florida coast, it encountered the water
disturbed by tropical storm Cindy just six days before (behold the
"clumpiness" of hurricanes!). The rough seas mixed colder water
with the hotter surface layer, reducing the heat energy available
to maintain Dennis, who shriveled faster than a (provide
inappropriate metaphor here).
Hurricanes require hot water. But in years where there's a lot
of early activity, there's going to be less of that around, which
means, in general, that later storms will tend to be a bit less
frequent or weaker. At least that's the theory. Reality is more
complicated, as the relationship between water temperature and
hurricane intensity is far rougher than indicated by simplistic
computer models with warmer oceans.
Here's the bottom line: Since World War II, there is no
significant relationship between what happens in the entire
hurricane season and what happened early in that season.
But then there's 1933, the same year that Mexico got pounded.
Five storms were detected before August 1, which ties the record
(observed three times) in the postwar era. The entire season saw 21
tropical storms and hurricanes, a record that still stands
today.
topics:
Global Warming, Energy
