Atlantic Ocean Conveyer, not global temperatures, determine the frequency of
hurricanes. It is predicted that we will see an increase in the number of
Atlantic hurricanes for the next 10-15 years compared to recent years. This is
nothing new -- the last peak occurred between 1930 and 1970.
What is the Atlantic Ocean Conveyer and why does it affect hurricanes?
In the North Atlantic in the Labrador and Gin Sea, water is cooled and sinks to
the bottom in winter. The sinking water is replaced by warm Gulf Stream water
that leaves the North Carolina coast and proceeds toward the Northeast to
Ireland and Norway. The Gulf Stream draws water from the South Atlantic. The
rest of the conveyer belt is in the other oceans, but the Atlantic portion is
the dominant feature.
This oceanic phenomenon varies and changes the sea-surface temperatures. When
it is strong -- 1930-1970; 1995-2000-now -- there are more Atlantic hurricanes.
When it is weak -- 1905-1925; 1975-1994 -- the hurricane season is mild.
And what about intensity?
Hurricanes have to keep moving or they die by upwelling the cold water below
the warmer sea-surface temperatures. Hurricane Mitch, 1998, is a textbook
example of this. Mitch stalled off Honduras when it lost its upper atmospheric
steering currents. This caused it to die and drop over 6 feet of rain on the
poor folks in Honduras and Nicaragua!
A hurricane will also grow stronger as it moves over water warmer than 80
degrees F (26.5 C). This is why some people believe that global warming will
increase hurricane intensity, but there are no scientific calculations that
show the areas of water this warm increasing in size.
However, if one graphs the ocean environment for Category 3, 4 and 5 storms,
there is no difference in ocean-surface temperatures for tropical storms and
Category 1 and 2 storms. What's more, the scientific literature documents that
the western Atlantic off Africa is the prime breeding ground for the stronger
storms -- and many of those stronger storms never make it to landfall.
While it is tempting to blame the frequency or intensity of hurricanes on man,
we all must remember how variable nature is -- and specifically in this case,
the effect of natural variations on hurricanes' intensity and frequency is
extremely higher than the possibility of man's interference.