For decades NASA researchers
have helped refine our understanding of hurricanes and tropical cyclones.
Among their tools are computer models
that can help distinguish the roles played by sea surface temperature, pressure and wind speed.
These model studies show that what makes a busy or slow Atlantic hurricane season
largely relies on three ocean patterns.
These three factors contribute to either
a strong or a weak hurricane season, each acting a little like an on/off switch.
First, the El Niño Southern Oscillation’s two phases:
El Niño and La Niña affect sea surface temperature in the Pacific Ocean,
which then alters wind strength in the Atlantic.
During El Niño years, Pacific sea surface temperatures near the equator
are generally warmer than average, which drive winds that shear the tops off hurricanes,
making it harder for them to form. Cooler Pacific water during La Niña years
creates more favorable hurricane wind conditions.
The North Atlantic Oscillation’s two phases: positive and negative
describe the difference in pressure between two poles of a pressure system
– low pressure near Iceland and high pressure near the Azores Islands.
When the pressure difference between them is lower than average, hurricanes are more likely to form.
When the two pressure systems are more intense than average,
wind conditions are not conducive to hurricane formation.
Further south, the two phases of the Atlantic Meriodonal Mode describe sea surface temperatures:
either warmer or cooler. When the area north of the equator is warmer than average,
hurricanes are more likely to form.
When the same area is cooler, hurricanes are less likely to form.
Together, these factors can make or break a strong hurricane season,
and understanding how they interact can help better predict the number and strength
of hurricanes seen throughout the season.
For example, in 2005, the Pacific sea surface temperatures were average, so the El Niño Southern Oscillation played very little role.
The North Atlantic Oscillation was mildly negative,
which influenced winds more favorably for hurricanes.
The Atlantic Meridional Mode was the strongest influencer this season,
with very warm Atlantic water temperatures leading to a busy and powerful hurricane season,
which included Hurricanes Katrina, Rita and Wilma.
In 2013, the El Niño Southern Oscillation favored hurricane formation at the beginning of the season,
with a weak La Niña contributing cool water in the Pacific,
a neutral Atlantic Meridional Mode and a negative North Atlantic Oscillation
Oscillation setting up favorable hurricane conditions.
However, in mid-July, the North Atlantic Oscillation suddenly changed to a strongly positive intensity,
creating unfavorable wind conditions and limiting hurricane formation in the Atlantic.
(beeping)