July 17, 2006
Among the ocean and atmosphere “vital
signs†that forecasters monitor on a daily basis during
hurricane season are sea surface temperatures. Warm sea surface
temperatures, minimal wind shear, and some kind of atmospheric disturbance
are all required to start and strengthen hurricanes.
This image from Aqua's AMSR-E instrument shows sea surface temperature conditions
centered in the Atlantic and the Gulf of Mexico on July 17, 2006, compared
to average conditions (1985-1997) determined from data from the Advanced
Very High Resolution Radiometer (AVHRR) on satellites of the National Oceanic
and Atmospheric Administration (NOAA). Places where temperatures were cooler
than average are blue, places where they are warmer than average are red,
and places where temperatures were around average are white. A swath of
cooler-than-average temperatures arcs northeast away from southern North
America, while to the south, a wide region of warmer-than-normal temperatures
stretch between Africa and Central America. These warmer-than-normal
sea surface temperatures in what is known asHurricane Alley will become important
as the hurricane season progresses.
One explanation for the cool swath is a smaller-than-average southward extent
of the Bermuda High. The Bermuda High is a semi-permanent area of
high atmospheric pressure that shifts position in the Atlantic between Bermuda
and the Azores Islands, off the coast of Portugal. According to
indicators tracked by NOAA, the Bermuda High did not extend as far south
as it normally does, and it has been weaker than normal since mid-June.
Forecasters monitor the position and strength of the Bermuda High because
it influences hurricane formation and tracks. A high pressure zone over the
ocean surface calms winds, which allows sea surface temperatures to warm.
Air is generally sinking toward the surface in high-pressure areas, and sinking
air tends to suppress cloud formation, which can also allow sea surface
temperatures to rise. The calm winds in the area also reduce wind shear on
developing storms. Finally, tropical storms that form tend to travel around
the edge of the Bermuda High in a clockwise circle. When the Bermuda High
shifts south and west, as it did in 2005, the path of storms takes them right
into the Gulf of Mexico.
Forecasters must track the Bermuda High all season long because its location
and strength can shift very rapidly from a configuration that would decrease
the likelihood of land-falling hurricanes to one that would increase the
likelihood. The current predictions from the National Hurricane Center are
for a very active 2006 Atlantic hurricane season.
NASA image by Jesse Allen, based on data from Remote Sensing Systems. Text
by Rebecca Lindsey, with input from David Adamec.