Posted: Jul 8, 2010 11:16 AM by Melissa Canone
WASHINGTON (AP) - The climate phenomenon known as La Nina
appears to be developing, threatening more bad news in the efforts
to clean up the oil spill in the Gulf of Mexico.
When a La Nina occurs there tend to be more hurricanes than
normal in the Atlantic and Caribbean regions, which include the
Gulf of Mexico.
The federal Climate Prediction Center said Thursday that La Nina
conditions are likely to develop in July and August.
La Nina is marked by an unusual cooling of the sea surface in
the tropical Pacific Ocean. Water temperatures in that area can
affect air pressure and winds, resulting in changes in the weather
in many parts of the world.
In a La Nina, wind shear is increased over the Pacific and
reduced over the Atlantic. Wind shear is the difference in strength
of winds at low levels compared to higher level winds.
A strong wind shear reduces hurricanes by breaking up their
ability to rise into the air, while less shear means they can climb
Thus, the Climate Prediction Center notes, "there tend to be
more Atlantic hurricanes during La Nina because of this expanded
area of low vertical wind shear."
In addition, during a La Nina "more hurricanes form in the deep
tropics from African easterly waves. These systems have a much
greater likelihood of becoming major hurricanes, and of eventually
threatening the U.S. and Caribbean islands," according to the
center, a part of the National Oceanic and Atmospheric
The climate center's current hurricane forecast for this season
is for 14 to 23 named storms of which 8 to 14 are expected to be
hurricanes and 3 to 7 major hurricanes.
The center noted that during June, sea surface temperature
continued to decrease across the equatorial Pacific Ocean, with
cool areas expanding across the central and eastern Pacific. In
addition, increased rainfall persisted over Indonesia, while the
area of reduced rain expanded westward over the western and central
Combined with changes in the winds over the Pacific "these
oceanic and atmospheric anomalies reflect developing La Nina
conditions" which are likely to continue through early 2011, CPC
The last La Nina occurred from the fall of 2007 to the spring of
2008. The opposite mode, El Nino, with warm Pacific conditions, has
been in place since the spring of 2009.