[env-trinity] NOAA Says May Showed Transition From La Nina To El Nino/Southern Oscillation Neutral

[Sari Sommarstrom]

THE COLUMBIA BASIN BULLETIN:
Weekly Fish and Wildlife News www.cbbulletin.com
June 10, 2011  Issue No. 578


* NOAA Says May Showed Transition From La Nina To 
El Nino/Southern Oscillation Neutral

NOAA’s Climate Prediction Center this week said a 
transition from La Niña to El Nino/Southern 
Oscillation-Neutral conditions occurred during 
May 2011 as indicated by generally small sea 
surface temperature anomalies across the equatorial Pacific Ocean.

The subsurface oceanic heat content remained 
elevated, but relatively constant during the 
month.  Consistent with other transitions to 
ENSO-neutral conditions, the atmospheric 
circulation anomalies continued to show some 
features consistent with La Niña, albeit at weaker strength.

Current observed trends, along with forecasts 
from a majority of the ENSO models, indicate 
ENSO-neutral will continue through the Northern 
Hemisphere summer 2011. Thereafter, most models 
and all multi-model forecasts predict 
ENSO-neutral to continue through the remainder of 
2011. However, the status of ENSO beyond the 
Northern Hemisphere summer remains more uncertain 
due to lower model forecast skill at longer lead 
times, particularly during this time of year.

La Niña is defined as cooler than normal 
sea-surface temperatures in the central and 
eastern tropical Pacific ocean that impact global 
weather patterns. La Niña conditions recur every 
few years and can persist for as long as two years.

El Niño and La Niña are extreme phases of a 
naturally occurring climate cycle referred to as 
El Niño/Southern Oscillation. Both terms refer to 
large-scale changes in sea-surface temperature 
across the eastern tropical Pacific. Usually, 
sea-surface readings off South America's west 
coast range from the 60s to 70s F, while they 
exceed 80 degrees F in the "warm pool" located in 
the central and western Pacific. This warm pool 
expands to cover the tropics during El Niño, but 
during La Niña, the easterly trade winds 
strengthen and cold upwelling along the equator 
and the West coast of South America intensifies. 
Sea-surface temperatures along the equator can 
fall as much as 7 degrees F below normal.

El Niño and La Niña result from interaction 
between the surface of the ocean and the 
atmosphere in the tropical Pacific. Changes in 
the ocean impact the atmosphere and climate 
patterns around the globe. In turn, changes in 
the atmosphere impact the ocean temperatures and 
currents. The system oscillates between warm (El 
Niño) to neutral (or cold La Niña) conditions 
with an on average every 3-4 years.

Typically, a La Niña is preceded by a buildup of 
cooler-than-normal subsurface waters in the 
tropical Pacific. Eastward-moving atmospheric and 
oceanic waves help bring the cold water to the 
surface through a complex series of events still 
being studied. In time, the easterly trade winds 
strengthen, cold upwelling off Peru and Ecuador 
intensifies, and sea-surface temperatures drop 
below normal. During the 1988- 89 La Niña, SSTs 
fell to as much as 4 degrees C (7 degrees F) 
below normal. Both La Niña and El Niño tend to 
peak during the Northern Hemisphere winter.

La Niña often features drier than normal 
conditions in the Southwest in late summer 
through the subsequent winter. Drier than normal 
conditions also typically occur in the Central 
Plains in the fall and in the Southeast in the 
winter. In contrast, the Pacific Northwest is 
more likely to be wetter than normal in the late 
fall and early winter with the presence of a 
well-established La Niña. Additionally, on 
average La Niña winters are warmer than normal in 
the Southeast and colder than normal in the Northwest.

El Niño and La Niña occur on average every 3 to 5 
years. However, in the historical record the 
interval between events has varied from 2 to 7 
years. According to the National Centers for 
Environmental Prediction, this century's previous 
La Niñas began in 1903, 1906, 1909, 1916, 1924, 
1928, 1938, 1950, 1954, 1964, 1970, 1973, 1975, 
1988, and 1995. These events typically continued 
into the following spring. Since 1975, La Niñas 
have been only half as frequent as El Niños

La Niña conditions typically last approximately 
9-12 months. Some episodes may persist for as long as two years.

Contrasting El Niño and La Niña winters, the jet 
stream over the United States is considerably 
different. During El Niño the jet stream is 
oriented from west to east over the northern Gulf 
of Mexico and northern Florida. Thus this region 
is most susceptible to severe weather. During La 
Niña the jet stream extends from the central 
Rockies east- northeastward to the eastern Great 
Lakes. Thus severe weather is likely to be 
further north and west during La Niña than El Niño.

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