The image to the right shows
ten years of annual banding from a Greenland ice core, where year to year
variation of precipitation can vary significantly. Note the variance between
the light summer bands (arrows) and dark winter bands. The distance between
arrows is an indicator of overall annual precipitation. Data from instruments
and paleo proxies allow scientists to
observe patterns that occur within the interannual scale.
In recent years,
scientists have become aware of a particular climatic pattern that occurs
between one and ten year time scales. Known as ENSO, (the El Niño
Southern Oscillation), this climate phenomenon-- an interannual variation
of atmospheric and oceanic conditions in the equatorial Pacific-- has been
described by climatologists as the largest single source of interannual
climate variability on a global scale. (See Chang,
Battisti, 1998), and ENSO's Irregular Beat).
An ENSO-like climate signature has been seen in paleo records going back
thousands of years (see Moy, 2002), but
in some cases the regional impact or teleconnections appear to vary over
time. (Alverson, 2001)
ENSO's Twin Pulses
The pattern of warming and cooling of the eastern and central Pacific
can be measured in the sea level pressure in the region. When the
pressure measured at Darwin, Australia is compared with that measured
at Tahiti, the differences between the regions can be used to generate
an "index" number. A positive number indicates La Niña
(eastern tropical Pacific ocean cooling) and a negative number indicates
El Niño (or ocean warming).
Click here for more detail on El
Niño's regional and seasonal impact.
The classic example of ENSO's climatic clout is the 1982-83 El Niño
event which had enormous economic impact on the fishing industries
in Ecuador and Peru due to failed anchovy harvest that occurred when
sardines unexpectedly migrated south into Chilean waters. Severe weather
hit Hawaii and Tahiti, which are unaccustomed to such severe weather.
Monsoon rains fell over the central Pacific instead of the western
Pacific, leading to droughts and disastrous forest fires in Indonesia
and Australia. Winter storms battered southern California, causing
widespread flooding across the southern United States. Click
on image above for more detail on La Niña's regional and seasonal
La Niña can
also have it's own influence, triggering droughts in the southern
U.S. and flooding in the Pacific Northwest. While both El Niño
and La Niña
have strong teleconnections (linkages over great distance between
the phenomenon and regional climate) around the world, not every region
seems to be impacted by ENSO dynamics. The Front Range region of Colorado,
for instance, shows no
strong ENSO signature, while southwestern Colorado shows indications
of a much stronger ENSO signal. Recent studies have identified Atlantic
hurricane frequency and damages increasing during La Niña events
and decreasing during El Niño events. See below.
Even though ENSO may not show up in a regional climate signature,
it does influence global climate and it overlaps with other climatic
processes such as the North Atlantic Oscillation. See the Climate
Prediction Center's Tutorial for more on how ENSO impacts the
global climate system. Also see:
The ocean's ability to retain and then release heat is central
to climate phenomenon such as ENSO.
The ocean's surface waters are separated from colder, deeper water
by a thermocline. Upwelling of cold waters, particularly along
the equator in the eastern half of the Pacific basin, can be blocked
when sea level is high, resulting in warm
When sea levels are low, the thermocline tends to be shallow,
allowing upwelling motions to bring cold water to the surface,
resulting in cold conditions.
How Measured Instruments used to track decadal variability and climate
patterns include thermometers, rain gauges, and stream gauges.
Sea Surface Temperature (SST) is particularly important in tracking
ENSO and other ocean oscillations.
ENSO-like signature also appears in some paleo records, including
cores taken from trees, corals and lake sediments in the Andes
mountains of South America. However, climate variability occurring
at an interannual-scale (between one and ten year periods) is
not well understood, in part because it has been studied for only
a few decades. Paleo records are now being overlapped with instrumental
records in order to better understand interannual variability
and related events such as ENSO.
scientist Dr. Julia Cole notes that "tropical climate variability
is dominated by ENSO, which strongly affects extratropical climate
patterns around the globe." See
Variability in Paleoclimate Records.
Scientists have observed that
during warm ENSO episodes (El Niño) there is an increased number
of tropical storms and hurricanes in the eastern Pacific and a decrease
in the Gulf of Mexico and the Caribbean Sea and that La Niña events
favor hurricane formation in the Atlantic. (Pielke,