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Climate Science: Investigating Climatic and Environmental Processes
Interannual Processes (<101years)
Image of ice core showing annual bandingENSO's Twin Pulses
Forcing Factors

Does ENSO Impact Hurricane Frequency?


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).
Image showing El Nino dynamics
Click here for more detail on E
l 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.
Image of La Nina dynamics
Click on image above for more detail on La Niña's regional and seasonal impact.

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:

FORCING FACTORS
Image of globe






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 events.

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.

An 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.

Paleoclimate scientist Dr. Julia Cole notes that "tropical climate variability is dominated by ENSO, which strongly affects extratropical climate patterns around the globe." See Decadal Variability in Paleoclimate Records.

Scientists are also looking at whether sunspots, which have an 11 year cycle, may play a significant role in climate processes. See Do Sunspots Influence Climate? and The Sun-Climate Connection- Did Sunspots Sink the Titanic? from NOAA OAR.

Does ENSO Impact Hurricane Frequency?
SOI Index
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, Landsea, 1998).

Number of Hurricanes ASO AtlanticSee La Niña, El Niño, and Atlantic Hurricane Damages in the United States by Roger A. Pielke, Jr. and Christopher W. Landsea for more on the relationship between ENSO and tropical storms and hurricanes.

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Images from NOAA's Paleoclimatology Program, NGDC's MGG, the Climate Diagnostic Center, PMDL and IRICP.

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