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felt further inland, but the extent and magnitude is much more variable
outside the Southwest.
Although El Niño events appear to have brought summer drought to
the southeastern United States in the past 40-50 years, this pattern was not
seen (and even reversed) during the late 19th and early 20th centuries. In
fact, there seem to be substantial decadal changes in the impacts of ENSO
on drought in the United States over the past 150 years (Cole et al., 1998).
EN
SO-Related Data
An ENSO event usually continues for 12-18 months. If we could use
ENSO-related data to monitor the development of an ENSO, it would
be possible to predict regional precipitation and hence drought. Usually
researchers make use of single “descriptor indices” to monitor an ENSO
event. These indices are simple, easy to handle, and they capture relevant
information about the events.
There are two main groups of ENSO indices: simple and composite. The
simple indices are derived from single properties of either the atmosphere
or the ocean. The most popular of such indices is the SOI, as defined earlier
in this chapter. Surface temperature anomalies at different regions can be
used to define these indices. Toward this end, the tropical Pacific has been
divided into a number of regions such as Niño 1, 2, 3, 4, and 3.4 (which
encompasses part of both regions 3 and 4). Niño 1 is the area defined by
80°-90° W and 5°-10° S, Niño 2 by 80°-90° W and 0°-5° S, Niño 3 by
90°-150° W and 5° N-5° S, Niño 4 by 150° W-160° E and 5° N-5° S, and
Niño 3.4 by 120° W-170° W and 5° N-5° S (figure 3.4).
In addition, some research or monitoring centers all over the world use
other more specific indices such as the outward long-wave radiation (OLR)
to monitor the shift of the deep convection over the western Pacific, the
zonal wind component at 850 Hpa level as a measure of the intensity of the
trade winds, and the temperatures of the subsurface ocean layers. Simple
indices may fail to capture a coupled ocean—atmosphere event, and their
selection could lead to wrong results.
The composite (or coupled) indices combine characteristics of the ocean
and atmosphere. The most successful index in this group is the multivari-
ate ENSO index (MEI). The MEI is a composite index using sea-surface
temperatures, surface air temperatures, sea-level pressure, zonal east-west
surface wind, meridional north-south surface wind, and the total amount
of cloudiness over the tropical Pacific. Positive MEI values are related to
warm-phase or El Niño events and negative values to the cool-phase or
La Niña events. The MEI can be understood as a weighted average of the
main ENSO features, and it is calculated as the first unrotated principal
component (PC) of all six observed fields combined. This is accomplished
by normalizing the total variance of each field first and then performing
the extraction of the first PC on the variance matrix of the combined fields
[35],
Line
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-0.1
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Shor
PgEn
[35],
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