Chapter 14 - Equatorial Processes
14.3 El Niño Teleconnections
statistically significant correlations between weather events that
occur at different places on the Earth. Figure 14.11 shows the dominant
global teleconnections associated with the El Niño-Southern
Oscillation (ENSO). It shows that
ENSO is an atmospheric perturbation influencing the entire Pacific.
|Figure 14.11 Sketch of regions receiving enhanced rain
(dashed lines) or drought (solid
lines) during an El Niño event. (0) indicates that rain changed during
the year in which
El Niño began, (+) indicates that rain changed during the year after El
Niño began. From
Ropelewski and Halpert (1987).
The influence of ENSO is through its influence on convection in the equatorial Pacific.
As the area of heavy rain moves east, it perturbs atmospheric pressure (Figure 14.12) and
influences the position of the jet stream at higher latitudes. This sequence of events
leads to some predictability of weather patterns a season in advance over North
America, Brazil, Australia, South Africa and other regions.
The ENSO perturbations to mid-latitude and tropical weather systems leads
to dramatic changes in rainfall in some regions (Figure 14.12). As the convective
regions migrate east along the equator, they bring rain to the normally arid,
central-Pacific islands. The lack of rain the the western Pacific leads to
in Indonesia and Australia.
|Figure 14.12 Changing patterns of convection in the equatorial
Pacific during an El Niño,
set up a pattern of pressure anomalies in the atmosphere (solid lines) which
extratropical atmosphere. From Rasmusson and Wallace (1983).
An Example: Variability of Texas Rainfall
Figure 14.11 shows a global
view of teleconnections. Let's zoom in to one region, Texas, that I chose only
because I live there. The global figure shows that the region should have higher
than normal rainfall in the winter season after El Niño begins. I therefore
correlated yearly averaged rainfall for the state of Texas to the Southern
Index (Figure 14.13). Wet years correspond to El Niño years in the equatorial
Pacific. During El Niño, convection normally found in the western equatorial
Pacific moved east into the central equatorial Pacific. The subtropical jet
east, carrying tropical moisture across Mexico to Texas and the Mississippi
Valley. Cold fronts in winter interact with the upper level moisture to produce
winter rains from Texas eastward.
|Figure 14.13 Correlation of yearly averaged rainfall
averaged over all Texas each year plotted as a function of the Southern
Oscillation Index averaged for the year.
From Stewart (1994).