Chapter 14 - Equatorial Processes

14.3 El Niño Teleconnections

Teleconnections are 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 drought 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 influence the 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 Oscillation 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 also moves east, carrying tropical moisture across Mexico to Texas and the Mississippi Valley. Cold fronts in winter interact with the upper level moisture to produce abundant 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).

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