Case Study: El Niño: A Link among Atmospheric, Oceanic, and Crustal Circulation?

Republished From: Environmental Science 2016

FIGURE 1  Correlation between El Niño Events and Seismic Activity  The timing of El Niño events is indicated by the shaded years. Notice that the shaded areas are correlated with months in which there are earthquakes along the East Pacific Rise with a magnitude of eight or more (triangles). A similar correlation exists between El Niño events and months with the greatest release of seismic energy along the East Pacific Rise (circles). Source: Redrawn from a figure in The New York Times.

Most scientists believe that El Niño events (discussed in this chapter) are generated by interactions between atmospheric and oceanic circulation. According to this hypothesis, El Niño events can be triggered by a large thermal input that raises sea surface temperatures. These thermal inputs can be driven by the normal pattern of oceanic circulation. Nonlinear and stochastic elements in the pattern of oceanic circulation cause variations in the rate at which heat accumulates and dissipates in various portions of the ocean. These variations can warm portions of the sea surface and trigger an El Niño event.

A geophysicist, Daniel A. Walker, hypothesizes that a different sequence of events produces an El Niño event. Walker says that the thermal input to the oceans comes from Earth’s interior. This hypothesis is based on an intriguing correlation of seismic activity under the eastern portion of the Pacific Ocean near Easter Island and the onset of El Niño events. This correlation can be used to illustrate possible linkages among the physical systems of planet Earth and the difference between statistical correlation and physical causation.

The East Pacific Rise is located west of Easter Island. Along this rise, tectonic plates move 160–170 mm (6.3–6.7 inches) per year. This rate is one of the most rapid in the world. As a result, seismic activity along the East Pacific Rise has been studied extensively for more than thirty years. During this period scientists have tracked the number of earthquakes and the amount of energy they release.

While plotting these data, Walker noticed an interesting pattern: Months with the greatest number of earthquakes or months with earthquakes that release the greatest amount of energy precede the onset of El Niño events. This hints at a relationship between seismic activity and El Niño events. A simple statistical analysis suggests that if the seismic events and the El Niño events occurred randomly, the probability of finding a sequence in which periods of heightened seismic activity precede El Niño events would be 1 out of 313. Such long odds imply that the sequence probably is not generated by random chance. Rather, there may be a physical connection between seismic activity and El Niño events.

But statistical correlation should not be confused with physical causation. Correlation indicates that the timing of the two events is similar but does not mean that the two events are connected. For example, some Wall Street analysts have noticed a correlation between the winner of the Super Bowl and the performance of the stock market. When the Super Bowl champions are a team from the old American Football League, stocks usually decline over the year; stocks rise over the year if the winner is a team from the old National Football League. Clearly the victor does not determine the performance of the stock market. So this relationship is simply a correlation. To show causation, scientists must describe a physical mechanism that connects events.

Scientists have come up with several explanations for how seismic activity may trigger El Niño events. These hypotheses are based on the relationship between air temperature and pressure. In general, warm air has less pressure than cold air, so hypotheses seek to explain how seismic activity along the East Pacific Rise could affect air temperature.

One explanation postulates that seismic activity is associated with volcanic activity on the seafloor that releases large amounts of heat. If this heat were to reach the ocean surface, it could raise sea surface temperature, which would raise air surface temperature, which would reduce atmospheric pressure. Many scientists are skeptical of this hypothesis because the thermocline probably would prevent the heated water from reaching the surface.

Another explanation focuses on changes in the rate at which surface waters absorb sunlight. Scientists find that water from ocean vents has a higher concentration of nutrients, which increases the local rate of photosynthesis. If seismic activity increases the concentration of materials critical to sea life and if those materials reach the surface, the resultant increase in biological activity could increase the amount of sunlight that is absorbed by the upper layers of the ocean. This could warm the sea surface, increase air temperature, and reduce atmospheric pressure. Another explanation hypothesizes that seismic activity along the East Pacific Rise alters circulation in the surface and intermediate layers of the eastern Pacific in a way that increases surface temperature and reduces pressure.

Most scientists disagree with the explanations for the statistical correlation between seismic activity and the onset of El Niño events offered by Walker and his colleagues. Their hypothesis may be validated by further research. But for now, the statistical relationship between El Niño events and seismic activity is nothing more than an interesting correlation.

Additonal Reading

Walker, D. “More Evidence Indicates Link between El Niños and Seismicity.” Eos 76 (1995): 33–36. Walker, D. “Seismicity of the Pacific Ocean Rise: Correlations with the Southern Oscillation Index?” Eos 69 (1988): 857–867. 

Student Learning Outcome

• Students will be able to explain how differences between correlations and causation affect our ability

Glossary

Citation

Cleveland, C. (2014). Case Study: El Niño: A Link among Atmospheric, Oceanic, and Crustal Circulation?. Retrieved from http://www.trunity.net/sam2/view/article/51cbf4007896bb431f6aeb04