Exercises: El Niño-Southern Oscillation and Global Weather

Define El Niño. In your answer, describe what happens to the trade winds and to sea-surface temperatures in the central and eastern tropical Pacific Ocean.

Compare La Niña with El Niño. State whether the sea-surface temperatures in the eastern tropical Pacific are warmer or cooler than average during each phase, and whether the trade winds are weaker or stronger than normal.

NOAA's Climate Prediction Center monitors the Niño 3.4 region of the tropical Pacific. An anomaly of $+0.5\,^\circ\text{C}$ or higher (sustained) indicates El Niño, and an anomaly of $-0.5\,^\circ\text{C}$ or lower indicates La Niña. A region shows an anomaly of $+1.2\,^\circ\text{C}$ for several months in a row. Classify the ENSO phase and explain what "anomaly" means.

The table gives monthly sea-surface temperature anomalies in the Niño 3.4 region:

Use the threshold of $\pm 0.5\,^\circ\text{C}$.

1. In which month did conditions first reach the El Niño threshold?

2. Calculate the average anomaly for September through November.

3. Describe the trend and predict what is likely to happen to rainfall in Peru and to the trade winds.

During El Niño winters, the subtropical jet stream often strengthens and shifts south across the Pacific. Explain how this change can lead to a wetter-than-average winter along the Gulf Coast and across the southern United States, including Texas. Then explain why the Pacific Northwest tends to be drier and warmer in many El Niño winters.

Coral reefs can bleach when sea-surface temperature rises about $1\,^\circ\text{C}$ above the usual maximum monthly mean for a sustained period. A reef near the Florida Keys has a maximum monthly mean of $29.0\,^\circ\text{C}$. Weekly average temperatures in late summer were $29.4$, $29.9$, $30.2$, and $30.4\,^\circ\text{C}$.

1. Which weeks reached or exceeded the bleaching threshold?

2. Explain what happens inside coral during bleaching and why the coral turns white.

3. Describe one effect of the loss of the coral on the reef ecosystem.

During one strong El Niño event, scientists report (illustrative values):

1. Calculate the temperature change at each location.

2. Explain the sea-level pattern in terms of weaker trade winds and the movement of warm water.

3. Predict how evaporation and rainfall change at each location and justify your reasoning.

4. Explain two ways warmer surface water at Peru could affect marine ecosystems, including coral.

A hypothetical table shows winter precipitation as a percent of the long-term average during a strong El Niño and a strong La Niña:

1. Describe the pattern shown by the data.

2. Use the position of the jet stream and the temperature of the equatorial Pacific to explain the differences between the two phases.

3. A water manager in Houston must plan for the coming winter and a strong La Niña is forecast. Write a short recommendation using the data, and state one limitation of using ENSO alone to predict local weather.