During an El Niño event, what happens to the trade winds and the distribution of warm water in the equatorial Pacific?
ATrade winds strengthen, pushing warm water further west and cooling the eastern Pacific
BTrade winds weaken, allowing warm water to slosh eastward and suppressing upwelling along South America
CTrade winds reverse direction, driving warm water from west to east at the surface
DTrade winds intensify, raising sea surface temperatures uniformly across the Pacific
Under normal conditions, easterly trade winds pile warm water in the western Pacific and drive upwelling of cold deep water along South America's coast. El Niño begins when those trade winds weaken — warm water spreads eastward, the thermocline flattens, and upwelling is suppressed. This is the positive phase of the ENSO cycle.
Question 2 True / False
The thermohaline circulation is driven primarily by wind stress at the ocean surface, just like surface ocean currents.
TTrue
FFalse
Answer: False
The thermohaline circulation (or overturning circulation) is driven by density differences, not wind. In the North Atlantic, warm surface water releases heat to the atmosphere, becomes saltier and denser, and sinks to form North Atlantic Deep Water. This density-driven sinking pulls the surface current northward. Wind drives the shallow, fast surface gyres; density drives the slow, deep overturning circulation.
Question 3 Short Answer
Peru experiences flooding during El Niño years, while Australia typically experiences drought. Explain why the same El Niño event produces opposite precipitation anomalies in these two regions.
Think about your answer, then reveal below.
Model answer: El Niño warms sea surface temperatures in the eastern Pacific near Peru. Warm, moist air rises over the warm water, fueling convection and heavy rainfall over coastal South America. Simultaneously, the western Pacific cools relative to normal — the warm pool retreats east — so atmospheric convection over Australia weakens, bringing drier-than-normal conditions.
Atmospheric convection follows warm sea surface temperatures. In the normal state, the western Pacific warm pool drives rainfall over Australia and Southeast Asia. During El Niño, that warm pool shifts east toward Peru, taking the rainfall with it. This 'teleconnection' — the transmission of a climate signal across thousands of kilometers via atmospheric circulation — is why one ENSO event can simultaneously flood one continent and drought another.