AICE Marine Science Practice Exam

During an El Niño event, one of the defining characteristics is the reversal of pressure gradients in the western Pacific Ocean. Typically, trade winds blow from east to west, causing warm surface waters to accumulate in the western Pacific, while cooler, nutrient-rich waters rise in the eastern Pacific due to upwelling. However, during an El Niño, these trade winds weaken or even reverse, resulting in a significant change in sea surface temperatures and weather patterns across the globe.

The pressure changes associated with El Niño influence ocean currents and can lead to decreased upwelling in the eastern Pacific, which normally supports significant marine life due to the influx of nutrients. This alteration can cause a range of climatic effects, including changes in rainfall patterns, increased storm activity, and impacts on marine ecosystems.

The other options represent aspects of normal ocean conditions or processes. Warm water is indeed pushed to the eastern Pacific during an El Niño, but this phenomenon is fundamentally linked to the reversal of pressure gradients and weakening of trade winds. Upwelling of nutrients is usually diminished during these events, and trade winds do not consistently blow from east to west—they become weaker. Therefore, the correct answer accurately captures the significant atmospheric changes that occur during an El Niño event.