El Nino and La Nina
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AP Environmental Science › El Nino and La Nina
A fisheries manager in Peru reports a sharp decline in anchovy populations coinciding with reduced nutrient upwelling and unusually warm coastal waters in the eastern Pacific. At the same time, trade winds across the equatorial Pacific are weaker than average. Which ENSO phase is most likely occurring?
La Niña, because stronger trade winds reduce upwelling and warm the eastern Pacific
Neutral ENSO, because upwelling changes are unrelated to trade winds
El Niño, because weaker trade winds reduce upwelling and warm the eastern Pacific
La Niña, because weaker trade winds enhance upwelling and cool the eastern Pacific
Explanation
El Niño occurs when SSTs warm in the eastern Pacific due to weakened trade winds, reducing upwelling, while La Niña cools the region with stronger winds and increased upwelling. The ocean-atmosphere interaction in El Niño deepens the thermocline in the east, limiting nutrient-rich cold water from surfacing, which harms fisheries like anchovies off Peru. This leads to declines in marine productivity and warmer coastal waters. Choice A correctly identifies El Niño and explains the mechanism of weaker winds reducing upwelling. It fits the observations, unlike choices suggesting La Niña or no relation to winds. Recognizing these links is key for managing fisheries during ENSO.
A climatology student notes that during an ENSO event, warm surface water shifts eastward across the equatorial Pacific, convection increases over the central/eastern Pacific, and the trade winds weaken. Which mechanistic explanation best links these observations to the Walker circulation?
Warm SSTs in the east increase the pressure gradient, accelerating trade winds and intensifying the Walker circulation
Stronger trade winds deepen the thermocline in the east, strengthening the Walker circulation and shifting convection westward
Weaker trade winds reduce the east–west pressure gradient, weakening the Walker circulation and allowing convection to shift eastward
Weaker trade winds increase upwelling off Peru, cooling the eastern Pacific and strengthening the Walker circulation
Explanation
El Niño is defined by warmer SSTs in the eastern Pacific and weakened trade winds, disrupting the normal Walker circulation, while La Niña has cooler SSTs and stronger winds, reinforcing it. The ocean-atmosphere coupling in El Niño reduces the east-west pressure gradient, allowing warm water and convection to shift eastward, which weakens the overall circulation. This mechanistic link explains the observed warm water movement and increased central/eastern Pacific convection. Choice B correctly captures this by noting weaker trade winds' role in diminishing the pressure gradient and Walker circulation. It best links the observations, as the other choices incorrectly suggest stronger winds or increased upwelling during such events. This explanation highlights how feedback loops amplify ENSO phases.
A fisheries scientist notes that anchovy populations off Peru often decline sharply during certain years. In those same years, SSTs in the eastern equatorial Pacific are above average and the Walker circulation is weakened. Which explanation best links these observations?
Neutral ENSO eliminates coastal currents, preventing anchovies from migrating and causing population collapse.
El Niño strengthens trade winds, increasing upwelling and causing anchovy declines due to excessive nutrients.
El Niño reduces nutrient-rich upwelling off Peru, lowering primary productivity and disrupting the food web that supports anchovies.
La Niña reduces upwelling off Peru by weakening trade winds, decreasing nutrients and reducing anchovy populations.
Explanation
El Niño is marked by warmer eastern Pacific SSTs and a weakened Walker circulation, reducing upwelling and nutrient supply, while La Niña enhances upwelling with cooler SSTs. This interaction affects marine ecosystems, as upwelling brings nutrients that support phytoplankton and fish like anchovies. During El Niño, diminished upwelling leads to lower primary productivity, disrupting the food web and causing anchovy populations to decline. The observations of above-average SSTs and weakened circulation point to El Niño as the cause. Choice A best links these by explaining the reduced upwelling and its ecological impact.
Which sequence best describes the causal chain during La Niña conditions in the equatorial Pacific?
Stronger trade winds → enhanced upwelling off Peru → cooler eastern Pacific SSTs → strengthened Walker circulation with convection focused in the west.
Weaker trade winds → enhanced upwelling off Peru → cooler eastern Pacific SSTs → increased rainfall along coastal Peru and Ecuador.
Stronger trade winds → reduced upwelling off Peru → warmer eastern Pacific SSTs → convection shifts eastward.
Weaker trade winds → reduced upwelling off Peru → warmer eastern Pacific SSTs → weakened Walker circulation.
Explanation
La Niña's causal chain starts with stronger trade winds enhancing upwelling, cooling eastern Pacific SSTs, and strengthening the Walker circulation with western-focused convection. This contrasts with El Niño's weaker winds and warming. The sequence reinforces the ocean-atmosphere feedback loop. El Niño sequences involve reduced upwelling and eastward convection. Choice B accurately outlines La Niña's chain, including cooler SSTs and western convection.
Meteorological data show enhanced convection over the western Pacific near Indonesia, stronger-than-normal easterly trade winds, and cooler SSTs in the central/eastern equatorial Pacific. Which set of impacts is most consistent with these conditions?
El Niño; stronger upwelling off Peru and increased Atlantic hurricane activity.
La Niña; weaker trade winds reduce upwelling, warming the eastern Pacific and shifting convection eastward.
La Niña; increased rainfall in Indonesia/Australia and a tendency toward drier conditions along the west coast of South America.
El Niño; increased rainfall in coastal Peru/Ecuador and decreased rainfall in Indonesia and Australia.
Explanation
La Niña involves cooler SSTs in the central/eastern Pacific and stronger trade winds, intensifying the Walker circulation, whereas El Niño has warmer SSTs and weaker winds. This strengthens convection over the western Pacific, leading to increased rainfall in Indonesia and Australia, and drier conditions in South America due to enhanced upwelling. In contrast, El Niño shifts rainfall eastward. The data showing enhanced western convection, stronger winds, and cooler SSTs are consistent with La Niña and its precipitation impacts. Choice B correctly identifies these impacts, including wetter Indonesia/Australia and drier western South America.
A forecaster sees an ENSO advisory indicating cooler-than-average SSTs in the eastern Pacific and stronger trade winds. Which change in the Walker circulation is expected relative to neutral conditions?
Walker circulation weakens, with rising motion shifting eastward into the central Pacific.
Walker circulation reverses direction, with surface winds blowing from east to west replaced by west to east in all years.
Walker circulation stops, eliminating convection across the tropical Pacific.
Walker circulation strengthens, with enhanced rising motion over the western Pacific and stronger subsidence over the eastern Pacific.
Explanation
La Niña features cooler eastern Pacific SSTs and stronger trade winds, strengthening the Walker circulation, while El Niño weakens it with warmer SSTs. The Walker circulation involves rising air over the warm western Pacific and sinking in the east, driven by the SST gradient. Cooler SSTs and stronger winds during La Niña enhance this circulation, increasing rising motion in the west and subsidence in the east. El Niño disrupts this pattern. Choice B correctly expects a strengthened Walker circulation under the given La Niña-like conditions.
A report notes that during a particular ENSO phase, the eastern Pacific thermocline becomes shallower than average, making it easier for cold water to reach the surface, and the Walker circulation strengthens. Which phase is described?
El Niño, because stronger trade winds and a strengthened Walker circulation are defining features of El Niño.
La Niña, because a shallower eastern thermocline supports enhanced upwelling and cooler SSTs.
El Niño, because a shallower eastern thermocline reduces upwelling and warms SSTs.
Neutral, because thermocline depth and Walker circulation are unrelated.
Explanation
La Niña involves a shallower eastern thermocline, enhancing upwelling and cooling SSTs, with a strengthened Walker circulation. El Niño deepens the thermocline, reducing upwelling. The described shallower thermocline and strong circulation indicate La Niña. Neutral lacks these changes. Choice B accurately describes La Niña's shallower thermocline supporting enhanced upwelling and cooler SSTs.
A climate summary states: “The equatorial Pacific shows persistent cool anomalies, strong trade winds, and a westward-shifted warm pool; thunderstorms are concentrated over the western Pacific.” Which impact is most consistent with this summary?
El Niño conditions, often associated with increased rainfall in Indonesia and Australia relative to average.
El Niño conditions, often associated with stronger upwelling and cooler coastal waters off Peru.
Neutral conditions, because trade wind anomalies do not occur during ENSO phases.
La Niña conditions, often associated with increased rainfall in Indonesia and Australia relative to average.
Explanation
La Niña is marked by cool Pacific anomalies, strong trade winds, and a westward-shifted warm pool, enhancing convection and rainfall in Indonesia and Australia. El Niño shifts convection eastward, often drying those regions. The summary describes La Niña conditions. Neutral lacks such anomalies. Choice A is consistent, linking La Niña to increased western rainfall.
An ENSO monitoring report shows that the SST anomaly in the Niño 3.4 region rises from $0.0^{\circ}C$ to $+1.0^{\circ}C$ over several months. At the same time, the pressure difference between Tahiti and Darwin decreases, indicating a weakening Walker circulation. Which set of impacts is most consistent with these observations?
El Niño: weaker trade winds, reduced upwelling off Peru, and increased rainfall risk along coastal western South America
El Niño: stronger trade winds, enhanced upwelling off Peru, and drier conditions along coastal Peru
La Niña: weaker trade winds, reduced upwelling off Peru, and increased rainfall in the central/eastern Pacific
La Niña: stronger trade winds, enhanced upwelling off Peru, and increased rainfall over Indonesia
Explanation
El Niño development is indicated by warming SSTs in the Niño 3.4 region (central equatorial Pacific) exceeding +0.5°C for several months, with the +1.0°C anomaly representing moderate El Niño conditions. The decreasing pressure difference between Tahiti and Darwin indicates a weakening Walker circulation, as the normal east-west pressure gradient diminishes. During El Niño, trade winds weaken because the reduced pressure gradient provides less driving force for easterly flow. Weaker trade winds reduce offshore Ekman transport along the South American coast, suppressing the upwelling that normally brings cold, nutrient-rich water to the surface off Peru. The combination of reduced upwelling and eastward spreading warm water increases SSTs in the eastern Pacific. As warm water and convection shift eastward, areas that normally experience dry conditions, like coastal Peru and Ecuador, face increased rainfall risk from the displaced convection. This represents a complete reversal of normal conditions where these regions are typically dry due to cold SSTs and subsiding air.
In a year with El Niño conditions, which change is most likely in the tropical Pacific compared with neutral years?
A deeper thermocline in the eastern Pacific and reduced nutrient delivery to surface waters near Peru.
Convection intensifies over Indonesia because warm water piles up farther west than normal.
Enhanced upwelling in the eastern Pacific due to stronger easterly trade winds.
Colder SSTs in the central/eastern Pacific and stronger Walker circulation.
Explanation
El Niño involves weakened trade winds, leading to a deeper eastern thermocline and reduced upwelling, warming SSTs, while La Niña has stronger winds and shallower thermocline. This ocean-atmosphere interaction alters nutrient delivery to surface waters. In El Niño, the deeper thermocline means less nutrient-rich cold water reaches the surface near Peru, affecting fisheries. La Niña enhances upwelling and nutrients. Choice B correctly identifies this change as typical of El Niño compared to neutral conditions.