AP Environmental Science Quiz: El Nino And La Nina
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El Nino And La NinaQuestion 1 of 20

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?

El Niño, because weaker trade winds reduce upwelling and warm the eastern Pacific
La Niña, because stronger trade winds reduce upwelling and warm the eastern Pacific
La Niña, because weaker trade winds enhance upwelling and cool the eastern Pacific
Neutral ENSO, because upwelling changes are unrelated to trade winds
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AP Environmental Science Quiz

AP Environmental Science Quiz: El Nino And La Nina

Practice El Nino And La Nina in AP Environmental Science with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

What this quiz covers

This quiz focuses on El Nino And La Nina, giving you a quick way to practice the rules, question types, and explanations that matter most for AP Environmental Science.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

All questions

Question 1

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?

  1. El Niño, because weaker trade winds reduce upwelling and warm the eastern Pacific (correct answer)
  2. La Niña, because stronger trade winds reduce upwelling and warm the eastern Pacific
  3. La Niña, because weaker trade winds enhance upwelling and cool the eastern Pacific
  4. Neutral ENSO, because upwelling changes are unrelated to trade winds

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.

Question 2

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?

  1. Stronger trade winds deepen the thermocline in the east, strengthening the Walker circulation and shifting convection westward
  2. Weaker trade winds reduce the east–west pressure gradient, weakening the Walker circulation and allowing convection to shift eastward (correct answer)
  3. Weaker trade winds increase upwelling off Peru, cooling the eastern Pacific and strengthening the Walker circulation
  4. Warm SSTs in the east increase the pressure gradient, accelerating trade winds and intensifying 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.

Question 3

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?

  1. El Niño reduces nutrient-rich upwelling off Peru, lowering primary productivity and disrupting the food web that supports anchovies. (correct answer)
  2. La Niña reduces upwelling off Peru by weakening trade winds, decreasing nutrients and reducing anchovy populations.
  3. El Niño strengthens trade winds, increasing upwelling and causing anchovy declines due to excessive nutrients.
  4. Neutral ENSO eliminates coastal currents, preventing anchovies from migrating and causing population collapse.

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.

Question 4

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?

  1. El Niño; increased rainfall in coastal Peru/Ecuador and decreased rainfall in Indonesia and Australia.
  2. La Niña; increased rainfall in Indonesia/Australia and a tendency toward drier conditions along the west coast of South America. (correct answer)
  3. El Niño; stronger upwelling off Peru and increased Atlantic hurricane activity.
  4. La Niña; weaker trade winds reduce upwelling, warming the eastern Pacific and shifting convection eastward.

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.

Question 5

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?

  1. Walker circulation weakens, with rising motion shifting eastward into the central Pacific.
  2. Walker circulation strengthens, with enhanced rising motion over the western Pacific and stronger subsidence over the eastern Pacific. (correct answer)
  3. Walker circulation reverses direction, with surface winds blowing from east to west replaced by west to east in all years.
  4. Walker circulation stops, eliminating convection across the tropical 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.

Question 6

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?

  1. El Niño, because a shallower eastern thermocline reduces upwelling and warms SSTs.
  2. La Niña, because a shallower eastern thermocline supports enhanced upwelling and cooler SSTs. (correct answer)
  3. Neutral, because thermocline depth and Walker circulation are unrelated.
  4. El Niño, because stronger trade winds and a strengthened Walker circulation are defining features of El Niño.

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.

Question 7

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?

  1. La Niña conditions, often associated with increased rainfall in Indonesia and Australia relative to average. (correct answer)
  2. El Niño conditions, often associated with increased rainfall in Indonesia and Australia relative to average.
  3. El Niño conditions, often associated with stronger upwelling and cooler coastal waters off Peru.
  4. Neutral conditions, because trade wind anomalies do not occur during ENSO phases.

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.

Question 8

An ENSO monitoring report shows that the SST anomaly in the Niño 3.4 region rises from 0.0∘C0.0^{\circ}C to +1.0∘C+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?

  1. La Niña: stronger trade winds, enhanced upwelling off Peru, and increased rainfall over Indonesia
  2. El Niño: weaker trade winds, reduced upwelling off Peru, and increased rainfall risk along coastal western South America (correct answer)
  3. El Niño: stronger trade winds, enhanced upwelling off Peru, and drier conditions along coastal Peru
  4. La Niña: weaker trade winds, reduced upwelling off Peru, and increased rainfall in the central/eastern Pacific

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.

Question 9

A weather station network reports that the southern United States experienced a wetter-than-average winter, while parts of Indonesia experienced reduced rainfall. Concurrently, SSTs in the eastern Pacific were above average and trade winds were weaker. Which ENSO phase best explains the observations?

  1. La Niña, because stronger trade winds shift convection eastward toward South America.
  2. El Niño, because warmer eastern Pacific waters shift convection eastward and alter jet stream patterns affecting North America. (correct answer)
  3. La Niña, because cooler eastern Pacific waters reduce the east–west pressure gradient and weaken the Walker circulation.
  4. Neutral ENSO, because global teleconnections do not occur when SSTs are anomalously warm.

Explanation: El Niño features warmer eastern Pacific SSTs and weaker trade winds, shifting convection eastward and influencing global weather via jet stream changes, unlike La Niña's westward shift. This often brings wetter winters to the southern U.S. and drier conditions in Indonesia. The reported wetter southern U.S. and drier Indonesia, with warm SSTs and weak winds, indicate El Niño. La Niña would likely cause drier southern U.S. Choice B correctly attributes these to El Niño's eastward convection shift and teleconnections.

Question 10

A student claims: "When the central/eastern Pacific warms, the trade winds strengthen and upwelling increases off South America." Which correction best reflects ENSO dynamics and the Walker circulation?

  1. Warming in the central/eastern Pacific typically weakens the trade winds, weakens the Walker circulation, and reduces upwelling off South America (correct answer)
  2. Warming in the central/eastern Pacific typically strengthens the trade winds, strengthens the Walker circulation, and increases upwelling off South America
  3. Warming in the central/eastern Pacific has no effect on trade winds or upwelling because the Walker circulation is controlled only by the Atlantic
  4. Warming in the central/eastern Pacific typically strengthens the trade winds but weakens the Walker circulation, increasing upwelling

Explanation: El Niño warms the central/eastern Pacific, weakening trade winds and the Walker circulation, reducing upwelling, while La Niña cools it with stronger winds and enhanced circulation. The student's claim incorrectly states stronger winds and increased upwelling during warming, which opposes El Niño dynamics. Correctly, warming weakens winds and reduces upwelling. Choice A provides the accurate correction, reflecting true ENSO interactions. It works because it aligns with observed feedback loops, unlike the mismatched options. This correction emphasizes proper understanding of circulation changes.

Question 11

A researcher observes that convection and thunderstorms have shifted from the western Pacific toward the central/eastern equatorial Pacific. The Walker circulation is weaker than normal. Which SST and wind pattern should accompany this shift?

  1. Cooler central/eastern Pacific SSTs with stronger easterly trade winds
  2. Warmer central/eastern Pacific SSTs with weaker easterly trade winds (correct answer)
  3. Cooler central/eastern Pacific SSTs with weaker easterly trade winds
  4. Warmer central/eastern Pacific SSTs with stronger easterly trade winds

Explanation: El Niño shifts convection eastward with warmer central/eastern SSTs and weaker trade winds, weakening the Walker circulation, while La Niña keeps it westward with cooler SSTs and stronger winds. This interaction drives thunderstorm patterns. Choice B accompanies the observed shift. It correctly pairs warmer SSTs and weaker winds, unlike others. Convection shifts are visible in satellite data. Walker circulation strength modulates these changes.

Question 12

A hurricane researcher notes that during a certain summer, Atlantic tropical cyclone formation is higher than average. At the same time, the eastern equatorial Pacific is cooler than normal and the trade winds are stronger. Which ENSO phase is most consistent with these observations?

  1. El Niño, because it reduces Atlantic wind shear and increases hurricane formation
  2. La Niña, because it is often associated with reduced Atlantic wind shear and increased hurricane formation (correct answer)
  3. El Niño, because it strengthens the Walker circulation and cools the eastern Pacific
  4. Neutral, because ENSO only affects the Pacific basin

Explanation: La Niña cools the eastern Pacific with stronger trade winds, reducing Atlantic wind shear and increasing hurricanes, while El Niño warms it, increasing shear and suppressing hurricanes. The ocean-atmosphere link in La Niña strengthens the Walker circulation, influencing distant Atlantic conditions. Higher cyclone formation aligns with La Niña's reduced shear. Choice B correctly states La Niña and the mechanism. It matches observations, unlike El Niño or neutral choices. This shows ENSO's impact on hurricane seasons.

Question 13

A climate report states: "The east–west sea-level pressure gradient across the equatorial Pacific has increased, and surface easterlies have strengthened, reinforcing rising air over the western Pacific and sinking air over the eastern Pacific." Which ENSO phase does this describe, and what SST pattern is expected?

  1. El Niño; warmer-than-average SSTs in the eastern equatorial Pacific
  2. La Niña; cooler-than-average SSTs in the eastern equatorial Pacific (correct answer)
  3. El Niño; cooler-than-average SSTs in the eastern equatorial Pacific
  4. La Niña; warmer-than-average SSTs in the eastern equatorial Pacific

Explanation: La Niña involves cooler SSTs in the eastern Pacific and strengthened trade winds, enhancing the Walker circulation with rising air in the west and sinking in the east, while El Niño warms the region and weakens the circulation. The increased pressure gradient in La Niña reinforces surface easterlies and the circulation pattern. This leads to expected cooler eastern Pacific SSTs. Choice B accurately describes La Niña and the SST pattern. It matches the report, whereas others reverse the phase or SSTs. This reinforces how pressure gradients drive ENSO.

Question 14

During boreal winter, satellite and buoy data show sea-surface temperatures (SSTs) in the central and eastern equatorial Pacific are about +1.5∘C+1.5^{\circ}C above average. Surface observations also indicate weakened easterly trade winds and a reduced east–west (Walker) circulation, with convection and heavy rainfall shifting from Indonesia toward the central Pacific. Which set of global weather impacts is most consistent with these conditions?

  1. La Niña conditions; stronger trade winds enhance upwelling off South America, causing wetter-than-average conditions in Indonesia and drier conditions along the west coast of South America.
  2. El Niño conditions; increased upwelling off Peru intensifies the cold tongue, leading to cooler eastern Pacific SSTs and reduced storminess in the southern United States.
  3. El Niño conditions; warmer eastern Pacific and weakened Walker circulation tend to bring wetter conditions to coastal Peru/Ecuador and a shifted jet stream that often increases winter precipitation in the southern United States. (correct answer)
  4. Neutral ENSO conditions; Pacific SST anomalies are small, so global precipitation and jet stream patterns remain near climatological averages with no systematic shifts.

Explanation: El Niño and La Niña are phases of the El Niño-Southern Oscillation (ENSO) cycle, which involves interactions between the ocean and atmosphere in the tropical Pacific. El Niño occurs when sea-surface temperatures (SSTs) in the central and eastern equatorial Pacific become warmer than average, typically by +0.5°C or more, leading to weakened easterly trade winds and reduced upwelling of cold water. In contrast, La Niña features cooler-than-average SSTs in the same region, with strengthened trade winds and enhanced upwelling. The ocean-atmosphere interaction is key: during El Niño, the weakened Walker circulation shifts convection and rainfall eastward from Indonesia toward the central Pacific, altering global weather patterns. The described conditions—+1.5°C SST anomalies, weakened trade winds, and shifted convection—clearly indicate an El Niño event. This matches choice C, as El Niño often brings wetter conditions to coastal Peru and Ecuador due to reduced upwelling and warmer waters, while the shifted jet stream increases winter precipitation in the southern United States. Understanding these patterns helps predict impacts like flooding or droughts in affected regions.

Question 15

In boreal winter, observations show a strengthened Walker circulation and unusually strong easterly trade winds. SST anomalies are negative in the Niño 3.4 region. Which paired impacts are most consistent with this ENSO phase?

  1. Wetter southern United States and fewer Atlantic hurricanes
  2. Drier Indonesia and increased rainfall along coastal Peru
  3. Wetter Indonesia/northern Australia and increased Atlantic hurricane activity (correct answer)
  4. Suppressed upwelling off South America and warmer eastern Pacific fisheries

Explanation: La Niña is characterized by cooler-than-average SSTs in the Niño 3.4 region and strengthened trade winds, enhancing the Walker circulation, whereas El Niño features warmer SSTs and weaker winds. The interaction between ocean and atmosphere in La Niña intensifies upwelling and concentrates convection in the western Pacific, leading to wetter Indonesia and northern Australia. It also tends to increase Atlantic hurricane activity due to reduced wind shear. The described conditions match La Niña, making choice C correct with its paired impacts of wetter western Pacific regions and more hurricanes. Other choices describe El Niño effects like drier Indonesia or suppressed upwelling. This illustrates how ENSO teleconnections affect distant weather patterns.

Question 16

In a particular year, the central/eastern equatorial Pacific is anomalously warm and the trade winds weaken. Which teleconnection is commonly associated with these conditions during Northern Hemisphere winter?

  1. Reduced vertical wind shear over the tropical Atlantic, typically increasing Atlantic hurricane frequency.
  2. A tendency for wetter conditions in the southern United States and drier conditions in parts of Australia/Indonesia. (correct answer)
  3. Colder-than-average coastal waters off Peru due to enhanced upwelling and stronger trade winds.
  4. Increased convection over Indonesia because warm water piles up farther west than normal.

Explanation: El Niño brings warm SSTs in the central/eastern Pacific and weakened trade winds, differing from La Niña's cooling and stronger winds. This alters atmospheric circulation, influencing remote weather via teleconnections like jet stream shifts. During Northern Hemisphere winter, El Niño often causes wetter conditions in the southern U.S. and drier in Australia/Indonesia due to eastward-shifted convection. It typically suppresses Atlantic hurricanes by increasing wind shear, unlike La Niña. Choice B is correct as it describes a common El Niño teleconnection matching the warm SSTs and weakened winds.

Question 17

In an ENSO event, observations show unusually warm SSTs near the coast of Peru and Ecuador, reduced coastal upwelling, and convection shifting toward the central/eastern Pacific. Which additional pattern is most likely during Northern Hemisphere winter?

  1. Weaker trade winds and increased winter precipitation across the southern United States compared with average. (correct answer)
  2. Stronger trade winds and increased Atlantic hurricane activity due to reduced vertical wind shear over the Atlantic.
  3. Stronger trade winds and drier-than-average conditions along coastal Peru due to intensified upwelling.
  4. No change in Walker circulation because SST anomalies near South America do not affect atmospheric circulation.

Explanation: El Niño occurs when warm SSTs develop in the eastern Pacific, weakening trade winds and reducing upwelling, unlike La Niña's cooling and stronger winds. This ocean-atmosphere coupling shifts convection eastward, altering global weather patterns through teleconnections. During Northern Hemisphere winter, El Niño often brings increased precipitation to the southern U.S. due to a stronger subtropical jet stream. The described warm SSTs and reduced upwelling indicate El Niño, making increased winter precipitation in the southern U.S. a likely additional pattern. Choice A is correct as it matches these expected atmospheric and weather impacts.

Question 18

A student claims that when the Walker circulation weakens, the eastern equatorial Pacific tends to cool because upwelling strengthens. In a year when the Walker circulation is observed to weaken and the thermocline deepens in the eastern Pacific, which statement best corrects the student and explains the mechanism?

  1. The student is correct; weaker Walker circulation strengthens trade winds, enhancing upwelling and cooling the eastern Pacific.
  2. The student is incorrect; weaker Walker circulation is associated with weaker trade winds, reduced upwelling, a deeper eastern thermocline, and warmer SSTs (El Niño). (correct answer)
  3. The student is incorrect; weaker Walker circulation is caused by colder SSTs in the east, which increases upwelling and strengthens the trades (La Niña).
  4. The student is correct; a deeper thermocline in the east always corresponds to colder SSTs because cold water is closer to the surface.

Explanation: El Niño features warmer SSTs in the eastern Pacific due to weakened trade winds and reduced upwelling, while La Niña has cooler SSTs from stronger winds and enhanced upwelling. The Walker circulation, an east-west atmospheric loop, weakens during El Niño as the temperature gradient across the Pacific diminishes. A deeper thermocline in the east during El Niño means less cold water is upwelled, leading to warmer SSTs. The student's claim is incorrect because a weaker Walker circulation corresponds to weaker winds and warmer SSTs, not cooling. Choice B correctly identifies this error and explains the mechanism involving trade winds, upwelling, and thermocline depth.

Question 19

A climate model output shows persistent positive SST anomalies in the Niño 3.4 region and a reduction in the east–west sea-level pressure gradient across the tropical Pacific. What change in surface winds is most likely, and what ENSO phase is this?

  1. Stronger easterly trade winds; La Niña
  2. Weaker easterly trade winds; El Niño (correct answer)
  3. Weaker westerly trade winds; La Niña
  4. Stronger westerly winds across the Atlantic; El Niño

Explanation: El Niño features positive SST anomalies and weaker trade winds, reducing pressure gradients, while La Niña has negative anomalies and stronger winds. This interaction slackens the circulation in El Niño. Choice B correctly pairs weaker winds with El Niño. It fits the model output, unlike stronger wind options. Pressure gradients drive wind changes. This confirms ENSO phase identification.

Question 20

During an ENSO phase, the western Pacific experiences lower-than-average sea level while the eastern Pacific experiences higher-than-average sea level, reflecting an eastward shift of warm surface water. Trade winds are weaker. Which ENSO phase is indicated and what happens to upwelling near Peru?

  1. La Niña; upwelling increases near Peru
  2. El Niño; upwelling decreases near Peru (correct answer)
  3. La Niña; upwelling decreases near Peru
  4. El Niño; upwelling increases near Peru

Explanation: El Niño features weaker trade winds, shifting warm water east, raising eastern sea levels, and reducing upwelling near Peru, while La Niña strengthens winds and reverses this. This interaction alters sea level and productivity. Choice B indicates El Niño and decreased upwelling. It fits the description, unlike reversed options. Sea level changes reflect water distribution. Upwelling impacts fisheries directly.