El Nino and La Nina - AP Environmental Science

1. In a given year, SSTs in the eastern equatorial Pacific are $-1.2^\circ\text{C}$ below average, easterly trade winds are stronger than normal, and the Walker circulation intensifies with enhanced upwelling along the South American coast. Which outcome is most likely?
   Answer: La Niña conditions leading to wetter-than-average Indonesia/northern Australia and a tendency toward drier southern U.S. El Niño involves warmer SSTs in the central and eastern Pacific with weakened trade winds, whereas La Niña features cooler SSTs and stronger trade winds, intensifying the Walker circulation. The ocean-atmosphere interaction in La Niña strengthens easterly winds, enhancing upwelling of cold water along South America and boosting convection over the western Pacific. This results in wetter conditions in Indonesia and northern Australia due to shifted rainfall patterns, and often drier weather in the southern U.S. The given observations of below-average SSTs, stronger winds, and enhanced upwelling indicate La Niña, aligning with choice C's description of the phase and its typical impacts. Choice C is correct because it properly identifies the likely outcomes, unlike the others which describe El Niño effects or mismatched conditions. Understanding these interactions helps predict regional weather anomalies during ENSO events.
2. During boreal winter, satellite and buoy data show sea-surface temperatures (SSTs) in the central and eastern equatorial Pacific are about $+1.5^{\circ}C$ 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?
   Answer: 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. 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.
3. 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?
   Answer: La Niña, because it is often associated with reduced Atlantic wind shear and increased hurricane formation 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.
4. 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?
   Answer: El Niño: weaker trade winds, reduced upwelling off Peru, and increased rainfall risk along coastal western South America 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.
5. 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?
   Answer: El Niño reduces nutrient-rich upwelling off Peru, lowering primary productivity and disrupting the food web that supports anchovies. 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.
6. A researcher compares two years: Year 1 has stronger-than-average easterly trade winds and cooler SSTs in the eastern equatorial Pacific; Year 2 has weaker trade winds and warmer SSTs in the eastern equatorial Pacific. Which pairing correctly matches each year to the expected rainfall pattern in the tropical Pacific?
   Answer: Year 1: increased rainfall near Indonesia; Year 2: increased rainfall near Peru and Ecuador La Niña and El Niño create opposite rainfall patterns across the tropical Pacific due to their contrasting effects on atmospheric convection. Year 1, with stronger trade winds and cooler eastern Pacific SSTs, represents La Niña conditions where enhanced trade winds pile warm water in the western Pacific and increase upwelling of cold water in the east. This concentrates atmospheric convection and heavy rainfall over the warm waters near Indonesia and the western Pacific. Year 2, with weaker trade winds and warmer eastern Pacific SSTs, represents El Niño conditions where reduced trade winds allow warm water to spread eastward and suppress upwelling off South America. This eastward shift of warm water causes convection and rainfall to follow, increasing precipitation near Peru and Ecuador while reducing it over Indonesia. The rainfall follows the warm water because tropical convection is primarily driven by sea surface temperatures, with rising air and precipitation occurring over the warmest waters.
7. 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?
   Answer: Weaker trade winds and increased winter precipitation across the southern United States compared with average. 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.
8. During an ENSO event, sea level pressure is higher than normal over the eastern tropical Pacific and lower than normal over the western tropical Pacific, with stronger easterly trade winds. What does this pressure pattern imply about SSTs and convection?
   Answer: La Niña: cooler SSTs in the east and convection is enhanced over the western Pacific/Indonesia. La Niña features higher pressure in the east and lower in the west, strengthening trade winds and the Walker circulation, leading to cooler eastern SSTs and western-enhanced convection. El Niño reverses this pressure pattern, warming the east and shifting convection eastward. The described pressure and wind pattern implies La Niña conditions. Neutral or El Niño would not show stronger easterly winds. Choice B correctly links this to cooler eastern SSTs and enhanced western convection.
9. A weather station network reports that the subtropical jet stream over the eastern Pacific and southern U.S. is stronger and displaced southward during winter. Concurrently, the central/eastern equatorial Pacific is warmer than average and trade winds are weaker. Which ENSO phase is indicated?
   Answer: El Niño El Niño warms the central/eastern Pacific, weakening trade winds and shifting the jet stream southward, strengthening it over the southern U.S., while La Niña cools the region with stronger winds. This interaction alters atmospheric circulation patterns. The described jet stream changes indicate El Niño. Choice B correctly names the phase. It matches the warmer SSTs and weaker winds, unlike La Niña descriptions. Jet stream shifts explain regional weather impacts.
10. A region in eastern Australia experiences above-average rainfall and flooding during a year when the eastern equatorial Pacific is cooler than average and the Walker circulation is stronger. Which ENSO phase and mechanism best explain this?
    Answer: La Niña; stronger trade winds enhance convection over the western Pacific, increasing rain over Australia La Niña cools the eastern Pacific with stronger trade winds, enhancing western Pacific convection and rainfall over Australia, while El Niño weakens this, causing drought. Ocean-atmosphere interactions strengthen the Walker circulation in La Niña, boosting western rainfall. Choice B correctly identifies La Niña and the mechanism. It explains the above-average Australian rain, unlike El Niño options. This shows regional teleconnections. Stronger winds enhance convection westward.
11. In the tropical Pacific, which condition most directly leads to reduced upwelling along the coast of South America during an El Niño event?
    Answer: Weakened easterly trade winds that reduce the westward push of surface waters and lessen the upwelling-favorable conditions near the eastern boundary. During El Niño, weakened easterly trade winds reduce the westward push of surface waters, diminishing upwelling along South America and warming SSTs. This contrasts with La Niña's stronger winds enhancing upwelling. The ocean-atmosphere feedback weakens the Walker circulation in El Niño. Stronger winds would increase upwelling, not reduce it. Choice B correctly identifies weakened trade winds as the direct cause of reduced upwelling in El Niño.
12. A coastal community in Peru experiences unusually warm coastal waters and increased rainfall/flooding during an ENSO event. Which atmospheric-oceanic configuration most likely produced these impacts?
    Answer: El Niño: weakened trade winds allow warm water to spread east, reducing upwelling and shifting convection toward the eastern Pacific. El Niño causes warm coastal waters and increased rainfall in Peru due to reduced upwelling and eastward-shifted convection, weakening the Walker circulation. La Niña enhances upwelling, cooling waters and drying the coast. The ocean-atmosphere interaction during El Niño spreads warm water east. This configuration matches the community's warm waters and flooding. Choice A best explains El Niño's weakened winds, reduced upwelling, and eastward convection shift.
13. In a particular winter, the southern tier of the United States experiences above-average rainfall, while Indonesia and northern Australia experience drought and increased wildfire risk. Pacific observations show weaker trade winds and warm SST anomalies in the eastern equatorial Pacific. Which ENSO phase best explains these patterns?
    Answer: El Niño El Niño features warmer eastern Pacific SSTs and weaker trade winds, leading to wetter southern U.S. and drier Indonesia/Australia, while La Niña has cooler SSTs and stronger winds, reversing these patterns. Ocean-atmosphere interactions in El Niño shift convection eastward, suppressing western Pacific rainfall and enhancing eastern Pacific activity. This explains the observed drought in Indonesia/Australia and wet southern U.S. Choice B correctly identifies El Niño as the phase. It fits the patterns, unlike La Niña or neutral options. Such teleconnections highlight ENSO's global reach.
14. 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?
    Answer: La Niña conditions, often associated with increased rainfall in Indonesia and Australia relative to average. 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.
15. A meteorology class compares two years: Year 1: Niño 3.4 SST anomaly $+1.0^\circ\text{C}$; weakened trade winds; convection shifted eastward. Year 2: Niño 3.4 SST anomaly $-1.0^\circ\text{C}$; strengthened trade winds; convection concentrated over the western Pacific. Which statement correctly identifies Year 1 and a likely winter impact in North America?
    Answer: Year 1 is El Niño; expect a tendency toward wetter conditions in the southern U.S. El Niño features positive SST anomalies and weakened trade winds, shifting convection eastward, while La Niña has negative anomalies and stronger winds, focusing convection westward. Ocean-atmosphere interactions in El Niño weaken the Walker circulation, influencing North American winter weather by shifting the jet stream southward for wetter southern U.S. conditions. Year 1's positive anomaly, weakened winds, and eastward convection indicate El Niño. Choice B correctly identifies Year 1 as El Niño and notes the wetter southern U.S. tendency. It matches, whereas others misidentify the phase or impacts like stronger hurricanes (more La Niña-like). This comparison aids in distinguishing ENSO phases.
16. 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?
    Answer: Warmer central/eastern Pacific SSTs with weaker easterly trade winds 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.
17. An ocean monitoring report finds SSTs in the eastern equatorial Pacific are 1.0°C below average, trade winds are stronger than normal, and the Walker circulation is intensified with rising air and thunderstorms concentrated over Indonesia. Which outcome is most consistent with these conditions?
    Answer: La Niña conditions; enhanced upwelling off South America and a tendency toward drier conditions in coastal Peru/Ecuador and wetter conditions in Indonesia. La Niña is defined by cooler-than-average SSTs in the eastern equatorial Pacific, contrasting with El Niño's warmer SSTs in the same area. The ocean-atmosphere interaction during La Niña involves stronger trade winds that enhance upwelling of cold, nutrient-rich water off South America and intensify the Walker circulation. This results in concentrated convection and heavy rainfall over Indonesia, leading to wetter conditions there and drier conditions along coastal Peru and Ecuador. In contrast, El Niño weakens these winds and shifts rainfall eastward. The given conditions of below-average SSTs, stronger winds, and intensified circulation align with La Niña and its typical impacts, confirming choice C as correct.
18. A drought develops in Indonesia and parts of Australia while unusually heavy rains occur along the west coast of South America. At the same time, the Walker circulation is weaker than average. Which Pacific Ocean condition most likely accompanies this pattern?
    Answer: Warmer-than-average SSTs in the eastern/central equatorial Pacific due to reduced upwelling (El Niño). El Niño features warmer-than-average SSTs in the eastern/central Pacific due to reduced upwelling, weakening the Walker circulation, unlike La Niña's cooler SSTs and stronger circulation. This shift causes drought in Indonesia/Australia and heavy rains in South America by moving convection eastward. La Niña would produce the opposite precipitation patterns. The described drought in the west and rains in the east, with weakened circulation, align with El Niño's warm SST pattern. Choice B correctly identifies this ocean condition and its link to reduced upwelling.
19. A set of observations includes: (1) warmer-than-average SSTs in the eastern equatorial Pacific, (2) reduced sea-level pressure over the eastern Pacific relative to the west, and (3) weakened trade winds. Which change in precipitation is most likely?
    Answer: Increased rainfall over the central/eastern equatorial Pacific and decreased rainfall over Indonesia El Niño warms the eastern Pacific, weakening trade winds and reducing pressure gradients, shifting convection eastward, while La Niña cools it with opposite effects. This interaction increases rainfall over the central/eastern Pacific and decreases it over Indonesia during El Niño. The observations indicate El Niño conditions. Choice A correctly predicts the precipitation change. It works due to the eastward convection shift, unlike choices suggesting the reverse or no shift. Precipitation patterns are key ENSO indicators.
20. An APES student is asked to connect ENSO to ocean productivity. During which ENSO phase is primary productivity along the Peruvian coast most likely to decrease, and why?
    Answer: El Niño, because weakened trade winds reduce upwelling of nutrient-rich water El Niño warms the eastern Pacific by weakening trade winds, reducing upwelling and nutrient availability, while La Niña enhances upwelling with stronger winds, boosting productivity. The ocean-atmosphere interaction in El Niño deepens the thermocline, limiting nutrient-rich water. This decreases primary productivity off Peru. Choice A correctly identifies El Niño and the reason. It fits because weaker winds directly reduce upwelling, unlike La Niña options. This connects ENSO to marine ecosystems.

Start this quiz