- An oceanographic research vessel in the equatorial Pacific records the following data: easterly trade winds are significantly stronger than average, the sea level is anomalously high near Indonesia and low near Peru, and the thermocline is much shallower than normal off the coast of Ecuador. Which conclusion is most supported by these data?
Answer: A strong La Niña event is in progress.
All the provided data points are classic indicators of a strong La Niña event. La Niña is an intensification of the normal Pacific conditions. This includes: stronger-than-average easterly trade winds; a 'piling up' of warm water in the western Pacific, leading to higher sea level there and lower sea level in the east; and enhanced upwelling in the east, which brings the thermocline closer to the surface. Since all indicators are consistent, the conclusion of a strong La Niña is well-supported.
- Which statement best describes the positive feedback loop that sustains and amplifies El Niño conditions in the tropical Pacific Ocean?
Answer: Weaker trade winds reduce upwelling in the eastern Pacific, which warms the surface water, further reducing the east-west pressure gradient and weakening the trade winds.
The correct answer describes the Bjerknes feedback loop for El Niño. It starts with weakened trade winds. This reduces the westward push of warm water and diminishes upwelling of cold water in the east. The eastern Pacific warms. This warming reduces the temperature and pressure difference across the Pacific, which in turn further weakens the trade winds, amplifying the initial change.
- While sustained by a powerful ocean-atmosphere feedback loop, the onset of an El Niño event is often preceded by specific oceanic triggers. Which of the following is considered a key initiator in the transition from neutral conditions to El Niño?
Answer: The propagation of an oceanic Kelvin wave eastward along the equator, which deepens the thermocline in the eastern Pacific.
The onset of an El Niño is often triggered by the eastward propagation of a 'downwelling' oceanic Kelvin wave. This wave is a bulge of warm water that travels along the thermocline. When it reaches the coast of South America, it pushes the thermocline deeper, suppressing upwelling and initiating the surface warming that kicks off the Bjerknes feedback loop. The other options describe effects of ENSO or triggers for La Niña.
- A meteorologist is developing a seasonal forecast for the winter following the confirmation of a strong El Niño event. Which prediction for North America is most consistent with established ENSO teleconnections?
Answer: Cooler and wetter than average conditions for the southeastern United States and warmer, drier conditions for the Pacific Northwest.
El Niño typically alters the path of the subtropical jet stream over the Pacific, steering it further south over the U.S. This brings increased storm activity and moisture to the southern tier of the U.S., including the Southeast, resulting in cooler and wetter winters. Conversely, the Pacific Northwest often experiences warmer and drier conditions as the polar jet stream is shifted northward.
- A climatologist observes that the Southern Oscillation Index (SOI) has been strongly negative for several consecutive months. Which set of conditions is most likely occurring as a result?
Answer: Unusually low atmospheric pressure over Tahiti and unusually high pressure over Darwin, Australia, leading to reduced upwelling of cold, nutrient-rich water off the coast of South America.
A negative Southern Oscillation Index (SOI) corresponds to El Niño conditions. This occurs when atmospheric pressure is lower than normal over Tahiti (eastern Pacific) and higher than normal over Darwin, Australia (western Pacific). This pressure pattern weakens the easterly trade winds, which in turn reduces the upwelling of deep, cold, nutrient-rich water along the coast of South America.
- The 'Southern Oscillation' component of ENSO refers specifically to a seesaw of atmospheric mass. Which statement accurately describes this oscillation?
Answer: It is an east-west shift in atmospheric pressure between a center of action over the western tropical Pacific/Maritime Continent and a center over the eastern tropical Pacific.
The Southern Oscillation is the atmospheric component of ENSO. It is characterized by an inverse relationship, or 'seesaw,' in atmospheric pressure between the western and eastern tropical Pacific. When pressure is high in the west (e.g., Darwin, Australia), it tends to be low in the east (e.g., Tahiti), and vice versa. This east-west pressure shift drives the changes in the strength of the Walker Circulation and trade winds.
- While ENSO is often referred to as a 'cycle,' it is more accurately described as a mode of natural climate variability. Which characteristic of ENSO best justifies the term 'variability' over 'cycle'?
Answer: The transition between El Niño, neutral, and La Niña phases occurs at irregular intervals and with variable intensity and duration.
A 'cycle' implies regularity and predictability, like the annual cycle of the seasons. ENSO, however, is not regular. The time between El Niño events can range from 2 to 7 years, and their strength can vary from weak to very strong. This irregularity in timing, intensity, and duration is why scientists prefer the term 'variability.' It is a recurring pattern, but not a predictable, clockwork cycle.
- During a mature La Niña event, how does the structure of the upper Pacific Ocean typically change compared to neutral conditions?
Answer: The thermocline becomes shallower in the eastern Pacific and deeper in the western Pacific due to stronger trade winds.
During La Niña, the easterly trade winds are stronger than normal. This pushes more warm surface water to the west, causing it to 'pile up' and depress the thermocline (the boundary between warm surface water and cold deep water) in the western Pacific. In the eastern Pacific, this enhanced westward flow increases the upwelling of cold deep water, causing the thermocline to become much shallower than normal.
- A key distinction between the atmospheric conditions of El Niño and La Niña is the pressure gradient across the equatorial Pacific. How does this gradient change?
Answer: During El Niño, the pressure gradient weakens or reverses because pressure rises over the western Pacific and falls over the eastern Pacific relative to normal.
Normally, there is high pressure in the eastern Pacific (e.g., near Tahiti) and low pressure in the western Pacific (e.g., near Australia), which drives the easterly trade winds. During El Niño, this pressure difference decreases (weakens) or even reverses, as pressure falls in the east and rises in the west. This weakened gradient is why the trade winds slacken. La Niña is the opposite: the normal pressure gradient strengthens.
- The name 'El Niño,' which is Spanish for 'the boy child,' was originally given to this climate phenomenon by Peruvian fishermen. This name reflects which key characteristic of the event?
Answer: The appearance of unusually warm coastal waters and subsequent changes in weather typically occurred around the Christmas season.
The name 'El Niño' was coined by fishermen off the coast of Peru who noticed that a warm southward ocean current would often appear for a few weeks, typically around late December. Because this timing coincided with Christmas, they named it 'El Niño' in reference to the Christ child. This local, seasonal warming event is now understood to be part of the much larger, basin-wide phenomenon we call El Niño.
- Both ENSO and anthropogenic global warming affect global average surface temperatures. Which statement correctly distinguishes the effect of ENSO from the long-term warming trend?
Answer: El Niño events cause a temporary increase in the global average surface temperature that rises above the long-term trend, while La Niña events cause a temporary decrease.
ENSO represents interannual variability superimposed on the long-term trend of global warming. During El Niño, a vast amount of heat is released from the Pacific Ocean into the atmosphere, causing a temporary spike in global average surface temperatures. During La Niña, the ocean takes up more heat, leading to a temporary dip in global temperatures relative to the trend line. These are oscillations, not permanent changes to the baseline.
- The collapse of the Peruvian anchovy fishery is a well-documented economic consequence often associated with strong El Niño events. What is the primary oceanographic mechanism responsible for this collapse?
Answer: The eastward movement of warm, nutrient-poor surface water suppresses the normal upwelling of cold, nutrient-rich deep water, starving the phytoplankton at the base of the food web.
The Peruvian anchovy population thrives in the cold, nutrient-rich waters brought to the surface by coastal upwelling. During an El Niño, the trade winds weaken, and the thermocline off the coast of Peru deepens. This suppresses upwelling, cutting off the supply of nutrients like phosphates and nitrates. Without these nutrients, phytoplankton populations crash, removing the primary food source for the anchovies and leading to the collapse of the fishery.
- During a strong La Niña, the Walker Circulation intensifies. This intensification is most directly characterized by:
Answer: Stronger convection (rising air) over the western Pacific warm pool and stronger subsidence (sinking air) over the cooler eastern Pacific.
The Walker Circulation consists of rising air (convection) over the warm waters of the western Pacific, upper-atmosphere flow to the east, sinking air (subsidence) over the cool waters of the eastern Pacific, and surface trade winds flowing to the west. La Niña is an enhancement of this normal pattern. Therefore, convection in the west becomes stronger (more rain for Indonesia) and subsidence in the east becomes stronger (drier conditions for Peru), strengthening the entire circulation loop.
- To forecast the potential development of an El Niño event several months in advance, scientists at agencies like NOAA pay close attention to subsurface ocean conditions. Which of the following subsurface features is a critical precursor?
Answer: An eastward-propagating pulse of anomalously warm water traveling along the thermocline (an oceanic Kelvin wave).
Forecasting ENSO relies heavily on observing the subsurface ocean. A key predictor of an impending El Niño is the build-up of warm water in the western Pacific followed by the generation of a 'downwelling' Kelvin wave. This wave is essentially a bulge of warm water that travels eastward below the surface, along the thermocline. Its arrival in the eastern Pacific deepens the thermocline and begins the surface warming characteristic of El Niño. Option B describes a mature El Niño, not a precursor.
- A strong La Niña event is developing. A risk assessment firm is evaluating potential impacts around the world. Which of the following represents an increased risk directly associated with La Niña conditions?
Answer: A wetter-than-average monsoon season in India and Southeast Asia.
La Niña enhances the normal Walker Circulation, leading to even warmer waters and stronger atmospheric convection over the western Pacific and Maritime Continent. This pattern can strengthen the summer monsoon circulation over India and Southeast Asia, often leading to above-average rainfall. Conversely, La Niña is associated with a more active Atlantic hurricane season (due to decreased wind shear), drought in Peru, and cooler, wetter winters in the U.S. Pacific Northwest.
- How does the process of coastal upwelling off the coast of Peru change during a strong El Niño, and what is the primary cause of this change?
Answer: Upwelling ceases or is greatly reduced, because the thermocline deepens, placing the cold, nutrient-rich water layer too deep for the winds to draw it to the surface.
Upwelling is a wind-driven process. During El Niño, the easterly trade winds weaken. This has two effects: 1) the direct wind-forcing of upwelling is reduced, and 2) the thermocline (the boundary between warm surface water and cold deep water) deepens significantly. This pushes the cold, nutrient-rich water much farther from the surface, making it very difficult for the weakened winds to lift it, thus suppressing or eliminating upwelling.
- Understanding ENSO requires a baseline for comparison. Which of the following accurately describes the 'normal' or 'neutral' state of the tropical Pacific ocean-atmosphere system from which ENSO events deviate?
Answer: Easterly trade winds maintain a pool of warm water in the west and cool, upwelled water in the east, creating a distinct east-west temperature and pressure gradient.
The neutral state is not a state of no action, but rather a baseline level of circulation. In this state, the easterly trade winds are moderately strong. They push warm surface water westward, creating the Western Pacific Warm Pool. This process also facilitates upwelling of cold, deep water in the eastern Pacific. This results in a significant east-west gradient in both sea surface temperature (warm in west, cool in east) and atmospheric pressure (low in west, high in east), which defines the Walker Circulation.
- Which sequence correctly orders the chain of events that leads from the initial oceanic changes of an El Niño to drought conditions in Australia?
1. The east-west atmospheric pressure gradient across the Pacific weakens.
2. The Walker Circulation's primary region of convection and rainfall shifts eastward, away from Australia.
3. Easterly trade winds slacken or reverse.
4. Sea surface temperatures increase anomalously in the central and eastern equatorial Pacific.
Answer: 4 → 3 → 1 → 2
The correct causal sequence for an El Niño event is as follows: An initial warming of the central/eastern Pacific (4) reduces the east-west temperature gradient. This warming of the overlying air causes the atmospheric pressure gradient to weaken (the pressure in the east falls, pressure in the west rises) (1), which in turn causes the easterly trade winds to slacken (3). With a weak pressure gradient and weak winds, the main area of atmospheric convection and rainfall is no longer anchored over the western Pacific warm pool and shifts eastward (2). This eastward shift leaves Australia and Indonesia under an area of sinking, dry air, leading to drought.
