What are El Niño and La Niña weather patterns?
El Niño and La Niña weather patterns can influence conditions all around the world, from temperature and surface pressure to wind and rainfall. And with the recent string of intense hurricanes making landfall in the United States, we are hearing these terms used more throughout all popular media. But what exactly are El Niño and La Niña?
Two Pieces of a Larger Puzzle
El Niño and La Niña are opposite phases of a broader climate pattern called El Niño-Southern Oscillation, or ENSO, that alternates every 3-7 years to warm and cool global atmospheric temperatures. ENSO moves in a repetitive cycle from an El Niño (warmer) phase, ENSO neutral phase, La Niña (cooler) phase, ENSO neutral phase, and so on. According to the National Oceanic and Atmospheric Administration (NOAA), El Niño and La Niña phases are characterized by how they circulate the Earth’s temperature between deep ocean water, surface ocean water, and the air above water.
During El Niño phases, trade winds moving east-west over the tropical Pacific Ocean are weak. These weak easterly trade winds allow the average ocean surface temperature to stay warmer, which means that the cooler ocean water stays below the surface. The warmer ocean surface causes global air temperatures to become warmer. This is why “hotter” years tend to occur during El Niño phases. The weaker winds and warmer temperatures also result in decreased rainfall on Indonesia and increased rainfall on the central and eastern portions of the Pacific.
During La Niña phases, trade winds moving east-west over the tropical Pacific Ocean are strong. Stronger easterly trade winds allow the average ocean surface temperature to cool down, and cooler ocean water makes its way to the surface. The cooler ocean surface causes global air temperatures to become lower. This is why “colder” years tend to occur during La Niña phases. The stronger winds and cooler temperatures result in increased rainfall on Indonesia and decreased rainfall on the central and eastern portions of the Pacific.
Why are they called El Niño and La Niña?
El Niño and La Niña are Spanish for “the boy” and “the girl”. Those from Catholic communities may recognize “El Niño” to mean “the Christ child”. NOAA’s Pacific Marine Environmental Laboratory credits South American fishermen who recognized this pattern of alternating warm and cool waters in the Pacific Ocean. The fishermen noticed that the warmer waters of El Niño typically arrived around Christmas time, so they named it in association with the Santo Niño, signifying the birth of Christ. La Niña phase was named to identify it as the opposite side of ENSO cycle.
Many climate scientists use the terms El Niño and ENSO to mean the same thing. You might also hear La Niña referred to as an “ENSO cool event.” To avoid confusion in this post, I use ENSO to refer to the broader cyclical climate pattern and El Niño or La Niña to mean the specific phases within that broader climate pattern.
Does Earth’s atmosphere take on more heat during El Niño phases?
Not exactly. During El Niño phases, less cold water comes to the west Pacific Ocean’s surface. This means that the surface temperature of the west Pacific Ocean stays warmer, which makes global air temperature warmer. The heat sent out into the air from the ocean is heat that was already contained by our planet. ENSO cycles on their own do not bring in more heat from the sun or let out more heat into space. They are essentially working with whatever heat the planet already has in store.
The topic of global warming will be discussed in a later Science in a Nutshell post. But I do not want to overlook how it may or may not interact with the topic covered in this post. Regarding the effect of global warming on Earth’s ENSO cycle, scientists cannot say how global warming impacts ENSO phases themselves but have high confidence that global warming greatly affects the weather events associated with El Niño and La Niña phases.
Can El Niño and La Niña cause more storms to occur or more intense storms to form?
El Niño and La Niña phases directly influence how many storms form in a given region. In the central and west Pacific, El Niño phases mean an increase in typhoon activity. During La Niña phases, however, typhoons are more likely to push further west towards the Philippines and other parts of island Southeast Asia. During El Niño phases in the mainland United States and Caribbean Islands, there may be an increase in hurricane activity in the Pacific basin and a decrease in the Atlantic basin. The reverse is observed during La Niña phases: there may be a decrease in hurricane activity in the Pacific basin and an increase in the Atlantic basin. Check out this Science in a Nutshell post to learn the differences between typhoons and hurricanes.
Although it is understood that ENSO effects the number of storms that occur, it is difficult to determine if El Niño and La Niña caused a specific storm to occur. NOAA uses this as an example: let’s say that a specific area of the mainland United States experiences 7 hurricanes during an ENSO neutral phase but then experiences 10 during the following El Niño phase. It is nearly impossible to identify which 7 would have been the typical hurricanes and which 3 were the extra hurricanes. But an attribution study can analyze current and historical data of hurricanes in that area to see if there is a correlation between El Niño phases and the formation of more intense storms. So it will not be possible to tell which of the 3 hurricanes were added on by El Niño; but El Niño may have played a role if there were only 2 extreme hurricanes during ENSO neutral phase while there were 6 extreme hurricanes during El Niño phase.
How do we know which ENSO phase we are in?
NOAA uses a complex 5-category alert system to track the cyclical ENSO pattern—taking into consideration observed conditions, computer model forecasts, and empirical scientific judgement—to estimate when the Earth should transition from one ENSO phase to the next. The criteria for El Niño include: (1) average surface temperatures of the equatorial Pacific Ocean at least 0.5°C (or 0.9°F) warmer than the previous month’s average; (2) a continuation of slightly warmer average surface temperatures on the tropical Pacific Ocean five 3-month seasons in a row; and (3) weaker than usual easterly trade winds, less cloudiness and rainfall in Indonesia combined with more average surface pressure, OR more cloudiness and rainfall central or eastern Pacific combined with less average surface pressure.
With the exception of Criteria No. 2, La Niña phases have the exact opposite characteristics of El Niño phases. The criteria for La Niña include: (1) average surface temperatures of the equatorial Pacific Ocean at least 0.5°C (or 0.9°F) cooler than the previous month’s average; (2) a continuation of slightly warmer average surface temperatures on the tropical Pacific Ocean five 3-month seasons in a row; and (3) stronger than usual easterly trade winds, more cloudiness and rainfall in Indonesia combined with less average surface pressure, OR less cloudiness and rainfall central or eastern Pacific combined with more average surface pressure.
Closing Thoughts
El Niño and La Niña are opposite phases within a broader climate cycle: El Niño-Southern Oscillation (ENSO). El Niño and La Niña phases are characterized by the intensity of easterly trade winds in the tropical Pacific, consistency of average ocean surface temperature across the equatorial Pacific Ocean, and amounts of rainfall and surface pressure in certain regions of the world. They effect the frequency of typhoons and hurricanes, depending on which region of the world is experiencing which phase of ENSO. It cannot be determined if a specific storm was caused by a certain ENSO phase, but attribute studies can be done to determine if storms are becoming more intense during those ENSO phases.
Dångkulo' na' saina ma'åse'! Thank you so much for reading this Science in a Nutshell post!
Notes:
Random Thoughts posts are brief summaries of much larger topics.
Underlined text are hyperlinked to referenced sources.
Text in bold brown font are important terms to remember.
Post Date: 12 October 2024
