El Niño has arrived. The last month has seen an atmospheric shift in response to the unusually warm surface of the tropical Pacific Ocean.
The National Oceanic and Atmospheric Administration (NOAA) forecasts that these conditions will extend into the winter season. The likelihood of this developing into a potent event stands at a solid 56%, with an 84% chance for at least a moderate event.
El Niño is the warm phase of the El Niño-La Niña climate oscillation. This meteorological phenomenon alters global atmospheric circulation in predictable ways, helping us anticipate potential future weather and climate patterns. As El Niño strengthens, its impact on global temperature, precipitation, and other patterns become increasingly evident.
Take the monthly Niño-3.4 index, for example. This measures the temperature of the tropical Pacific Ocean’s surface. It has been observed to be 0.5 °Celsius (0.9 ˚Fahrenheit) higher than the long-term average (1991–2020), according to the OISSTv2.1 monthly dataset.
A 0.5 °C anomaly signifies the arrival of El Niño, marking a remarkable transition from the recent La Niña phase. The NOAA recorded a temperature anomaly of 0.8°C over the last week, further signifying an upward trend.
These conditions, in line with climate model predictions and current tropical Pacific conditions, are expected to remain above the El Niño threshold for the next several months. This marks the fulfillment of the first two criteria of El Niño.
The third criterion involves the indication of a weaker Walker circulation, which refers to the typical atmospheric pattern over the equatorial Pacific. Under normal conditions, warm western Pacific waters fuel rising air and storms, with winds blowing from the west to east high in the atmosphere. These give rise to surface winds known as trade winds that blow from east to west, maintaining the concentration of warm water in the western Pacific.
El Niño disrupts this system. Its warm surface waters shift rainfall and convection towards the central and eastern Pacific, which weakens the trade winds, allows further warming, and results in a backflow of warmer water to the east, reinforcing the El Niño sea surface temperature pattern. This feedback loop is a crucial sign of El Niño.
Recent evidence of this disrupted Walker circulation includes weaker trade winds over the western Pacific and increased cloud cover and rainfall over the equatorial Pacific. Reduced convection over Indonesia is another characteristic indicative of a weaker Walker circulation.
To measure the atmospheric component of El Niño, the NOAA employs the Equatorial Southern Oscillation Index (EQSOI) and the Southern Oscillation Index (SOI). Both of these indexes compare the surface atmospheric pressure in the western Pacific to that in the eastern Pacific.
In May, they measured -1.0 standard deviations, meaning these indexes were lower than about two-thirds of all measurements. This implies a significant indication of the weaker Walker circulation and evidence that the ocean-atmosphere system has coupled, marking the development of El Niño conditions.
Why all this focus on El Niño (and La Niña)? Their impact on atmospheric circulation has global implications. Warm air rising near the equator moves towards the poles high in the atmosphere, descending near 30 °N and 30 °S, in a pattern known as the Hadley circulation. This process affects jet streams over the middle to high latitudes, which guide storms worldwide and separate cold and warm air masses.
As El Niño warms the atmosphere over the central and eastern tropical Pacific, it strengthens the Hadley circulation and alters the jet streams. For instance, an El Niño winter tends to bring more storms across the southern US and warmer air to the northern half of North America.
Given that El Niño can usually be predicted months in advance, the NOAA can anticipate these changes and their potential impacts. When this weather pattern intensifies, warming the sea surface temperature significantly above average, it wields greater influence on global circulation, making the impact patterns more likely.
However, Mother Nature never fails to surprise. While El Niño increases the likelihood of certain patterns, exact predictions are elusive. In the words of two poignant post titles: “Not what I ordered: How El Niño is like a bad bartender,” and “No, you can’t blame it all on El Niño… but it’s still a seasonal forecaster’s best friend.”
Adding to the atmospheric signs, the subsurface ocean also lends support to the persistence of El Niño. Currently, the Pacific holds significantly warmer-than-average water beneath the surface. This is due to the passage of one downwelling Kelvin wave (an area of warm water that moves from west to east under the surface) and the emergence of another.
In fact, the average subsurface temperature for May 2023 was the fourth-warmest May value on record (1979–2023). This isn’t a guaranteed predictor of a strong El Niño, but it certainly is suggestive. The top two May values were in 1997 and 2015, both of which preceded strong El Niño events, but the third in 1980 did not.
As always, nature’s unpredictability makes it challenging to forecast several months ahead. Even though El Niño conditions have formed, there’s a slim chance (4-7%) that it might fizzle out. NOAA considers this unlikely but not impossible. The global oceans’ unprecedented warmth could throw us off track. Another potential, though less probable, outcome is a weak El Niño, with a 12% chance.
In the upcoming months, updates on this El Niño’s development will be provided. We’ll delve deeper into some of the impacts of El Niño, including a post on El Niño’s interaction with global temperature in a couple of weeks. As noted in the previous month, global temperatures are significantly above average and have been steadily rising. Thank you for joining us on this journey into the heart of our changing climate!
El Niño is a climate pattern that describes the unusual warming of surface waters along the tropical west coast of South America. It’s part of what’s known as the Southern Oscillation, a larger climatic phenomenon encompassing both the ocean and the atmosphere. Together, they’re often referred to as ENSO (El Niño-Southern Oscillation).
Typically, trade winds in the Pacific blow from east to west, pushing warm surface water towards Indonesia. When this happens, cold water from deep in the ocean rises up to replace it along the west coast of South America, a process known as upwelling. This cool, nutrient-rich water supports a rich ecosystem, including a thriving fishing industry.
During an El Niño event, the trade winds weaken or may even reverse direction. This change allows the warm water that was “piled up” in the western Pacific to slosh back towards the east. With the retreat of cooler water, sea surface temperatures along the western coast of South and Central America can rise significantly.
This significant warming of ocean water has substantial effects on global weather and climate patterns due to the large amount of heat energy transferred from the ocean to the atmosphere. For instance, it can lead to increased rainfall across the southern tier of the US and in Peru, which has caused destructive flooding, and drought in the West Pacific, affecting Australia and Indonesia.
These effects can also disrupt the marine ecosystem. The warmer surface waters of the Pacific Ocean during an El Niño can deplete the nutrients in the water, disrupting the food chain from its very base. For example, the anchovy industry in Peru and Ecuador can suffer during El Niño years as the anchovies, which prefer cooler water, disappear from their usual coastal habitats.
The frequency and intensity of these events vary. They tend to happen every two to seven years and can last anywhere from nine months to two years. Some can be more intense than others, and their effects can vary accordingly. However, predicting the exact timing and details of these events remains a significant scientific challenge.
On the flip side of El Niño is La Niña, which is essentially the opposite phase of ENSO. During La Niña, the trade winds are stronger than usual, pushing more warm water towards the west and causing cooler waters to rise along the eastern Pacific. This scenario leads to weather patterns that are generally the reverse of those during El Niño events.
It’s important to note that while we understand the general patterns of El Niño and La Niña, each event is unique and can bring its own surprises, which makes studying and predicting them an ongoing endeavor for climatologists.