El Niño is on its way

Predicted ramp-up of this well-known teleconnection could create headaches for pilots.

By Karsten Shein
Comm-Inst
Climate Scientist

For a few months now, media outlets have run headlines announcing that a strong El Niño is on its way. During the strong 2015–16 El Niño, it was being blamed for everything, from dips in the stock market to increases in arthritis pain. Spurious correlations aside, forecasts currently suggest, with almost complete certainty, that a strong – perhaps record-setting – El Niño event is taking shape this summer and extending through the remainder of the year.

While pilots won’t need to worry about more aching joints due to this event, they may want to keep acetaminophen on hand, as the effects this major climate teleconnection can have on aviation weather conditions can be enough to give pilots headaches.

What is El Niño?

El Niño is a specific phase in a large-scale teleconnection that occurs in the tropical South Pacific Ocean. Teleconnections describe any atmospheric conditions that are related across large distances. In the case of El Niño, the teleconnection is known as the Southern Oscillation. This oscillation is the connected, inverse relationship between barometric pressure on either side of the ocean that was first discovered in the 1920s by analyzing barometric pressure from Tahiti and Darwin NT, Australia.

Development of the 2026 El Niño event as observed in tropical Pacific sea surface temperature departures from normal. Usually, anomalies don’t become this large until later in the year, suggesting this may be a strong event.

Later studies connected the pressure relationship to changes in surface ocean currents. Normally, lower pressure over the western Pacific and higher pressure to the east produces a westerly current that raises cooler deep water along the South American coast and moves it across the ocean. Those normal, or “neutral,” conditions bring seasonal rain to Australia, the Pacific Islands, and Southeast Asia, while the west coast of South America remains relatively dry.

However, sometimes the east–west pressure difference decreases, slowing the surface ocean currents and the associated supply of cold water upwelling along the South American coast.

As Spanish sailors first documented in the 1500s, when that happens, surface water temperature rises and onshore airflow rising into the Andes produces heavy rains and flooding across coastal South America. Because these conditions appeared in December, those early accounts called the phenomenon El Niño, for “The (Christ) Child.” The opposite, strengthening of the pressure difference has since been labeled La Niña.

Oceanic Niño Index

Because the term El Niño is so widely known and associated with the Southern Oscillation, the phenomenon is often labeled as El Niño – Southern Oscillation (ENSO). Meteorologically, it is monitored by satellite and a series of buoys arrayed across the Pacific Ocean tracking sea surface temperature (SST). One of the more commonly used indices for ENSO is known as the Oceanic Niño Index (ONI).

It is produced by subtracting the long-term average SST over the past 3 months for a particular “Niño 3.4” region (5°N – 5°S and 120°W – 170°W) from the current 3-month average Niño 3.4 SST. An ONI around zero represents neutral conditions. When it is 0.5° C or more above the long-term average, it signals an El Niño. La Niña conditions exist when the ONI is -0.5° C or less below the normal.

The strength of El Niño/La Niña varies by how far above or below zero the ONI is. Strong events exceed +/– 1.5, while very strong events are at least +/– 2.0. The forecast ONI for this emerging El Niño is forecast to exceed 2.0. For comparison, the 2015–16 El Niño reached 2.8, and the 1982–83 event, which caused major atmospheric disruptions and sparked modern research into ENSO, topped at 2.2.

Similar to ONI, the length of an ENSO event will affect its impact on weather around the world. Most ENSO events start between May and August, slowly strengthening until peaking in December or January. Well-developed events may last over 12 months, only dissipating the following March–May. Events can be as short as a month, while the longest were the 24-month 1957–59 El Niño and the 36-month 1973–76 La Niña.

Shifting weather patterns

The main reason ENSO is more than just an interesting topic for researchers is that, as a teleconnection operating in a dynamic fluid (the atmosphere), it has the potential to affect the general circulation of the atmosphere and alter weather patterns worldwide.

For many people, this means weather that is significantly different from normal. During El Niño, places which rely on monsoon rains for agriculture may see extended drought and famine-inducing crop failures, while other, normally dry, regions receive flooding rains that produce major flooding and disease outbreaks. In a La Niña, many “normal” conditions can be amplified – ie, wet regions get wetter and dry regions become drier.

One reason for this is that the changes in global atmospheric circulation affect jet streams around the world. Although it is not a simple relationship, as several other teleconnections have an influence, ENSO events can dampen or amplify waves in the jet streams, producing hotter, colder, wetter, or drier climate patterns throughout many major flight corridors.

Shift in tropical Pacific weather patterns during El Niño and La Niña. These teleconnection events can affect weather worldwide.

For example, by winter, when El Niño reaches peak strength, the polar jet is often pushed equatorward and the storm track amplifies, bringing hotter and drier conditions to northwestern North America and cooler, wetter seasons with more severe weather across much of the southern US and Mexico.

A quieter Atlantic hurricane season is also a common result of El Niño. Worldwide, El Niño frequently contributes to hotter conditions in South America and southeast Asia, and drier conditions in the Caribbean, India, and southern Africa.

While some of these relationships are strong enough to be considered reliable, such as a significant increase in winter rainfall over Florida during El Niños, most connections are tenuous, particularly the further one is from the tropical Pacific.

So, while they can be good for general weather guidance, meteorologists consider El Niño as only one of many factors that can affect the weather in a given location. In general, however, a strong ENSO event can normally be expected to produce larger shifts in expected or “normal” weather conditions.

The presence of an El Niño or a La Niña – even a “super” or “record-setting” event – does not in any way diminish the need for a proper, comprehensive weather briefing and sound pilot judgement when operating around adverse weather activity.

It does, however, give pilots hints that certain weather conditions may become more or less likely over the places they fly. This can affect seasonal planning such as budgeting for deicing or anticipating more frequent severe weather deviations and delays. Regardless of the state of ENSO, if you encounter weather that your fellow pilots should be aware of, file a pirep.