After 3 years with a La Niña in place, its counterpart, El Niño, has officially made a comeback. Models are hinting that this event could turn into a strong one by the end of the year. Let’s explain how it works, what exactly this weather pattern is, and the impacts.

First, its name. El Niño is named after “El Niño de Navidad”, the name given by South American fishermen after they notice that periods of warm water peaked during the month of December in the 1600s.

Like La Niña, El Niño is not a weather phenomenon that can be specifically forecasted when it will start and end. Models often suggest a possibility of El Niño happening as the trends show warming waters, but it is impossible to know exactly when it starts or ends.

Sea surface waters over the tropical Pacific, specifically over the region called Niño 3.4, show positive temperature anomalies or temperatures above average, of at least 0.5C. This anomaly is the mean from five consecutive 3-month measurements of sea surface temperature anomalies. The warmest the anomaly, the strongest it is considered.

The last time we had a strong El Niño, with a warm anomaly at 2.6C or 4.7F, was during the winter of 2015/2016. The last El Niño happened in 2018/2019 and was weak. It was followed by neutral conditions for a year, then 3-consecutive La Niña years.

The trade winds blow east to west along the tropical Pacific. These trade winds push the warmer waters closer to Asia, which enhances rain over that region. Cooler waters rise from the depths of the ocean over the eastern portion of the Pacific, usually keeping storm activity lower. But when El Niño is present, these winds tend to be weaker, keeping the warm waters closer to South America which enhances shower activity over the central and eastern Pacific.

Since El Niño tends to peak in the winter months (northern hemisphere), this season tends to be the most affected globally, but there are impacts all over the globe year around if this phenomenon lasts for a year or longer.

During the late summer months, when El Niño is present, tropical activity tends to increase over the central Pacific. Luckily these storms do not encounter land, except in Hawaii, as the Pacific is vast. With an El Niño present, the correlation to tropical activity over the Eastern Pacific is not well defined. There could be activity as there is not an exact line that cuts off where the low or high shear is.

Overall, in the Atlantic, activity tends to decrease as wind shear increases. Wind shear serves as a mechanism to kill the top of storms as they develop, not allowing their formation cycle to continue to grow.

If El Niño is present in the winter months, lots of moisture is injected over the southern regions of the United States. The Pacific Jetstream tends to be extended which amplifies the storms’ path, allowing the storms to track across southern regions, and allowing the southern half of the United States to stay wetter and cooler than average.

Drier and warmer conditions are more common across the Pacific Northwest, northern Plains, Midwest, Ohio Valley, and parts of the interior Mid-Atlantic. The Caribbean usually suffers from big droughts during El Niño phases, which can put a dent in their economies.