What is El Nino and why does it matter during hurricane season?

The forecast follows an above-average season in 2018 that saw a Category 5 hurricane – Michael – make landfall and devastate Florida’s Gulf Coast.

In releasing the forecast Thursday, acting NOAA Administrator Neil Jacobs said several factors led to the agency’s estimate, including a current El Nino, or periodic warming in the Pacific Ocean.

While in most cases an El Nino condition would be good news for the suppression of the formation of hurricanes in the Atlantic, this year could be different, Jacobs warned.

Other influences, such as warmer than normal water temperatures, could blunt the effect of this year’s weak El Nino, according to Jacobs.

What is El Nino, and why does it matter during hurricane season? Here’s a look at the naturally occurring phenomenon.

What is El Nino?

An El Nino is a change in the climate pattern that leads to unusual warming of surface waters and the atmosphere. The change happens in the Pacific Ocean near the equator and is temporary.

What causes El Nino?

Strong trade winds usually blow from east to west across the Pacific near the equator, causing warm water to be pulled westward and pile up in the western part of the Pacific. The winds also pull the deeper, colder water in the eastern Pacific to the surface as they move westward.

During an El Nino, the usually strong trade winds are weaker. The weaker winds don’t pull warm waters westward and the warm water they do pull tends to fall back toward the east, making the waters near South America warmer.

What does it matter if the water is warmer in the eastern part of the Pacific?

Warmer water in the eastern Pacific tends to make trade winds weaker. Weaker westerly winds lead to increased rainfall in the eastern Pacific, while at the same time causing dryer conditions in the western Pacific.

How does that affect hurricane season in the Atlantic?

If the El Nino effect is strong in a particular year, it can suppress the formation of hurricanes in the Atlantic.

The main way that happens is by wind shear. Wind patterns produced by El Nino align to cut off a tropical storm's source of power – warm water and air.
Wind shear, a change in wind speed and/or direction with altitude, is increased over the Caribbean and Atlantic in years where the El Nino is strong.

Jacobs said that while there is a current El Nino, it appears to be a mild one, offering less protection against tropical systems that form in the Atlantic. The weak El Nino, coupled with warmer water temperatures, is likely to produce an average Atlantic hurricane season.

NOAA is predicting a near-normal Atlantic season with nine to 15 named storms, four to eight of which will become hurricanes. Of those, two to four are expected to grow to Category 3 or stronger storms. Category 3 storms have winds of 110 mph or higher.

What is La Nina?

La Nina is the opposite of El Nino. Click here for an explanation.

How often do El Ninos happen?

They happen about every three to seven years.