What happens during El Niño and La Niña events?

During El Niño (above, left), the trade winds decrease, ocean water piles up off South America, the sea surface temperature increases in the eastern Pacific, and there is a shift in the prevailing rain pattern from the western Pacific to the western Central Pacific. During La Niña (above, right), the trade winds increase, ocean water piles up in the western Pacific, the sea surface temperature decreases in the eastern Pacific, and the prevailing rain pattern also shifts farther west than normal. (Courtesy of NOAA National Weather Service.)

The effects of El Niño and La Niña on the U.S. Pacific Islands region are significant and  vary by location.

What El Niño and La Niña mean for…

Hawai‘i

During an El Niño event, Hawai‘i’s weather starts out much wetter than normal (August-October), but later dries out. Rainfall is usually well below average by February of the following year. The intensity of the dryness depends on the intensity of the El Niño event. Rainfall typically returns to normal by July in the year following the onset of El Niño.  The risk of tropical storm and hurricane activity around Hawai‘i is higher during El Niño in the hurricane season.  Sea levels are sometimes slightly above average, and exceptionally large swells are associated with winter storms north of Hawai‘i.

During a La Niña event, trade winds are stronger than normal and rainfall is usually above average across the Hawaiian Islands.

The western North Pacific (Guam, the Federated States of Micronesia, the Marshall Islands, the Northern Mariana Islands, and Palau)

During El Niño, a “wave” of conditions progress from west to east across the region. Above average rainfall tends to occur during the initial stages of the event.  During the peak of the event sea level is below normal and sea surface temperature above normal.  During the final stages of the event rainfall is below normal and can lead to drought.  This wave starts as early as March/April in the western Pacific (e.g., Palau, Guam) and May/June in the eastern Pacific (e.g. RMI).  It effects continue as late as December of the following year.  Tropical cyclone risk is reduced as typhoons are less likely than normal.

La Niña can cause heavy rainfall or dry conditions depending on the strength of the particular event and the specific location in the western North Pacific. Wetter than normal conditions during the dry season are likely during a La Niña event in the Northern Mariana Islands, Palau and Micronesia. The West Pacific Monsoon is pulled westward during La Niña, resulting in a drier-than-normal rainy season in Micronesia.  La Niña brings higher sea levels that tend to cause more flooding in the western North Pacific.

The South Pacific (American Samoa)

The El Niño signal is less distinct in American Samoa.  Rainfall tends to be lower than normal in American Samoa during the developing and mature stages of a strong El Niño (August through June). However, these dry conditions may be punctuated with intermittent wet conditions.  Sea level usually starts dropping during October or November of an El Niño year and can remain lower than average through the following year.  In a strong El Niño event, American Samoa is at an elevated risk of damaging tropical cyclones from November through January.

La Niña brings stronger east to west trade winds, which increase the sea level, and along with King Tides (the highest tides in the year) can cause flooding and coastal erosion.  American Samoa tends to be wetter than normal and flooding can occur during a La Niña event.