La Nina forms, but considered weak

Forecasters announced Thursday that the La Nina phenomenon has begun in the equatorial Pacific.

Hawaii’s wet season likely will yield average or above-average rainfall as a result.

La Nina frequently, but not always, occurs after an El Nino event. The two are characterized by opposite oceanic patterns: El Nino by warming sea surface temperatures and La Nina by cooling ones.

The strongest El Nino event in the past two decades occurred last year and contributed to the busiest Central Pacific hurricane season on record, as well as an abnormally dry winter in Hawaii.

La Nina is expected to have less impact statewide.

“It’s a weak La Nina, (and) it’s not expected to last too long,” said Kevin Kodama, a hydrologist with the National Weather Service in Honolulu. “It’s such a marginal event that it’s expected to stay weak and end in early 2017.”

When La Nina is weak, Hawaii tends to get average seasonal rainfall.

The last time La Nina occurred was during the winter of 2011-12. That, too, was a “weak to moderate event,” Kodama said.

Weak events are “tougher to forecast,” than moderate or strong ones, Kodama said.

Climate forecasters look at several factors when deciding whether La Nina has begun, he said. The primary factor is cooling anomalies in sea surface temperatures at the Pacific equator.

“They’re looking to see if it’s a half a degree (Celsius) or cooler,” Kodama said. “Not just for a week or two; it has to sustain itself for a while.” These conditions have been in effect for the past month.

Forecasters look for shifts in thunderstorm activity on the western side of the Pacific; there has been an increase in thunderstorms in Indonesia. Sea levels also have been higher than normal in the Western Pacific.

On the mainland, La Nina can exacerbate drought. It is expected to do so this year in the southern United States.

Email Ivy Ashe at iashe@hawaiitribune-herald.com.