Sunday, November 15, 2015

Wet years now dry: the transition to an ever more arid Hawai`i

While El Nino events often mean winter drought in the Islands, it has long been understood that the counterpart, La Nina, brought increased rainfall.

But that balance, which helped restore depleted groundwater reservoirs, appears to have been broken, according to Hawai`i and China researchers.

La Nina—the cold cycle that often occurs between El Nino warm cycles—now no longer brings additional rain, according to a paper in the Journal of Climate. 

The paper is entitled. “Variability of Hawaiian Winter Rainfall during La Niña Events since 1956.” Its authors are Christopher F. O’Connor and Pao-Shin Chu, of the University of Hawai`i Department of Atmospheric Sciences, School of Ocean and Earth Science and Technology, Pang-Chi Hsu, of China’s Nanjing University, and Kevin Kodama, of the Honolulu National Weather Service.

“Rainfall in Hawaii during La Niña years has undergone abnormal variability since the early 1980s. Traditionally, Hawaii receives greater-than-normal precipitation during the La Niña wet seasons. Recently, La Niña years have experienced less-than-normal rainfall,” they write.

The reason seems to be that La Nina itself is changing, they said.

"Variations in tropical sea surface temperatures and circulation features in the northern Pacific Ocean have changed during La Niña wet seasons, thus changing La Niña–year rainfall.”

A lot of our moisture in winter comes in the form of storm systems that blow through. They’re not blowing through as consistently, the authors write.

“A storm-track analysis reveals that the changes … are creating a less favorable environment for the development of Kona lows and midlatitude fronts in the vicinity of Hawaii.”

The fact that the Islands are drier than they used to be is not news. This latest El Nino work helps explain the mechanics of why rainfall is dropping. 

The National Climate Assessment of 2014 reviewed some of the bad news. 

“Freshwater supplies are already constrained and will become more limited on many islands,” that report said. “On most islands, increased temperatures coupled with decreased rainfall and increased drought will reduce the amount of freshwater available for drinking and crop irrigation.”

RaisingIslands has been covering this dryness trend for years. 

Here is a report on weakening tradewinds and their impact on rainfall. 

Here is a report reviewing reduced rainfall on Haleakala over the past quarter-century. 

Here is a report on the increasing frequency of El Nino andLa Nina events.

© Jan TenBruggencate 2015

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