While El Nino events often mean winter drought in the
Islands, it has long been understood that the counterpart, La Nina, brought
increased rainfall.
Sunday, November 15, 2015
Wet years now dry: the transition to an ever more arid Hawai`i
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
Posted by Jan T at 10:36 AM
Labels: Agriculture, Birds, Botany, Climate Change, Conservation, Oceanography, Sustainability, Weather, Wind, Zoology
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