[env-trinity] CBB: California 'Rain Debt' Equal To Full Year Of Precip; Will Strengthening El Nino Bring Relief?

Mon Aug 3 10:34:49 PDT 2015

THE COLUMBIA BASIN BULLETIN: Weekly Fish and Wildlife News

www.cbbulletin.com

July 31, 2015       Issue No. 758

California ‘Rain Debt’ Equal To Full Year Of Precip; Will Strengthening El
Nino Bring Relief? 

A new NASA study has concluded California accumulated a debt of about 20
inches of precipitation between 2012 and 2015 -- the average amount expected
to fall in the state in a single year. The deficit was driven primarily by a
lack of air currents moving inland from the Pacific Ocean that are rich in
water vapor.

In an average year, 20 to 50 percent of California's precipitation comes
from relatively few, but extreme events called atmospheric rivers that move
from over the Pacific Ocean to the California coast.

"When they say that an atmospheric river makes landfall, it's almost like a
hurricane, without the winds. They cause extreme precipitation," said study
lead author Andrey Savtchenko at NASA's Goddard Space Flight Center in
Greenbelt, Maryland.

Savtchenko and his colleagues examined data from 17 years of satellite
observations and 36 years of combined observations and model data to
understand how precipitation has varied in California since 1979. The
results were published Thursday in Journal of Geophysical Research -
Atmospheres, a journal of the American Geophysical Union
http://agupubs.onlinelibrary.wiley.com/agu/jgr/journal/10.1002/(ISSN)2169-89
96/

The state as a whole can expect an average of about 20 inches of
precipitation each year, with regional differences. But, the total amount
can vary as much as 30 percent from year to year, according to the study.

In non-drought periods, wet years often alternate with dry years to balance
out in the short term. However, from 2012 to 2014, California accumulated a
deficit of almost 13 inches, and the 2014-2015 wet season increased the debt
another seven inches, for a total 20 inches accumulated deficit during the
course of three dry years.

The majority of that precipitation loss is attributed to a high-pressure
system in the atmosphere over the eastern Pacific Ocean that has interfered
with the formation of atmospheric rivers since 2011.

Atmospheric rivers occur all over the world. They are narrow, concentrated
tendrils of water vapor that travel through the atmosphere similar to, and
sometimes with, the winds of a jet stream. Like a jet stream, they typically
travel from west to east. The ones destined for California originate over
the tropical Pacific, where warm ocean water evaporates a lot of moisture
into the air. The moisture-rich atmospheric rivers, informally known as the
Pineapple Express, then break northward toward North America.

Earlier this year, a NASA research aircraft participated in the CalWater
2015 field campaign to improve understanding of when and how atmospheric
rivers reach California.

Some of the water vapor rains out over the ocean, but the show really begins
when an atmospheric river reaches land.

Two reached California around Dec. 1 and 10, 2014, and brought more than
three inches of rain, according to NASA's Tropical Rainfall Measuring
Mission (TRMM)'s multi-satellite dataset.

The inland terrain, particularly mountains, force the moist air to higher
altitudes where lower pressure causes it to expand and cool. The cooler air
condenses the concentrated pool of water vapor into torrential rains, or
snowfall as happens over the Sierra Nevada Mountains, where water is stored
in the snowpack until the spring melt just before the growing season.

The current drought isn't the first for California. Savtchenko and his
colleagues recreated a climate record for 1979 to the present using the
Modern-Era Retrospective Analysis for Research and Applications, or MERRA.
Their efforts show that a 27.5 inch deficit of rain and snow occurred in the
state between 1986 and 1994.

"Drought has happened here before. It will happen again, and some research
groups have presented evidence it will happen more frequently as the planet
warms," Savtchenko said. "But, even if the climate doesn't change, are our
demands for fresh water sustainable?"

The current drought has been notably severe because, since the late 1980s,
California's population, industry and agriculture have experienced
tremendous growth, with a correlating growth in their demand for water.
Human consumption has depleted California's reservoirs and groundwater
reserves, as shown by data from NASA's Gravity Recovery and Climate
Experiment (GRACE) mission, leading to mandatory water rationing.

"The history of the American West is written in great decade-long droughts
followed by multi-year wet periods," said climatologist Bill Patzert at
NASA's Jet Propulsion Laboratory in Pasadena, California. He was not
involved in the research. "Savtchenko and his team have shown how variable
California rainfall is."

According to Patzert, this study added nuance to how scientists may
interpret the atmospheric conditions that cause atmospheric rivers and an El
Niño's capacity to bust the drought. Since March, rising sea surface
temperatures in the central equatorial Pacific have indicated the formation
of El Niño conditions.

El Niño conditions are often associated with higher rainfall to the western
United States, but it's not guaranteed.

Savtchenko and his colleagues show that El Niño contributes only six percent
to California's precipitation variability and is one factor among other,
more random effects that influence how much rainfall the state receives.

While it's more likely El Niño increases precipitation in California, it's
still possible it will have no, or even a drying, effect.

A strong El Niño that lasts through the rainy months, from November to
March, is more likely to increase the amount of rain that reaches
California, and Savtchenko noted the current El Niño is quickly
strengthening.

The National Oceanic and Atmospheric Administration, which monitors El Niño
events, ranks it as the third strongest in the past 65 years for May and
June. Still, it will likely take several years of higher than normal rain
and snowfall to recover from the current drought.

"If this El Niño holds through winter, California's chances to recoup some
of the precipitation increase. Unfortunately, so do the chances of floods
and landslides," Savtchenko said. "Most likely the effects would be felt in
late 2015-2016."

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