Geoscience Reference
In-Depth Information
that falls each year in California, so they are now recognized as a crucial
source of water for the state. h ey also contribute a signii cant amount (10 to
40 percent) of the total annual precipitation in other western states, includ-
ing Oregon, Washington, Nevada, and Idaho.
Floods
Atmospheric river storms are a double-edged sword: they provide critical
water resources to the region, but they also produce the largest and most
destructive l oods along the West Coast. Climatologist Mike Dettinger of
the U.S. Geological Survey and his colleagues have studied the past sixty
years of climate and l ood history in California and have shown that the
years with the largest “Pineapple Express” storms (a type of atmospheric
river storm that draws heat and vapor from the tropics near Hawaii and
transports it to the West Coast) correlate with the largest l oods. Dettinger
also observed that these storms are tied to both the tropical Pacii c (ENSO
events) and the North Pacii c (PDO events). His analysis reveals that, during
El Niño when the PDO is in a positive (warm) phase, a higher proportion
of precipitation in south-central California and western Washington comes
from Pineapple Express storms.
Atmospheric river storms bring high volumes of precipitation—the
equivalent of ten to i t een Mississippi Rivers—from the tropics to the mid-
latitudes over relatively short periods of time, and, because they are warm,
more of that precipitation falls as rain high in the mountains rather than as
snow, leading to severe l oods. On the West Coast, atmospheric river storms
occurred during the stormiest years on record—1861-62, 1997, and 2006—
and other stormy years in between (1937, 1955, 1964, 1969, 1986, and 1994).
h e relationship between unusually warm storms and severe l ooding was
observed as early as 1900 by John Muir, who wrote: “h e Sierra Rivers are
l ooded every spring by the melting of the snow as regularly as the famous old
Nile. Strange to say, the greatest l oods occur in winter, when one would sup-
pose all the wild waters would be mul ed and chained in frost and snow. . . .
But at rare intervals, warm rains and warm winds invade the mountains, and
push back the snow line from 2000 t to 8000 t , or even higher, and then
come the big l oods” (chap. 11).
Atmospheric river storms are likely responsible for the 1861-62 l oods,
according to scientists at the U.S. Geological Survey. But just how common
are these storms and resulting l oods? Was this a freak event that is unlikely
