California Drought (Weather Discussion 5 September 2014)
xkcd comic
 Note the roughly three-year cycle of droughts: 2002-04, 2007-09, 2012-present
 This begs the question: what influences weather and climate on such time scales?
Current conditions
 Drought is as bad as it has been all year, yet alone in the past fifteen years per the archives.
1971-2000 precipitation climatology
 California peaks from Nov through Mar, particularly Jan and Feb
 “On average, about 30-50% of annual precipitation in the west coast states occurs in just a few
atmospheric river events, thus contributing to water supply. Atmospheric rivers are a primary
feature in the entire global water cycle, and are tied closely to both water supply and flood risks,
particularly in the Western U.S.” (NOAA/ESRL, http://www.esrl.noaa.gov/psd/atmrivers/)
 Example atmospheric river event, schematic (Ralph et al. 2011, MWR)
How did we get here?
 2014 year-to-date precipitation ranks: below to much below average, but not record-setting
 2013 precipitation ranks: much below average to record-setting across the entire state
 2012 precipitation ranks: near average; interior slightly above north, slightly below south
 Wintertime (Dec-Feb) precipitation ranks are slightly different, with 2011-12 and 2013-14 being
quite dry and 2012-13 being near-average. While winter is when most precipitation occurs in
California, other months can work in concert with or against what actually happened then.
Larger-Scale Retrospective
 2013, 2012: focus on Plate 2.1 in each (page S8 in 2012, S6 in 2013)
 Generally low precipitation and low total column water vapor from the central/eastern Pacific
northeastward toward the southwest United States, consistent with La Nina to neutral conditions
(cooler than normal sea surface temperatures, stronger than normal subsidence in Walker
circulation).
 Note tie to atmospheric rivers and the subtropical jet; while there exists synoptic-scale
variability, major rainfall events associated with landfalling atmospheric rivers have been
notably absent.
El Nino Southern Oscillation
 Modulation of sea surface temperatures in the equatorial Pacific by abnormal trade winds; leads
to modulations in the location of the rising branch of the zonal Walker circulation and, by
extension, latent heat release (in deep, moist convection) and its impacts on the downstream
mid-latitude circulation
 Historical SOI index: 2002-04 was neutral to weak El Nino, 2007-09 was generally La Nina,
and 2012-present has generally been neutral or weak La Nina
 Continental US temperature/precipitation composites by ENSO phase
o El Nino is generally wet in California from fall through spring, excepting northern
California in fall.
o La Nina is generally dry in California in winter and spring and wet in fall, especially
northern CA.
 To first order, it would seem that ENSO could be a factor, but there seems to be some variability
within this small sample set, suggestive of other influences.
What about the Pacific Decadal Oscillation, an ENSO-like phenomenon across the subtropical and
mid-latitude northern Pacific that operates on longer time scales (20-30 years)?
 What is the PDO? Nominally, a manifestation of sea-surface temperature variability in the
northern Pacific Ocean, particularly the mid-latitudes south of the Aleutians.
 Warm phase is +PDO, cold phase is -PDO.
 Causes of the PDO are unknown.
The reflection of the PDO to the equatorial Pacific is of like sign; +PDO indicates warmer-than-normal
equatorial waters while -PDO indicates cooler than normal equatorial waters. One could say, then, that
+PDO predisposes the equatorial Pacific base state to El Nino, while -PDO predisposes the equatorial
Pacific base state to La Nina.
 Correlation of PDO index to SST: positive correlation indicates +PDO with higher SST, -PDO
with lower SSTs.
Historical PDO index
 2002-04 was generally positive, 2007-09 and 2012-present are generally negative
 Correlation of 500 hPa geopotential height (Dec-Feb, 1948-2011) to PDO index
o Low-high-low pattern: +PDO indicates anomalous trough-ridge-trough (pointing
anomalous moisture feed on east side of trough into west coast), -PDO indicates
anomalous ridge-trough-ridge (pointing anomalous moisture feed over central US)
 Correlation of total column precipitable water (a measure of moisture) to PDO index
o Low-high-low pattern: +PDO indicates anomalously dry-wet-dry, -PDO indicates
anomalously wet-dry-wet
 Correlation of Dec-Feb precipitation to Dec-Feb PDO index
o Generally weakly positive, albeit not statistically significant, with -PDO indicating drier
conditions.
To first order, it would seem that the ongoing -PDO conditions could also be a factor, but again 200204 doesn't really seem to fit the mold. Further investigation is warranted.
What about into the future?
 National Climate Assessment: http://nca2014.globalchange.gov/report/our-changingclimate/precipitation-change
o First figure (2.12): 1991-2012, precipitation is actually up over California, but it is down
for 2000-2012 (click on southwest US for bar graph to illustrate second point)
o Precip change by season (2.14): generally negative, dominated by a statisticallysignificant decrease in spring
o Newer guidance (2.15) is similar with minor differences. Note differences between
various emission scenarios: similar to slightly wetter with cutbacks, drier with continued
growth.
 Why might we expect to see these changes? (In other words, how does climate change influence
climate modes of variability, in turn influencing synoptic-scale events?)
Societal impacts?
 Pacific Institute website: http://www.californiadrought.org/
 Pacific Institute report on 2007-09 drought
 Keep in mind the veracity of various sources of information, particularly online. Does the
source have an agenda that they are trying to advance? If so, how does that influence the
information that you see – or, as the case may be, do not see?
 Pacific Institute claims to be an independent non-governmental organization, but I personally
cannot vouch for or against their independence...