The prominence of decadal variability
in North American precipitation
S c ott St. G eorge
Geological Survey of Canada
Toby A ult
University of Arizona

Greg McCabe, Michael Palecki, and
Julio Betancourt, 2004:
Pacific and Atlantic Ocean
influences on multidecadal
drought frequency in the
United States
PNAS 12, 4136 - 4141

Here be
decadal variability
Decadal variability in precipitation or
drought as a signal detection problem
Terrestrial climate records often
processed to emphasize D2M timescales4

Does it matter at the regional scale?
5

1920 1940 1960 1980 2000
Decadal variance
30%
Decadal variance
5%
1920 1940 1960 1980 2000

Where is decadal variability
in precipitation strong?

Dan Cayan, Michael Dettinger,
Henry Diaz, and Nick Graham,1998:
Decadal variability of
precipitation over
western North America.
Journal of Climate 11, 3148-3166

Methods
Photo: Christos_m2001

PDSI
Temperature
Precipitation Streamflow
Storms
Rain/snow

Gridded (0.5 deg) monthly
precipitation, Climate Research
Unit, University of East Anglia
Period of record: 1901 - 2002

Decadal variability
Decadal variabil
Decadal varia
Decadal v

Noise
Interannual
Trend
Multidecadal
Decadal

What did we find?
Photo: futureshape

Slides missing*
* Thank goodness

Winter decadal
0% 10% 20% 30% variance

Autumn decadal
0% 10% 20% 30% variance

Annual multidecadal
0% 10% 20% 30% variance

Northern California precipitation

Northern California precipitation
Decadal variance = ~30 percent

D2M ‘hotspots’

“Decadal-scale fluctuations are
crucial because they control water
supplies, affect biota, and may
modulate higher-frequency events
such as floods and drought.”
Dan Cayan and colleagues [1998]

Decadal hotspot - Oregon/California

Northern California

Sacramento River
Photo: Patty Colmer

Northern California

Northern California

Photo: California Department of Water Resources
Levee break, Sacramento basin

1915 levee break
Photo: California Department of Public Works
Courtesy Joan Florsheim

Northern California

Northern California

Decadal hotspot - Minnesota

Minnesota

Mississippi River at Minneapolis
Photo: Mulad

Minnesota

Minnesota

Minnesota

Mech et al. (1987):
Does high snow over
consecutive winters have a
cumulative, negative, effect
on the viability of moose
offspring?

Minnesota

Minnesota

Minnesota

Associations with
major climate modes

It’s complicated.
46

Winter precipitation and
the PDO

Winter precipitation and
the PDO

Three main points
49

location and
seasonality
make a big difference
50

D2M propagates into
hydrology* and
ecology**
* clearly
** maybe
51

Is it a coincidence that D2M
variability is strongest in
the ENSO-PDO
null?
52

Thank you
Julio Betancourt Dave Meko
Julia Cole Joellen Russell
Susan Pullan Mike Wallace
Mike Mann Connie Woodhouse
Support
Manitoba Hydro
Natural Resources Canada
National Science Foundation

S co t t St. G e o r g e Toby Au lt
Geological Survey of Canada University of Arizona
sstgeorg@nrcan.gc.ca tault@email.arizona.edu