ClimateVariabilityClimate
Variability
Climate Impacts Group&
Department of Atmospheric SciencesUniversity of Washington
Eric Salathé
Thanks to Nathan Mantua
Northwest Climate: the mean
Factors that influence local/regional climate:
1. Latitude
• day length, intensity of sunlight
2. Altitude
3. Mountain Barriers
4. Proximity to the ocean
• ocean currents
5. location relative to prevailing winds
Mean SLP fields
• the dominant feature shifts from the subtropical High in summer to the Aleutian Low in winter
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
Oregon Climate Servicehttp://www.ocs.orst.edu
Northwest terrain maps the big-picture windsand storms onto a complex landscape
• localized cold air outbreaks
• the Puget Sound Convergence Zone
• rain shadows
“Arctic Blasts”
The Puget Sound Convergence Zone
Annual average rain+snowfall: 1961-1990
The predictable part: seasonal rhythms
Puget Sound Precip
Upwelling winds at 48N
Amphitrite Pt SST
Oct Feb Jun
Oct Feb Jun
Oct Feb Jun Jan May Sep
InsolationInsolation
Year to year variations on the seasonal rhythms
Monthly Puget Sound Precip
Daily Upwelling winds
Monthly Amphitrite Pt SST
Northwest Climate Variability
Pollen records on the Olympic Peninsula
Crocker Lake
McLachlan, J. S. and L. B. Brubaker. 1995 Local and regional vegetation change on the northeastern Olympic Peninsula during the Holocene. Canadian J. of Botany.
alder
cedars
pines
df
cool
fires: hot-dry
cool-wet
1750 1775 1800 1825 1850 1875 1900 1925 1950 1975 2000Year
5.0
5.1
5.2
5.3
5.4
5.5
Log10 mean flow, The Dalles, OR (cfs)
Source: Gedalof, Z., D.L. Peterson and Nathan J. Mantua. (2004). Columbia River Flow and Drought Since 1750. Journal of the American
Water Resources Association.
The Dust Bowl (1929-1931) was probably not the worst drought sequence in the past 250 years
(based on Columbia Basin Tree-ring chronologies)(based on Columbia Basin Tree-ring chronologies)
red = observed, blue = reconstructed
PNW climate variability
1. What does our region’s climate history tell us about “natural variability”?
2. How is climate variability experienced in the Pacific Northwest? * are there patterns within the region? * are there preferred frequencies of change (year to year, decade to decade, etc.)
3. Why does our climate vary?
Warm and cool (or “wet” and “dry”) halves of the year: oct-mar versus apr-sep
Characteristics of variability?
• Lots of year-to-year variability in both halves of the year; longer-term variations– Multi-decadal “cycles” and century long trends
• temperatures and precipitation are more variable in cool season than in warm season
Washington State Oct-Sept Total Precip
36
48
19871967194719271907 2007
Washington State Oct-Sept Average Temperature
46
48
19871967194719271907 2007
50
Riffe Lake, west slopes of the Cascades
Spring 2001
March 15 Snow depth anomalies at Paradise, Mt Rainier
Avg ~ 4 meters (170 inches)January 5, 2005: 48 inchesJanuary 6, 2007: 130 inches
Avg=4 meters
Water Year Columbia River streamflow
Average annual runoff at The Dalles, Oregon ~ 150 Million Acre-Feet (MAF);
Oct 2000-September 2001 ~ 100 MAF
NW Climate variability
• Why the strong climate changes?– The chaotic nature of the climate system– big volcanic eruptions– natural modes of climate variability internal to
the climate system: • in the Pacific sector, changes in ENSO and PDO
are important factors
• Circulation changes are sensitive to the intensity of tropical El Nino events
• Contrast the “average” event with the extreme winters of 1982-83 and 1997-98
Oct 97-Mar 98:El Niño
Oct 98-Mar 99:La Niña
El Niño year precip anomalies Oct 1997- Mar 1998
La Niña year precip anomalies Oct 1998- Mar 1999
Regional patterns?
• Typically, cool-season (oct-mar) climate anomalies are coherent throughout most of the PNW region
• warm-season climate anomalies also tend to be regionally coherent, but to a lesser degree
Regional patterns?
Observations Regional Simulation
Leung et al 2003
Dry
wet
Accumulated daily rainfall: Oct 1 1998-Sept 20 1999 A very wet year everywhere but Yakima!
http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/ensostuff/current_impacts/global_precip_accum.html
“composite avg” PNW temperature and precipitation
during El Niño and La Niña
(based on averages of past century’s events)
EN-LN
The Pacific Decadal Oscillation
• an El Niño-like pattern of climate variability
• 20 to 30 year periods of persistence in North American and Pacific Basin climate
• warm extremes prevailed from 1925-46, and again from 1977-98; a prologed cold era spanned 1947-76
1998?1925 1947 1977
Mantua et al. 1997, BAMS
Figures produced by Todd Mitchell, UW/JISAO
October-March PDO Regression fieldsOctober-March PDO Regression fieldsMaps show typical warm PDO climate anomaliesMaps show typical warm PDO climate anomalies
Surface Air Temperature Precipitation
Strong Strong
Aleutian Aleutian
LowLow
Strong Strong
Aleutian Aleutian
LowLow
Warm
Warm
drydry
drydry
wetwet
drydry
drydry
A history of ENSO
1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000
warm coolwarm
A history of the PDO
Real time “nowcasts” of the PDO?
http://jisao.washington.edu/pdo
Monthly PDO index: 1900-Jan 2008
Because we don’t know how the PDO works (key mechanisms for decadal patterns remain mysterious), we can’t be sure that the SST pattern (and PDO index) is a good indicator for where we are with this pattern. Recent years have a variable PDO index…but perhaps no moreso than the late 1980s…
PDO and PNW monthly temperatures and precipitation
PDO and Cascades snowpack
0
100000
200000
300000
400000
500000
600000
700000
Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep
Month
Average Flow (cfs)
ColdPDO
WarmPDO
Water year stream flow composites for Columbia River “natural” flows at The Dalles, Oregon
PDO/ENSO and NW hydrology
• Because extremes in ENSO and PDO tend to favor either “warm and wet” or “cool and dry” conditions, these combinations lead to amplified responses in snowpack and streamflow – Ex: cold wet weather, lower snowline, more
precipitation, more snow, less evaporation and more runoff
Cool/Warm PDO and Paradise snowdepth histograms
From the National Climate Data Center: www.ncdc.noaa.gov
1977
19941944
October-March OR-ID-WA Temperature and Precipitation
*
A regionally averaged view of PNW cool season Temps and precip
Major drought years
2001
Winter windsand pressure over the North Pacific
Summer windsand pressure over the North Pacific
“Aleutian Low” “Subtropical High”
HH
LL
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