The Role Of The Pacific North American Pattern On The Pace Of Future Winter

Warming Across Western North America

John AbatzoglouUniversity of Idaho

2nd Annual PNW Climate Science ConferenceSeattle, WA, Sep 14, 2011

Warming Trajectory

Annual Mean Temperature Change for the Pacific NW (Mote and Salathe, 2010)

What drives the pace of winter warming in the West?

Linear trend in 1 Apr SWE (1960-2002)

Mote (2006)

+.8 to +1.2°C

Pacific North American Pattern (PNA)

What is it?• Preferred atmospheric circulation regime• Proxy of “ridginess” of large scale waves• Positive phase: warm air into the West• Comingles with ENSO and PDO

Why does it matter?• Strong influence on surface air

temperature and snow-hydrology• Weather regimes and associated impacts

in PNW often tied to PNA phase

500hPa height pattern

Associated Time Series

Example #1: Seasonal differences“Spring Warming, Autumn Not”

Spring:

+0.28˚C/decade

Autumn: +0.07˚C/decade

Abatzoglou andRedmond (2007)

Mean Temperature 11 Western States

40% of the spring warming attributed to circulation changes

Variability has masked regional warming in autumnGCM 20C3M +0.15 to +0.2˚C/decade

Example #2: Loss of Mountain Snowpack

http://www.wrcc.dri.edu/cwd/products/

+ 60m/decade ~ +0.37 ˚C/decade, 50% more than projected changes

Pacific North American PatternResponsible for about a third of snowpack loss in Cascades since 1958

Freezing level Rain + Melt

SnowNo Melt

±400m per PNAINDEX

Abatzoglou (2011)Similar results to Casola et al., 2010

JFM Trend (%/decade, 1958-2005) in Percent of Precipitation as Snow

Observed Daily PNA Removed

-5 0 5

Fundamental Research Question

How will climate variability alter the pace of future winter warming across western North America?

• Can models simulate the PNA in today’s climate?

• Is the PNA sensitive to 21st century anthropogenic forcing?

• What are the implications for mountain snowpack loss?

Methodology

Models– 500hPa height, 12 CMIP3 GCMs, 1971-2000, 2001-2100 A1B– Downscaled daily temperature and precipitation

Diagnostics– Modified pointwise methods & one point correlation– Rotated Empirical Orthogonal Functions– Linear Least Squares Trend Analysis

Similar results using REOF

Can GCMs Simulate the PNA in today’s climate?One-point correlation maps of JFM 500hPa height to NW N. America node

NCEP-NCARReanalysis (1948-2010)Different GCMs

(1950-2000)

Most GCMs simulate PNA variance within 20% of observations.

Is the PNA sensitive to anthropogenic forcing?Monthly Linear Trends in the PNA (1961-2100)

Majority of models suggest a shift towards positive PNA regime in winter with anthropogenic forcing

PNA as a Pacemaker:Explained Variance of Winter Warming

r=0.83

Implications for Mountain Snowpack% Change in Precipitation as Snow, Jan-Mar (2046-2065 vs. 1971-2000)

Composite of four GCM with largest PNA trend

Composite of four GCM with smallest PNA trend

“Allied” forcing could double the loss of mountain snowfall

Downscaled output from MACA (Abatzoglou and Brown, 2011)

Summary

• Climate variability has played a significant role in modifying the pace of climate change and associated impacts in the western US

• GCMs suggest a shift toward the positive phase of the PNA with anthropogenic forcing: amplifier of regional change

• PNA explains a majority of variance in late-winter intermodel temperature variability across western North America

• Richer understanding of atm-ocean dynamics with anthropogenic forcing may lead to refinements in regional climate projections.

Similar results using REOF

Can GCMs Simulate the PNA in today’s climate?One-point correlation maps

Taylor Diagram: visual means of synthesizing pattern similarity

Power-spectra of PNA needs to be analyzed

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