Seattle Public Library Talk

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Decadal Trends in Extreme Precipitation, Winds,
and Snowpack over the Northwest. Cliff
Mass University of Washington
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There has been a lot discussion in the media and
in some of the literature suggesting that western
U.S. extreme precipitation events have already
increased in intensity or will soon increase
under anthropogenic global warming
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Press Advisory National Wildlife
Federation November 17, 2009 Contacts XXXX
(name removed) YYYY (you know her!) Senior
Environmental Policy Specialist Climate
Scientist National Wildlife Federation, Pacific
Region National Wildlife Federation Global
warming is exacerbating extremely heavy rainfall
events, and recent climate change modeling
suggests that these pineapple express storms are
no exception. Heavier rainfall events combined
with significant snowmelt in midwinter is just
what the Pacific Northwest should expect from
global warming, said Dr. YYYY, climate
scientist, National Wildlife Federation. .
Recent data1 suggests that as a result of
climate change, the largest storms (i.e., those
that are most likely to cause flooding) will
trend toward producing increasingly larger
quantities of precipitation.
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Others Have Suggested We Are Now Experiencing
Extremes in Low Northwest Snowpack and That
Anthropogenic Warming is A Significant Cause
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XXXXXXXXX
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And some have suggested that cold season storms,
like our big windstorms, will also get stronger
There is also evidence of an increase in the
intensity of storms in both the mid- and
high-latitude areas of the Northern Hemisphere,
State X
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This talk will suggest that some of these claims
are inconsistent with observations and the latest
modeling results.The truth is more complex and
nuanced.
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Our credibility is at stake

• Anthropogenic global warming IS a serious problem
  that must be dealt with.
• But by claiming excessive impacts on extremes we
  undermine our credibility and our ability to
  enhance societys efforts to reduce greenhouse
  gas emissions and to precede with
  adaptation/mitigation.

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Extreme Precipitation over the NWThere are
several studies in the literature
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Trends of 7-day extreme (1 yr or gt) 1931-963
and 1-day extremes were similar
Kunkel, Andsager, and Easterling, J. of Climate,
1999
Little trend in the NW. Suggesting of small
increases in western WA and decreases in western
Oregon (tail indicates significant at 5 level)
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When it Rains it Pours (not reviewed) Used the
Kunkel Approach for 1-day rainfall (1948-2006)

• More over Wa, LESS over Oregon, little trend, N.
  CA.

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Are there trends in major precipitation events?

• Examined top 20, 40, 60 two-day precipitation
  events at stations along the coast for 1950-2008.

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Trends on Unregulated Rivers1950-2009 Max
Annual Daily Discharge
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Most modeling studies are inconclusive about
whether global warming has influenced trends of
extreme precipitation during the past century.
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• They ran contemporary dynamical downscaling
  (1970-2007)
• Hadley regional model forced by Hadley Center
  GCM
• WRF forced by ECHAM5 GCM

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The lack of correspondence between observed and
simulated trends for extreme precipitation likely
results from the dominance of natural variability
over anthropogenic trends during the period
1970-2007.
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Nearly are all heavy Northwest precipitation
events are associated with the Pineapple
Express (a.k.a. atmospheric rivers)

• A relatively narrow current of warm, moist
  air from the subtropicsoften starting near or
  just north of Hawaii.

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Atmospheric Rivers

• Atmospheric rivers are closely associated with
  the jet stream (on its southern side)
• Most General Circulation Models suggest the jet
  stream will move north and weaken under global
  warming.
• Thus, the atmospheric rivers may well move
  northward.
• Could our region thus experience a DECREASE in
  extreme precipitation?

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Regional and Global Climate Simulations Indicate
Substantial Uncertainty for Extreme Precipitation
over the Northwest Under Global Warming

• Some even suggest a decline in winter
  precipitation

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DJF
Simulated seasonal precipitation changes (mm/day)
from 1989-1999 to 2045-2055 from the ECHAM5- MM5
regional model for a) DJF b) MAM c) JJA and d) SON
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Other Research on Future Precipitation Trends
over the NW No Agreement

• Chen et al. (2003) examined the impacts of
  doubling CO2 using MM5 and RegCM2 regional models
  to downscale the CCSM GCM simulation.
• Found an increase in average precipitation over
  central California and a decline over the Pacific
  Northwest.

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Studies

• Kim (2005) used the MAS regional model
  downscaled from the HadCM2 GCM for 19902000 and
  204049. Found the largest increases in extreme
  precipitation over northern California and
  Oregon, with little trend over Washington and
  southern California.
• Tebaldi et al. (2006) examined nine general
  circulation models found a general increase of
  extreme precipitation over the NW, with the
  magnitude of the positive trend increasing to the
  north, while decreasing or constant intensity was
  predicted over central and southern California.

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Studies

• Duffy et al. (2006) used four regional climate
  models nested within two global oceanatmosphere
  climate models, finding that the spatial
  distributions of precipitation vary substantially.

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Extreme Precipitation Bottom Line

• The uncertainty of what will happen over the NW
  is huge.
• Simplistic talk about warming temperatures,
  causing more water vapor resulting in more
  extreme precipitation here should be avoided.

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West Coast Windstorms

• Increasing number of major windstorms from
  northern Oregon into southern BC
• Decreasing numbers to the south.

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West Coast Windstorms

• Northwest windstorms are closely associated with
  the strength and position of the jet stream.
• If the jet stream weakens and moves northward,
  what will happen to our winter storms?
• Will the decline in windstorms over the southern
  PNW move northward?

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Snowpack Little Hint of Decline Due to
Anthropogenic Warming
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1950-2000
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Washington-wide snowpack since 1950, Relative to
1971-2000 normal
Snowpack in usual maximum month
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Washington-wide snowpack period of
record, Relative to 1971-2000 normal
Snowpack in usual maximum month
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April 1 Snowpack
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Remove Natural Variability
Slow steady decline over the past 80 yearsno
indication of anthropogenic global warming signal
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The NW is a location in which AGW temperature
effects will be weaker and delayed
1976-2007 temperature trends
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Conclusions

• Changes in weather extremes over the Northwest
  have been highly heterogeneous, increasing some
  places and decreasing in others.
• Natural variability appears to be dominant.
• There is no compelling evidence that
  anthropogenic warming has had any influence on
  changes in Northwest weather extremes.

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Conclusions

• The future impact of anthropogenic global warming
  on NW extremes is highly uncertain.
• It may well be that extremes in precipitation and
  winds could decline over the NW under global
  warming.
• We must be very careful in communicating such
  uncertainties to our user communities.

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The End
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60-yearTrendsfor Top-60Events
Individual Stations
Two-degree bands