Decadal Trends in Extreme Precipitation, Winds, and Snowpack over the Northwest. Cliff Mass University of Washington

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

Press Advisory National Wildlife Federation November 17, 2009 Contacts: XXXX (name removed) YYYY (you know her!) Senior Environmental Policy SpecialistClimate Scientist National Wildlife Federation, Pacific RegionNational 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 data[1] 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.

Others Have Suggested We Are Now Experiencing Extremes in Low Northwest Snowpack and That Anthropogenic Warming is A Significant Cause

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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

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.

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 society’s efforts to reduce greenhouse gas emissions and to precede with adaptation/mitigation.

Extreme Precipitation over the NW There are several studies in the literature

Trends of 7-day extreme (1 yr or >): and 1-day extremes were similar Little trend in the NW. Suggesting of small increases in western WA and decreases in western Oregon (tail indicates significant at 5% level) Kunkel, Andsager, and Easterling, J. of Climate, 1999

When it Rains it Pours (not reviewed) Used the Kunkel Approach for 1-day rainfall ( ) More over Wa, LESS over Oregon, little trend, N. CA.

Are there trends in major precipitation events? Examined top 20, 40, 60 two-day precipitation events at stations along the coast for

Trends on Unregulated Rivers : Max Annual Daily Discharge

Most modeling studies are inconclusive about whether global warming has influenced trends of extreme precipitation during the past century.

They ran contemporary dynamical downscaling ( ): Hadley regional model forced by Hadley Center GCM WRF forced by ECHAM5 GCM

“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

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 subtropics…often starting near or just north of Hawaii.

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?

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

Simulated seasonal precipitation changes (mm/day) from to from the ECHAM5- MM5 regional model for a) DJF b) MAM c) JJA and d) SON DJF

Other Research on Future Precipitation Trends over the NW: No Agreement Chen et al. (2003) examined the impacts of doubling CO 2 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.

Studies Kim (2005) used the MAS regional model downscaled from the HadCM2 GCM for 1990–2000 and 2040–49. 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.

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

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.

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

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?

Snowpack Little Hint of Decline Due to Anthropogenic Warming

Washington-wide snowpack since 1950, Relative to normal Snowpack in usual maximum month

Washington-wide snowpack period of record, Relative to normal Snowpack in usual maximum month

April 1 Snowpack

Remove Natural Variability Slow steady decline over the past 80 years—no indication of anthropogenic global warming signal

The NW is a location in which AGW temperature effects will be weaker and delayed temperature trends

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.

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.

The End

60- year Trends for Top-60 Events Individual Stations Two-degree bands