Climate Variability. Eric Salathé. Climate Impacts Group & Department of Atmospheric Sciences University of Washington. Thanks to Nathan Mantua

Transcription

1 Climate Variability Eric Salathé Climate Impacts Group & Department of Atmospheric Sciences University of Washington Thanks to Nathan Mantua

2 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

3 Mean SLP Summer fields Subtropical High the dominant feature shifts from the subtropical High in summer to the Aleutian Low in winter Winter L Aleutian Low

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5 Oregon Climate Service

6 Northwest terrain maps the big-picture winds and storms onto a complex landscape localized cold air outbreaks the Puget Sound Convergence Zone rain shadows

7 Arctic Blasts

8 The Puget Sound Convergence Zone

9 Annual average rain+snowfall:

10 The predictable part: seasonal rhythms Puget Sound Precip Insolation Insolation Oct Feb Jun Upwelling winds at 48N Jan May Sep Oct Feb Jun Amphitrite Pt SST Oct Feb Jun

11 Year to year variations on the seasonal rhythms Monthly Puget Sound Precip Daily Upwelling winds Monthly Amphitrite Pt SST

12 Northwest Climate Variability

13 Pollen records on the Olympic Peninsula Crocker Lake cedars cool-wet df alder fires: hot-dry pines cool McLachlan, J. S. and L. B. Brubaker Local and regional vegetation change on the northeastern Olympic Peninsula during the Holocene. Canadian J. of Botany.

14 Log10 mean flow, The Dalles, OR (cfs) The Dust Bowl ( ) was probably not the worst drought sequence in the past 250 years (based on Columbia Basin Tree-ring chronologies) red = observed, blue = reconstructed Year Source: Gedalof, Z., D.L. Peterson and Nathan J. Mantua. (2004). Columbia River Flow and Drought Since Journal of the American Water Resources Association.

15 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?

16 Warm and cool (or wet and dry ) halves of the year: oct-mar versus aprsep

17 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

18 Washington State Oct-Sept Average Temperature Washington State Oct-Sept Total Precip

19 Riffe Lake, west slopes of the Cascades Spring 2001

20 March 15 Snow depth anomalies at Paradise, Mt Rainier Avg=4 meters Avg ~ 4 meters (170 inches) January 5, 2005: 48 inches January 6, 2007: 130 inches

21 Water Year Columbia River streamflow Average annual runoff at The Dalles, Oregon ~ 150 Million Acre-Feet (MAF); Oct 2000-September 2001 ~ 100 MAF

22 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

23 Winter Aleutian Low Contrast the average event with the extreme winters of and

24 Oct 97-Mar 98: El Niño Oct 98-Mar 99: La Niña

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26 El Niño year precip anomalies Oct Mar 1998 La Niña year precip anomalies Oct Mar 1999

27 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

28 Regional patterns? Observations Regional Simulation Dry wet Leung et al 2003

29 composite avg PNW temperature and precipitation during El Niño and La Niña (based on averages of past century s events) EN-LN

30 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 , and again from ; a prologed cold era spanned ? Mantua et al. 1997, BAMS

31 October-March PDO Regression fields Maps show typical warm PDO climate anomalies Surface Air Temperature Precipitation dry Strong Aleutian Low Warm Warm Strong Aleutian Low dry wet dry dry Figures produced by Todd Mitchell, UW/JISAO

32 A history of the PDO warm cool warm A history of ENSO

33 Real time nowcasts of the PDO? 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 Monthly PDO index: 1900-Jan

34 PDO and PNW monthly temperatures and precipitation

35 PDO and Cascades snowpack

36 Water year stream flow composites for Columbia River natural flows at The Dalles, Oregon Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Average Flow (cfs) ColdPDO WarmPDO Month

37 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

38 Cool/Warm PDO and Paradise snowdepth histograms

39 Conclusions Primary modes of Pacific variability ENSO (El Niño/La Niña) PDO Both vary between Cool-wet Warm-dry Large impact on snowpack Large impact on streamflow ENSO is predictable with a few month lead PDO is not well understood or predictable Effects of climate change not well understood