poleward amplification of northern hemisphere weekly snowcover extent trends
DESCRIPTION
Poleward amplification of Northern Hemisphere weekly snowcover extent trends. Stephen D éry & Ross Brown ENSC 454/654 – “Snow and Ice”. Outline. Background Motivation & Goals Data, Methods & Data Issues Results Discussion Conclusions. Background. - PowerPoint PPT PresentationTRANSCRIPT
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Poleward amplification of Northern Hemisphere weekly snowcover extent
trends
Stephen Déry & Ross Brown
ENSC 454/654 – “Snow and Ice”
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Outline
• Background• Motivation & Goals• Data, Methods & Data Issues• Results• Discussion• Conclusions
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Background
• Northern Hemisphere snowcover extent (SCE) varies between 4-46 x 106 km2.
• Its distinct properties makes snow a key component of global climate.
• Snow responds to changes in surface air temperatures & precipitation, thus providing another indicator of climate change.
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• Previous studies reveal a 5% per decade decline in Northern Hemisphere SCE (Frei & Robinson 1999).
• Declining mountain snowpacks & earlier spring freshets have been observed in recent decades over western North America (Mote et al. 2005; Stewart et al. 2005).
• Changes in snow depth & snowcover duration have also been recorded (Brown and Braaten 1998; Stone et al. 2000).
Background
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Motivation & Goals
• In light of near-record warmth in 2006 & the recent changes observed in the cryosphere, there is an urgent need to better understand SCE trends.
• Objective: To develop & interpret weekly trends in Northern Hemisphere (NH), North American (NA) & Eurasian (EU) SCE for the period 1972-2006.
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• Weekly values of SCE from January 1972 to December 2006 from Rutgers University.
• Monotonic trends in weekly SCE assessed with Mann-Kendall test (MKT) over NH, NA (excluding Greenland) & EU.
• MKT assumes a linear trend in the form: S = mt + b (1)
• Where S is SCE, t is time (year) & m is the slope of the linear trend given by:
Data & Methods
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mk = (Sj – Si)/(tj – ti),
k = 1, 2, …, n(n-1)/2
i = 1, …, n-1
j = 2, 3, …, n
• All of the slopes mk are then ranked, with the median value representing the slope m of the linear trend.
• The coefficient b is found by substituting the median values of SCE & time in Eq. (1) & solving for b.
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Strength of the MKT
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Strength of the MKT
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• Trends expressed in absolute values (× 106 km2), as a % from initial (1972) values, in standardized units, & insolation-weighted anomalies.
• Time series of weekly SCE data (Si) are standardized by:
SSi = (Si – Si)/σi , (i =1-53)
• Insolation-weighted anomalies are computed by multiplying the absolute values of SCE by the ratio of the average & maximum weekly incoming solar radiation at 60oN.
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Data Issues• Continental snowcovers exhibit temporal
persistence.• This implies positive autocorrelation of
SCE values, meaning that time series of subsequent weekly SCE values do not form independent datasets.
• Thus methodologies must be developed to reduce/remove the effect of serial correlation on trend analyses.
• Trends & correlations are considered statistically-significant when p < 0.05.
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Number of weeks with significant autocorrelations of continental SCE
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Year-to-year autocorrelation in continental SCE
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Results
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Summary of Trend Analysis
Statistic N.H. N.A. Eurasia
Mean SCE (×106 km2)
23.3 8.4 14.9
SCE Trend (×106 km2)/35 years
-1.32 -0.80 -0.49
Positive significant trends
2 0 4
Negative significant trends
24 23 20
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Discussion
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Coherent variability & signal
• Correlation between standardized NA & EU weekly SCE is r = 0.41 (p < 0.001).
• Standardized weekly SCE are of the same sign 64% of the time (88% when greater than ±1 standard deviation).
• Correlation between NA & EU trends in standardized SCE is r = 0.83 (p < 0.001).
• This implies a hemispheric-scale process may be acting on continental snowcovers.
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Poleward amplification of trends• Linear regressions on standardized SCE
trends (January to early August) yield correlation coefficients of -0.89 to -0.96.
• This suggests a poleward amplification of SCE anomalies owing to persistence in the cryospheric system.
• Negative trends in early spring SCE amplify during late spring & summer, with implications to the growing season, vegetation growth, species composition, …
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Poleward Amplification
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Snow-albedo feedback
• Trends in insolation-weighted SCE values show greatest changes near the summer solstice.
• This feature, in addition to the spatial coherence of the intercontinental snowcovers & temporal persistence on weekly & annual time scales, are possible manifestations of snow-albedo feedback.
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Conclusions• Strong negative trends in NH, NA, & EU
weekly SCE (1972-2006) are observed.• These trends are influenced by temporal
persistence (i.e. serial correlation) in the cryospheric system.
• Similar behaviour in NA & EU snowcovers, including covariability, persistence, & amplified trends in spring/summer provides evidence of the snow-albedo feedback acting on a hemispheric-scale.
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Postscript: Autocorrelation
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