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Global analysis of recent frequency component changes in interannual climate variability Murray Peel1 & Tom McMahon1 1 Civil & Environmental Engineering, The University of Melbourne, Victoria, Australia Outline Background Temporal changes in frequency components – Empirical Mode Decomposition – Data set – Results for dividing year = 1970 – Sensitivity of results to the dividing year Conclusions EGU 2006 - Session AS1.07/CL040/CL007 Climate change impact on climate variability Mainly assessed at daily, monthly & seasonal scales – Changes in extreme event frequency – Changes in the shape parameter of the daily frequency distribution Less attention has been paid to the annual scale EGU 2006 - Session AS1.07/CL040/CL007 Background Potential modification of interannual climate variability is important – Multi-year drought severity – Reservoir reliability – Ecosystem dynamics EGU 2006 - Session AS1.07/CL040/CL007 Interannual Climate Variability What drives changes in the mean and variance of an annual time series? – Look at the components of a time series using spectral analysis Source: IPCC, Climate Change 2001 EGU 2006 - Session AS1.07/CL040/CL007 Spectral Analysis (EMD) Empirical Mode Decomposition (EMD) – Decomposes a time series into Intrinsic Mode Function(s) (IMFs) A residual (Trend) – Locally adaptive algorithm Robust to non-linear / non-stationary data – No data pre-processing (like removal of “trend”) EGU 2006 - Session AS1.07/CL040/CL007 Spectral Components EMD spectral components – High Frequency (<10 years; Intra-Decadal) Sum of IMFs with average period < 10 years – Low Frequency (>10 years; Inter-Decadal) Sum of IMFs with average period >= 10 years + the residual – Effectively using EMD as a high/low pass filter EGU 2006 - Session AS1.07/CL040/CL007 EMD Example Robe, South Australia Annual Precipitation (mm) . 1000 800 600 Observed High Component Low Component 400 200 0 -200 -400 1860 1880 1900 1920 1940 1960 Year Can assess temporal changes in component behaviour (pre and post a dividing date) EGU 2006 - Session AS1.07/CL040/CL007 1980 2000 Data Set Annual temperature and precipitation data from the GHCN (version 2) Choose 1970 as the dividing year – To maximise the number and spatial distribution of stations with >= 15 years of unbroken record pre- and post- the dividing year (N >= 30) – Annual temperature Stations = 1,524, average N = 63 years, ~1930 - 1993 – Annual precipitation Stations = 2,814, average N = 74 years, ~1920 - 1993) EGU 2006 - Session AS1.07/CL040/CL007 Results Robe Example VarRatio1970 = Variance>=1970 / Variance<1970 – Obs. = 0.65, High = 0.80, Low = 0.64 1000 Annual Precipitation (mm) . 800 600 Observed High Component Low Component 400 200 0 -200 -400 1860 1880 1900 1920 1940 Year EGU 2006 - Session AS1.07/CL040/CL007 1960 1980 2000 Results Temperature – 1,524 Stations VarRatio1970 Percentiles % Stations Obs. High Low 5% 0.41 0.45 0.09 50% 0.95 1.04 0.67 95% 1.97 2.08 5.36 >=1 44.3 53.3 38.3 <1 55.7 46.7 61.7 EGU 2006 - Session AS1.07/CL040/CL007 Temperature Observed VarRatio1970 EGU 2006 - Session AS1.07/CL040/CL007 No. Stations (>=1) > (<1) No. Stations (>=1) = (<1) No. Stations (>=1) < (<1) Results Precipitation – 2,814 Stations VarRatio1970 Percentiles % Stations Obs. High Low 5% 0.46 0.45 0.11 50% 0.95 0.98 0.79 95% 2.03 2.21 4.43 >=1 44.8 48.6 40.7 <1 55.2 51.4 59.3 EGU 2006 - Session AS1.07/CL040/CL007 Precipitation High Component VarRatio1970 EGU 2006 - Session AS1.07/CL040/CL007 No. Stations (>=1) > (<1) No. Stations (>=1) = (<1) No. Stations (>=1) < (<1) Sensitivity to dividing year Temperature 100% 80% Obs High Low Stations 1440 1280 70% 1120 60% 960 50% 800 40% 640 30% 480 20% 320 10% 160 0% 0 1920 1925 1930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 Dividing Year EGU 2006 - Session AS1.07/CL040/CL007 Stations % of Stations with VarRatio (>=1) 90% 1600 Sensitivity to dividing year Precipitation 4000 Obs High Low Stations % of Stations with VarRatio (>=1) 90% 80% 3600 3200 70% 2800 60% 2400 50% 2000 40% 1600 30% 1200 20% 800 10% 400 0% 0 1920 1925 1930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 Dividing Year EGU 2006 - Session AS1.07/CL040/CL007 Stations 100% Conclusions – Temperature VarRatio1970 – Observed: slight decrease – High Frequency (intra-decadal): slight increase – Low Frequency (inter-decadal): large decrease Variance moving from low to high frequency component, over much of the last century Decreasing the long-term memory Increasing the degree of randomness EGU 2006 - Session AS1.07/CL040/CL007 Conclusions – Precipitation VarRatio1970 – Observed: slight decrease – High Frequency (intra-decadal): slight decrease – Low Frequency (inter-decadal): large decrease Variance moving from low to high frequency component, over much of the last century Decreasing the long-term memory Increasing the degree of randomness – To a lesser extent than the temperature results EGU 2006 - Session AS1.07/CL040/CL007 Overall Conclusions Recent increase in intra-decadal fluctuations maybe due to climate change – Consistent with other research indicating that the degree of randomness will increase under a warmer climate Recent decrease in inter-decadal fluctuations may reduce the usefulness of teleconnection based forecasting systems EGU 2006 - Session AS1.07/CL040/CL007 Acknowledgements The analysis presented forms part of a paper under review at – Geophysical Research Letters Funded by – Australian Research Council Discovery Grant Useful Discussions – Geoff Pegram EGU 2006 - Session AS1.07/CL040/CL007