1. Cooperative Research Programs (CoRP) Satellite Climate Studies Branch (SCSB) 1 1 Reconstruction of Near-Global Precipitation Variations Thomas Smith 1 Phillip Arkin 2 1. NOAA/NESDIS/STAR SCSB and CICS, College Park, Maryland 2. CICS/ESSIC/University of Maryland, College Park, Maryland

2. Cooperative Research Programs (CoRP) Satellite Climate Studies Branch (SCSB) 2 2 Precipitation Reconstructions Beginning 1900 Want to understand precipitation change with climate change –Need ocean-area precipitation anomalies Satellite data for global analyses since 1979 –Defines statistical properties of anomalies Gauges over land and islands since 1900 –For reconstructions with spatial covariance –EOFs define covariance (called REOF) –First regional-monthly REOF, later test global-annual REOF Analyses of SST and SLP available since 1900 or earlier –For reconstructions using anomaly correlations between precip. and SST+SLP –CCA used (called RCCA), for global-annual analysis Merging REOF and RCCA can give an improved reconstruction Other improvements being developed and tested

3. Cooperative Research Programs (CoRP) Satellite Climate Studies Branch (SCSB) 3 3 REOF Method Find historical time series for a set of covariance EOF maps Covariance maps based on modern data (satellites) Fit the historical data to the set of modes, minimize the error of the fit Data, D, at spatial point, x, and time, t, is anomaly, a, minus a first guess REOF anomaly is first guess + weighted sum of covariance modes Best fit to get weighted sum, Minimizes squared error over spatial locations,

4. Cooperative Research Programs (CoRP) Satellite Climate Studies Branch (SCSB) 4 4 System of Equations For Best-Fit Weights Differentiate squared error w.r.t. each weight System of equations to solve to get the best-fit weights: Screen out modes not adequately sampled Cross validation used to tune number of modes & screening parameters Can use since monthly anomalies are approximately normal

5. Cooperative Research Programs (CoRP) Satellite Climate Studies Branch (SCSB) 5 5 REOF Data Monthly Gauge Historical Analyses Global Historical Climatology Network (GHCN) –Longest record, least filling Global Precipitation Climatology Center (GPCC) –Used in GPCP modern analysis Climate Research Unit (CRU) –Shortest record, most filling Gauge-Satellite: Global Precipitation Climatology Project (GPCP) Merges GPCC gauges and multi-satellite estimates Global-monthly analysis since 1979, used for statistics

6. Cooperative Research Programs (CoRP) Satellite Climate Studies Branch (SCSB) 6 6 Against SOI Against NAO (Dec-Mar) REOF Climate-Mode Regressions: Consistent Interannual Variations

7. Cooperative Research Programs (CoRP) Satellite Climate Studies Branch (SCSB) 7 7 REOF Spatial Standard Deviation Global spatial standard deviation –Similar interannual changes, REOF(GHCN) lower before 1940 (sampling) –REOF(CRU) strong most of record, blended with REOF(GPCP) and used in merged analysis

8. Cooperative Research Programs (CoRP) Satellite Climate Studies Branch (SCSB) 8 8 Reconstructions Using CCA (RCCA) REOF interannual variations consistent, but multi- decadal variations less consistent Annual Canonical Correlation Analysis (CCA) to reconstruct longer-period variations –Fields of predictors (SST and SLP anomalies) correlated with a predictand field (precipitation anomalies) –Train CCA using GPCP data and SST+SLP analyses –RCCA to reconstruct annual-average global anomalies beginning 1900 RCCA only uses gauges in training period

9. Cooperative Research Programs (CoRP) Satellite Climate Studies Branch (SCSB) 9 9 Ocean Comparisons (7-Year Filtered) RCCA & REOF differ before 1980 –1970s climate shift in RCCA –REOF does not resolve trend in RCCA & in AR4 ensemble RCCA & REOF(GPCP) similar

10. Cooperative Research Programs (CoRP) Satellite Climate Studies Branch (SCSB) 10 Merged Reconstructions Over land –REOF Over oceans –REOF interannual and RCCA multi-decadal Comparison to AR4 Ensemble –1 st mode, compares model and reconstruction response to climate change Merged reconstruction available at http://cics.umd.edu/~tsmith/recpr/

11. Cooperative Research Programs (CoRP) Satellite Climate Studies Branch (SCSB) 11 GPCP-Period Trends Full GPCP trends (upper) show the most spatial variation GPCP filtered with monthly- regional REOF smoother RCCA is similar to GPCP, but with larger spatial scales and stronger local trends Testing to evaluate differences in trends

12. Cooperative Research Programs (CoRP) Satellite Climate Studies Branch (SCSB) 12 Global-Annual EOF Tests: Ocean Comparisons 20 mode global-annual EOF –GHCN only & GHCN + PSST (pseudo data from regression against SST) –No PSST: Global EOF multi-decadal change similar to RCCA –With PSST: The trend is stronger and spatial scales of the trend are larger

13. Cooperative Research Programs (CoRP) Satellite Climate Studies Branch (SCSB) 13 Summary Regional-monthly REOF resolves oceanic interannual variations, but is less consistent for oceanic multi-decadal variations Global-annual RCCA using SST & SLP indicates oceanic multi-decadal increasing precipitation, roughly consistent with climate models Merged analysis combining the regional-monthly REOF and the RCCA has been developed and is available at http://cics.umd.edu/~tsmith/recpr/http://cics.umd.edu/~tsmith/recpr/ Tests using a global-annual EOF show: –Global multi-decadal variations in RCCA are similar to those from a global-annual EOF reconstruction computed from only gauges –Trends are stronger and spatial scales are larger using SST information Reconstruction testing and improvements are continuing

14. Cooperative Research Programs (CoRP) Satellite Climate Studies Branch (SCSB) 14 Extra Slides

15. Cooperative Research Programs (CoRP) Satellite Climate Studies Branch (SCSB) 15 Anomaly Distribution Relative frequency distribution for common months and locations (1992-2001) Both anomalies approximately normal –reconstruction methods should work

16. Cooperative Research Programs (CoRP) Satellite Climate Studies Branch (SCSB) 16 Trend Maps: Global EOF and RCCA Global EOF using only gauges: – Trend has smaller-scale patterns Global EOF using gauges and pseudo-observations from SST: –Trend has larger-scale spatial features RCCA –Large-scale features, roughly similar to those in EOF with pseudo-observations Many large-scale features similar in all –Use of correlated data (SST or SLP or both) causing trend spatial scales to expand

17. Cooperative Research Programs (CoRP) Satellite Climate Studies Branch (SCSB) 17 Global-Annual EOF Tests: Gauge Area 20 mode global-annual EOF –GHCN only & GHCN + PSST –Gauge average: heavy black dashed line No PSST: EOF similar to gauge average With PSST: negative trend before 1950