Statistical Analyses of Historical Monthly Precipitation Anomalies Beginning 1900 Phil Arkin, Cooperative Institute for Climate and Satellites Earth System Science Interdisciplinary Center, University of Maryland And Tom Smith, Matt Sapiano and Ching-Yee Chang

Global Precipitation Datasets GPCP (left)/CMAP (right) mean annual cycle and global mean time series Monthly/5-day; 2.5° lat/long global; both based on microwave/IR combined with gauges Both used in IPCC AR4

Climate Model-Based Precipitation A number of climate models have been used to simulate the 20 th Century and precipitation from those runs can be compared to global precipitation datasets A number of climate models have been used to simulate the 20 th Century and precipitation from those runs can be compared to global precipitation datasets 30 years available from GPCP and other global precipitation datasets not long enough to confidently validate model simulations 30 years available from GPCP and other global precipitation datasets not long enough to confidently validate model simulations

Reconstruction of Near-Global Precipitation Variations for the 20 th Century Base Satellite Data Base Satellite Data Need global satellite analyses for reconstruction statistics Need global satellite analyses for reconstruction statistics GPCP, CMAP and MSAP tested; GPCP works best GPCP, CMAP and MSAP tested; GPCP works best Direct Reconstructions: fitting data to Empirical Orthogonal Functions (REOF) Direct Reconstructions: fitting data to Empirical Orthogonal Functions (REOF) EOF (or PC) analysis, for covariance maps EOF (or PC) analysis, for covariance maps Fit available gauge-station data to a set of covariance maps Fit available gauge-station data to a set of covariance maps Yields monthly gauge-based 5-degree analyses available beginning 1900 Yields monthly gauge-based 5-degree analyses available beginning 1900 Indirect Reconstructions: using Canonical Correlation Analysis (RCCA) Indirect Reconstructions: using Canonical Correlation Analysis (RCCA) Correlate fields of sea-surface temperature (SST) and sea-level pressure (SLP) with fields of precipitation during satellite era Correlate fields of sea-surface temperature (SST) and sea-level pressure (SLP) with fields of precipitation during satellite era Both SST and SLP analyzed for the 20 th century Both SST and SLP analyzed for the 20 th century

Oceanic Precipitation EOF and CCA-based reconstructions from 75°N - 75°S EOF and CCA-based reconstructions from 75°N - 75°S Major differences on long time scales evident before 1980 Major differences on long time scales evident before 1980 REOF does not contain the trend seen in RCCA & in AR4 ensemble REOF does not contain the trend seen in RCCA & in AR4 ensemble Interannual variability similar in both Interannual variability similar in both AR4 ensemble has upward trend as does RCCA, although smaller in magnitude AR4 ensemble has upward trend as does RCCA, although smaller in magnitude

Merged Reconstructions REOF reliable over land where gauges are available REOF reliable over land where gauges are available Interannual REOF reliable over oceans, but multi-decadal REOF less reliable over oceans Interannual REOF reliable over oceans, but multi-decadal REOF less reliable over oceans Multi-decadal RCCA appears to be more reliable over oceans Multi-decadal RCCA appears to be more reliable over oceans Merge by replacing ocean multi-decadal REOF with ocean multi-decadal from RCCA Merge by replacing ocean multi-decadal REOF with ocean multi-decadal from RCCA For recent period, use REOF(GPCP) For recent period, use REOF(GPCP)

Merged Reconstruction Near-Global Averages Filtered Reconstructions for All Areas and Ocean Areas Filtered Reconstructions for All Areas and Ocean Areas Ocean average changes most Ocean average changes most Big increase in mid 1970s; suggestion of something similar around 1920 Big increase in mid 1970s; suggestion of something similar around 1920 Including land greatly smoothes interannual variations Including land greatly smoothes interannual variations

Normalized Joint EOF Merged Reconstruction and AR4 Ensemble Merged Reconstruction and AR4 Ensemble Both annual averaged and filtered before computing JEOFs Both annual averaged and filtered before computing JEOFs First mode indicates joint trend-like variations First mode indicates joint trend-like variations Tropical ENSO-like increase Tropical ENSO-like increase Mid-latitude decrease Mid-latitude decrease High-latitude increase High-latitude increase Pattern differences may reflect model biases Pattern differences may reflect model biases

Summary EOF-based reconstructions resolve oceanic interannual variations through the 20 th century EOF-based reconstructions resolve oceanic interannual variations through the 20 th century Direct reconstruction using the available gauge data Direct reconstruction using the available gauge data Over land REOF does best for all variations Over land REOF does best for all variations CCA-based reconstructions resolve oceanic multi-decadal variations through the 20 th century CCA-based reconstructions resolve oceanic multi-decadal variations through the 20 th century Indirect method using correlations with better sampled variables Indirect method using correlations with better sampled variables Merged analysis takes advantage of the best qualities of both Merged analysis takes advantage of the best qualities of both Future improvements possible with new data or refined reconstruction methods Future improvements possible with new data or refined reconstruction methods Extended reanalyses may yield independent precipitation information Extended reanalyses may yield independent precipitation information The merged reconstruction has some important potential applications The merged reconstruction has some important potential applications

Potential Uses of Reconstructed Precipitation Diagnostic/descriptive studies of global precipitation variations on interannual to multi- decadal time scales Diagnostic/descriptive studies of global precipitation variations on interannual to multi- decadal time scales Changes in ENSO, PDO, NAO, AMO over the 20 th century can be better described and understood Changes in ENSO, PDO, NAO, AMO over the 20 th century can be better described and understood Oceanic influence on dry and wet regimes, particularly multi-year droughts, can be more clearly diagnosed Oceanic influence on dry and wet regimes, particularly multi-year droughts, can be more clearly diagnosed Validate and improve climate model simulations/projections of precipitation Validate and improve climate model simulations/projections of precipitation Longer baseline of observed precipitation should facilitate improvement of the models Longer baseline of observed precipitation should facilitate improvement of the models And might be used to enable statistical adjustment of model output And might be used to enable statistical adjustment of model output Data available at