Systematic characterization of CMIP5 model spread in simulated tropical Pacific rainfall

Current-generation climate models exhibit various errors or biases in the spatial distribution and intensity of climatological rainfall relative to observations. Here, empirical orthogonal function analysis is applied to the space-model index domain of precipitation climatologies over the Pacific from Coupled Model Intercomparison Project Phase 5 (CMIP5) simulations to explore systematic variation across the ensemble. Two patterns, generically termed principal uncertainty patterns (PUPs), are identified in the December-January-February climatology: the leading PUP is associated with spread in the meridional width of deep convection, while the second is associated with tradeoffs in precipitation intensity along the South Pacific Convergence Zone (SPCZ), the Intertropical Convergence Zone (ITCZ), and the spurious Southern Hemisphere ITCZ. These PUPs manifest physically plausible relationships to spread in simulated sea surface temperatures (SSTs), circulation, and moisture. Further analysis of atmosphere-only, prescribed SST simulations demonstrates decreased spread in the spatial distribution of precipitation, while substantial spread in intensity remains.