Sources of predictability for decadal drought in western North America in GCMs

Understanding the mechanisms of drought variability and persistence is critical for the eventual development of effective forecasting methods, and is motivated by the potential economic and social cost of future events. Global climate models are used to explore the dynamics and characteristics of long-term drought in western North America and low-frequency climate variability seen in proxy records, such as tree cores. Potential sources of predictability are identified, based on the relationships of drought with other climate variables, and the skill of the National Center for Atmospheric Research's (NCAR) Community Earth System Model (CESM) hindcasts relative to a control run. Decadal precipitation variability in western North America is hypothesized to be partly attributable to recurrent Pacific or Atlantic Ocean Sea Surface Temperature (SST) anomalies, as atmospheric fluctuations vary on much shorter time scales. Teleconnections from the Pacific Ocean to low-frequency precipitation in California and Nevada in winter are robustly represented in CMIP5 historical simulations. However, only 20 per cent of low-frequency precipitation variability in California is attributed to SST anomalies in the north Pacific Ocean. Precipitation anomalies coincide with atmospheric circulation and SST anomalies, but can also occur in the absence of the typical anomaly patterns. This suggests that some long-term droughts are driven by stochastic processes, and do not necessarily require ocean feedback for decadal memory. The respective roles of oceanic forcing, atmospheric noise and land feedbacks in persisting long-term drought-like conditions is an outstanding research question. Soil moisture in Texas and Oklahoma in April is shown to be positively correlated with North American monsoon precipitation for the following summer in southwest US in the CMIP5 models, indicating a potential source of non-oceanic interannual persistence. Ensemble simulations are used to examine the sensitivity to antecedent land conditions and atmospheric noise for the initiation, persistence and termination of megadroughts in western North America. Altering the soil moisture or snowpack conditions in CESM illustrates the effect of the changing the sources of memory in the land-atmosphere feedback on drought persistence.