Ensemble experiments examining the role of Pacific Ocean forcing for multi-decadal drought in southwestern US

The role of Pacific Ocean SST anomalies for multi-decadal droughts in southwestern US was examined for case-studies within a 900 year global climate model simulation. Decadal precipitation variability in southwestern US is partly attributable to SST variability. In this region, however, atmospheric intrinsic variability can also induce long-term, low-frequency precipitation variability without ocean feedbacks. This suggests that some long-term droughts are driven by stochastic processes, and are not predictable based on the pattern of SST forcing. This study examines individual drought events in a perfect model scenario, in order to resolve the relative role of ocean forcing as compared to internal variability and land feedbacks.

Ensemble experiments with NCAR's Community Earth System Model (CESM) were used to explore the reproducibility of long-term droughts for southwestern US. Potential predictability of four multi-decadal droughts identified in the control simulation depends on the region or seasonality of precipitation anomalies, and the dominance of local moisture recycling as compared to oceanic driven circulation patterns. When ocean feedbacks are permitted, ensemble predictions initialized at the beginning of multi-decadal droughts fail to capture the upcoming precipitation anomaly, suggesting that atmospheric internal variability and/or land-feedbacks dominate the generation of these low-frequency precipitation anomalies. However, when western US multi-decadal droughts occur near the coast during winter, coincident with negative tropical and positive north Pacific Ocean SST anomalies, the precipitation anomalies can be reproduced by prescribing the SST forcing pattern for the atmospheric model. These case-studies suggest some predictability from recurrent SST anomalies for long-term droughts of certain characteristics. This has implications for analysis of the most recent drought in California, brought about by a La Nina-like Pacific Ocean SST pattern, and resulting in more than a decade of devastating impacts. Future development of effective forecasting methods requires further understanding of the relative roles of internal variability, ocean forcing and land feedbacks in the generation and persistence of various long-term droughts in southwestern US.