ESRL/PSD Seminar Series

The role of SST anomalies, soil moisture and internal variability in decadal drought in western North America

Sally Langford
CIRES/ATOC, University of Colorado, Boulder


Western North America is susceptible to severe impacts of decadal to multi-decadal droughts, as evidenced by tree-core or lake sediment records. Future predictions also suggest that this region will become more arid. Understanding the mechanisms of drought variability and persistence is critical for the eventual development of effective forecasting methods for water resource planning. The ocean is expected to be the main source of potentially predictable decadal memory in the system, as the atmosphere varies on a much shorter timescale. However, low-frequency precipitation anomalies in western North America can occur in the absence of ocean feedbacks. Sea surface temperature anomalies in north Pacific Ocean are associated with around 20 per cent of low-frequency winter precipitation in California in the CMIP5 historical runs. Therefore, long-term drought in western North America may be generated by unpredictable atmospheric noise, or persisted by other sources of low-frequency variability, such as land processes and feedbacks. Water storage, vegetation and fire frequency are more predictable on longer timescales than precipitation, as measured by anomaly correlation for hindcasts compared to a 'perfect model' control run with CESM1.0.3. The importance of SST anomalies or antecedent land conditions in initiating and persisting decadal droughts in western North America is explored with ensemble simulations of CESM1.0.3. The model results confirm the importance of internal variability, SST forcing and land processes in projections of future decadal hydroclimate; the relative role of each process differs for droughts with varying characteristics.

Wednesday, June 11th
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