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Research Project: Sustaining Irrigated Agriculture in an Era of Increasing Water Scarcity and Reduced Water Quality

Location: Agricultural Water Efficiency and Salinity Research Unit

Title: Importance of the El Nino teleconnection to the 21st century California wintertime extreme precipitation increase

item ZECCA, KATHERINA - University Of California
item ALLEN, ROBERT - University Of California
item Anderson, Raymond - Ray

Submitted to: Geophysical Research Letters
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/26/2018
Publication Date: 10/1/2018
Citation: Zecca, K., Allen, R.J., Anderson, R.G. 2018. Importance of the El Nino teleconnection to the 21st century California wintertime extreme precipitation increase. Geophysical Research Letters. 45(19):10,648-10,655.

Interpretive Summary: Understanding how precipitation may change in the future with climatic variations is critical for long-term water resource planning for farmers and irrigation. In California, this is especially critical as most precipitation arrives in relatively few, extreme, wintertime events. These extreme precipitation events can be challenging for irrigation managers to capture, particularly if they are warmer, since managers must balance retaining stream flow in reservoirs for irrigation with flood protection. Ensembles of multiple climate models can assess future changes in extreme precipitation events. One commonly used model ensemble, the Coupled Model Intercomparison Project Phase 5 (CMIP5), has shown an overall increase in extreme winter precipitation while overall precipitation decreased. However, CMIP5 has large differences in extreme precipitation between different model runs, resulting in high uncertainty. Therefore, in this study, we evaluated extreme precipitation outcomes by evaluating CMIP5 model runs by how closely they simulate the El Niño ocean temperature phenomenon, which is closely linked to wetter winters in California. CMIP5 models that better simulate El Niño show that all changes in precipitation come from increased prevalence of extreme precipitation, rather than increased frequency of non-extreme precipitation events. These increased precipitation events result from enhance water vapor transport from the tropics. Comparison of daily, historic Pacific Ocean sea surface temperatures with model outputs show that model runs that better simulate El Niño also match observed sea surface temperatures on days that California experiences extreme precipitation. The results are of importance for irrigation and water managers who are using climate models to assess long-term water availability, flood risk, and potential need for additional storage infrastructure to capture extreme precipitation events. The results are also of importance for farmers who may need to use alternative techniques (such as conjunctive recharge flooding of fields) to capture more intense precipitation and who may need to mitigate additional flood risk.

Technical Abstract: Under continued climate warming, California (CA) hydrological projections, particularly precipitation, exhibit significant uncertainty. Recent analyses, however, have indicated a tendency for increased CA precipitation through the 21st century, particularly during December-January-February (DJF). Here, we show that this increase is due entirely to an increase in extreme (>90th percentile) daily precipitation. This response is consistent with enhanced horizontal vapor transport off the CA coast, most of which is caused by thermodynamical effects due to increases in atmospheric moisture. Furthermore, observations over the late-20th century show that CA DJF extreme precipitation is associated with an El Niño-like sea surface temperature (SST) anomaly pattern. Models that better simulate the observed El Niño teleconnection with CA DJF precipitation also better reproduce the El Niño-like SST anomaly pattern associated with extreme precipitation, including the associated thermodynamical and dynamical atmospheric responses. In turn, these models simulate a significantly larger increase in extreme precipitation under warming.