21st century California drought risk linked to model fidelity of the El Nino teleconnection

Authors: ALLEN, ROBERT - University Of California; Anderson, Raymond - Ray

Submitted to: npj Climate and Atmospheric Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/27/2018
Publication Date: 9/3/2018
Citation: Allen, R.J., Anderson, R.G. 2018. 21st century California drought risk linked to model fidelity of the El Nino teleconnection. npj Climate and Atmospheric Science. 1:21. https://doi.org/10.1038/s41612-018-0032-x.
DOI: https://doi.org/10.1038/s41612-018-0032-x

Interpretive Summary: Understanding future water availability is critical for helping farmers and irrigation managers make long term plans for their operation. This is particularly true in California, where large inter-annual variations in precipitation result in large seasonal and annual differences in irrigation water availability. To assess water availability under future climate scenarios, researchers have used ensembles of multiple climate models to assess a range of likely outcomes for water availability. One commonly used model ensemble, the Coupled Model Intercomparison Project Phase 5 (CMIP5), has shown significant drying in California when the mean of the model outputs is used to assess drought risk. However, CMIP5 has large differences in hydrologic outputs between different model runs, resulting in high uncertainty over future hydrologic changes in California. Therefore, in this study, we evaluate individual 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 reduced risk of drought compared to CMIP5 models that poorly simulate El Niño and increased intensity of rain during the winter months (December-February). The results are of importance for irrigation and water managers who are using climate models to make long term projections of water availability as well as for farmers who may need to use alternative techniques (such as conjunctive recharge flooding of fields) to capture intensified winter runoff for summer irrigation.

Technical Abstract: Greenhouse gas induced climate change is expected to lead to negative hydrological impacts for southwestern North America, including California (CA). This includes a decrease in the amount and frequency of precipitation and an increase in evapotranspiration, both of which imply a decline in surface water availability, and an increase in drought and stress on water resources. However, a recent study showed the importance of tropical Pacific sea surface temperature (SST) warming and an El Nino-like teleconnection in driving an increase in CA precipitation through the 21st century, particularly during winter (DJF). Here, we extend this prior work and show wetter (drier) CA conditions, based on several hydrological metrics, are associated with an El Nino (La Nina)-like SST pattern on decadal to multi-decadal time scales. Models that poorly simulate the observed DJF El Nin˜o-CA precipitation teleconnection also poorly simulate these longer-term relationships, and yield a significant increase in 21st century annual and multi-annual CA drought risk. In contrast, models that better simulate the observed teleconnection also better simulate the longer-term relationships, and yield negligible change in the risk of 21st century annual and multi-annual CA drought. Seasonally, however, CA drought risk for most metrics is projected to significantly increase during the non-winter months, particularly in the models that poorly simulate the observed teleconnection. Thus, our results show that future projections of CA drought are dependent on the ability of models to simulate the El Nino CA precipitation teleconnection. Although the increase in CA drought risk is muted in models that better simulate the teleconnection, continued concern is warranted, particularly during the dry season.