Understanding the Failed Southern California Rains during the 2016 El Ni?o winter

Dr. Tao Zhang

Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, and Physical Sciences Division, NOAA/Earth System Research Laboratory (ESRL)

Room 319,Building No. 40

10:00am July 12, 2017

Abstract

　　Failure of Southern California (SCAL) winter rains during the 2015-16 strong El Ni?o came as a surprise and a disappointment. Similarities were drawn to very wet winters during several historical strong El Ni?o events, leading to heightened expectations that SCAL’s multi-year drought would abate in 2016. Atmospheric model simulations and coupled model seasonal forecasts are diagnosed to determine both the potential predictability and actual prediction skill of the failed rains, with a focus on understanding the striking contrast of SCAL precipitation between the 2016 and 1998 strong El Ni?o events. Ensemble mean simulations indicate that the December-February 2016 winter dryness was not a response to global boundary forcings, which instead generated a wet SCAL signal. Nor was the extreme magnitude of observed 1998 wetness entirely reconcilable with a boundary-forced signal, indicating it was not a particularly precise analogue for 2016.

　　Furthermore, model simulations indicate the SCAL 2016 wet signal was 20-50% less intense than its simulated 1998 counterpart. Such a weaker signal was captured in November 2015 initialized seasonal forecasts, indicating dynamical model skill in predicting a less prolific 2016 rainy season and a capability to forewarn that 2016 would unlikely experience the prolific flooding rains of 1998. Analysis of ensemble spreads indicate that 2016 dryness was an extreme climate event having less than 5% likelihood in the presence of 2016 global forcings. However, its probability of occurrence was 3-4-fold greater in 2016 compared to 1998. The failed seasonal rains themselves are mainly argued to be symptoms of sub-seasonal variability whose predictability remains to be explored. The role of SST forcings over different regions on 2016 SCAL Rains is also discussed.