UC Irvine

As multi-year (MY) ENSO events become increasingly frequent in the 21st century, our traditional understanding of ENSO climate impacts is no longer sufficient. This dissertation investigates global climate impacts and associated mechanisms induced by MY and single-year (SY) ENSOs using a 2,200-year CESM1 pre-industrial simulation, forced Atmospheric General Circulation Model (AGCM) experiments, and observational datasets. The study finds that the most distinct impacts produced by MY ENSO compared to SY ENSO occurred on Antarctic sea ice concentration (SIC) during austral winter and on surface air temperature (SAT) over middle-to-high latitude continents during boreal winter. Therefore, the first three chapters of the dissertation focus on these two particular regions to provide a comprehensive understanding of the distinct impacts and the teleconnection mechanisms that enable these effects. Furthermore, this dissertation study observes that the distinct climate impacts of MY ENSO extend beyond the surface to affect soil moisture variations over four specific land areas of the world. The differential soil moisture responses between MY and SY ENSO events are documented, and the associated impact mechanisms are identified in the fourth chapter of the dissertation.In the Southern Hemisphere during austral winter, MY La Niñas induce a zonal shift in the tri-polar SIC anomaly pattern typically induced by the SY La Niñas (Chapter 2). This shift arises from the unique pre-onset conditions associated with MY La Niñas that interact with the Indian Ocean and subsequently trigger atmospheric circulation modes that affect the Antarctic SIC anomaly pattern. The dissertation further explores the Antarctic SIC response to MY El Niños and finds that MY El Niño's impacts on Antarctic SIC are not symmetric to MY La Niña's impacts on Antarctic SIC (Chapter 3). The asymmetry arises from the different pre-onset conditions between the El Niño and La Niña phases of MY ENSO events, which enable MY La Niña to only produce unique Indian Ocean conditions compared to SY La Niña, while MY El Niño can produce unique Atlantic and Indian Ocean conditions compared to SY El Niño. The findings obtained from the CESM1 simulations were verified, and the distinctive zonally-shifted SIC anomaly pattern is confirmed by observed ENSO events during the period from 1979 to 2020. In the Northern Hemisphere during boreal winter, the dissertation reveals that SY La Niña events typically induce anomalous warming over Europe and Western & Eastern Siberia, and cooling over North America. In contrast, MY La Niña events diminish the cooling over North America and the warming over Western & Eastern Siberia while intensifying warming over Europe (Chapter 4). These distinct impacts are more pronounced during the first winter, except for Western Siberia where the effect is stronger in the second winter. The dissertation conducts a series of statistical analyses to uncover how the diverse sensitivities of the four continent sectors are driven by sea surface temperature anomaly (SSTA) differences between SY and MY La Niñas in the Pacific, Atlantic, and Indian Oceans through various teleconnection mechanisms. The unique effects of MY La Niña on mid-to-high latitude continents, as revealed by the CESM1 simulation, are further verified by observed La Niña events. Finally, the dissertation examines the global soil moisture response to SY and MY La Niña events (Chapter 5) and finds that MY La Niñas can amplify the soil moisture impacts produced by the SY La Niña over four specific regions: drying over North America and the Middle East, and wetting regions over Australia and the Sahel desert. The study further reveals that the amplification effects are caused by different reasons in these four regions. The amplified wetting over Australia is attributed to both increased precipitation induced by the MY La Niña in the same winter and accumulation from the first winter, while the amplified wetting soil moisture over the Middle East is only attributed to accumulated soil moisture anomalies from the first winters. In North America, the amplified drying of soil moisture during the second winter is also attributed to soil moisture accumulation. The amplified wetting of soil moisture in the Sahel is linked to non-local moisture transport from the surrounding North Atlantic and Indian Oceans. Furthermore, the contributions of the three oceans to the soil moisture response are investigated by performing a series of forced AGCM experiments. These findings underscore the importance of considering the different duration of El Niño and La Niña events in understanding their impacts on various climate variables. Understanding these mechanisms is crucial for improving climate models, predicting future climate changes, and developing adaptation strategies to mitigate their impacts on ecosystems and societies.