Modulation of El Niño - Southern Oscillation and its impacts by the mean climate state

This doctoral thesis aims to understand how changes in the background climate state can affect the dynamics and regional impacts of El Niño-Southern Oscillation (ENSO). Given the global reach of ENSO teleconnections, it is key to understand how natural and anthropogenic sources of climate variability modulate its dynamics, in order to improve the resilience of ecosystems and populations to ENSO impacts. Climate interactions between the Atlantic and Pacific Oceans
at different timescales modulate the tropical Pacific mean state and ENSO variability. Firstly I use the NCAR-CESM1 model to investigate how Atlantic Multidecadal Variability (AMV) modulates ENSO. Under warm AMV conditions (AMV+), ENSO variability weakens, relative to AMV-. In AMV+, the thermocline is less reactive to surface wind stress anomalies, which are more confined over the western equatorial Pacific. Building on the first results chapter, I explore the AMV modulation of ENSO impacts over Australia in austral summer, and find that AMV strengthens the climate response to ENSO in western Australia and weakens it over the east. AMV alters the Hadley circulation response to ENSO, ultimately modulating its extratropical teleconnections. Finally, a set of pacemaker simulations are performed with the EC-Earth3-CC model to understand how the impacts of very strong El Niño, such as the ones observed in 1982/83, 1997/98 and 2015/16 would manifest in a future warmer climate. The simulations reveal that the impacts of extreme El Niño events will amplify in the future in some regions, including North America and Australia, where the cold and hot anomalies during winter and summer, respectively, will get amplified during very strong El Niño events. In
other regions, like Southern Africa and southeastern South America, shifts in atmospheric circulation lead to a weakening of the simulated impacts of very strong El Niño events. Overall, this thesis has further demonstrated that both ENSO variability and teleconnections are sensitive to changes in the background climate state induced by AMV and anthropogenic emissions of greenhouse gases.