Understanding Forced Pacific Climate Responses with Large Ensembles

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ABSTRACT

Human activities reshape the climate system by altering both the mean state and its variability. The Pacific—home to Pacific Decadal Variability (PDV) and the El Niño–Southern Oscillation (ENSO)—mediates global hydroclimate, yet the forced component of these modes remains contested. This dissertation uses climate model large ensembles, including single-forcing experiments, to disentangle externally forced responses from internal variability. First, I show that the common PDV diagnostic, which removes only the global mean SST, aliases mean state trends into PDV and falsely implies a strong forced response. Using the ensemble mean to represent mean state change reveals no significant forced shift in PDV modes. Second, I examine solar geoengineering impacts on ENSO. Subtropical Pacific marine cloud brightening reduces ENSO variance by two-thirds, while stratospheric aerosol injection has little effect. A mixed-layer heat budget links suppression to a La Niña–like mean state that weakens the Bjerknes feedback. Third, I quantify intensification of global marine heatwaves, finding it is dominated by mean SST warming by late century, with variability playing a minor role. In the Northeastern Pacific, single-forcing ensembles show aerosols drove SST trends before ~1980 and greenhouse gases thereafter, while the recent abrupt warming reflects internal variability rather than Chinese aerosol reductions.
 

BIO

Chen Xing is a PhD candidate at the Bren School of Environmental Science & Management, where she studies climate variability and dynamics, with a focus on their past behavior and future response to human influence. Her doctoral research investigates how anthropogenic forcing influences Pacific decadal variability and marine heatwaves, and how solar geoengineering affects the response of the El Niño–Southern Oscillation. Following her PhD, she will join the University of Texas Institute for Geophysics as a distinguished postdoctoral fellow.