UC San Diego

Decades of research have shown that the accumulation of greenhouse gases in the atmosphere due to anthropogenic emissions strongly influences global and regional climate. However, natural variability associated with intrinsic climate modes introduces substantial noise across myriad timescales, posing a significant challenge for the accurate simulation and projection of future climate variability and change.

In this dissertation, we utilize state-of-the-art climate model simulations in tandem with modern observational data sets to: 1. Investigate the physical mechanisms governing the evolution of internal climate modes on seasonal-to-decadal timescales, 2. Analyze the impact of those modes on remote climates through ocean-atmosphere teleconnections, and 3. Assess the interplay of internal and forced variability in order to better interpret observed climate trends. We show that the observed cooling of the tropical Pacific Ocean in recent decades primarily stems from a decadal intensification of the equatorial wind-driven ocean circulation during boreal winter. We then provide model evidence that internal variations associated with this decadal cooling superposed with anthropogenic trends to produce a period of accelerated Hadley Cell expansion that strongly mirrors observed trends since 1980.

We also consider the influence of extratropical climate variability on tropical climate. Specifically, we develop a novel statistical technique to isolate the spatiotemporal evolution of the Atlantic Meridional Mode (AMM) in observations. We show that the AMM integrates extratropical atmospheric noise into a deterministic pattern of propagating sea surface temperature (SST), wind, and latent heat flux anomalies. A comparison to CMIP5 reveals that most models tend to poorly simulate the relevant coupled feedbacks. We further explore these mechanisms using a fully-coupled climate model to produce an ensemble of North Pacific SST pacemaker experiments. We find that North Pacific SST has significantly influenced the observed trajectory of observed ENSO variability, particularly the 2014-2015 and 2015-2016 El Niños. These interactions are made possible through two physical pathways that arise from the evolution of the Pacific Meridional Mode (PMM).

The research presented in this dissertation improves understanding of the physical mechanisms controlling key internal climate modes, and takes a significant step towards quantifying tropical-extratropical interactions.