The role of meridional modes in pacific climate variability and change
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Decadal changes in Pacific climate affect long-term transitions in marine ecosystems and influence the statistics of weather including ocean and atmosphere extremes such as strong droughts, hurricanes and marine heatwaves. However, the physical mechanisms that generate decadal variance and how these dynamics will change under anthropogenic forcing remains unknown. This research utilizes statistical and physical modeling to investigate the role of extratropical El Niño-Southern Oscillation (ENSO) precursors such as Pacific Meridional Modes (PMMs) in driving Pacific decadal variability and change. In the first part of this dissertation, we develop an interpretative framework for the Pacific decadal variability (PDV) in which the stochastic variability of the North Pacific Oscillation (NPO) energizes the decadal-scale sea surface temperature anomalies (SSTa) variability in the extratropics through PMM dynamics. The spatial evolution of the North PMM (NPMM) dynamics injects decadal variance into the tropics where the SSTa anomalies are amplified by the feedback dynamics characteristic of the ENSO system along the equatorial plane. After the SSTa are amplified in the tropics, ENSO teleconnections project the decadal-scale variance onto the climate modes of the extratropics (e.g., Pacific Decadal Oscillation and North Pacific Gyre Oscillation). The coupling between extra-tropics/tropics/extra-tropics acts as a primary mechanism to redden (e.g. energize the low frequency) the Pacific climate spectrum by prolonging the memory of the system to stochastic perturbation from the ENSO precursors. In the second part of this dissertation, we use the interpretative framework to explore how the PDV has changed in the observational record, and how it is expected to change according to the Community Earth System Model (CESM) Large Ensemble (LENS) under the RCP8.5 radiative forcing scenario. After designing a set of diagnostics to measure the interaction dynamics between NPMM and ENSO, we found that the variance of the NPMM increases in a warming climate because of an intensification of the thermodynamic coupling of the ocean and atmosphere, also known as the wind-evaporations-SST (WES) feedback. This feedback is dependent on the mean background state (i.e., warming of SST generates a stronger response in evaporation) and modulates the amplitude of the growth of NPMM. In the third part of this dissertation, we investigate the relative importance of the NPMM and South PMM (SPMM) in ENSO and tropical Pacific decadal variability (TPDV) by performing experiments with the CESM model in which the NPMM and SPMM are selectively suppressed. We find that both meridional modes energize the tropical variance independently on different timescales. The absence of NPMM leads to a significant reduction of the tropical interannual variability (~35%), while the absence of the SPMM has no appreciable impact on ENSO but significantly reduces the TPDV (~30%). While the relative importance of the NPMM and SPMM may be model dependent, the stochastic atmospheric variability in the extra-tropics that energizes the meridional modes emerges as a key source of TPDV.