Date of Award

Fall 2022

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Geology and Geophysics

First Advisor

Fedorov, Alexey

Abstract

The emerging warming patterns in the tropical Pacific are key to understanding earth’s climate response to increasing greenhouse gases (GHGs). This is true on interannual timescales dominated by El Niño/Southern Oscillation (ENSO), and on decadal and longer timescales, as Pacific trade wind variations and ENSO decadal modulation can influence temporary global warming trend slowdowns with links to climate sensitivity. While it is imperative to understand and predict changes in the tropical Pacific, progress is hindered by discrepancies between models and observations, diverging theories and uncertainty in projections. My thesis tackles these outstanding problems through five chapters. The first chapter provides a framework through which different theories about how the mean state tropical Pacific responds to warming can be investigated and reconciled. Through a hierarchy of modelling experiments, ranging from idealized box models to fully coupled global climate models (GCMs), I illustrate how the transient ocean-thermostat (OT) response to warming, which creates a strengthening of the Indo-Pacific SST gradient, is in competition with atmospheric effects that act to weaken the gradient. Over time, the OT weakens and gives way to eastern equatorial (EP) warming, which may in turn be amplified via extra-tropical warming and slowdown of meridional cells. In my second chapter, I investigate what drives differences in these responses across models. I compare two versions of the same GCM with different magnitudes of fast and slow responses and show how the strength of the initial response is linked to the mean state winds, which shapes the shift of the thermocline as well as wind-evaporation feedbacks off-equator. My third chapter applies this framework to investigate tropical Pacific climate change in the Coupled Model Intercomparison Project phase 6 (CMIP6). I compare change in the tropical Pacific mean state gradient across 40 different models and 5 different warming experiments. An investigation of historical simulations points to the role of aerosols, in combination with a possible OT, in delaying warming of the eastern equatorial Pacific throughout the historical era. These chapters show how the tropical Pacific mean state response to warming should be understood in two regimes: a fast and a slow response, and the balance between the regimes is sensitive to the model and the nature of forcing. My fourth chapter is concerned with understanding discrepancies between the observational record and historical model simulations. I show that observed sea surface temperature (SST) trends for 1980-2020 are dominated by three signals: uniform warming, a negative PDO pattern, and an enhanced northern hemisphere (ENH) warming pattern. CMIP6 historical simulations generally overestimate North Pacific warming but underestimate the western Pacific and Indian ocean warming, contributing to the models’ inability to replicate the Walker cell strengthening with implications for accurately projecting a future trend reversal. The fifth chapter is concerned with links between mean state changes and changes to ENSO. I show that a large majority of the models predict a stronger ENSO in a range of warming experiment, yet in the majority of models, the strongest forcing is not associated with the strongest ENSO response. Furthermore, changes in ENSO SST variability is poorly correlated with the mean state SST changes, while changes in ENSO rainfall variability is tied to changes in both mean state and ENSO SST variability. The Bjerknes Stability Index is generally a poor predictor for a small to moderate increase in ENSO strength, suggesting that changes to the mean state are offset by increases in atmospheric noise or/and potential nonlinear effects. These findings challenge the framework in which we should understand how ENSO responds to changes in the tropical Pacific mean state.

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