Recent findings suggest that anthropogenic forcing is changing the relative importance of the processes that energize the modes of Pacific Decadal Variability (PDV) and their impact on climate extremes over North America. Specifically, the principal investigators (PIs) of this project have documented (1) statistically significant changes in the Kuroshio-Oyashio Extension (KOE) region and its air-sea coupling dynamics, and (2) the dynamics of two-way tropical-extratropical coupling between the North Pacific and Central Tropical Pacific (NP-CP), which involve the extratropical precursor dynamics of El Niño. There is growing evidence that changes in these processes are directly linked to the recent intensification of marine heatwaves and precipitation extremes observed over western North America in the last decade. However, it remains unclear how these processes are represented in the Modes of Variability (MOVs) in Earth System Models (ESMs), and how they are projected to change in a changing climate. This project will develop new understanding and novel approaches aimed at diagnosing and comparing the dynamical processes of PDV in ESMs, going beyond the analysis of the dominant variance patterns, and will also expand our ability to implement performance evaluation metrics that better capture key processes of ESM behavior.

Approach & Hypothesis: By combining existing re-analysis products and climate model ensembles of Earth System Models with empirical dynamical models (EDMs) and high-resolution simulations of the E3SM, this proposal will examine the following hypothesis. Hypothesis-1: The KOE global teleconnections to PDV and the dynamics of two-way coupling between tropics and extratropics energize two dominant Pacific “dynamical” modes, the KOE Mode and the NP-CP Mode, which are changing under anthropogenic forcing (addressed in TASK 1: Diagnose “dynamical” modes of variability in a changing climate using EDMs).  Hypothesis-2: The KOE air-sea interactions and resulting large-scale teleconnections are critical for the KOE and NP-CP mode. Increasing the resolution of climate models used in CMIP projections over the KOE region is necessary to improve the representation and predictability of North Pacific climate modes and extremes (addressed in TASK 2: Assess the effects of KOE air-sea interactions in the representation of PDV in ESMs and high-resolution simulations of E3SM). Hypothesis-3: The dynamics and changes in the NP-CP mode have led to the observed positive trend in climate variance in the Northeast Pacific and Central Tropical Pacific SSTs, which is linked to the recent marine heatwaves and precipitation extremes over North America (addressed in TASK 3: Using EDM dynamic filtering on observations and ESMs simulations to diagnose the impacts of the NP-CP mode on climate extremes).

Science Team: The PIs bring complementary expertise in the theory and EDM diagnostic modeling of Pacific climate variability (Di Lorenzo, Capotondi), ENSO dynamics (Capotondi, Stevenson), E3SM modeling (Stevenson), and coupled climate modeling with ESMs (Stevenson, Di Lorenzo and Capotondi). The PIs will also collaborate with Luke Van Roekel, an E3SM developer at Los Alamos National Laboratory, to conduct the E3SM high-resolution sensitivity simulations. Dr. Matthew Newman (UC Boulder) and Dr. Dillon Amaya (NOAA) will also collaborate on the climate extremes diagnostic for the EDM, E3SM, and ESMs simulations.

Broader Implications: The PDV and climate change control both large-amplitude transitions in marine ecosystems and the statistics of climate and weather extremes in the Pacific basin. More recently, the interactions between PDV and climate change have been implicated in the dynamics and evolutions of prolonged extreme conditions over North America, yet the mechanisms controlling the PDV phase and predictability remain unclear because of the wide range of model-dependent expressions that have not been diagnosed from a mechanistic point of view in ESMs. This proposal will provide the basis for a dynamical interpretation of the differences in the current suite of coupled climate models and associated representation errors. The improved understanding and modeling of the dynamics of PDV will provide a mechanistic-based framework for predicting societally relevant indicators linked to droughts, heatwaves, and ecosystem services. All tools and diagnostics developed in this project will become available to the ESM community.