(1) One-way teleconnections from tropics to extra-tropics (C1). Ocean and atmospheric teleconnections from the tropics to the extra-tropics forced by Eastern Pacific ENSO (EP-ENSO) are known to energize the Aleutian Low (AL) and PDO system on timescales of 6m-1yr. In turn, the oceanic adjustment to the PDO excites long ocean Rossby waves that propagate the signal into the Kuroshio-Oyashio Extension (KOE) region on timescales of 2-4yr. Collectively, this quasi-deterministic cycle EP-ENSO -> PDO -> KOE (C1 in the schematic) carries a multi-year memory that energizes the North Pacific Decadal Variability (PDV).

(2) Two-way coupling between tropics and extra-tropics (C2). While C1 emphasizes the one-way coupling from the tropics to the extra-tropics, results from this DOE project show that two-way coupling between tropics and extra-tropics is likely a more important mechanism for explaining South, North, and Tropical PDV (C2 in the schematic). By combining Linear Inverse Models (LIMs) with reanalysis and large-ensembles of CMIP5 & 6 models, including an E3SM1 ensemble generated by this team (Stevenson et al. submitted*), the PIs show that extra-tropical ENSO precursor dynamics excited by the weakening of the off-equatorial trade winds energize the tropical Pacific decadal variability, specifically the Central Pacific ENSO pattern (CP-ENSO). In the Northern Hemisphere, these wind fluctuations are forced by the North Pacific Oscillation (NPO) and its oceanic expression -- the North Pacific Gyre Oscillation (NPGO). The resulting tropical PDV is then exported again to the extra-tropics through the ENSO teleconnections NPO/NPGO (extra-tropics) -> CP-ENSO (tropics) -> NPO/NPGO (extra-tropics) (C2 in the schematic) leading to the canonical PDV pattern over the entire Pacific basin (Zhao and Di Lorenzo, 2020*; Zhao et al. 2021a*). Beyond its implication for PDV, this two-way coupling is also an important mechanism for multi-year ENSO in CMIP6 NPO -> CP-ENSO -> NPO -> CP-ENSO (Ding, Di Lorenzo et al. 2022*). Furthermore, given that the NPO leads to different flavors of ENSO, the PIs found that the two-way coupling NPO/NPGO -> CP-ENSO -> EP-ENSO -> AL/PDO is a primary mechanism supporting the evolutions and persistence of multi-year climate extremes of the North Pacific such as the recent 2013-2015 marine heatwaves and droughts over North America (Capotondi et al., 2019*; Xu et al, 2020*). In model simulations, when this coupling is interrupted, the spatial and temporal structure of the PDO is significantly changed featuring a reduction in variance in the eastern North Pacific and the emergence of a stronger variance pattern in the KOE (Zhao et al. 2021b*). 

(3) Coupling between KOE and basin-scale PDV in a changing climate (C3). Further analyses reveal that the KOE may play a bigger role in controlling the basin-scale PDV, especially as the climate is changing towards warmer conditions. While several studies have explored how ocean variability in the KOE can trigger downstream atmospheric teleconnections in the central and eastern North Pacific, the PIs are able to show that these downstream teleconnections dynamics can also impact the tropical Pacific and give rise to PDV with a preferred quasi-decadal timescale. Specifically, the atmospheric response to the low-frequency oceanic forcing of the KOE acts as a nudging of the NPO conditions on timescale of 1-2yr and energize the KOE -> NPO -> CP-ENSO -> NPO/NPGO -> KOE (Joh and Di Lorenzo, 2019*; C3 in the schematic). This mechanism has been preliminarily confirmed in a high-resolution ocean-atmosphere model (Joh, Di Lorenzo, Kirtman et al. 2021*) with suggestions that C3 may be explaining a larger fraction of PDV under global warming. The exact mechanism linking the KOE-driven PDV and climate change is still unknown but may be linked to changes in the average location of the storm tracks with significant effects on the statistics of extremes over North America. Analyses that use a LIM to decompose Pacific basin dynamics in both nature (Newman et al. 2016; Capotondi et al., submitted*) and CMIP models (Wu, Di Lorenzo, et al., in review*) clearly indicate that this tropical-extra-tropical coupling emerges as one of the dominant dynamical modes of Pacific climate, with areas of high amplitude in the KOE and Central Tropical Pacific. We refer to this new pattern as the North Pacific-Central Pacific Mode (NP-CP). Although previous studies of statistical variability patterns have identified various elements of the NP-CP mode, the PIs have shown it is a dynamical mode of the Pacific system, suggesting it is generated by a sequence of dynamical events (e.g. C3 + C2), that impacts predictability of phenomena including the PDO, NE Pacific marine heatwaves, and CP ENSO events. The emergence of the NP-CP mode along with the intensification of Pacific low frequency variability seen in observations and climate models (Joh and Di Lorenzo, 2017; 2021*; Liguori and Di Lorenzo, 2018) raises key questions about its underlying mechanisms and their relation to climate change.