Multi-year El Niño events tied to the North Pacific Oscillation

The El Niño-Southern Oscillation (ENSO) is a dominant climate phenomenon influencing global weather patterns and extreme events. Typically, El Niño and La Niña episodes last 9-12 months, peaking in early winter. However, some events persist for two years or more, significantly amplifying their impacts. Multi-year El Niño events, though less frequent than their La Niña counterparts, cause prolonged floods and droughts with substantial socioeconomic consequences. The mechanisms behind the extended duration of these events remain unclear. While existing theories focus on tropical ocean-atmosphere interactions, this study explores the influence of extratropical atmospheric variability, particularly the North Pacific Oscillation (NPO), which is known to affect various aspects of ENSO. The research question centers on whether the NPO is dynamically linked to multi-year El Niño events and, if so, how. The importance of understanding this link lies in its potential to improve multi-year ENSO prediction and projection capabilities, particularly considering the surprising and impactful 2014/15/16 El Niño event, which was poorly predicted by many models. This study aims to address this gap by investigating the two-way feedback mechanism between tropical and extratropical processes, focusing on the role of the NPO in generating multi-year El Niño events. This work is crucial because accurate prediction of multi-year ENSO events is vital for mitigating their significant socio-economic impacts globally.

Existing literature extensively documents the characteristics and impacts of ENSO, highlighting the asymmetry in the duration of El Niño and La Niña events, with La Niña tending to be more persistent. Several hypotheses attempt to explain this asymmetry, primarily focusing on tropical coupled ocean-atmosphere processes in the Pacific, Indian, and Atlantic Oceans. However, few theories adequately address the multi-year persistence of both El Niño and La Niña events. The role of extratropical forcings in multi-year ENSO has been largely overlooked, despite some studies suggesting the importance of extratropical atmospheric variability, such as the NPO, in influencing various aspects of ENSO development, pattern, phase transition, and decadal variability. While these studies suggest a connection, a clear understanding of the dynamic linkage between the NPO and multi-year ENSO, especially multi-year El Niño, remains elusive. This research bridges this gap by directly exploring this previously understudied link.

This study employs a combination of observational analyses and climate model experiments to investigate the relationship between the NPO and multi-year El Niño events. Observational data from multiple sources, including HadISST, ERSST, Kaplan SST, COBE SST for sea surface temperature (SST), and NCEP1 and HadSLP2 for sea level pressure (SLP), are analyzed. Five multi-year El Niño events since 1950 are identified based on a defined threshold for the Niño3.4 index, which tracks SST anomalies in the central equatorial Pacific. The study focuses on the sea level pressure (SLP) anomalies during the boreal winter preceding these events to assess the NPO's influence. The North Pacific Oscillation (NPO) index is calculated using the second leading Empirical Orthogonal Function (EOF) of JFM (January-February-March) SLP anomalies north of 15°N. The North Pacific Meridional Mode (NPMM) index is calculated using SST anomalies in the subtropical northeastern Pacific. The analysis examines the temporal evolution of SST and SLP anomalies composited over the three-year period encompassing each multi-year El Niño event. To investigate the two-way feedback mechanism between the tropics and extratropics, the study examines the relationship between NPO-induced SST anomalies and the subsequent changes in SLP over the Hawaiian region. The study also examines the role of the Central Pacific (CP) El Niño, characterizing its patterns and relationship to the NPO-induced SST anomalies. CMIP5/6 models are used to evaluate the robustness of the observational findings and analyze the historical and future frequency of multi-year El Niño events. A coupled general circulation model (CGCM), the Flexible Global Ocean-Atmosphere-Land System Model Grid-point version 2 (FGOALS-g2), is used for sensitivity experiments with and without imposed positive NPO forcing. The NPO forcing is simulated by assimilating observed atmospheric temperature and wind anomalies associated with the NPO into the model, thus constraining the atmospheric component. Bootstrap methods and statistical tests are used to assess the significance of the results, including the correlation between the changes in the number/ratio of multi-year El Niño events and changes in NPO variance.

The study reveals a robust link between the NPO and multi-year El Niño events. All five observed multi-year El Niño events were preceded by a distinct NPO pattern during the preceding boreal winter. The analysis reveals a two-way feedback mechanism: the NPO forces a Central Pacific (CP) El Niño in the subsequent winter through the North Pacific Meridional Mode (NPMM); this CP El Niño, in turn, feeds back to the extratropics, reinforcing the NPO variability, thus prolonging El Niño conditions. The composite analysis of five multi-year El Niño events demonstrates the temporal evolution of this feedback loop. CMIP5/6 model simulations support this finding, showing that a high percentage of multi-year El Niño events are preceded by positive NPO events. The CGCM experiments confirm the role of the NPO forcing in prolonging El Niño events. The NPO forcing increases the frequency of multi-year El Niño events in the model simulations. Analysis of CMIP5/6 projections under RCP8.5 scenario indicates a significant increase in the frequency of multi-year El Niño events in the future climate. A positive correlation is found between increased NPO variance and more frequent multi-year El Niño events in these future climate projections. The study also shows that NPO can play a role in the development of multi-year La Niña events, although the relationship is less direct than for El Niño events. The results suggest that the preceding NPO and El Niño can, together or separately, influence the occurrence of multi-year La Niña events.

The findings strongly support the hypothesis that the NPO plays a crucial role in generating multi-year El Niño events. The two-way feedback mechanism identified, involving extratropical-tropical teleconnections, significantly contributes to understanding the prolonged duration of these events. The study highlights the importance of considering extratropical influences, specifically the NPO, when predicting and projecting ENSO events. The observed increase in multi-year El Niño events in recent decades and the projected increase under future climate change emphasize the significance of this finding. The results underscore the need to incorporate the NPO's influence in ENSO prediction models to improve accuracy and lead times. The close connection between increased NPO variance and more frequent multi-year El Niño events in future climate projections highlights the importance of understanding the mechanisms driving NPO variability under anthropogenic forcing. This suggests that improvements in NPO prediction may translate into improvements in predicting multi-year El Niño events.

This study demonstrates the crucial role of the North Pacific Oscillation (NPO) in generating multi-year El Niño events. A two-way feedback mechanism between the NPO and the tropical Pacific, involving the North Pacific Meridional Mode (NPMM) and Central Pacific El Niño, explains the prolonged duration of these events. Future climate projections suggest a likely increase in multi-year El Niño frequency due to enhanced NPO variability. Incorporating the NPO's influence into ENSO prediction models is essential for improving forecasting accuracy. Future research should focus on refining the understanding of the mechanisms driving NPO variability under climate change and investigating the relative contributions of other factors to the duration of ENSO events.

The study is limited by the relatively small number of multi-year El Niño events observed since 1950, which could potentially affect statistical power and the generalization of findings. The influence of other factors, such as tropical Atlantic and Indian Ocean variability, on multi-year ENSO events, is not fully explored in this study. Further research is needed to fully understand the complex interactions between various modes of variability in the global climate system.