Where the winds clash: what is really triggering El Niño initiation?

El Niño-Southern Oscillation (ENSO) is a phenomenon causing heat redistribution in the tropical Pacific, resulting in irregular sea surface temperature (SST) oscillations between warm (El Niño) and cold (La Niña) phases every 2-7 years. ENSO's impacts are global, affecting weather patterns and impacting crop yields, prices, and even contributing to social unrest and conflict. Improving ENSO predictability is crucial. Current ENSO understanding focuses on the thermocline's depth, heat content in the water column, and oceanic feedback on zonal wind patterns. Existing theories highlight the ocean's subsurface memory, sustained by the balance between internal pressure gradients and easterly winds, leading to westward heat transfer and the “charging” of the western Pacific, discharged via equatorial Kelvin waves. Studies also focus on equatorial internal waves associated with the Pacific Warm Pool (WP) displacements, a region of warm waters that drives deep atmospheric convection. The convergence of westerly winds (WW) in the west/central Pacific and easterly winds (EW) in the central/eastern Pacific at the WP's eastern edge, displaces rigidly in phase with the Southern Oscillation Index (SOI). Previous research highlights equatorial waves (Kelvin and Rossby waves) associated with WP displacements, but their contributions to SST changes in the western and eastern Pacific differ. Investigating the Easterly/Westerly Wind Convergence Zone (EWCZ) lateral displacements is crucial for better understanding and predicting ENSO. The intensity, location, and timing of WW events are key for El Niño development, as are EW patterns. This study introduces the EWCZ longitude as a synthetic parameter summarizing WW and EW anomalies’ collective action, defining it as the region of minimal zonal derivative of the meridional mean wind velocity. We hypothesize that this parameter can significantly improve our understanding of El Niño initiation.

The existing literature extensively covers the role of equatorial waves (Kelvin and Rossby waves) in El Niño development, focusing on the interactions between ocean and atmosphere dynamics. Several studies have demonstrated the importance of the Pacific Warm Pool (WP) and its zonal displacements in triggering El Niño events. The role of westerly wind bursts (WWBs) has been highlighted as a key factor in preconditioning the ocean for El Niño development, triggering the generation of equatorial waves and leading to the eastward displacement of the WP. However, there is still ongoing debate on the relative importance of different factors and the exact mechanisms that lead to El Niño initiation. Some studies have focused on the role of easterly wind anomalies, while others emphasize the importance of the interaction between westerly and easterly wind patterns. This paper aims to bridge the gap by proposing a new perspective that considers the combined effect of westerly and easterly winds and their convergence zone, as a primary factor in determining El Niño initiation and intensity.

This study utilizes wind data from the ERA-Interim project, including zonal, meridional, and vertical components of monthly and daily winds (1 January 1979–31 August 2019) over the region 130°E–80°W, 5°S–5°N. The EWCZ longitude was determined by fitting the meridional mean of the zonal wind with a step function within the 130°E–100°W zonal region. The best-fit step function minimized the root-mean-square difference from the original meridional mean. SOI data was obtained from the Climatic Research Unit at the University of East Anglia. To detect propagating Kelvin waves (KWs), daily Sea Surface Height (SSH) data (1 January 1993–31 December 2019) with a 0.25° × 0.25° spatial resolution were obtained from orbiting altimeters and processed by the Copernicus European Marine Service. KW amplitudes were computed based on the method described by Boulanger and Menkes (1995, 1999), and Boulanger et al. (2003), expressing SSH as a linear superposition of freely propagating Rossby and Kelvin waves. Monthly in situ Temperature-Salinity (TS) data (16 January 1993–16 December 2018) covering 50 vertical levels (0–5500 m) were obtained from the Copernicus Marine Environment Monitoring Service to compute sea water density profiles using the CSIRO MATLAB library. Data on the position of maximum vertical atmospheric convection was determined from wind data. El Niño events were identified using data from https://ggweather.com/enso/oni.htm.

The study found a strong negative correlation (~−0.9) between the EWCZ longitude and the SOI, confirming previous findings that show the eastward displacement of the WP is well correlated to SOI. Analysis of ocean water density and temperature anomalies beneath the EWCZ showed that wind convergence pushes less dense surface waters downward before an El Niño event. After the El Niño peak, the convergence weakens, and denser waters replenish the interior water column. Kelvin wave amplitude analysis revealed that KWs form regularly below the wind convergence. However, only when the convergence is east of approximately 175°E do KWs reach the eastern Pacific boundary, triggering an El Niño event. The further east the convergence zone extends, the more severe the El Niño. The 2014 “false” El Niño alarm is explained; a KW propagated across the Pacific, but the EWCZ remained west of 175°E. In 2015, the EWCZ extended far into the central/eastern tropical Pacific, resulting in a super El Niño. Analysis of zonal displacements of maximum vertical atmospheric convection showed a correlation coefficient of ~-0.6 with the SOI, consistent with the eastward shift of the WP and atmospheric convection observed during El Niño events. The study concludes that westerly wind events, when interacting with trade winds east of 175°E, generate Rossby and Kelvin waves. East of 175°E, the sloping thermocline causes Rossby waves to deviate poleward, ceasing interaction with Kelvin waves, allowing Kelvin waves to modify the thermocline depth effectively and trigger El Niño. The zonal shifts of atmospheric convection and EWCZ are correlated with SOI, showing that westerly wind events are modulated by the state of the tropical Pacific climate system.

This study offers a novel perspective on El Niño initiation, emphasizing the EWCZ's location as a critical factor. The findings highlight the interplay between westerly and easterly winds, their convergence, and the resulting wave dynamics. The westward propagation of Kelvin waves and their interaction with Rossby waves, especially east of 175°E where the thermocline slopes, explains why only eastward-shifted convergence triggers El Niño. The strong correlation between EWCZ position, SOI, and atmospheric convection zone displacement supports the idea that El Niño isn't solely triggered by stochastic westerly wind events but is an integral part of the tropical Pacific climate system's dynamic behavior. This improved understanding is crucial for improving El Niño prediction models and anticipating future climate changes.

This research provides a novel perspective on El Niño initiation, highlighting the importance of the EWCZ's longitudinal position and its influence on Kelvin and Rossby wave dynamics. The study emphasizes the EWCZ’s role as a crucial parameter for El Niño initiation and intensity. Future research should focus on the long-term changes in EWCZ position and isothermal surface shapes to better understand ENSO in a changing climate and enhance predictability.

The study primarily uses observational data, limiting the ability to fully elucidate causal relationships. The analysis relies on specific data sets and methodologies, potentially impacting the generalizability of findings. Future studies should incorporate more comprehensive datasets, explore other factors affecting El Niño, and use advanced modeling techniques to strengthen the causal links between EWCZ location and El Niño development.