A Mechanism for the Generation of a Warm SST Anomaly in the Western Equatorial Pacific: A Pathway Perspective

• Published in: Journal of geophysical research. Oceans 2023-11, Vol.128 (11), p.n/a
• Main Authors: Gao, Chuan, Zhang, Rong‐Hua
• Format: Article
• Language: English
• Subjects: Anomalies; Boundary currents; El Nino; El Nino events; El Nino phenomena; El Nino-Southern Oscillation event; El Niño onset; Equator; Equatorial countercurrents; Equatorial regions; Evolution; General circulation models; Geophysics; Kelvin waves; La Nina; La Nina events; Mathematical models; NECC pathways; Ocean circulation; Ocean currents; ocean modeling; Ocean models; Ocean temperature; Ocean warming; Oceanic general circulation model; Oceans; Perturbation; Perturbations; Rossby waves; Sea surface; Sea surface temperature; Sea surface temperature anomalies; Southern Oscillation; SST anomaly generation; Surface temperature; Surface wind; Water circulation; Wave propagation; Wave reflection; Westerlies; Wind; Wind effects; wind response; Wind stress
• ISSN: 2169-9275; 2169-9291
• DOI: 10.1029/2023JC020119

Processes leading to the onset and development of an El Niño event in the tropical Pacific remain elusive. Observed data and Ocean General Circulation Model (OGCM) simulations are used to reveal a well‐defined pattern of sea surface temperature (SST) perturbations along the mean North Equatorial Countercurrent (NECC) pathways in association with the onset and evolution of some El Niño events. The OGCM‐based sensitivity experiments are conducted to illustrate how a warm SST anomaly (SSTA) on the equator can result from a thermal forcing that is prescribed north of 10°N, similar to observed SST anomalies in December 1988. Within approximately one year, the imposed SST anomaly north of 10°N tends to be transported to the dateline region on the equator by the mean ocean circulation in the western Pacific (the low‐latitude western boundary current and the NECC). In due course, an upper‐layer ocean warming is generated off the equator at 6–10°N and then on the equator, which acts to induce a westerly wind anomaly response; a simple statistical atmospheric wind stress model is then used to depict an expected westerly wind response. These resultant SST and surface wind perturbations can couple together over the western tropical Pacific, forming air‐sea interactions and setting up a stage for El Niño onset. As such, this pathway mechanism can reasonably well explain the first appearance of a warm SST anomaly on the equator in the dateline region and the corresponding development of westerly wind anomalies over the western Pacific in association with El Niño onset. Plain Language Summary Classical ENSO theories still cannot adequately explain the onset mechanism for some El Niño events. Taking 1991–92 El Niño as an example, its evolution is strikingly different from the delayed oscillator theory which is involved in propagating equatorial Rossby and Kelvin waves and their reflections at the western boundary in the tropical Pacific. During the 1988–89 La Niña event, large amount of warm waters are accumulated in the western Pacific, which is expected for an El Niño to occur in 1990–91. But, there was no El Niño until 1991–92, when it was initiated without the wave reflections at western boundaries. On the other hand, there exists a well‐defined structure of the mean water pathways in the western tropical Pacific, which can play a role in initiating a warm SSTA on the equator near the dateline. Here, an idealized ocean model experiment is conducted to illustrate a mec