An overlooked aspect concerning the effect of the spatial pattern of zonal wind stress anomalies on El Niño evolution and diversity

Abstract

Anomalous zonal wind stress in the central-to-western Pacific plays a crucial role in ENSO evolution by exciting oceanic waves that propagate eastward or westward. However, compared to its intensity, the importance of its spatial pattern is an overlooked aspect in ENSO theory. Using a linear regression model and numerical simulations with the Zebiak-Cane model, we here show that the zonal wind stress anomaly pattern significantly affects the development of El Niño and its type. Specifically, if the westerly wind stress is closer to the western Pacific (WP), the excited upwelling Rossby waves will take less time to reach the WP coast before they are reflected as Kelvin waves. This significantly weakens sea surface temperature anomalies in the eastern Pacific region since less time is provided for their amplification due to positive feedbacks. This causes the anomalous warming center to be closer to the central Pacific (CP) region, leading to a CP-type El Niño.