MJO-induced Ocean Density Currents and Their Contribution to El Niño Onset in reanalysis data

This study investigates the spatio-temporal characteristics of eastward Ocean Density Currents (ODCs) induced by the Madden- Julian Oscillation (MJO) and their influence on El Niño onset. Building on the work of Kerns and Chen (2021) and Jauregui and Chen (2024), we analyze upper 30-m averaged eastward currents associated with MJO precipitation events along the equatorial Pacific, using 30 years (1993–2022) of ERA5 and ORAS5 reanalysis data. Our results highlight the critical role of freshwater forcing on the zonal pressure gradient (ZPG) force in accelerating eastward currents and enhancing the eastward expansion of the warm pool.

Over the period 1993-2022, analysis of 177 MJO precipitation events over the western Pacific and eight El Niño onset events indicates that the MJO precipitation events prior to El Niño influence the warm pool's eastward extension due to horizontal advection of temperature by ODCs. On average, ODCs are primarily located west of the density front, confined within the upper 30 meters of the ocean, and propagate eastward at a rate of 0.4 m s⁻¹. Strong equatorial density currents (∼ 4000 km, 30-40 days, > 30 cm s⁻¹) typically occur associated with cumulative and strong MJO forcing in boreal winter and spring when the equatorial Pacific is transitioning to El Niño from neutral conditions.