The Warm pool eastward extension associated with the MJO events occurred prior to the Onset of El Niño event 2018

The ocean-atmosphere interactions in the western Pacific warm pool (WP) are of fundamental importance to the interannual variability associated with ENSO that affect the global weather and climate. The ENSO prediction has been a great challenge, especially the timing of the onset of El Niño. Previous studies have shown that the Madden-Julian Oscillation (MJO) can serve as “trigger” to the El Niño onset. However, the mechanism(s) responsible for the MJO-ENSO interaction is not well understood. This study is aimed to quantify the MJO-induced WP eastward extension (WPEE) in time and space using an unprecedented 20 years of satellite observations of SST and Precipitation (OISST, TRMM-GPM), which include over 98 MJO events over the WP. We found that about 65% of these events had a significant impact in WPEE (from 100’s Km to more than 2,000 Km) occurring mostly during the first 15 days post-MJO, but can extend beyond 25 days in major and/or multiple MJO events. A large eastward extension is often associated with strong and/or consecutive MJO events with heavy precipitation and strong westerly winds, and they occur before the onset of EL Niño, especially during boreal spring. These finding motive the present study, which aims to improve our understanding of the physical mechanism(s) through which the MJO influence the eastward extension of the WP that might lead to the onset of El Niño.

Motivated by the Tropical Pacific Observing System (TPOS) 2020 objectives, we conducted a relatively high-resolution coupled atmosphere-ocean model simulation of the MJO events occurred in January-May prior to the onset of 2018 El Niño. We validate the model using observational datasets (TRMM-GPM, CCMP surface winds, in situ-moorings, Argo floats, and HYCOM analysis fields) by examining the MJO-induce WPEE observed in satellite data. The goal of this presentation is to use the high temporal and spatial resolution to look for the mechanism that explain the WPEE and the zonal SST gradient relaxation leading to the onset of EL Niño. Previous studies have pointed out two main mechanisms in warming the central Pacific. 1) Ocean Kelvin waves deepening the thermocline and 2) Upper ocean stratification building up heat. We examine these mechanism as well as the heat flux anomalies at the air-sea interface in the vicinity of the WP eastern edge.