MJO-induced Warm Pool Eastward Extension Prior to the Onset of El Niño: Observations from 1998-2019

Abstract The Madden-Julian Oscillation (MJO) and El Niño Southern Oscillation (ENSO) are the two most important tropical phenomena that affect global weather and climate on intraseasonal and interannual timescales. Although they occur on different timescales, the MJO-induced sea surface temperature (SST) anomalies over the equatorial Pacific have spatial scales similar to SST anomalies prior to El Niño. This study aims to address the question of whether the MJO plays an important role in the warm pool eastward extension (WPEE) leading up to El Niño. We use over 20 years of satellite observations, including optimum interpolated SST, TRMM-GPM precipitation, and the cross-calibrated multi-platform (CCMP) surface winds from 1998-2019, to quantify the spatial structure and duration of the MJO-induced warm SST anomalies over the equatorial Pacific (130°E-180°E, 10°S–10°N). The intensity of the MJO is measured by the total rain volume and average surface westerly wind speed throughout its convectively active phase. Results show that 1) 61% of the MJO events induced a WPEE over 1000-3000 km along the equator, which can last beyond 15 days; 2) the MJO events prior to El Niño are generally stronger and produce significant WPEE far beyond its annual cycle and increasing SST warming in the Niño 3.4 region; 3) consecutive MJO events can produce much stronger WPEE prior to El Niño, which are observed in all El Niño events from 1998-2019; and 4) more frequent and stronger MJO-induced WPEE occur in March-May than other seasons. These results can help better understand the MJO-ENSO interaction and, ultimately, improve the prediction of El Niño onset.