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

The Madden-Julian oscillation (MJO) and El Nino-Southern Oscillation (ENSO) are the two most impor-tant tropical phenomena that affect global weather and climate on intraseasonal and interannual time scales. Although they occur on different time scales, the MJO-induced sea surface temperature (SST) anomalies over the equatorial Pacific have spatial scales similar to SST anomalies prior to El Nino. 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 Nino. We use over 20 years of satel-lite observations, including optimum interpolated SST, TRMM-GPM precipitation, and the cross-calibrated multiplatform (CCMP) surface winds from 1998 to 2019, to quantify the spatial structure and duration of the MJO-induced warm SST anomalies over the equatorial Pacific (10 degrees S-10 degrees N, 130 degrees E-180 degrees). The intensity of the MJO is measured by the total rain vol-ume and average surface westerly wind speed throughout its convectively active phase. Results show that 1) 61% of the 98 MJO events induced a WPEE over 1000-3000 km along the equator, which can last beyond 15-30 days after the MJO precipitation ended; 2) the MJO events prior to El Nino are generally stronger and produce significant WPEE far beyond its annual cycle and increasing SST warming in the Nino-3.4 region; 3) consecutive MJO events can produce much stronger WPEE prior to El Nino, which are observed in all El Nino events from 1998 to 2019; and 4) more frequent and stronger MJO-induced WPEE occurs in March-May than other seasons. These results can help better understand the MJO-ENSO interaction and, ultimately, improve the prediction of El Nino onset.