Seasonal dependence of the MJO-ENSO relationship

Observations of the development of recent El Nino events suggest a pivotal role for the Madden-Julian oscillation (MJO). Previous attempts to uncover a systematic relationship between MJO activity and the El Nino-Southern Oscillation (ENSO), however, have yielded conflicting results. In this study the MJO-ENSO relationship is stratified by season, and the focus is on MJO activity in the equatorial western Pacific. The results demonstrate that MJO activity in late boreal spring leads El Nino in the subsequent autumn-winter for the period 1979-2005. Spring is the season when MJO activity is least asymmetric with respect to the equator and displays the most sensitivity to SST variations at the eastern edge of the warm pool. Enhanced MJO activity in the western Pacific in spring is associated with an eastward-expanded warm pool and low-frequency westerly surface zonal wind anomalies. These sustained westerly anomalies in the western Pacific are hypothesized to project favorably onto a developing El Nino in spring.