Seasonal dependence of the MJO-ENSO relationship

Observations of the development of recent El Niño events suggest a pivotal role for the Madden-Julian Oscillation (MJO). Attempts to uncover a systematic relationship between MJO activity and El Niño, however, have yielded conflicting results. Some earlier studies that failed to find a relationship were either based on boreal winter data, when the MJO is strongest but shifted into the Southern Hemisphere, or used all months together that biased the results to boreal winter. By stratifying by season and focusing on MJO activity in the equatorial western Pacific, we demonstrate a robust lagged relationship between MJO activity in late spring and El Niño in the subsequent autumn/winter for the period 1979-2005. Spring is the season when the MJO is most focused near the equator and is most sensitive to SST variations at the eastern edge of the warm pool. Enhanced MJO activity in the western Pacific in spring is shown to be associated with an eastward-expanded warm pool and associated westerly surface zonal wind anomalies. These westerly anomalies in the western Pacific are hypothesized to project most favourably onto a developing El Niño in spring.