The impact of trade surges on the Madden–Julian Oscillation under different ENSO conditions

The tropical–extratropical interaction associated with the Madden–Julian Oscillation (MJO) and El Niño–Southern Oscillation (ENSO) in boreal winter is examined. When MJO convection activates over the Indian Ocean, an extratropical high appears over the north Pacific. This high shifts westward in the El Niño developing (EV) phase but eastward in the normal (NR) and El Niño decaying (EC) phases. Northeasterly trade surges that originate from the high intrude the tropical western north Pacific only during EV phases and moisturize the area through convergence, resulting in the activation of MJO convection not only south of the equator but also north of the equator. Meridionally spread moist fields in EV phases can facilitate the development of convective disturbances straddling the equator causing equatorial westerly wind bursts from the MJO, which are known to trigger El Niño. On the other hand, extratropical fluctuations and MJO activity do not interact during the other ENSO phases. These results suggest that tropical–extratropical feedback, between the MJO and the north Pacific high accompanied by the trade surges, occurs preferentially during EV phases.