The Impact of Trade Surges on the Madden-Julian Oscillation under Different ENSO Conditions

Abstract

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 is activated over the Indian Ocean, an anomalous extratropical high appears over the north Pacific. In this study, a zonal shift of the high is found to depend on ENSO phases, and the high shifts westward in the El Niño developing (EV) 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. Although convective activities are centered south of the equator in boreal winter, the moisturization by the surges results in the activation of MJO convection not only south of the equator but also north of the equator, facilitating the formation of the twin cyclonic disturbances straddling the equator. During the other ENSO phases, on the other hand, extratropical fluctuations and MJO activity do not interact each other. It has been shown in previous studies that these twin cyclonic disturbances produce westerly wind bursts, which can trigger El Niño. These results suggest that tropical-extratropical interaction, between the MJO and the north Pacific high accompanied by the trade surges, occurs preferentially during EV phases, and feeds back to the development of El Niño.