Abstract
In this paper, we report the triggering effect of extratropical perturbation on the onset of an atypical Madden–Julian Oscillation (MJO) and onset of the 2015–16 El Niño in March 2015. The MJO exhibited several unique characteristics: the effect of extratropical forcing, atypical genesis location and timing in the equatorial western Pacific, and the extremity of amplitudes in many aspects. The southward-penetrating northerly associated with the extratropical disturbances in the extratropical western North Pacific contributed to triggering the deep convection and westerly wind burst (WWB) and onset of the MJO over the anomalously warm tropical western Pacific in early March. The persisting strong WWB forced downwelling Kelvin wave-like oceanic perturbation that propagated eastward and led to the onset of the 2015–16 El Niño. The proposed novel extratropical forcing mechanism explaining the unique extratropics–MJO–El Niño association, based on both data diagnostics and numerical experiments, warrants further attention for a more detailed understanding of the onset of the MJO and its potential effect on El Niño.
Highlights
An El Niño event had been expected to occur since spring 2014 because of the high ocean heat content in the equatorial western Pacific
Before the Madden–Julian Oscillation (MJO) onset in early March, an atypically strong high-pressure system persisted over the extratropical western North Pacific in late February 2015
An atypical MJO initiated to the west of the dateline in early March 2015 and rapidly amplified to an unprecedented magnitude over the warm sea surface temperature (SST) in the central and eastern Pacific on March 16
Summary
An El Niño event had been expected to occur since spring 2014 because of the high ocean heat content in the equatorial western Pacific. In March 2015, a strong MJO occurred in the equatorial western Pacific, a characteristic that is distinctively different from previous MJO–El Niño events (e.g., the 1997–98 El Niño event) in which the MJO often originated in the Indian Ocean (IO) and propagated eastward into the equatorial western Pacific. We explored this unique feature and found that the March 2015 MJO might have developed because of forcing originating in the extratropical western North Pacific. We conducted data diagnostics and numerical simulations to provide supporting evidence
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