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Abstract Eastward-moving moist deep convection and atmospheric circulation signals associated with the tropical Madden–Julian oscillation (MJO) sometimes break down as they cross the Maritime Continent region, but other times, the signal propagates across the region maintaining amplitude or regaining it over the west Pacific basin. This paper assesses the hypothesis that upper-tropospheric zonal diffluence of the background wind over the Maritime Continent causes much of this Maritime Continent barrier effect and its variation over time, through two mechanisms: 1) by slowing down the MJO as stronger-than-average background upper-tropospheric zonal wind over the Indian Ocean advects the MJO circulation signal westward, slowing its eastward advance, and 2) through the zonal advection of the background wind by subseasonal zonal wind across a region of zonal diffluence of the background wind, which advects the background wind of the opposite sign to the MJO wind. Advection of the opposite-signed background wind counteracts the MJO wind and reduces its associated upper-tropospheric mass divergence, weakening the mechanisms of the upper-tropospheric Kelvin wave component of the MJO circulation. Composites of MJO-associated zonal wind and outgoing longwave radiation signals diminish as they cross the Maritime Continent region when the region’s background zonal winds are diffluent, and composites of data reconstructing the relevant advection terms reveal the direct action of the advection mechanisms. Significance StatementThe Madden–Julian oscillation (MJO) is the leading subseasonal variation of the tropical atmosphere. This project addresses how diffluence of the upper-tropospheric background zonal wind can break down MJO events through advection of and by the background wind.
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This content will become publicly available on August 1, 2026
How the GEFS forecast model represents the dry dynamical component of the maritime continent barrier effect on the MJO
Previous research has shown that the equatorial upper tropospheric circulation signal associated with the Madden Julian Oscillation (MJO) over the Indian Ocean behaves like a Kelvin wave, with the eastward-propagation of the associated zonal wind anomaly caused by acceleration by the geopotential gradient force in quadrature with the wind anomaly, with the resultant signal amplified or decayed as the MJO wind advects background zonal wind in regions of background confluent or diffluent flow, with its phase speed adjusted by Doppler shifting by background zonal wind. This paper assesses these previously diagnosed mechanisms in the GEFS V12 forecast model, showing that similar mechanisms occur with the model MJO, but weaker and less organized. Results suggest that, relative to the validation data, the model stalls the MJO upper tropospheric zonal wind anomaly during diffluent background conditions near the Maritime Continent and weakens its amplitude more rapidly than validation data. The stalled propagation leads the model MJO to persist the signals of advection of and by the background wind, but at a reduced rate as the scale of the model MJO wind anomaly diminishes. Results show that beyond the stronger Doppler effect in the model, stalling the model MJO results from filling of the relative geopotential trough collocated with the MJO easterly wind anomaly, leading to breakdown of the geopotential gradient force term that is responsible for propagation of the wind anomaly. Thus, when the Maritime Continent region experiences zonally diffluent flow, the reason for the stronger Maritime Continent barrier effect in the model is that the model does not persist the Kelvin wave propagation mechanism that continues the eastward movement of the easterly wind anomaly in observations.
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- PAR ID:
- 10633443
- Editor(s):
- Duplessy, Jean-Claude
- Publisher / Repository:
- Springer Nature
- Date Published:
- Journal Name:
- Climate Dynamics
- Volume:
- 63
- Issue:
- 8
- ISSN:
- 0930-7575
- Subject(s) / Keyword(s):
- Madden Julian Oscillation Kelvin waves Maritime Continent Barrier Effect
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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