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Abstract The Madden–Julian oscillation (MJO) propagates eastward as a disturbance of mostly zonal wind and precipitation along the equator. The initial diagnosis of the MJO spectral peak at 40–50-day periods suggests a reduction in amplitude associated with slower MJO events that occur at lower frequencies. If events on the low-frequency side of the spectral peak continued to grow in amplitude with reduced phase speed, the spectrum would just be red. Wavelet regression analysis of slow and fast eastward-propagating MJO signals during northern winter assesses how associated moisture and wind patterns could explain why slow MJO events achieve lower amplitude in tracers of moist convection. Results suggest that slow MJO events favor a ridge anomaly over Europe, which drives cool dry air equatorward over Africa and Arabia as the active convection develops over the Indian Ocean. We hypothesize that dry air tracing back to this source, together with a longer duration of the events, leads to associated convection diminishing along the equator and instead concentrating in the Rossby gyres off the equator. Significance StatementThe Madden–Julian oscillation (MJO) dominates the subseasonal variability of the tropical atmosphere. This work suggests that it favors maximum convective activity in the 40–50-day period range because lower-frequency MJO signals tend to import more cool dry air from the extratropics and along the equator, thereby weakening the slower events.
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On the MJO Phase Speed among Different Background Moisture and Zonal Wind Base States
Abstract The variability of the phase speed of the Madden–Julian oscillation (MJO) is poorly understood. The authors assess how the phase speed of the convective signal of the MJO associates with the background states over eastern Africa and the Indian Ocean. Relaxation of the coupling between tropical modes and their circulation has been previously linked to faster propagation; for example, the MJO speeds up over the eastern Pacific where its convective signal decouples from the circulation. In contrast, our results show that fast MJO events happen to exist during periods of wetter background states (>90 days) from East Africa across the Indian Ocean, whereas slow MJO is associated with dry background states. We found that fast MJO exhibits strong active and inactive phases with a structure suggesting more hierarchical convection. Results indicate that the association of the phase speed of the MJO as seen in the integrated filtered moist static energy with its tendency is stronger than the association of the phase speed as observed in the dry static energy with its tendency which is consistent with the acceleration of the MJO during wet background states. Also, our results indicate that the MJO may be faster during periods of enhanced low-level moisture because these periods have anomalously weak upper-tropospheric easterly background winds, which reduce the westward advection of the MJO by the background easterly wind, resulting in higher eastward phase speed of the MJO. The acceleration of the MJO by the background zonal wind overwhelms the deceleration associated with the moist-wave dynamics. Significance StatementThis study shows that the Madden–Julian oscillation (MJO), which is the dominant subseasonal weather signal in the tropics, moves eastward more quickly across eastern Africa and the Indian Ocean when the region is abnormally moist. The faster propagation does not appear to result from the higher moisture but instead from encountering weaker-than-normal upper-air winds from the east that tend to occur during moist periods.
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- PAR ID:
- 10568185
- Publisher / Repository:
- American Meteorological Society
- Date Published:
- Journal Name:
- Journal of the Atmospheric Sciences
- Volume:
- 82
- Issue:
- 2
- ISSN:
- 0022-4928
- Format(s):
- Medium: X Size: p. 301-317
- Size(s):
- p. 301-317
- Sponsoring Org:
- National Science Foundation
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