- Award ID(s):
- Publication Date:
- NSF-PAR ID:
- Journal Name:
- Journal of Climate
- Page Range or eLocation-ID:
- 8523 to 8536
- Sponsoring Org:
- National Science Foundation
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Reexamining the Moisture Mode Theories of the Madden–Julian Oscillation Based on Observational AnalysesAbstract Two existing moisture mode theories of the MJO, one emphasizing boundary layer moisture asymmetry (MA) and the other emphasizing column-integrated moist static energy (MSE) tendency asymmetry (TA), were validated with the diagnosis of observational data during 1979–2012. A total of 2343 MJO days are selected. While all these days show a clear phase leading of the boundary layer moisture, 20% of these days do not show a positive column-integrated MSE tendency in front of MJO convection (non-TA). A further MSE budget analysis indicates that the difference between the non-TA composite and the TA composite lies in the zonal extent of anomalously vertical overturning circulation in front of the MJO convection. A background mean precipitation modulation mechanism is proposed to explain the distinctive circulation responses. Dependent on the MJO location, an anomalous Gill response to the heating is greatly modulated by the seasonal mean and ENSO induced precipitation fields. Despite the negative MSE tendency in front of MJO convection in the non-TA group, the system continues moving eastward due to the effect of the boundary layer moistening, which promotes a convectively unstable stratification ahead of MJO convection. The analysis result suggests that the first type of moisture mode theories, themore »
Abstract The diversity of the Madden-Julian Oscillation (MJO) in terms of its maximum intensity, zonal extent and phase speed was explored using a cluster analysis method. The zonal extent is found to be significantly correlated to the phase speed. A longer zonal extent is often associated with a faster phase speed. The diversities of zonal extent and speed are connected with distinctive interannual sea surface temperature anomaly (SSTA) distributions and associated moisture and circulation patterns over the equatorial Pacific. An El Niño–like background SSTA leads to enhanced precipitation over the central Pacific, allowing a stronger vertically overturning circulation to the east of the MJO. This promotes both a larger east-west asymmetry of column-integrated moist static energy (MSE) tendency and a greater boundary-layer moisture leading, serving as potential causes of the faster phase speed. The El Niño–like SSTA also favors the MJOs intruding further into the Pacific, causing a larger zonal extent. The intensity diversity is associated with the interannual SSTA over the Maritime Continent and background moisture condition over the tropical Indian Ocean. An observed warm SSTA over the Maritime Continent excites a local Walker cell with a subsidence over the Indian Ocean, which could decrease the background moisture, weakeningmore »
Effects of MJO Vertically Tilted Structure on Its Phase Speed from the Moisture Mode Theory PerspectiveAbstract The effect of vertically tilted structure (VTS) of the MJO on its phase propagation speed was investigated through the diagnosis of ERA-Interim reanalysis data during 1979–2012. A total of 84 eastward propagating MJO events were selected. It was found that all MJO events averaged throughout their life cycles exhibited a clear VTS, and the tilting strength was significantly positively correlated to the phase speed. The physical mechanism through which the VTS influenced the phase speed was investigated. On the one hand, a stronger VTS led to a stronger vertical overturning circulation and a stronger descent in the front, which caused a greater positive moist static energy (MSE) tendency in situ through enhanced vertical MSE advection. The stronger MSE tendency gradient led to a faster eastward phase speed. On the other hand, the enhanced overturning circulation in front of MJO convection led to a stronger easterly/low pressure anomaly at the top of the boundary layer, which induced a stronger boundary layer convergence and stronger ascent in the lower troposphere. This strengthened the boundary layer moisture asymmetry and favored a faster eastward propagation speed.
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Enhanced Feedback between Shallow Convection and Low-Level Moisture Convergence Leads to Improved Simulation of MJO Eastward PropagationAbstract Recent study indicates that the non-instantaneous interaction of convection and circulation is essential for evolution of large-scale convective systems. It is incorporated into cumulus parameterization (CP) by relating cloud-base mass flux of shallow convection to a composite of subcloud moisture convergence in the past 6 h. Three pairs of 19-yr simulations with original and modified CP schemes are conducted in a tropical channel model to verify their ability to reproduce the Madden–Julian oscillation (MJO). More coherent tropical precipitation and improved eastward propagation signal are observed in the simulations with the modified CP schemes based on the non-instantaneous interaction. It is found that enhanced feedback between shallow convection and low-level moisture convergence results in amplified shallow convective heating, and then generates reinforced moisture convergence, which transports more moisture upward. The improved simulations of eastward propagation of the MJO are largely attributed to higher specific humidity below 600 hPa in the free troposphere to the east of maximum rainfall center, which is related to stronger boundary layer moisture convergence forced by shallow convection. Large-scale horizontal advection causes asymmetric moisture tendencies relative to rainfall center (positive to the east and negative to the west) and also gives rise to eastward propagation. Themore »