Moisture Modes and the Eastward Propagation of the MJO

• Published in: Journal of the atmospheric sciences Vol. 70; no. 1; pp. 187 - 192
• Main Authors: Sobel, Adam, Maloney, Eric
• Format: Journal Article
• Language: English
• Published: Boston, MA American Meteorological Society 01.01.2013
• Subjects: Advection; Climatology. Bioclimatology. Climate change; Convergence; Damping; Diffusion rate; Disturbances; Drying; Earth, ocean, space; Easterlies; Evaporation; Exact sciences and technology; External geophysics; General circulation models; Grants; Growth rate; Heat; Horizontal; Horizontal diffusion; Humidity; Madden-Julian oscillation; Marine; Meteorology; Modes; Modulation; Moist static energy; Moisture; Moisture effects; Moisture gradient; Perturbation; Perturbation methods; Propagation; Propagation modes; Wavelengths; Wind; Indian Ocean; Propagation characteristic; climate variability; Wind effect; Intraseasonal oscillation; evaporation; linear models; Madden Julian oscillation; Atmospheric humidity; Atmospheric disturbance
• ISSN: 0022-4928; 1520-0469
• DOI: 10.1175/JAS-D-12-0189.1

The authors discuss modifications to a simple linear model of intraseasonal moisture modes. Wind–evaporation feedbacks were shown in an earlier study to induce westward propagation in an eastward mean low-level flow in this model. Here additional processes, which provide effective sources of moist static energy to the disturbances and which also depend on the low-level wind, are considered. Several processes can act as positive sources in perturbation easterlies: zonal advection (if the mean zonal moisture gradient is eastward), modulation of synoptic eddy drying by the MJO-scale wind perturbations, and frictional convergence. If the sum of these is stronger than the wind–evaporation feedback—as observations suggest may be the case, though with considerable uncertainty—the model produces unstable modes that propagate weakly eastward relative to the mean flow. With a small amount of horizontal diffusion or other scale-selective damping, the growth rate is greatest at the largest horizontal scales and decreases monotonically with wavenumber.