Linear Simulations of Madden-Julian Oscillation Wind and Temperature Perturbations

The authors examine the linear response to an idealized Madden-Julian Oscillation (MJO) heating function in a primitive equation model using a basic state of rest. Simulation results are compared with the composite structure of the COARE MJO and reanalysis data. The model is able to reproduce most of the gross temperature and circulation patterns, such as the low level westerly and westward tilted disturbance structure. The two major problems in the simulation are the lower tropospheric temperature response is too strong, and the low level zonal wind perturbations are too shallow.

A mode by mode comparison by projecting both the observed and simulated MJO structures onto the first two baroclinic modes review that the difference mainly comes from the second mode. By shifting the second mode heating phase the model generates a deeper zonal wind response. Further, two modes of idealized explicit momentum forcing representing convective momentum transport (CMT) by synoptic scale or mesoscale convective systems are applied to the model to compensate the discrepancies in the temperature structure. This modeling study shows the different contributions of heating and momentum forcing to the large-scale circulations. Apparently, CMT plays an important role in generating the observed vertical structure of the MJO.