MM5 Modeling of the Madden–Julian Oscillation in the Indian and West Pacific Oceans: Implications of 30–70-Day Boundary Effects on MJO Development

Abstract

The results of an experiment designed to isolate the initiation phase of the Madden?Julian oscillation (MJO) from 30?70-day boundary effects is presented. The technique used to accomplish this involves employing the fifth-generation Pennsylvania State University?National Center for Atmospheric Research (PSU?NCAR) Mesoscale Model (MM5), as first presented in the companion paper to this paper. Two runs, each 2 yr long, are integrated forward from 1 June 1990. The first run, called the control, uses the unmodified National Centers for Environmental Prediction (NCEP)?NCAR reanalysis (NRA) dataset for boundary conditions. The second run, called the notched, uses the same NRA dataset for the boundary conditions, with the exception that all signals with periodicities in the 30?70-day range have been removed. Any signals in the 30?70-day range subsequently generated by the notched run are then solely due to signals generated from within the model domain or from signals entering through the domain boundaries with frequencies outside of the MJO band. Comparisons between 2-yr means from each run indicate that filtering the boundaries does not significantly modify the model climatology. The mean wind structure, thermodynamic state, and outgoing longwave radiation (OLR) are almost identical in the control and notched runs. A 30?70-day bandpass filter is used to isolate MJO-like signals in the runs. Comparisons of 30?70-day bandpassed zonal wind, moist static energy (MSE), and OLR reveal that the notched run develops many of the expected characteristics of MJO episodes, but with a weaker signal. Large-scale, organized structures develop that possess seasonal shifts in amplitude, mirroring observed MJO activity, have opposite wind directions in the upper and lower troposphere, and propagate eastward during most strong episodes. The results suggest that neither remnants from previous MJO episodes nor extratropical feedbacks within the MJO time band are necessary for MJO initiation. However, the control run is more organized than the notched run, implying that 30?70 signals outside the model domain influence the MJO signal. There is also some evidence that the recharge?discharge mechanism plays a role in MJO formation.