Regional scale modeling of the Madden-Julian Oscillation using MM5: An investigation of the importance of 30--70 day boundary effects on MJO development.

摘要

A new methodology to study the Madden-Julian Oscillation (MJO) investigates using a regional model with prescribed boundary conditions. This differs from previous MJO studies that typically involve simplified mathematical models or general circulation models. This study uses the Fifth Generation Pennsylvania State/NCAR Mesoscale Model (MM5) with boundaries forced by the NCEP/NCAR Reanalysis data set (NRA). The experimental domain extends from the western Indian Ocean to the Dateline and the runs last 24 months.; First, a comparison is made against observations to assess the model climatology and MJO signature in a Control run. Two-year means indicate realistic wind patterns, and outgoing longwave radiation (OLR) results are good for the model interior with larger discrepancies near the western and eastern boundaries. In the MJO time band, the Control and comparison data 30–70 day zonal wind and OLR have similar periodicities, exhibit eastward propagation, and possess the observed seasonal and vertical structure of the MJO.; Next, a multi-run experiment investigates the importance of feedbacks between the MJO and 30–70 day signals outside of the Tropics and MJO initiation region. The first run, the Control, uses unmodified NRA data for boundary conditions. The second run, the Notched, uses the same NRA data for the boundaries with the exception that signals with 30–70 day periodicities are removed. Any signals in the 30–70 day range subsequently generated by the Notched run are solely due to mechanisms within the model domain or from signals entering the domain with frequencies outside the MJO band. Comparisons between two-year means for each run indicate that the filtering does not significantly alter the model climatology. Comparisons of 30–70 day bandpassed zonal wind, moist static energy, and OLR reveal that the Notched run develops many of the expected MJO characteristics including large scale, organized structures that possess seasonal shifts, have the expected vertical zonal wind shifts, and propagate eastward during most strong episodes. The results suggest remnants from previous MJO episodes and extratropical feedbacks within the MJO time band are unnecessary for initial MJO development and hint that a recharge-discharge mechanism plays a role in MJO formation.