Atmospheric Overturning across Multiple Scales of an MJO Event during the CINDY/DYNAMO Campaign

Abstract

Abstract This study investigates the atmospheric overturning of the October 2011 MJO event observed during the Cooperative Indian Ocean Experiment on Intraseasonal Variability in the Year 2011 (CINDY)/DYNAMO field experiment using a cloud-permitting numerical model. The isentropic analysis is used to sort the vertical mass transport in terms of the equivalent potential temperature of the air parcels, which naturally decomposes the atmospheric overturning between ascending air with high entropy and subsiding air with low entropy. The circulation is further decomposed into contributions of four main scales: basinwide ascent, meridional overturning, regional overturning, and convection. Results show that the convective scale dominates the upward mass transport while larger scales play an important role both by allowing a deeper overturning and by modulating convective activity. There are substantial changes in the atmospheric overturning during different phases of this MJO event. Increased convective activity at low levels precedes the onset of the MJO by several days. The initiation of the MJO itself is associated with a substantial increase in the atmospheric overturning over the Indian Ocean. The subsequent eastward propagation of the MJO event can be clearly captured by the evolutions of convective-scale vertical mass fluxes at different altitudes. The equivalent potential temperatures of the rising and subsiding air parcels in the convective-scale overturning are also increased in the troposphere during the active phase of the MJO.