Potential Vorticity Accumulation Following Atmospheric Kelvin Waves in the Active Convective Region of the MJO

This study investigates the potential vorticity (PV) generated by convectively coupled atmospheric Kelvin waves (CCKW) that propagate through the local active convective phase of the Madden-Julian Oscillation (MJO). Lag composites are created to illustrate PV generation. The MJO is tracked using a modified real time multivariate MJO (RMM) index that uses outgoing longwave radiation (OLR) and zonal winds that have been filtered for the MJO in the zonal wavenumber frequency domain. This filtering reduces the influence of noise that often projects onto the standard RMM index. CCKWs are tracked using Kelvin filtered OLR. These data are used to select CCKW events that occur within each phase of the MJO. Lag composites are centered on the peak of CCKW convection. PV on an isentropic surface from the ECMWF interim reanalysis is used to track variations in PV associated with the waves.

An analysis of TRMM 3B42 rainfall estimates within these events reveals that most of the rainfall within the active MJO occurs within the convectively active region of the Kelvin wave. This observation is consistent with the notion that the Kelvin wave is the dominant source of PV in the MJO. Since most of the rainfall embedded in the Kelvin waves occurs in smaller scale higher frequency events, most PV in the MJO is created by relatively high frequency convection modulated by CCKWs. Once generated, the convection remains in the environment for the lifetime of the low level westerly phase of the MJO.