Dependence of MJO Phase Speed on Background Flow

Three lines of evidence are presented in support of the view that a large fraction of the variance in the phase speeds of Madden Julian oscillation (MJO) events is governed by advection by the background circulation in which anomalous MJO convection and circulation are embedded. First, regression of fields of data against outgoing longwave radiation (OLR) signals at zonal wavenumber 2 filtered for a variety of phase speeds via the space-time wavelet transform shows the preferred spatial/temporal structures of the MJO at those phase speeds. Reconstruction of the momentum budget terms regressed against indexes wavelet filtered for the different phase speeds suggests that advection of and by the background wind in regions of stronger upper tropospheric background flow is associated with faster eastward propagation. Second, a new algorithm of lag regression is applied, relating OLR anomalies in the equatorial plane to a rotated RMM index, to analyze the relationship between MJO phase speed and the background circulation. In standard linear regression, the slope coefficient is the ratio between the covariance between the predictor and the predictand to the variance of the predictor. In the new algorithm, these variance and covariance quantities are modeled as they respond to background wind signals in different parts of the world, yielding regression slope coefficients between the RMM index and field variables as the relationship varies with the background flow. The result demonstrates how MJO phase speed varies with these background quantities. Finally, third, simple observations of the background wind associated with individual MJO events that are fast or slow are consistent with the general conclusions: Anomalous background westerly winds in the upper troposphere near the longitudes of the convection but over a broad range of latitudes in the tropics are associated with more rapid eastward propagation while anomalous easterly wind in the same region is associated with slow propagation. Results suggest that the slowest MJO convective events over the Pacific basin occur to the east of exit regions in the background jet stream when the regional base states continue to favor convection.

This analysis is part of broader study of the association of the global atmospheric circulation with the phase speed of MJO convection.