Northward Propagation Mechanisms of the Asian Summer Monsoon in the ERA-Interim Renalysis and the SP-CCSM

The Asian Summer Monsoon (ASM) spans the region from the Arabian Sea to the western North Pacific Ocean. It is characterized by intermittent onset and break periods of precipitation that occur on intraseasonal timescales. Monsoon rain onset is associated with northward-propagating convective anomalies that originate near the equator and move northward at a speed of about 1 degree latitude per day. Northward propagation (NP) of monsoon rainfall is often (but not always) associated with the eastward-propagating Madden-Julian Oscillation (MJO), where the northward-propagating component results from the interaction of deep convection with the mean deep easterly shear.

Several theories have been proposed to explain the process of NP, including boundary layer convergence forced by convectively-induced barotropic vorticity anomalies and boundary layer moisture advection. SST anomalies and associated surface fluxes and gradients have also received considerable attention for their role in NP since it has been observed that ocean-atmosphere coupled general circulation models (CGCMs) tend to produce more realistic intraseasonal variability than their atmosphere-only counterparts. Questions remaining concerning which of these processes dominate NP and whether or not a given process is active over the entire remain.

In this study, we analyze each of the proposed NP mechanisms in 20 June-August rainy seasons in the ASM for both ERA-Interim data and SP-CCSM model output. SP-CCSM simulates many features of the ASM, and we wish to understand if the simulated NP occurs via the processes observed in the reanalysis data. We find that MJO-linked NP events originate on the trailing (westward) end of the MJO convective envelope, and are initially associated with boundary layer advection, and then the barotopic vorticity mechanism. SST-linked NP mechanisms are generally of secondary importance, but non-negligible. SP-CCSM simulates observed NP mechanisms with remarkable fidelity over most of the ASM domain, but NP is deficient over the Arabian Sea. Our analysis suggests that the weaker-than-observed easterly shear in the Arabian Sea is the culprit. Since SP-CCSM is adept at producing the spectrum of observed convection and wave types in the ASM region, this finding emphasizes that fact that GCMs must simulate both realistic mean states and variability in order to produce realistic monsoon precipitation.