Model performance metrics and process diagnostics for boreal summer intraseasonal variability

Abstract

Representation of the boreal summer intraseasonal oscillations (BSISO) is evaluated in the 20-year climate simulations from 27 general circulation models (GCMs), produced as part of a global multi-model evaluation project coordinated to study the vertical structure and physical processes of the Madden–Julian oscillation (MJO). Model performance metrics are developed to assess the simulated BSISO characteristics, with a special focus on its northward propagation over the Asian monsoon domain. Several process-oriented diagnostics developed by the MJO community are also tested for the BSISO. Simulating the phase speed and meridional extent of BSISO northward propagation, the northwest–southeast tilted rain-band structure and the quasi-biweekly mode are identified as some of the persisting problems for many GCMs. Interestingly, many of the GCMs, which capture BSISO eastward propagation, also show good fidelity in simulating BSISO northward propagation. Meridional vertical profiles of anomalous wind, temperature and diabatic heating of BSISO are better simulated in the GCMs that simulate the northward propagation. Process-oriented diagnostics based on seasonal mean vertical shear of zonal and meridional wind, large-scale rain fraction and relative humidity are also examined, but it still remains challenge to find a process diagnostic which is strongly linked to BSISO northward propagation. The complex spatial structure and presence of multi-scale disturbances, demand the development of more focused GCM evaluation metrics and process diagnostics specifically for the BSISO.