Boreal summer synoptic-scale waves over the western North Pacific in multi-model simulations

During boreal summer, vigorous synoptic-scale waves (SSWs), often evident as southeast-northwestward oriented wave trains, prevail over the western north Pacific (WNP). In spite of their active role for regional weather and climate, modeling studies on SSWs are rather limited. In this study, a comprehensive survey on climate model capability in representing the WNP SSWs is conducted by analyzing simulations from 27 recent general circulation models (GCMs). Results illustrate that it is greatly challenging for GCMs to realistically represent the observed SSWs over the WNP. Only two models out of the 27 GCMs generally well simulate both intensity and spatial pattern of the observed SSW mode. Plausible key processes for realistic simulations of SSW activity are further explored based on multi-model simulations. It is found that GCM skill in representing spatial pattern of the SSW over the WNP is highly correlated to model skill in simulating summer mean patterns of the low-level convergence associated with the WNP monsoon trough, and conversion from eddy available potential energy (EAPE) to eddy kinetic energy (EKE). Meanwhile, simulated SSW intensity is found to be highly correlated to amplitude of 850hPa vorticity, divergence, and conversion from EAPE to EKE over the WNP. All these analyses confirm the two main energy sources in sustaining the SSW over the WNP as previously suggested, namely, the barotropic conversions by both rotational and divergent low-level mean flow, and conversions from EAPE to EKE. Results also illustrate that the observed modulations of SSW activity by the Madden-Julian oscillation are able to be captured in model simulations.