17:15 〜 18:45
[AAS03-P01] MJO simulated with a global kilometer-scale climate simulation: Implication for the cross-scale interaction
キーワード：全球雲解像気候シミュレーション、マッデン・ジュリアン振動、スケール間相互作用
The Madden–Julian oscillation (MJO) is the predominant tropical intraseasonal variability and is known to modulate global weather patterns. An accurate simulation of the MJO still remains a challenge for general circulation models (GCMs), and climate simulations with GCMs often struggle with the mean state-variability tradeoff. Global storm-resolving climate simulations made possible by a recent increase in computing power are expected to be useful for investigating this issue because of its merit of direct coupling between moist processes and dynamics.
The present study examines the MJO representation in an AMIP-type ~10-year simulation with the Nonhydrostatic Icosahedral Atmospheric Model (NICAM) at 3.5-km horizontal resolution, in comparison with that of a conventional GCM, MIROC6. The NICAM simulation successfully reproduces realistic initiation frequency, propagation, and hierarchical structure of MJO convection, as well as realistic mean states (e.g., mean tropical precipitation), whereas MIROC6 underestimates the number of robust MJO events, and the activity of westward-propagating synoptic-scale waves embedded within MJO convective envelopes. As specific processes, the enhanced mixed Rossby-gravity wave-like systems seem to be a precursor and building blocks of the MJO simulated with NICAM at least over the Indian Ocean, consistent with several observational studies. In addition, NICAM-MJO propagation into the western Pacific is supported by high-frequency intermittent advective moistening that can be triggered by the upper-tropospheric PV intrusion from the extratropics. This feature is also found in some observed MJO events.
Our results suggest that good MJO simulations can be attributed to the representation of the feedback from synoptic-scale waves to MJO convective envelopes, and that the extratropics sometimes plays an active role in MJO dynamics. A success in simulating these cross-scale interactions associated with the MJO is one of the advantages of kilometer-scale climate simulations without the assumption of a priori scale separation and quasi-equilibrium characteristics.
The present study examines the MJO representation in an AMIP-type ~10-year simulation with the Nonhydrostatic Icosahedral Atmospheric Model (NICAM) at 3.5-km horizontal resolution, in comparison with that of a conventional GCM, MIROC6. The NICAM simulation successfully reproduces realistic initiation frequency, propagation, and hierarchical structure of MJO convection, as well as realistic mean states (e.g., mean tropical precipitation), whereas MIROC6 underestimates the number of robust MJO events, and the activity of westward-propagating synoptic-scale waves embedded within MJO convective envelopes. As specific processes, the enhanced mixed Rossby-gravity wave-like systems seem to be a precursor and building blocks of the MJO simulated with NICAM at least over the Indian Ocean, consistent with several observational studies. In addition, NICAM-MJO propagation into the western Pacific is supported by high-frequency intermittent advective moistening that can be triggered by the upper-tropospheric PV intrusion from the extratropics. This feature is also found in some observed MJO events.
Our results suggest that good MJO simulations can be attributed to the representation of the feedback from synoptic-scale waves to MJO convective envelopes, and that the extratropics sometimes plays an active role in MJO dynamics. A success in simulating these cross-scale interactions associated with the MJO is one of the advantages of kilometer-scale climate simulations without the assumption of a priori scale separation and quasi-equilibrium characteristics.