The CMIP5 models projected a weakening of horizontal divergence over the Maritime Continent: the mechanism of the weakening is well understood by many previous studies, and the change is considered to be robust. In addition to the weakening, a wave-like pattern of stationary waves from East Asia to the North Pacific was projected. We found a close relationship between the changes in the divergence and in the waves: coupled models which predicted the former changes stronger projected the bigger amplitudes of the latter waves, and vice versa. This result suggests that the latter change was forced by the former robust change.
To clarify dynamical relationships among the two changes, we performed four different LBM experiments using the zonal mean basic state and zonally asymmetric thermal forcing in the tropics. For each experiment, we used the present/ future basic state and the present/future thermal forcing from the CMIP5 models. The acronym of each experiment is as follows: EXP01) present basic state and present thermal forcing, EXP02) present basic state and future thermal forcing, EXP03) future basic state and present thermal forcing, EXP04) future basic state and future thermal forcing. The differences between the future and present experiments (EXP04-EXP01) were similar to the changes projected by the CMIP5 models, although positions and amplitudes differed slightly. In addition, the EXP03 showed that the change in the basic state and the resultant weakening of horizontal divergence over the Maritime Continent explained most of the changes in the stationary wave. On the other hand, EXP02 showed that the change in thermal forcing accounted for the eastward shift of the stationary waves.
Additionally, we conducted storm track experiments with the LBM to investigate the role of transient eddy feedback on stationary wave changes. The results suggested that the feedback shifts the thermally forced stationary waves northeastward. This shift may explain the difference between the LBM experiments (EXP04-EXP01) and the CMIP5 future projection.
This study implies that the robust weakening of the tropical circulation due to global warming causes change in the mid-latitude circulation by inducing changes in stationary waves which propagate from the tropics to the mid-latitudes.
*This presentation is based on the following two papers.
Harada, M., S. Hirahara, S. Hagiya, H. Murai, Y. Oikawa, and S. Maeda, 2013: Intensification of the southern side of the Asian jet stream during the northern winter in CMIP5 models. SOLA, 9, 94-97, doi:10.2151/sola.2013-021.
Harada, M., S. Wakamatsu, S.Hirahara, H. Murai, Y. Oikawa, and S. Maeda, 2014:Impacts of slowed tropical circulation on winter stationary waves in East Asia and the North Pacific, SOLA, 10, 180-184, doi:10.2151/sola.2014-038.