Effects of subseasonal basic state changes on Rossby wave propagation during northern summer

Abstract

We investigated the impact of subseasonal basic-state changes on Rossby wave propagation between July and August. Our investigation was based on an experimental study that used a linear baroclinic model. Two sets of diabatic heating were imposed at the midlatitudes and the tropics. It was found that, by a principal component analysis, the atmospheric response to diabatic heating reaches a steady state on day 15. Thus, the response to midlatitude forcing reveals eastward Rossby wave propagation along the Asian jet, while that to tropical forcing shows a Gill-type response followed by propagation into the extratropics. The basic state differences between July and August are mainly revealed by thermal and dynamical effects: a strong land-sea thermal contrast and meridional shift of the Asian jet. With regard to midlatitude forcing, the Rossby wave response under the August basic state is higher than that under the July basic state. This can be attributed primarily to the dynamical effect of the basic state. On the other hand, the Rossby wave response to tropical forcing shows stronger amplitude under the July basic state than the August basic state. The response is caused mainly by both the thermal and dynamical effects of the basic state. In addition, the interdecadal change in the basic state induces a significant change in the Rossby wave propagation. The difference is most evident in the change to midlatitude forcing that is due to the dynamical effect of the basic state. Copyright 2011 by the American Geophysical Union.