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Poleward propagation of boreal summer intraseasonal oscillations in a coupled model: role of internal processes |
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| Authors: | R S Ajayamohan H Annamalai Jing-Jia Luo Jan Hafner Toshio Yamagata |
| Institution: | 1. Canadian Centre for Climate Modelling and Analysis, University of Victoria, P.O. Box 3065, STN CSC, Victoria, BC, V8W 3V6, Canada 2. International Pacific Research Center, University of Hawaii, Honolulu, USA 3. Frontier Research Centre for Global Change, Japan Agency for Marine-Earth Science and Technology, Yokohama, Japan 4. Department of Earth and Planetary Science, The University of Tokyo, Tokyo, Japan |
| Abstract: | The study compares the simulated poleward migration characteristics of boreal summer intraseasonal oscillations (BSISO) in a suite of coupled ocean?Catmospheric model sensitivity integrations. The sensitivity experiments are designed in such a manner to allow full coupling in specific ocean basins but forced by temporally varying monthly climatological sea surface temperature (SST) adopted from the fully coupled model control runs (ES10). While the local air?Csea interaction is suppressed in the tropical Indian Ocean and allowed in the other oceans in the ESdI run, it is suppressed in the tropical Pacific and allowed in the other oceans in the ESdP run. Our diagnostics show that the basic mean state in precipitation and easterly vertical shear as well as the BSISO properties remain unchanged due to either inclusion or exclusion of local air?Csea interaction. In the presence of realistic easterly vertical shear, the continuous emanation of Rossby waves from the equatorial convection is trapped over the monsoon region that enables the poleward propagation of BSISO anomalies in all the model sensitivity experiments. To explore the internal processes that maintain the tropospheric moisture anomalies ahead of BSISO precipitation anomalies, moisture and moist static energy budgets are performed. In all model experiments, advection of anomalous moisture by climatological winds anchors the moisture anomalies that in turn promote the northward migration of BSISO precipitation. While the results indicate the need for realistic simulation of all aspects of the basic state, our model results need to be taken with caution because in the ECHAM family of coupled models the internal variance at intraseasonal timescales is indeed very high, and therefore local air?Csea interactions may not play a pivotal role. |
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