Department of Urban and Environmental Engineering (Environmental Science and Engineering)This thesis suggests dynamical mechanisms for the decadal change of the MJO teleconnection pattern over the Northern Hemisphere during boreal winter by investigating the influence of the decadal change of background states in the mid-latitude and tropical forcing on MJO teleconnection pattern and interpreting this relationship.
Thus, it is important to understand how these changes influence on MJO teleconnection pattern to represent the MJO teleconnection pattern caused by changes of background states and MJO activities over the tropics. In boreal winter, OLR variance with intraseasonal variability variance is strengthened over the Maritime Continent and weakened over the southern Indian Ocean and the western Pacific. This is consistent with the results of the MJO amplitude at each MJO phase based on OMI. These changes of MJO activity are associated with the La Nina-like change of background states in tropics. There is wet and warm anomaly over the Maritime Continent and the western Pacific, and ascending motion over the Maritime Continent, whereas dry, cold and descending anomaly over the central and the eastern Pacific. The location of MJO teleconnection patterns can be determined by seasonal mean upper-level zonal wind acted as a waveguide. The obvious change of the jet stream is found, especially over the eastern Pacific. Over this region, stationary Rossby wavenumber is expanded east and northward in the recent period than that in the past period. This change modifies the location and intensity of the teleconnection patterns based on the analysis of zonal wavenumber.
Based on the model experiments, the decadal change in the background states, especially zonal wind at the upper level, leads to strengthening the intensity of the MJO teleconnection pattern over the jet exit region but does not modify the pattern itself. On the other hand, the tropical heating modifies the teleconnection patterns over the North Pacific and North America, and these changes are similar to the observed difference maps between the two periods. Thus, the results indicate that the decadal change of the MJO teleconnection pattern is caused by the change of tropical diabatic heating rather than the change of zonal wind in the mid-latitude as background states. These changes are associated with the background states are changed into La Nina-like pattern in the recent period. This leads to moisture and SST increase over the warm pool region. In addition, the ascending motion of the vertical circulation in the background states strengthens over the eastern region of Maritime Continent, and then the convective anomaly becomes stronger over this region in the recent period than in the past period. It is thought that these background changes including the intensification of the vertical circulation lead to the enhancement of the MJO teleconnection pattern and its teleconnectivity over the upstream regions. Furthermore, there is close relationship of the MJO teleconnection patterns between P1 and El Nino years or P2 and La Nina years.clos
