Interannual variations of the boreal summer intraseasonal oscillation

Autor: Teng, Haiyan
Rok vydání: 2003
Druh dokumentu: Diplomová práce
Popis: A new diagnostic approach based on wavenumber-frequency spectral analysis was proposed for the boreal summer intraseasonal oscillation (ISO). The wave characteristics of the MJO, the northwestward-propagating ISO in the western North Pacific (WNP) and the northward-propagating ISO in the Indian Ocean are subjectively measured. The strongest interannual variations of the ISO are found in the off-equatorial western North Pacific. In summers when El Nifw is developing, both the westward and northward propagating waves with periods of 15-40 days and 8-10 days are enhanced in July-October. ENSO affects the northwestward-propagating ISO in the WNP by changing the mean vertical shear in the zonal wind. During July-October in El Nino developing years, the easterly vertical zonal-wind shears over the tropical western Pacific are considerably increased, in contrast to a westerly-verticaI-shear mean state in the La Nina years. The easterly vertical shear promotes development and northwestward emanation of Rossby waves away from the equatorial western-central Pacific in the lower troposphere, reinforcing the WNP ISO. The stronger WNP ISO in El Nino summers cools the local SST. The WNP SST drops under the climatology at the climatological wet phases in the warm summers, while it is sustained above the climatology in the cold summers. In the Indian summer monsoon region, the life cycle of the northward-propagating ISO was revealed by QuikSCAT and TRMM satellite data. The composite scenario suggests that the intraseasonal convective anomalies are first generated in the equatorial Indian Ocean between 60 E-70 E, associated with the low-level convergence and upper-level divergence at the monsoon's break phase. The anomalies then propagate eastward and intensify at 70 E-100 E about 10 days later. Thereafter, the northward propagation takes place in two steps: At the first step, Rossby waves are emanated from the equatorial convection and propagate northwestward to 15°N, 70-90 E in less than 10 days. There is also a Rossby-wave response center in the southern hemisphere. During the second step, the southeast-northwestward titled rainband moves northeastward to the inner land region and decays. The second step takes about 20 days. In order to measure the seasonal activity of the northward-propagating ISO, an index was constructed from 1948 to 2001 based on the 20-50-day wavenumberone northward propagating u850 anomalies averaged at 65°E-90°E. The weak-ISO years are characterized by isolated intraseasonal convection at the equator and 20 N, whereas the two regions are connected by the Rossby-wave-emanation process in the strong ISO years. The seasonal activity of the northward-propagating ISO is found significantly influenced by the mean convection in the equatorial eastern Indian Ocean (EEIO), which is the source for the Rossby-wave emanation. ENSO modifies the northwardpropagating ISO mainly through influencing the mean convection in the EEIO. The ISO is significantly weaker in El Nino developing May-Sept, and stronger in the La Nina summer. The seasonal activity of the northward-propagating ISO is not related to the overall performance of the Indian summer monsoon. Averaged rainfall at the 29 Indian subdivisions were examined from 1948 to 2001. During 1979-2001, two divisions in the southern Peninsular have a positive correlation with the ISO intensity, and 4 divisions in the west central India and central Northeast India have a negative correlation with the ISO intensity. This confirms the ISO-mean rainfall relationship revealed by the CMAP rainfall. But neither the All Indian rainfall index or the monsoon circulation index (Wang et al. 2001) has a significant correlation with the seasonal activity of the northward propagating ISO. The interannual variations of the MJO have a seasonal dependence. The May-Jul MJO is closely related with ENSO. At El Nino's developing stage, the MJO is enhanced. This is in a sharp contrast with the wintertime MJO, which is not sensitive to the ENSO condition.
xvi, 169 leaves
Databáze: Networked Digital Library of Theses & Dissertations
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