| Summary: | 碩士 === 國立臺灣大學 === 大氣科學研究所 === 103 === Equatorial Rossby wave (ER) is most active in western North Pacific (WNP) than in other tropical regions. Its active season starts from mid-May to mid-November when warm SST, cyclonic vorticity, zonal wind convergence and easterly vertical wind shear provide a preferable environment for ER to amplify in WNP. We perform an analysis of space-time filtered ER based on 10 years of data to extract ER signals. Two dominant types of ER are identified through an EOF analysis on 850mb ER vorticity and a subjective inspection of strong ER cases related to tropical disturbances.
The 850mb horizontal structure of type-I ER features a wave structure along 7.5°N and 17.5°N. The ER vorticity and divergence are out of phase between the two latitudes within the wave train. The amplitude of the northern center is larger than that of the southern center. Stronger convection appears in the northern side exhibiting a first baroclinic structure. The southern side is associated with weaker convection and an equivalent barotropic vertical structure. The 850mb horizontal structure of type-II ER shows a dominant southwest-northeast tilted wave pattern indicating an unstable ER. For both types of ER, the positive (negative) phase of vorticity is accompanied with convergence (divergence) and convection (suppressed convection).
The variance of vorticity for the two types of ER shows that they are most active within 90°E to 160°E and 5°N to 25°N. Type-II ER has a larger amplitude than that of type-I ER. Type-II ER is associated with stronger LF westerly wind anomaly (relative to summer climatology) than type-I ER; therefore LF zonal wind convergence is stronger and located more eastward for type-II ER.
ER kinetic energy (KE) budget of multi-scale interaction is used to discuss the relationship between the two types of ER and their low frequency background state (LF). To derive ER KE budget equation, all variables are decomposed into three bands: LF, ER and high frequency field (HF). The contribution to the tendency of ER in the ER band (KT) is separated into generation from LF-ER interaction (KTEL) and HF-related interaction (KTH). KT gains via KTEL and loses through KTH. Two predominant processes contributing to KTEL are barotropic energy conversion (BC) and eddy geopotential flux convergence (GF). BC is mainly contributed by an accumulation of ER kinetic energy through low-frequency zonal wind convergence. Thus ER-II amplifies in a broader zonal extent east of that of ER-I. In addition, the ER momentum flux uv in northeast-southwest tilted type-II ER waves converts low-frequency kinetic energy with zonal wind shear into ER kinetic energy.. GF has positive contribution to KTEL in lower troposphere
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