探討西北太平洋暖季赤道羅士比波的特性

Characteristics of Equatorial Rossby Wave in western North Pacific during Warm Season

徐世裴(Shih-Pei Hsu)

指導教授 ： 隋中興

國立臺灣大學/理學院/大氣科學研究所/碩士(2015年)

https://doi.org/10.6342/NTU.2015.02488

摘要

赤道羅士比波(equatorial Rossby wave，ER)在西北太平洋(WNP)熱帶地區最活躍，WNP的ER活躍季為5月中到11月中。活躍季時，ER的振幅增強且訊號集中在北半球。本研究使用波數-頻率頻譜分析法找出ER訊號。無論對WNP的ER 850mb渦度場做EOF分析，或是分析伴隨熱帶擾動(以下簡稱擾動)的ER個案，皆可將WNP活躍季的ER歸納出兩種不同水平結構的類別(ER-I與ER-II)。
ER-I水平結構沿經度方向在7.5˚N有波狀的渦度中心分佈，西側為正渦度、東側為負渦度，在同樣經度範圍的17.5˚N附近亦有波狀結構，而且跟7.5˚N的渦度中心反號，17.5˚N的渦度中心伴隨較強的對流，其垂直結構為第一斜壓，上下層反號，南側伴隨的對流不明顯，其垂直結構為相當正壓。ER-II水平結構呈現東北-西南傾斜並拉長，伴隨較強的對流，且對流與正渦度幾乎同相位。ER渦度變異數分佈顯示兩個類別ER的主要活動區域從90˚E到160˚E，5˚N到25˚N，ER-II的振幅較ER-I大而且偏東。追蹤兩個類別ER的生命週期，發現兩個類別ER分別從生成到消散的水平結構皆無太大變化，僅振幅的增減。兩個類別ER對應的低頻場(30天以上)有低頻的西風距平(相較夏季氣候場)、正渦度距平以及緯向風輻合距平，而ER-II的低頻西風較ER-I強，因此ER-II的低頻緯向風輻合較強且其位置偏東。
我們使用多重尺度交互作用的ER動能收支，診斷兩個類別ER與低頻背景場的關係。將變數分成低頻場(LF)、ER以及剩餘的高頻場(HF)三個部分，推導出ER動能收支方程式，ER動能趨勢項可分成LF與ER交互作用的貢獻項以及與HF相關的貢獻項。聚焦於低對流層(850mb)，動能收支結果顯示兩個類別ER皆透過LF與ER交互作用的貢獻獲得動能，而獲得的大部分動能將傳遞給HF。LF與ER交互作用的貢獻項最主要的來源為正壓能量轉換項(BC)以及ER重力位通量輻合項(GF)。BC主要是透過低頻場緯向風輻合將ER動能累積，ER-II伴隨較強低頻場緯向風輻合，比ER-I增強快且區域比較偏東。另外，ER-II的水平結構具有東北-西南傾斜，搭配較強低頻緯向風切與ER波動uv通量,也是ER-II發展較強的主要貢獻項。而GF的主要貢獻來自ER風場水平平流ER重力位重新分配ER動能。

關鍵字

赤道羅士比波 ； 低頻背景場 ； ER動能趨勢項 ； 正壓能量轉換過程

並列摘要

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

並列關鍵字

Equatorial Rossby wave ； low frequency field ； ER kinetic energy tendency ； barotropic energy conversion

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